Species Interactions during Diversification and Community Assembly in an Island Radiation of Shrews

Abstract

BackgroundClosely related, ecologically similar species often have adjacent distributions, suggesting competitive exclusion may contribute to the structure of some natural communities. In systems such as island archipelagos, where speciation is often tightly associated with dispersal over oceanic barriers, competitive exclusion may prevent population establishment following inter-island dispersal and subsequent cladogenesis.Methodology/Principal FindingsUsing a combination of tools, we test the hypothesis that the distributions of shrew (Crocidura) species in the Philippines are the result of competitive exclusion preventing secondary invasion of occupied islands. We first compare ecological niche models between two widespread, allopatric species and find statistical support for their ecological similarity, implying that competition for habitat between these species is possible. We then examine dispersion patterns among sympatric species and find some signal for overdispersion of body size, but not for phylogenetic branch length. Finally, we simulate the process of inter-island colonization under a stochastic model of dispersal lacking ecological forces. Results are dependent on the geographic scope and colonization probability employed. However, some combinations suggest that the number of inter-island dispersal events necessary to populate the archipelago may be much higher than the minimum number of colonization events necessary to explain current estimates of species richness and phylogenetic relationships. If our model is appropriate, these results imply that alternative factors, such as competitive exclusion, may have influenced the process of inter-island colonization and subsequent cladogenesis.Conclusions/SignificanceWe interpret the combined results as providing tenuous evidence that similarity in body size may prevent co-occurrence in Philippine shrews and that competitive exclusion among ecologically similar species, rather than an inability to disperse among islands, may have limited diversification in this group, and, possibly other clades endemic to island archipelagos.

Archboldomys(Muridae: Murinae) Reconsidered: A New Genus and Three New Species of Shrew Mice from Luzon Island, Philippines

“…The above discoveries are consistent with and further strengthen the pattern revealed in previous studies in the Philippines (Esselstyn et al, 2009(Esselstyn et al, , 2011Heaney et al, 2009Heaney et al, , 2011Jansa et al, 2006Jansa et al, , 2009Musser and Heaney, 1992;Steppan et al, 2003) and other large, geologically old oceanic islands (e.g., Madagascar: Olson and Goodman, 2003;Yoder et al, 1996) that their mammalian biodiversity arose from extensive in situ diversification. The discoveries reported here further illustrate the insularizing effect of isolated mountaintops in the Philippines on the diversification of its murid rodents Heaney et al, 1989Heaney et al, , 2009Heaney et al, , 2011Heaney and Rickart, 1990;Rickart et al, 1998Rickart et al, , 2003Rickart et al, , 2005Steppan et al, 2003).…”

Biodiversity Dynamics on Islands: Explicitly Accounting for Causality in Mechanistic Models

“…These are characterized by employing stochasticity in at least one of their mechanisms. This might be, e.g., the selection of the number of offspring in a reproduction event (e.g., [22]) or dispersal decisions (e.g., [23]). In contrast, deterministic models are often described purely analytically by a set of differential equations [24,25], although in some deterministic models, stochasticity can be easily switched on [26].…”

“…For instance, Birand and Howard [38] and Rosindell and Phillimore [39] use parameter screening to obtain realistic rates of extinction, reproduction, speciation and migration compared with archipelago species numbers (including information on endemics) from plants and arthropods, or birds, respectively. Two other studies used representations of real geography as their model arena [23,34] for the investigation of radiation or speciation histories. Only a single study actually fitted their model to data, using a maximum likelihood approach and dated phylogenies of Galápagos land birds to obtain rates of diversification and radiation [29].…”

“…The models that simulate real-world systems [23,29,34] shed light on the mechanisms behind distribution patterns of real-world species; for instance, diversification of Philippine shrews shaped by competitive exclusion over dispersal ability [23], bluebirds on Bermuda as cryptic introductions [34] or presently increasing species richness in some bird lineages on the Galápagos, while other lineages already attained equilibrium dynamics [29]. The models of Birand and Howard [38] and Rosindell and Phillimore [39] were able to produce realistic species distribution and endemism patterns, when compared to empirical data of species numbers, while Valente et al [29] used Bayesian information criterion weights for selection of the best model that explains current diversification patterns on the basis of phylogenetic data.…”

“…Furthermore, hypothetical systems are more easily employed, because they do not need data to setup simulation arenas and have parameters set by assumptions and study design, which follow the model's theoretical background and study question, respectively. However, more specific questions can only be answered by simulating real-world systems [23,34]. The gained knowledge can also be used to extrapolate the development of the system into the future, taking into account different climate or disturbance scenarios.…”