Environmental filtering drives the assembly of mammal communities in a heterogeneous Mediterranean region

Agricultural expansion and intensification are major drivers of global change. Quantifying the importance of different processes governing the assembly of local communities in agroecosystems is essential to guide the conservation effort allocated to enhancing habitat connectivity, improving habitat quality or managing species interactions. We used multiple detection methods to record the occurrence of medium-sized and large-sized mammals in three managed landscapes of a heterogeneous Mediterranean region. Then we used a joint species distribution model to evaluate the relative influence of dispersal limitation, environmental filtering, and interspecific interactions on the local assembly of mammal communities in 4-km² plots. The partitioning of the explained variation in species occurrence was attributed on average 99% to environmental filters and 1% to dispersal filters. No role was attributed to biotic filters, in agreement with the scarce support for strong competition or other negative interactions found after a literature review. Four principal environmental factors explained on average 63% of variance in species occurrence and operated mainly at the landscape scale. The amount of shrub cover in the neighboring landscape was the most influential factor favoring mammal occurrence and accounted for nearly one-third of the total variance. The proportion of intensively managed croplands and proxies of human activity within landscape samples limited mammal presence. At the microhabitat scale (~80 m² plots) the mean percentage area deprived of woody vegetation also had a negative effect. Functional traits such as body mass or social behavior accounted for a substantial fraction of the variation attributed to environmental factors. We concluded that multiscale environmental filtering governed local community assembly, whereas the role of dispersal limitation and interspecific interactions was negligible. Our results suggest that further removal of shrubland, the expansion of intensive agriculture, and the increase of human activity are expected to result in species losses. The fact that community integrity responds to a single type of ecological process simplifies practical recommendations. Management strategies should focus on the conservation and restoration of shrubland, adopting alternatives to intensive schemes of agricultural production, and minimizing recreational and other human activities in remnant natural habitats within agroecosystems or mosaic landscapes.

Niche partitioning in small mammals: interspecific and biome-level analyses using stable isotopes

Small mammal assemblages from South America provide a unique opportunity to measure coexistence and niche partitioning between marsupials and placentals. We tested how these two major clades partition environmental resources by comparing stable isotopic ratios of similar sized Didelphidae and Sigmodontinae in four Brazilian biomes: Pampas grassland, Pantanal wetland, Cerrado woodland savanna, and Atlantic Forest. Generally, didelphid isotopic niche follows a scaling law, because we found an association between δ15N enrichment and body mass. Sigmodontines that primarily partition the environment via forest strata showed a greater intake of C4 or/and crassulacean acid metabolism (CAM) plants than didelphids, as reflected by their wider trophic niche. Values of δ13C were highest in savannas and grasslands (Cerrado and Pampas biomes), and values of δ15N were highest in the Atlantic Forest (in sigmodontines) and Pampas (in didelphids). While assessing patterns between the two major Brazilian biomes (Atlantic Forest and Cerrado), we found evidence of a broader trophic niche for both clades in the Cerrado. In the Atlantic Forest, niche occupation by Didelphidae was completely enclosed within the Sigmodontinae trophic niche. Both clades showed less overlap in the Cerrado, a less productive environment. Our results highlight the importance of a comparative framework and the use of stable isotopes for testing ecological questions related to how small mammalian communities partition their niche.