Our results suggest a consistent tendency for mammalian assemblages to be more phylogenetically overdispersed than expected by chance, and that this tendency is only detectable by pooling many assemblages. This pattern is seen both across the whole phylogeny (NRI/NRI results) and at the tips (NTI/NTI results). Additionally, the three clades analysed do not have significantly different NRI% or NTI values suggesting that this could be a general, rather than a clade-specific mammalian pattern.
The traditional interpretation of phylogenetic overdispersion is that competition among ecologically similar close relatives has led to exclusion of the inferior competitors and hence an assemblage with more distantly related species than expected. This fits with the predictions of earlier non-phylogenetic studies looking at how competition structures mammalian communities and with the results of Houle (1997)
, who found that primates that were phylogenetically ‘too close’ did not coexist. However, if traits allowing a species to exist in an area have evolved convergently in more distant relatives, then habitat filtering could also cause overdispersion (Cavender-Bares et al. 2004
; Kraft et al. 2007
). Determining whether competition or habitat filtering is more likely would require an analysis integrating data on the traits mediating competition and resource use. For example, Davies et al. (2007)
demonstrated a role for competition in structuring carnivoran assemblages, by analysing geographical range overlap together with differences in traits implicated in interspecific competition (body size and tooth morphology). In the absence of information about which traits in our groups most strongly influence competition, it is only possible to make weak inferences from ancillary information. Habitat filtering may be more likely in squirrels, because convergence is hypothesized in a range of skeletal traits (Roth 1996
), and because our assemblages come from a mixture of grassland, desert and woodland habitat types. Furthermore, the species pools for squirrels tended to be relatively large (median=15 species, first quartile=5.5 and third quartile=22), increasing power to detect habitat filtering but decreasing power to detect competition (Kraft et al. 2007
). Competition may be more likely in monkeys: they had smaller species pools (median=8 species, first quartile=5.75 and third quartile=11.5) and all the species come from broadly similar habitat types, perhaps reducing the scope for habitat filtering. Possum assemblages may be generally assembled at random with respect to phylogeny, perhaps because the traits involved in habitat filtering and competition in this group are independent of phylogeny, or because both mechanisms are acting and have cancelled each other out (e.g. Helmus et al. 2007
). However, these results are based on only 10 assemblages, so may not permit any robust conclusions.
Interestingly, few individual assemblages had significantly overdispersed NRI or NTI values. This could be due to low statistical power, since both our assemblages and species pools tended to be small, and most assemblages were either too small or too large with respect to the pool for maximum power (Kraft et al. 2007
). In addition, competition does not always lead to competitive exclusion. Some species may be temporally or spatially segregated within the same habitat. Other species show behavioural plasticity, which allows similar species to coexist (Houle 1997
; Lovette & Hochachka 2006
), or rapidly evolve different ecotypes (Harmon et al. 2003
). Finally, our results focus on how competition between close relatives may limit species distributions in some mammalian clades. However, other factors also influence where species occur, such as geographical boundaries, limits on dispersal, differential extinction (human mediated or otherwise), the distribution of resources (e.g. food and shelter) and interactions with species from different clades (e.g. predation). Although the narrow taxonomic focus of our approach should increase the likelihood of detecting competition in our clades (Darwin 1859
), distant relatives may also compete. For example, in the neotropics, frugivorous bats compete with birds (Palmeirim 1989
) and potentially with monkeys as well.
Our assemblage lists come from species checklists, which may be incomplete. This incompleteness could only undermine our results if, for some reason, the omissions caused the species on the lists to appear phylogenetically overdispersed. An alternative approach would be to use species range map overlap to determine the assemblage membership (e.g. Davies et al. 2007
). However, that approach is also problematic as maps tend to overestimate species ranges and hence species overlap (Hurlbert & Jetz 2007
). Species checklists, while imperfect, are much more likely to capture sets of interacting species. Here we use range maps only to delimit source pools, so any errors will affect only the pools; and we show that varying the threshold for inclusion in the source pool has no qualitative impact on the results. Our NRI results also appear to be influenced by the size of the species pool and our NTI results by the ratio of the number of species in the assemblage to the number of species in the pool (appendix S3, table S3 in electronic supplementary material). Since where NRI was affected by these factors, NTI was not, and vice versa, we believe that the general pattern of overdispersion in our assemblages was not merely due to methodological bias. However, these factors should be considered in future studies.
Our results suggest that species distributions are influenced by those of other closely related species. Since species geographical range maps do not reflect the local heterogeneity in distributions that may be caused by such interactions, our knowledge of where species actually occur may be unreliable. Previous authors have recognized this and its implications for biodiversity research and conservation (Hurlbert & Jetz 2007
). Likewise, the current trend for mapping how geographical ranges will shift under particular climate change scenarios may underestimate how species will be affected if these among-species interactions are ignored. Such interactions may make it much harder to predict how species will respond to their rapidly changing environment.