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A great challenge in ecology is to link patterns in nature with the factors that determine species coexistence and community structure. In general, these patterns have been associated with different environmental conditions and species traits. The coexistence of ant species could be affected by the availability of food and nesting resources, which depend on vegetation diversity and structural complexity. In this study, we attempt to reproduce, through null models, the properties of ant community structure in areas with different physiognomy of vegetation associated to different wildfire regimes. The null model construction considered ant traits such as occurrence frequency, body size, and nest type; and site characteristics such as vegetation height and extra-floral nectar availability, and their combinations. The null models were compared to observed species segregation and nestedness patterns. Ant species were more aggregated in space than expected by chance. Vegetation height and extra-floral nectar availability were included in the most successful models in predicting ant segregation and aggregation pattern. Furthermore, ants’ body size was enough to reproduce the nestedness of species distribution in sites. Our results suggest that under post-fire conditions, habitat complexity, resource availability and species traits such as body size may be the determinants of ant community structure.

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Community Ecology
Authors: R. J. Pakeman, R. J. Pakeman, S. A. Hinsley, S. A. Hinsley, P. E. Bellamy, and P. E. Bellamy

Oikos 358 370 Gotelli, N.J. 2000: Null model analysis of species co-occurrence patterns. Ecology 81 :2606-2621. Null model

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. 1986. Resource utilization, overlap and temporal community dynamics: a null model analysis of an epiphytic chironomid community. J. Anim. Ecol. 55: 491-506 Resource utilization, overlap and temporal community dynamics: a null

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579 582 Wilson, J.B. 1989. A null model of guild proportionality, applied to stratification of a New Zealand temperate rain forest. Oecologia 80: 263

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. Null Models in Ecology . Smithsonian Institution Press, Washington, USA. Graves GR Null Models in Ecology 1997

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noted evidence for an inverse correlation between trunk fat and HDLC/TC and found evidence for the null with respect to the correlation between thigh fat and HDLC/TC. Therefore, we added thigh fat to the null model to produce Distribution Model 2. Within

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Abundance-occupancy relationships were determined for desert plants in the northwestern Red Sea region, at both the whole landscape, and individual habitat levels. Some 58 stands (having a total of 66 species) were studied, using ten quadrats (10 × 10 m 2 ) per stand. The relation was positive and highly significant at both scales, but stronger at habitat level than across the regional landscape. Niche-breadth was estimated as the number of habitats occupied regionally by a species, and was significantly related to both abundance and occupancy. Niche breadth explained just 10.1%of variation in abundance but some 56.2% of variation in occupancy. Using empirical data, we tested whether those abundance-occupancy relationships diverged significantly from a theoretical null model. Relationships diverged significantly from the null model at both regional landscape and habitat levels. Applications of abundance-occupancy relationships for plant conservation showed that 36% of the species in the study region is at risk of extinction.

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Tree species richness is remarkably high in many tropical forests, even at very fine spatial scales. However, the study of fine-scale richness is complicated by the rarefaction effect: that is, a trivial correlation between the number of individuals and the number of species. We developed null models to test whether fine-scale species richness differs from random expectation, and applied these models to a dataset of 1170 100 m2circular plots in the old-growth portion of La Selva Biological Station in the Atlantic Lowlands of Costa Rica. Although species richness in these plots was close to its theoretical maximum, we found that it was frequently lower than null expectation. This was a result of slightly clumped distributions within species. We found no relationships between species richness at the 100 m2scale and soil type or topography, after accounting for the effects of density

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Competition and facilitation are expected to leave different signatures in the pattern of species co-occurrence. Competition may result in a given species pair occurring less often than expected by chance, whereas facilitation may result in a given species pair occurring more often than expected by chance. We assessed the co-occurrence of pairs of herbaceous and shrubby species in Brazilian savannas, asking (1) whether a given species pair occurs more often than expected by chance, (2) whether the number of species pairs in sites with frequent fires is higher than expected by chance, (3) whether the difference in the functional traits of heterospecific pairs is lower in sites with frequent fires, and (4) whether small environmental variations in each site — instead of species interactions — could explain the co-occurrence of species. We used null models to answer the first two questions, analyses of variance to answer the third question, and detrended correspondence analyses to answer the fourth question. In all studied sites, approximately half of the heterospecific pairs occurred more often than expected by chance. So, facilitation seems to be important in determining the co-occurrence of some species in Brazilian savannas. However, high fire frequencies changed the pattern of occurrence of the species pairs, resulting in a spatial signature indistinguishable from random. Frequent fires also promoted phenotypic clustering of species. Nevertheless, wherever fire frequency is reduced, competition may lead to phenotypic overdispersion of plant species. Thus, less harsh environmental conditions in savannas may increase the competition among plant species.

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Niche theory predicts that coexisting species will partition resources to limit the effects of interspecific competition. We examined microhabitat partitioning in six sets of steppe birds associated to agroecosystems in central Spain (female and male Great Bustards Otis tarda, female and male Little Bustards Tetrax tetrax, Red-legged Partridges Alectoris rufa and Eurasian Stone-curlews Burhinus oedicnemus) to estimate realized niche breadth, overlap and segregation. Principal Components Analysis on data from used and random microhabitat locations produced two axes we retained for analysis related with two key factors: cover-visibility and food availability. Non-parametric kernel density functions were calculated for each of the PCA axes and species (or sexes), and niche overlap estimated as the area shared between species’ density functions. Null models were run to evaluate overlap significance. In analyses of microhabitat selection by the six sets of birds, 13 out of 15 pairs had significant resource partitioning and niche segregation, except for the pairs partridge and female Great Bustard and the two sexes of Great Bustard. Eurasian Stone-curlew showed wider trophic niche breadth, although segregated from the other species, probably because of its higher invertebrate requirements. Great and Little Bustards segregated in both niche axes, selecting microhabitat according to their body size. Accessibility to food resources and shelter seems to be similar for partridges and female Great Bustards, overlapping in their selection, which may indicate the existence of segregation in other niche factors (e.g., feeding habits). Great Bustard males showed niche overlap with females. Little Bustard males showed feeding microhabitat selection patterns similar to those of females, although they preferred more open microhabitats to meet their sexual display requirements. The entire assemblage had significantly less overlap than expected by chance, suggesting that differential microhabitat selection and realized niche partitioning may explain coexistence in steppe bird communities. Our results suggest that the maintenance of different microhabitat structure should be a priority in the management of agricultural environments.

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