>>Department of Ecological Modelling >> Personal homepage Thorsten Wiegand >> Abstracts
A simulation model for a shrub ecosystem in the semiarid Karoo, South Africa
Wiegand, T., S. J. Milton, C. Wissel. 1995. Ecology 6:2205-2211
Plant community dynamics in semi-arid regions appear to be "event-driven'". The aim of our model is to attain an understanding of the main processes determining the spatial and temporal dynamics of a shrub community in the semi-arid Karoo on a large temporal scale and to identify the significant events which drive this shrub community. Vegetation at the study site covers 15-20% of the soil surface and is dominated by five shrubs: Brownanthus ciliatus (Mesembryanthemoideae), Ruschia spinosa (Ruschioideae), Galenia fruticosa (Aizoaceae), Pteronia pallens (Asteraceae) and Osteospermum sinuatum (Asteraceae). Grasses and annuals play little part in the dynamics of this vegetation. The model is based on detailed life-history data for the five dominant species and on monthly long-term rainfall data for this region. The method of "dynamic automata" is employed to model individual plants. Growth, death, seed production, germination and seedling establishment are modelled over long time scales in annual time-steps under the influence of the stochastic and unpredictable rainfall in ungrazed rangeland. In the absence of grazing, survival of seedlings depends on their competitive ability during the seedling stage and their ability to compete with established plants in neighboring cells.
The model shows that the dynamics of this shrub community are typified by episodic and discontinuous changes in species composition with intervening quasistable phases lasting some decades. The reason for this episodic behaviour is that both recruitment and mortality of plants depend on particular conditions: Using 93 complete data sets with monthly rainfall data taken at the weather station in Prince Albert we show that rainfall is only sufficient for seedling recruitment in 44% of all years for B. ciliatus and in less than 30% of all years for the four other species. We identify two types of abrupt and discontinuous changes in species composition: (1) big recruitment events which can only occur if plant density is low and if rainfall conditions are extraordinarilly favourable and (2) big mortality events which can only occur if cohorts, originating from big recruitment events, die within a short period of time and if little further recruitment has taken place. However, this behavior is not a property of the biota but is generated by rainfall input to the model. By using a different rainfall input, the model can also display regular cyclic succession. Therefore there appeared to be no contradiction between sudden, discontinuous changes and gradual, continuous and reversible changes in vegetation composition.
Vegetation change in semiarid communities: simulating
probabilities and time scales
In arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event-driven dynamic behavior, unpredictable and low rainfall and complicated interactions between species make it difficult to assess probabilities and time scales of vegetation change. To gain a better understanding of the main processes and mechanisms involved in vegetation change, we have developed a spatially explicit individual based model that simulates changes in plant communities over long time spans. The model, based on life-history attributes of the five dominant component plant species of a typical Karoo shrub community, follows the fate of each individual plant within the community, the sum of which is community dynamics. The model explores the differential effects of a realistic range of rainfall pattern on the abilities of these species to compete, survive, grow and reproduce. The specific aim of the model is to identify key processes of vegetation change and to calculate probabilities and timespans for transitions between different vegetation states. Such knowledge is needed for species conservation and sustained animal production.
We show that the time-scale for changes of the dynamic state of the system are long compared with human lifespans. Employing the full range of possible rainfall scenarios showed that short-term community dynamics (years to decades) and species composition depend strongly on the short-term (years) sequence of rainfall events. In all simulation experiments the final vegetation state varied by more than 37% after a 60 year simulation period. Simulating resting of an overgrazed part of the shrub community indicated that little improvement in rangland condition was likely during a period of 60 years. Even such active management, as (simulated) clearing of unpalatable shrubs, resulted in only a 66% probability that degraded shrubland would be in good condition after 60 years resting. Simulated overgrazing of a rangeland in good initial condition only became obvious 40 or 50 years after the initiation of heavy grazing, and after 70 years the mean vegetation state eventually reached that of an overgrazed rangeland.
Simulated plant population responses to small scale
disturbance in semi-arid shrublands
We used a spatially explicit simulation model to examine the impact of small scale disturbance on the temporal and spatial dynamics of a typical Karoo shrub plant community, and to gather insight into the interplay between disturbance architecture and population dynamics. Establishment, growth, mortality, seed dispersal and competitive interactions were modelled over long time scales in annual time-steps under the influence of stochastic and unpredictable rainfall. Disturbances were created by the digging of aardvarks (Orycteropus afer) and bat-eared foxes (Otocyon megalotis) or by humans as management action.
We examined the impact of three different disturbance regimes on long-term population dynamics by varying the type, rate and size of the small scale disturbances. The impact that a disturbance regime has on long-term community dynamics depends on complex interactions between disturbance characteristics and life-history attributes of component species. Plant density decreased with overall disturbance rates; this effect was independent of the type of disturbance. A given type and rate of disturbance did not influence all species within a guild (e.g. coloniser species) in the same way. The reason for these differences was that species were responding not only to the disturbance but to changes in competition intensity from other species and changes in their reproductive potential relative to other species. Such interactions resulted in a sequential change in dominant species within guilds as disturbance rates increased. An increase in the overall disturbance rate did not always produce the trend in evenness expected from the intermediate disturbance hypothesis, but was influenced by the relative abundance of different types of disturbances.
Population dynamics, disturbance, and pattern evolution:
identifying the fundamental scales of organization in a model ecosystem.
We used auto- and crosscorrelation analysis and L function analysis to analyze spatio-temporal pattern evolution in a spatially explicit simulation model of a semiarid shrubland (Karoo, South Africa), and to determine the impact of small-scale disturbances on system dynamics. Without disturbance, local dynamics were driven by a pattern of cyclic succession, where “colonizer” and “successor” species alternately replaced each other. This results in a strong pattern of negative correlation in the temporal distribution of colonizer and successor species. As disturbance rates were increased, the relationship shifted from being negatively correlated in time to being positively correlated — the dynamics became decoupled from the ecologically-driven cyclic succession and were increasingly influenced by abiotic factors (e.g., rainfall events). Further analysis of the spatial relationships among colonizer and successor species showed that without disturbance periods of attraction and repulsion between colonizer and successor species alternate cyclically at intermediate spatial scales. This was due to the spatial “memory” embedded in the system through the process of cyclic succession. With the addition of disturbance, this pattern breaks down, although there is some indication of increasing ecological organization at broader spatial scales. We suggest that many of the insights that can be gained through spatially explicit models will only be obtained through a direct analysis of the spatial patterns produced.
Key words: individual-based simulation model, disturbance, Karoo shrubland, Ripley’s L-Analysis, spatio-temporal dynamics, time-series correlation analysis.
The status of the brown bear (Ursus arctos) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, Northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management. The specially constructed simulation model, based on long-term field investigations on the western brown bear population in the Cordillera Cantabrica, includes detailed life history data and information on environmental variations in food abundance. The method of individual-based modeling is employed to simulate the fate of individual bears. Reproduction, family breakup, and mortalities are modeled in annual time steps under the influence of environmental variations in food abundance, mortality rates and reproductive parameters. In parallel, we develop an analytical model that describes the mean behavior of the population and that enables us to perform a detailed sensitivity analysis.
We determine current population parameters by iterating the model with plausible values and compare simulation results with the 1982-1995 time pattern of observed number of females with cubs of the years. Our results indicate that the population suffered a mean annual decrease of ~4-5% during the study period 1982-1995. We found that this decrease could be explained by a coincidence of high poaching pressure with a series of climatically unfavorable years during the period 1982-1988. Thereafter, population size probably stabilized. We estimated that the population currently consist of 25 or 26 independent females and a total of 50-60 individuals. However, our viability analysis showed that the population does not satisfy the criterion of a minimum viable population if mortalities remain at the level of the last few years of 1988-1995. The "salvation" of at least one independent female every three years is required. The population retains relatively high reproductive parameters, indicating good nutritive conditions of the habitat, but mortality rates are higher than those known in other brown bear populations. The most sensitive parameters, adult and subadult mortality of females, form the principal management target. Our model shows that the series of females with cubs contains valuable information on the state of the population. We recommend monitoring of females with cubs as the most important management action, both for collecting data and for safeguarding the most sensitive part of the population.
In semi-arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event driven dynamic behaviour, unpredictable and low rainfall and complicated interactions between species make it difficult to gather sufficient understanding of vegetation dynamics to be able to develop guidelines for sustainable management. Simulation models that consider the essential processes that determine vegetation dynamics offer scope for quantitatively exploring long-term vegetation dynamics of arid and semi-arid rangelands. In this paper we review three models that were aimed to provide an understanding of the vegetation dynamics and management of different typical vegetation types in South Africa; including the Karoo shrubland, the shrub-grassland of the southern Kalahari, and pure semi-arid grasslands.
We compile and exemplify a general modeling framework that allows for a systematic investigation of the impact of changes in landscape structure on population dynamics. The essential parts of the framework are a landscape generator with independent control over landscape composition and physiognomy, an individual-based spatially explicit population model that simulates population dynamics within heterogeneous landscapes, and scale-dependent landscape indices that depict the essential aspects of landscape which interact with dispersal and demographic processes. Landscape maps are represented by a grid of 50 x 50 cells and consist of good-quality, poor-quality or (inhabitable) matrix habitat cells. The population model was shaped in accordance to the biology of European brown bears (Ursus arctos), and demographic parameters were adjusted to yield a source-sink configuration. Results obtained with the spatially explicit model do not confirm results of earlier non-spatial source-sink models where addition of sink habitat resulted in a decrease of total population size because of dilution of high-quality habitat. Our landscape indices, that describe scale-dependent correlation between and within habitat types, were able to explain variations in variables of population dynamics (mean no. of females with sink home-ranges, the mean no. of females with source home-ranges, and mean dispersal distance) due to different landscape structure. When landscape structure changed, changes in these variables generally followed the corresponding change of an appropriate landscape index in a linear way. Our general approach incorporates source-sink dynamics as well as metapopulation dynamics, and the population model can easily be modified for other species groups.
Live fast, die young: estimating size-age relations
and mortality pattern of shrubs species in the semi-arid Karoo, South Africa.
We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling.
Rule-based assessment of suitable habitat and patch
connectivity for the Eurasian lynx in Germany
Conservation biologists often have to take decisions for which little information derived from detailed field studies is available. The urgency of most conservation issues and scarcity of financial resources make it impossible to obtain empirical data, but rather require that conservationists make the best out of the existing information to improve current management. In Germany lynx recovery and reintroduction initiatives are controversially discussed, one concern being that there is not enough suitable habitat left to support a viable population. The extent and the spatial arrangement of potentially suitable habitat is unknown and almost no data are available to address this question. The aim of this study was to predict the location of potentially suitable habitat for lynx in Germany, to obtain rough estimates of the potential maximum number of resident lynx, and to estimate connectivity between patches of suitable habitat. In a rule-based model habitat preferences of lynx were described as availability of forest cover, defined by patch size, degree of fragmentation and spatial structure. The model rules were implemented on a Geographic Information System (GIS) to produce a map showing the resulting patches of suitable habitat. Optimal corridors between patches were modeled with a cost-path analysis, based upon habitat type specific probabilities for lynx to cross. Our results show that patches of suitable habitat vary greatly in size, with 10 areas large enough to sustain more than 20 resident lynx. Overall, the 10 areas could sustain about 380 resident lynx. Uncertainty analyses of various critical model assumptions and parameters show that the predictions of our model are more or less stable, mainly due to the strong constraining impact of forest distribution. Our analyses suggest that reintroduction programs should focus less on small and isolated areas, but should include large scale connectivity into the decision making process. Our model is an example of a cross-boundary analysis of landscape structure based upon the needs of a species with large spatial requirements, an approach suitable as a basis for co-ordinating reintroduction initiatives and focus conservation efforts on the most promising areas.
Key words: Eurasian lynx, Lynx lynx, rule-based model, predictive
habitat model, patch connectivity, GIS, costpath-analysis, limited resources,
large-scale approach, decision making process, conservation, species reintroduction.
Assessing the suitability of
central European landscapes for the reintroduction of Eurasian lynx.
1. After an absence of almost 100 years the Eurasian lynx Lynx lynx is slowly recovering in Germany along the German-Czech border. Additionally, many reintroduction schemes have been discussed controversially in various locations. We present a habitat suitability model for lynx in this region as a basis for further management and conservation efforts aimed at recolonisation and population development.
2. We developed a statistical habitat model using logistic regression to quantify the factors that describe lynx home ranges in a fragmented landscape. As no data were available for lynx distribution in Germany, we used data from the Swiss Jura Mountains for model development and validated the habitat model with telemetry data from the Czech Republic and Slovenia. We derived several variables describing land use and fragmentation, introducing also variables that described the connectivity of forested and non-forested semi-natural areas on a larger scale than the map resolution.
3. We obtained a model with only one significant variable that described the connectivity of forested and non-forested semi-natural areas on a scale of about 80 km². This result is biologically meaningful, reflecting the absence of intensive human land use on the scale of an average female home range. Model testing at a cut level P > 0.5 correctly classified more than 80% of the Czech and Slovenian telemetry location data of resident lynx. Application of the model to Germany showed that the most suitable habitats for lynx were large forested low mountain ranges and the large forests in east Germany.
4. Our approach illustrates how information on habitat fragmentation on a large-scale can be linked with local data to the potential benefit of lynx conservation in central Europe. Spatially-explicit models like ours can form a basis for further assessing population viability of species of conservation concern in suitable patches.
Key-words: GIS, large-scale approach,
logistic regression, Lynx lynx, spatially explicit connectivity index, species
reintroduction, statistical habitat model.Key-words: GIS, large-scale approach,
logistic regression, Lynx lynx, spatially explicit connectivity index, species
reintroduction, statistical habitat model.
We suggest that the conscious use of information that is “hidden” in distinct structures in nature itself and in data extracted from nature (= pattern) during the process of modeling (= pattern-oriented modeling) can substantially improve models in ecological application and conservation. Observed patterns, such as time-series patterns and spatial patterns of presence/absence in habitat patches, contain a great deal of data on scales, site-history, parameters and processes. Use of these data provides criteria for aggregating the biological information in the model, relates the model explicitly to the relevant scales of the system, facilitates the use of helpful techniques of indirect parameter estimation with independent data, and helps detect underlying ecological processes. Additionally, pattern-oriented models produce comparative predictions that can be tested in the field.
We developed a step-by-step protocol for pattern-oriented modeling and illustrate the potential of this protocol by discussing three pattern-oriented population models: (1) a population viability analysis for brown bears (Ursus arctos) in northern Spain using time-series data on females with cubs of the year to adjust unknown model parameters; (2) a savanna model for detecting underlying ecological processes from spatial patterns of tree distribution; and (3) the incidence function model of metapopulation dynamics as an example of process integration and model generalization.
We conclude that using the pattern-oriented approach to its full potential will require a major paradigm shift in the strategies of modeling and data collection, and we argue that more emphasis must be placed on observing and documenting relevant patterns in addition to attempts to obtain direct estimates of model parameters.
Keywords: Brown bear; conservation biology; ecological application;
error propagation; incidence function model; indirect parameter adjustment;
Kalahari; pattern-oriented modeling; population models; savanna, uncertainty,
Endangered species balancing between natural and human constrains: the case of
brown bears (Ursus arctos) in northern Spain
We developed a conceptual framework for classifying habitat quality that requires the construction of separate habitat models for each demographic key-feature, to be applied when the factors that determine different demographic processes differ substantially. For example, survival of large carnivores is mainly determined by human-induced mortality, while nutritional condition determines the reproductive rate. Hence, a two-dimensional habitat model built for reproduction and survival yields five hypothetical habitat categories: matrix (no reproduction and/or very high mortality), sink (low reproduction, high mortality), refuge (low reproduction, low mortality), attractive sink (high reproduction, high mortality) and source (high reproduction, low mortality). We applied this framework to two endangered brown bear sub-populations in the Cantabrian Mountains, Spain. Our aim was to generate working hypotheses on the quality and the spatial arrangement of bear habitat for analyzing the present conditions of the different population nuclei, for identifying core areas of high conservation value, conflictive areas, or areas with unoccupied potential habitat. We used a geographic information system (GIS) and two spatial long-term data sets on presence and reproduction, and performed logistic regressions for building a two-dimensional habitat model. The analysis reveals that both populations seem to exist under sub-optimal habitat conditions: the eastern population occupies mainly areas of sub-optimal natural habitat and relatively low human impact, while the western population is mainly located in areas with high human impact, but otherwise good natural quality. To test hypotheses on demographic features of the obtained habitat categories we classified data on historic extinction in the north of Spain (14th to 19th centuries) within the two-dimensional model. Extinction probabilities within each habitat category confirmed the hypotheses: most extinctions occurred in matrix habitat, and the least in source habitat.
Keywords: attractive sink; endangered species; extinction; habitat model; human impact; logistic regression; matrix; refuge; source-sink theory; Ursus arctos
Expansion of brown bears (Ursus
arctos) into the eastern Alps: a spatially explicit population model.
We present a spatially-explicit population model for analysing the expansion of brown bears (Ursus arctos) after the reintroduction program in central Austria. The model is based on field investigations into brown bears in Austria and Slovenia and on current knowledge of brown bears. The landscape of the eastern Alps is represented by a GIS-derived raster map defining local habitat suitability and five major spatial barriers to dispersal. The population model follows the fate of individual bears and simulates reproduction, dispersal, home range establishment, and mortality in annual time steps. We indirectly adjust unknown or uncertain model parameters with 10-year data on the number of females with cubs in central Austria and determine key variables of population dynamics, such as population sizes and growth rates within different population nuclei, dispersal distances, or mortality rates, for model parameterisations that reproduces the data on females with cubs.
We estimated a current (1996-2000) growth rate of the population in Austria and adjacent parts of Italy of some 14%; a high proportion of this growth was due to immigration from Slovenia. Consequently, the growth rate of the subpopulation in central Austria, which probably is isolated functionally (i.e., no exchange of females) from the nuclei along the Austrian-Slovenian border, yielded some 7%. This subpopulation may comprise seven residents, and we estimated for females a 33% risk of extinction during the 1992-2000 period. Validation and confirmation of our model results with data on bear densities that were not used for model construction and parameterization supported our findings. The high female mortality rates together with the vulnerability of the small population to chance events (i.e., demographic stochasticity) are the most pressing threat for the population in the eastern Alps. Our approach could be widely applied for analysing dynamics of rare and endangered species in which the paucity of data precludes an appraisal of the state of the population using standard methods
It has been argued that spatially explicit population models (SEPMs) cannot provide reliable guidance for conservation biology because of the difficulty of obtaining direct estimates for their demographic and dispersal parameters and because of error propagation. We argue that appropriate model calibration procedures can access additional sources of information, compensating the lack of direct parameter estimates. Our objective is to show how model calibration using population-level data can facilitate the construction of SEPMs that produce reliable predictions for conservation even when direct parameter estimates are inadequate. We constructed a spatially explicit and individual based population model for the dynamics of brown bears (Ursus arctos) after a reintroduction program in Austria. To calibrate the model we developed a procedure that compared the simulated population dynamics with distinct features of the known population dynamics (= patterns). This procedure detected model parameterizations that did not reproduce the known dynamics. Global sensitivity analysis of the uncalibrated model revealed high uncertainty in most model predictions due to large parameter uncertainties (coefficients of variation CV » 0.8). However, the calibrated model yielded predictions with considerably reduced uncertainty (CV » 0.2). A pattern or a combination of various patterns that embed information on the entire model dynamics can reduce the uncertainty in model predictions, and the application of different patterns with high information content yields the same model predictions. In contrast, a pattern that does not embed information on the entire population dynamics (e.g., bear observations taken from sub-areas of the study area), does not reduce uncertainty in model predictions. Because population-level data for defining (multiple) patterns are often available our approach could be applied widely.
Keywords: individual-based model; model calibration; pattern-oriented modeling; population dynamics; spatially explicit population model; Ursus arctos; uncertainty
We analyzed data sets on phytomass production, basal cover, and monthly
precipitation of a semiarid grassland in South Africa for good, medium, and poor
rangeland condition (1) to investigate whether phytomass production per unit of
basal cover differed among rangeland conditions, (2) to quantify the time-scales
of a carry-over effect from production in previous months, and (3) to construct
predictive models for monthly phytomass. Finally, we applied the best models to
a 73-year data set of monthly precipitation data to study the long-term
variability of grassland production.
Rings, Circles and
Null-Models for Point Pattern Analysis in Ecology
We used a spatially explicit population model that was
generalized to produce nine ecological profiles of long-lived species with
stable home ranges and natal dispersal to investigate the effects of habitat
loss and fragmentation on population dynamics. We simulated population dynamics
in landscapes composed of three habitat types (good-quality habitat ranging from
10-25%, poor-quality habitat ranging from 10-70%, and matrix). Landscape
structures varied from highly fragmented to completely contiguous. The specific
aims of our model were (1) to investigate under which biological circumstances
the traditional approach of using two types only (habitat and matrix) failed and
assess the potential impact of restoring matrix to poorquality habitat, (2) to
investigate how much of the variation in population size was explained by
landscape composition alone and which key attributes of landscape structure can
serve as predictors of population response, and (3) to estimate the maximum
fragmentation effects expressed in equivalent pure loss of goodquality habitat.
Poor-quality habitat mattered most in situations when it was generally not
considered (i.e., for metapopulations or spatially structured populations when
it provides dispersal habitat). Population size increased up to 3 times after
restoring matrix to poor-quality habitat. Overall, habitat amount accounted for
68% of the variation in population size, whereas ecological profile and
fragmentation accounted for approximately 13% each. The maximal effect of (good-quality)
habitat fragmentation was equivalent to a pure loss of up to 15% of good-quality
habitat, and the maximal loss of individuals resulting from maximal
fragmentation reached 80%. Abundant dispersal habitat and sufficiently large
dispersal potential, however, resulted in functionally connected landscapes, and
maximal fragmentation had no effect at all. Our findings suggest that predicting
fragmentation effects requires a good understanding of the biology and habitat
use of the species in question and that the uniqueness of species and the
landscapes in which they live confound simple analysis.
Fragmented landscapes, road mortality and patch
connectivity: modeling dispersal for the Eurasian lynx in Germany
1. Although many reintroduction schemes for the Eurasian lynx Lynx lynx in Germany have been discussed, the implications of connectivity between suitable patches have not been assessed.
2. We introduce an individual-based, spatially explicit dispersal model to assess the probability of a dispersing animal reaching another suitable patch in the complex heterogeneous German landscape, with its dense transport system. The dispersal model was calibrated using telemetric data from the Swiss Jura and based on a map of potential lynx dispersal habitat.
3. Most suitable patches could be interconnected by movements of dispersing lynx within 10 years of reintroduction. However, when realistic levels of mortality risks on roads were applied, most patches become isolated except along the German–Czech border. Consequently, patch connectivity is limited not so much by the distribution of dispersal habitat but by the high mortality of dispersing lynx. Accordingly, rather than solely investing in habitat restoration, management efforts should try to reduce road mortality.
4. Synthesis and applications. Our approach illustrates how spatially explicit dispersal models can guide conservation efforts and reintroduction programmes even where data are scarce. Clear limits imposed by substantial road mortality will affect dispersing lynx as well as other large carnivores, unless offset by careful road-crossing management or by the careful selection of release points in reintroduction programmes.
Key-words: conservation, large carnivores, large-scale approach, Lynx lynx, movement, spatially explicit individual-based model, species reintroduction
*'This is an electronic version of an article published in Journal of Applied Ecology: complete citation information for the final version of the paper, as published in the print edition of Journal of Applied Ecology, is available on the Blackwell Synergy online delivery service, accessible via the journal's website at http://www.blackwellpublishing.com/journals/jpe or http://www.blackwell-synergy.com.
Effects of matrix heterogeneity on animal dispersal: from
individual behavior to metapopulation-level parameters.
Key-words: autocorrelated random walk, Lynx pardinus, matrix fragmentation, individual-based spatially explicit simulations, interpatch connectivity, standard of plausibility.
Rhodes, J. R., T. Wiegand, C. A. McAlpine, J. Callaghan, D. Lunney, M. Bowen, and H. P. Possingham. 2006. Conservation Biology 20:449-459
Species distribution models are commonly used to inform landscape and conservation planning. In urban and semiurban landscapes, the distributions of species are often determined by a combination of natural habitat and anthropogenic impacts. Understanding the spatial influence of these two processes is crucial for making spatially explicit decisions about conservation actions. We present a logistic regression model for the distribution of koalas (Phascolarctos cinereus), in a semiurban landscape in eastern Australia, that explicitly separates the effect of natural habitat quality and anthropogenic impacts on koala distributions. We achieved this by comparing the predicted distributions with the predicted distributions assuming anthropogenic variables are fixed at their mean values. Similar approaches have relied on making predictions assuming anthropogenic variables are zero, which will be unreliable if the training data set does not include anthropogenic variables close to zero. Our approach is novel because it can be applied to landscapes where anthropogenic variables are never close to zero. Our model showed that, averaged across the study area, natural habitat was the main determinant of koala presence. However, at a local scale, anthropogenic impacts could be more important, with consequent implications for conservation planning. We demonstrate that, by using this modeling approach and presenting predictions visually as a map, provides important information for making decisions on how different conservation actions should be spatially allocated. This approach is particularly useful for areas where wildlife and human populations exist in close proximity.
Key words: anthropogenic impacts, conservation planning, logistic regression, natural habitat quality, mixed effects model, spatial distribution model
Kramer-Schadt, S.,.E. Revilla, and T. Wiegand. 2005. Biological Conservation 125: 169-182
Keywords: large carnivores, Lynx lynx L., mortality scenarios, population viability analysis, spatially explicit individual-based model, species reintroduction
D. Vezzani, D., A. Rubio, S. M. Velazquez, N. Schweigmann and T. Wiegand. 2005. Acta Tropica 95: 123-131
Little information is available on the ecology of Aedes aegypti Linnaeus at the southern extreme of its distribution (Buenos Aires, Argentina), particularly on microhabitat suitability. The aim of our study was to identify at a detailed scale, microhabitat factors that correlate with the presence of preimaginal stages of the mosquito. In March 2001 we performed a spatial census of all containers located in a 1-ha patch within a cemetery in Buenos Aires City. On a reference map (1:700) we plotted the position of graves and surrounding corridors, the location of containers, the shade projected by each plant between 10:00 and 16:00 hours and vegetation cover. We classified vegetation by height, substrate by composition and shadow by level of exposure to sunlight. We performed univariate and multivariate logistic regression analyses with 9 constructed independent variables, some of them at scales of 0.5, 1, 2, 3, and 10 m. Of 850 receptacles examined, 101 contained preimaginal stages of Ae. aegypti. Level of exposure to sunlight, type of substratum, vegetation height and distance of containers to vegetation were significantly associated with the presence of breeding sites at the studied scales. Final multivariate models were significant at scales of 2 (c2(3)=25.693, p<0.001) and 3 m (c2(3)=26.440, p<0.001), and 65.9% and 66.8% of our data were correctly classified respectively for each scale. Our results suggest that sites less exposed to sunlight, with taller and closer vegetation, and in shaded and vegetated neighbourhoods were the most favourable microhabitats for Ae. aegypti breeding.
Key words: mosquitoes - Aedes aegypti - microhabitat suitability - vegetation - shade - statistical habitat model - Argentina
Riginos, C., S. J. Milton, and T. Wiegand. 2005. Journal of Vegetation Science 16:331-340
Question: In semi-arid systems, rainfall gradients can cause plant-plant interactions to shift from negative to positive or vice versa. However, the importance of a second major abiotic factor, soil nutrients, has rarely been considered. We consider different combinations of both factors and ask: do net adult-seedling interactions become less competitive and more facilitative with increasing overall abiotic harshness?
Location: Succulent Karoo, Western Cape, South Africa.
Methods: We examined the interactions between seedlings and adult shrubs at two sites. Sites differ in rainfall, and each contain two habitats: nutrient-rich mounds associated with underground termitaria and a relatively nutrient-poor matrix. We carried out a spatial pattern analysis of community-wide seedling-adult associations. We then conducted field and greenhouse experiments to test the effects of soil and the presence of neighboring shrubs on the growth and survival of six seedling species.
Results: At the higher rainfall site, both competitive and facilitative effects of adults on seedlings were found but did not differ by habitat, despite the more benign conditions in the mound habitat. At the lower rainfall site, adult shrubs generally had neutral effects on seedlings in the matrix habitat. In the nutrient-rich mound habitat, however, adult shrubs had strong and consistently competitive effects on seedlings.
Conclusion: Seedling-adult interactions could not be predicted by a simple overall gradient of abiotic harshness, demonstrating the need for more complex, mechanistic models to predict plant-plant interactions. We suggest that rainfall and soil nutrients affect seedling-adult relations through their interactive effects on the life-history attributes of the species involved.
Key words: Competition; facilitation; nurse-plant effect; point pattern analysis; Succulent Karoo
Volker Grimm, Eloy Revilla, Uta Berger, Florian Jeltsch, Wolf M. Mooij, Steven F. Railsback, Hans-Hermann Thulke, Jacob Weiner, Thorsten Wiegand, Donald L. DeAngelis, Science 310:987-991
Agent-based complex systems are dynamic networks of many interacting agents; examples include ecosystems, the immune system, financial markets, and cities. In the search for general principles underlying the internal organization of such systems, bottom-up simulation models such as cellular automata and agent-based models are widely used. So far, however, no general framework for designing, testing, and analyzing bottom-up models has been established. However, recent advances in bottom-up ecological modeling have come together in a general strategy we call Pattern-oriented Modeling. This strategy provides a unifying framework for decoding the internal organization of agent-based complex systems and may lead toward unifying algorithmic theories of the relationship between adaptive behavior and system complexity.
Extending Point Pattern Analysis for Objects of Finite Size
and Irregular Shape*
2 We use a mapped area of a grass-shrub steppe in semiarid Patagonia, Argentina,
to show that the shrub community is essentially randomly structured, but that
shrubs facilitate grasses in their immediate neighbourhood.
3 The occurrence of this random spatial structure provides important new
information on the biology of shrub populations. In general, previous data from
semiarid and arid ecosystems have showed that adult shrubs tend to show
over-dispersed patterns, whereas juveniles are clumped.
4 We find that the point approximation may produce misleading results (1) if
plant size varies greatly, (2) if the scale of interest is of the same order of
magnitude as the size of the plants and (3) if the plants of a given pattern are
constrained through competition for space by the presence of other plants. The
point approximation worked well in all other cases, but usually depicted weaker
significant effects than when the size and shape of plants were taken into
5 Our approach to quantifying small-scale spatial patterns in plant communities
has broad applications including the study of facilitation and competition.
Ecologists will be able to use the software available to take advantage of these
*This is an electronic version of an article published in Journal of Ecology: complete citation information for the final version of the paper, as published in the print edition of Journal of Ecology, is available on the Blackwell Synergy online delivery service, accessible via the journal's website at http://www.blackwellpublishing.com/journals/jec or http://www.blackwell-synergy.com.
Abrupt population changes in treeline ecotones along smooth
1 We developed a spatially explicit and individual-based simulation model describing the dynamics of tree populations across treeline ecotones. Our aims were to identify minimal factors and processes able to generate treeline types with abrupt vs. smooth transitions in different variables (tree height, age, density), to investigate the role of positive feedback in pattern generation, and to find out why krummholz appears at some treelines, but not at all. We hypothesised that a different balance between smooth growth and mortality gradients across the treeline ecotone could account for differences between commonly observed treeline types.
an electronic version of an article published in Journal of Ecology: complete
citation information for the final version of the paper, as published in the
print edition of Journal of Ecology, is available on the Blackwell Synergy
online delivery service, accessible via the journal's website at
Getzin, S., C. Dean, F. He, T. Trofymow, K. Wiegand, and T. Wiegand. Ecography 29: 671-682.
Douglas-fir forest is a major forest in the Pacific Northwest region. While the successional dynamics and large scale structure of the forest is well studied, the fine-scale spatial characteristics at the stand level are still poorly understood. Here we investigated four hypotheses on the fine-scale spatial structure of the forest on Vancouver Island. Our primary purpose is to understand how the three dominant species, Douglas-fir, western hemlock and western redcedar, coexist and partition space along a chronosequence comprising of three stands of immature, mature and old-growth. We used the O-ring statistic and Ripley’s L-function to quantify the change in spatial distribution and association of the species along the chronosequence. Evidence on intra- and inter-specific competition was also inferred from correlations between nearest-neighbor distances and tree size. Our results show that (1) variation in local site characteristics was highly important for the unexpected aggregated pattern of Douglas-fir in old-growth, (2) surviving trees of the species were less aggregated than their pre-mortality patterns, thus rejecting the random mortality hypothesis and being strong evidence for intra-specific competition, (3) tree death was a random process among larger overstorey trees in the plots, and (4) inter-specific competition declined with increasing plot age as a result of spatial resource partitioning. Here we highlight the importance of spatial heterogeneity for the long-term coexistence of shade-intolerant pioneer Douglas-fir and shade-tolerant colonizer species during forest succession.
Watson, D.M., Roshier, D.A., and T. Wiegand. Austral Ecology 32: 359-369.
Occurrence patterns of parasitic plants are constrained by the distribution of suitable hosts and movement patterns of seed vectors and, accordingly, represent a simplified system to study many aspects of spatial ecology and determinants of distribution. Previous work has focused on the aerially hemiparasitic mistletoes, and it is unclear whether root parasites are affected by similar factors. Here, we evaluate spatial patterns in the root-parasitic Santalum lanceolatum in an arid shrubland in northwestern New South Wales, central Australia. In this region, the principal host is a long lived nitrogen fixing shrub Acacia tetragonophylla closely associated with ephemeral creek-lines. The location of 765 individuals of both species was mapped along a 250 m section of creek-line using a total survey station, and occurrence patterns of the root parasite related to host distribution and landscape context. We used Ripley’s K-function and the O-ring statistic to determine whether the distribution of S. lanceolatum was random, aggregated or regular; the spatial scales at which these patterns occurred; and to quantify any spatial associations between the parasite and its host, A. tetragonophylla. While acacias were closely associated with the creek-line, S. lanceolatum plants were more tightly clustered, displaying significant clustering at two spatial scales (1.2 m and 8.8 m). We suggest that host quality may act as an important constraint, with only those acacias growing in or near the creek-line being physiologically capable of supporting a parasite to maturity. Insights gained from spatial analysis are used to guide ongoing research in this system, and highlight the utility of the O-ring statistic for understanding patterns of distribution affected by multiple processes operating at critical scales.
Keywords: Dispersal, Distribution, Host quality, O-ring statistic, Santalum.
Aguayo, M., T. Wiegand, G. Azócar, K. Wiegand, and C. Vega. Ecology and Society: 12 (1): 13.
[online] URL: http://www.ecologyandsociety.org/vol12/iss1/art13/
City growth and changes in land use patterns cause several important social and environmental impacts. In order to understand the spatial and temporal dynamics of these processes, we must identify and analyze the factors that drive urban development, especially those that can be used to predict future changes and their potential environmental effects. The objectives of this research were to quantify the relationship between the urban growth and its driving forces, exemplified for Los Angeles, a city in central Chile, and to predict the spatial growth pattern based on historical land use changes. This involved (i) analysis of a set of multi-temporal images (1978, 1992, 1998) and characterization of the land use change spatial pattern; (ii) the construction of digital coverage in GIS; (iii) the selection of predictive variables through univariate analysis; (iv) construction of logistic regression models using 1978 - 1992 growth vs. non growth as dependent variable; and (v) the prediction of the probability of land use change through application of the regression model for the 1992-1998 period. To investigate the influence of spatial scale we constructed several blocks of models i) comprising only distance variables (e.g., distance to highway), ii) comprising only scale-dependent density variables (e.g., density of urban area within a 600m radius), iii) combining distance and density variables and iv) combining distance and density variables at several spatial scales. All blocks included the environmental variables. We found that combining distance and density variables at several scales are required to appropriately capture the multi-scale urban growth process. The best models correctly predict about 90% of the observed 1992-1998 land use changes. Distance to access roads, the densities of the urban road system and urbanized areas at different scales, and soil type were the strongest driving forces of the growth pattern. In turn, other environmental variables were less important or not significant for explaining the urban growth process. Our approach which combines spatial modeling tools and GIS significantly advances our understanding of urban growth patterns, it provides an important contribution to urban planning and management, and can be widely applied.
Kramer-Schadt, S., E. Revilla, T. Wiegand, and V. Grimm.
Analysis of spatial pattern in early stages of primary succession on former lignite mining sites.
Felinks, B. and T. Wiegand 2008. Journal of Vegetation Science
Methods: Individual plants were mapped along a 2 x 28m transect during three successive years and classified into two groups (1) the pioneer Corynephorus canescens and (2) 'all other species'. Using the pair-correlation function, univariate point pattern analysis was carried out by applying a heterogeneous Poisson process as null model. Bivariate analysis and a toroidal shift null model were applied to test for independence between the spatial patterns of the two groups separately for each year, as well by exploring spatiotemporal patterns from different years.
Results: Corynephorus canescens and 'all other species' showed in the first year an aggregated pattern on a spatial scale > 40cm and in the second and third year a significant attraction for distances between 4 – 12cm, with an increasing radius in the third year. The analyses of interspecific spatiotemporal dynamics revealed a change from independence to attraction between distances of 4 and 16cm when using Corynephorus canescens as focal species. However, applying 'all other species' as focal points result in a significant attraction at distances up to 60cm in the first year and a diminishing attraction in the second and third year with distances £ 6cm. Conclusions: Facilitative species-species interactions are present in early stages of primary succession, resulting mainly from pioneer species acting as physical barriers and their ability to capture diaspores being drifted by secondary dispersal along the substrate surface. However, due to gradual establishment of perennial species and their ability of lateral extension by vegetative dispersal, facilitation may influence spatial pattern formation predominantly on rather short temporal and fine spatial scales.
Keywords: Facilitation, Pair-correlation function, Point pattern analysis, Species interactions, Succession
Spatial patterns in the sociable weaver (Philetairus socius).
Giesselmann, U.C., T. Wiegand, J. Meyer, R. Brandl, and M. Vogel. in press.
Land use impact on Vitellaria paradoxa C.F. Gaerten. stand structure and distribution patterns: a comparison of Biosphere Reserve of Pendjari in Atacora district in Benin. Agroforestry Systems
Djossa, B.A., Fahr, J., Wiegand, T., Ayihouénou, B.E., Kalko, E.K.V., and B. A. Sinsin. 2008. Agroforestry Systems 72: 205-220
The shea tree, Vitellaria paradoxa, is a
socio-economically important tree for the rural population in parts of West
Africa. Our study assessed the current status of this native tree species with
regard to increasing human pressure in northern Benin. We compared distribution
of adult shea trees, seedlings and saplings in farmed lands with protected areas
in the Biosphere Reserve of Pendjari (BRP). At our study site near BRP,
agricultural activities foster recruitment of shea trees by regularly cropping
of vegetation cover. Furthermore, traditional farming practices preserve adult
individuals thus permitting regular fruit harvests. Consequently, most of the
tallest and largest individuals of shea trees are found in framed lands. In
contrast, the highest density of juvenile trees including seedlings (dbh <5 cm)
and saplings (dbh 5-10 cm) occurred within BRP. Saplings were negatively
affected by farming activities. Furthermore, spatial point pattern analysis
revealed differences in the spatial structure of juveniles. Juveniles showed
significant aggregations at small scale (< 20 m) in BRP as well as significant
and positive small-scale associations with adult trees. This contrasts with
farmed lands where we did not find such spatial patterns at similar small scale
but only a weak aggregation between juveniles and absence of association
(attraction) of adults to juveniles. Although our analyses indicate that shea
trees are rather well preserved, we conclude that the observed severe reduction
of saplings in farmed lands is likely to negatively impact the long-term
viability of the tree population. Therefore agroforestry practices must consider
the preservation of sapling populations in farming areas for long-term
A spatially-explicit analysis of seedling recruitment in the terrestrial orchid Orchis purpurea.
Jacquemyn, H., R. Brys, K. Vandepitte, O. Honnay, I. Roldán-Ruiz and T. Wiegand.
2007. New Phytologist
Spatial patterns of seedling recruitment were investigated in two populations of the terrestrial orchid Orchis purpurea using both univariate and bivariate point pattern analysis, parentage analysis and seed germination experiments. • Both adults and recruits showed a clustered spatial distribution with cluster radii of about 4-5m. Results of the parentage analysis showed that offspring-dispersal distances were slightly larger than distances obtained from the point pattern analyses. The suitability of microsites for germination differed among sites, with strong constraints in one site and almost no constraints in the other.
Our results provide a clear and coherent picture of recruitment patterns in a tuberous, perennial orchid. Seed dispersal is limited to a few meters from the mother plant, whereas the availability of suitable germination conditions may vary strongly from one site to the next. Because of a time-lag of three to four years between seed dispersal and actual recruitment and irregular flowering and fruiting patterns of adult plants, interpretation of recruitment patterns using point patterns analyses ideally should take into account the demographic properties of orchid populations.
Key-words: micro-site limitation, Orchidaceae, Orchis purpurea, seed germination, seed limitation, spatial point pattern analysis.
Wiegand, T, C.V.S. Gunatilleke, I.A.U.N. Gunatilleke, and T. Okuda. Ecology88: 3088–3102
Clustering at multiple critical scales may be common for plants since many different factors and processes may cause clustering. This is especially true for tropical rain forests where theories for explaining species coexistence and community structure rest heavily on spatial patterns. We used point-pattern analysis to analyze the spatial structure of Shorea congestiflora, a dominant species at a 25-ha forest dynamics plot in a rain forest at Sinharaja World Heritage Site (Sri Lanka), which apparently shows clustering at several scales. We developed cluster processes incorporating two critical scales of clustering for exploring the spatial structure of S. congestiflora and interpret it in relation to factors such as competition, dispersal limitation, recruitment limitation, and Janzen–Connell effects. All size classes showed consistent large-scale clustering with a cluster radius of approximately 25m. Inside the larger clusters, small-scale clusters with a radius of 8m were evident for recruits and saplings, weak for intermediates, but disappeared for adults. The pattern of all trees could be divided into two independent patterns, a random pattern (nearest neighbor distance > 8m) comprising about 12% of the trees and a nested double-cluster pattern. This finding suggests two independent recruitment and/or seed dispersal mechanisms. Saplings were several times as abundant as recruits and may accumulate several recruit generations. Recruits were only weakly associated to adults and occupied about half of the large-scale clusters, but saplings almost all. This is consistent with recruitment limitation. For about 70% (95%) of all juveniles the nearest adult was less than 26m (53m), suggesting a dispersal limitation which may also be related with the critical large-scale clustering. Our example illustrates how the use of specific and complex null hypothesis of spatial structure in point pattern analysis can help to better understand the biology of a species and to generate specific hypotheses to be further investigated in the field.
Key words: Janzen–Connell, multiple clustering, pair-correlation function, point-pattern analysis, Ripley’s K-function, Sinharaja Forest Dynamics Plot, spatial point processes
Wiegand, T, C.V.S. Gunatilleke, and I.A.U.N. Gunatilleke. 2007. The American Naturalist
Keywords: coexistence, habitat association, pair-correlation function, plant-plant interactions, point pattern analysis, tropical forest.
How individual species structure diversity in tropical forests.
Wiegand, T, C.V.S. Gunatilleke, I.A.U.N. Gunatilleke, and A. Huth. 2007.
Keywords: biodiversity, spatial patterns, spatial statistic, species–area relationship.
Wiegand, T., J. Naves and M. Garbulsky, and N. Fernández.
Many animal species have developed specific
evolutionary adaptations to survive prolonged periods of low energy availability
that characterize seasonal environments. The seasonal course of primary
production, a major aspect of ecosystem functioning, should therefore be an
important factor determining the habitat quality of such species. We tested this
hypothesis by analyzing the relationship between habitat quality and ecosystem
functioning for brown bears (Ursus arctos), a species showing hyperphagia
and hibernation as evolutionary adaptation to seasonal peaks and bottlenecks in
ecosystem productivity,respectively. Our unique long-term data set comprised
data from two brown bear populations in northern Spain on historical presence,
current presence, and reproduction. The data were classified on a grid of 5 x 5
km pixels into five classes: frequent reproduction, sporadic reproduction,
frequent presence, sporadic presence, and recent extinction. We used the
longterm average of the seasonal course of NDVI (normalized difference
vegetation index) as a proxy for ecosystem functioning and investigated the
relationship between habitat quality and ecosystem functioning with methods
borrowed from statistical point-pattern analysis.
Grazing impacts in vegetated dune fields: predictions from spatial pattern analysis
Blanco, P.D., C. M. Rostagno, H. F.
del Valle, A. M. Beeskow, and T. Wiegand.
Rangeland Ecology and Management
This study deals with the changes induced by
grazing on soil erosion processes in vegetated dune fields of Península Valdés,
in the Patagonia region of Argentina. We performed a spatial analysis to assess
erosion features’ patterns. Blowouts, used as main indicators of aeolian erosion
processes, as well as dune crests, which are susceptible to erosion, were mapped
on aerial photographs and images from Landsat 7 Enhanced Thematic Mapper Plus,
in eight paddocks under two grazing conditions: lightly (0.4 sheep/ha) and
heavily grazed (0.8 sheep/ha). From the mapped locations of water points,
crests, and blowouts we calculated a spatial statistic (O-ring statistic), which
gives the expected intensity of blowouts within the area covered by crests as
function of distance away from water points. Additionally, to explore if the
density of crests around water points influences the density of blowouts, we
estimated the intensity of dune crests in the neighborhood of water points and
compared the densities of blowouts among water points with low, medium, and high
densities of crests. For the heavy grazing treatment we found highly significant
(P<0.05) aggregation of blowouts around water points with peak densities
threefold higher than expected at random occurring between 90 and 210 m.
However, the aggregation was only weakly significant for the light grazing
treatment and occurred only at distances of about 30 m away from the water
point. We found that the impact of grazing on soil stability
Heterogeneity influences spatial patterns and demographics in forest stands
Getzin, S., T. Wiegand, K. Wiegand, and F.
Journal of Ecology,
1 The spatial pattern of tree species retains signatures of factors and processes such as dispersal, available resource patches for establishment, competition, and demographics. Comparison of the spatial pattern of different size classes can thus help to reveal the importance and characteristics of the underlying processes. However, tree dynamics may be masked by large-scale heterogeneous site conditions, e.g., when the restricting size of regeneration sites superimposes emergent patterns.
Key words:case-control, dispersal strategies, inhomogeneous pair-correlation function, large-scale heterogeneity, point pattern analysis, succession, western hemlock
Temporal and spatial differentiation in seedling emergence may promote species coexistence in Mediterranean fire-prone ecosystems
De Luis, M., J. Raentós, T. Wiegand, and
J. C. González-Hidalgo
Ecography, conditionally accepted
Mediterranean ecosystems are hotspots of species richness where fire is one of the key processes influencing their structure, composition and function. Post-fire seedling emergence constitutes a crucial event in the life cycle of plants and species-specific temporal and spatial patterns of seedling emergence have been hypothesized to contribute to the high diversity in these ecosystems. Here we study the temporal and spatial patterns of seedling emergence observed for the four dominant species (Cistus albidus, Ulex parviflorus, Helianthemum marifolium, Ononis fruticosa) after an experimental fire in a Mediterranean gorse shrubland. In a first analysis we compared the timing of emergence of each species using the Kaplan-Meier method. The spatial component of seedling emergence and the spatiotemporal relationship between different cohorts of the same species were analyzed using recent techniques of spatial point pattern analyses. We found a bimodal temporal pattern of emergence. Emergences of Cistaceae species (H. marifolium and C. albidus) occurred predominantly early after the fire while Fabaceae (O. fruticosa and U. parviflorus) emerged mainly during the next autumn. All species individually showed an aggregated spatial pattern and when testing for pair interactions we found that the clusters of individual species were spatially segregated. Additionally, the clusters of individual species showed an internal spatial structure where seedlings of different cohorts were spatially segregated. These patterns are expected by theoretical models to promote species coexistence. We identify a number of mechanisms that all have the potential to contribute to the observed pattern formation. However, the potential interactions among these mechanisms are complex and not easy to predict. Our analyses are a significant step forward in studying seedlings emergence in fire prone ecosystems because they consider, to our knowledge, the first time both spatial and temporal patterns of all dominant species together.
Key words:Cistus albidus, Ulex parviflorus, Helianthemum marifolium, Ononis fruticosa, Fabaceae, Cistaceae, Mediterranean gorse shrubland, Kaplan-Meier method, O-ring statistics, point-pattern analysis, segregation hypothesis
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