Mass Effects Mediate Coexistence in Competing Shrews

Abstract

Recent developments in metacommunity theory have raised awareness that processes occurring at regional scales might interfere with local dynamics and affect conditions for the local coexistence of competing species. Four main paradigms are recognized in this context (namely, neutral, patch-dynamics, species-sorting, and mass-effect), which differ according to the role assigned to ecological or life-history differences among competing species, as well as to the relative time scale of regional vs. local dynamics. We investigated the patterns of regional and local coexistence of two species of shrews (Crocidura russula and Sorex coronatus) sharing a similar diet (generalist insectivores) over four generations, in a spatially structured habitat at the altitudinal limit of their distributions. Local populations were small, and regional dynamics were strong, with high rates of extinction and recolonization. Niche analysis revealed significant habitat differentiation on a few important variables, including temperature and availability of winter resting sites. In sites suitable for both species, we found instances of local coexistence with no evidence of competitive exclusion. Patterns of temporal succession did not differ from random, with no suggestion of a colonization-competition trade-off. Altogether, our data provide support for the mass-effect paradigm, where regional coexistence is mediated by specialization on different habitat types, and local coexistence by rescue effects from source sites. The strong regional dynamics and demographic stochasticity, together with high dispersal rates, presumably contributed to mass effects by overriding local differences in specific competitive abilities.

Invading and Expanding: Range Dynamics and Ecological Consequences of the Greater White-Toothed Shrew (Crocidura russula) Invasion in Ireland

“…C. russula is known to have had negative impacts on another shrew species, Crocidura leucodon , displacing it in Switzerland in association with the range expansion of C. russula during the 20 th century [65]. However, C. russula did not competitively exclude another similarly sized shrew ( Sorex coronatus ) in the same region [15]. Regional coexistence of S. coronatus and C. russula was maintained by a degree of habitat specialization, with local coexistence facilitated by dispersal from these source habitats [15].…”

“…1A), it is much more likely that these are ‘jumps’ occurring within Ireland. As in continental Europe, the species reaches very high densities in the summer [20] and can occupy human dwellings and farm buildings in large numbers [15] within its invasive range in Ireland (AD McDevitt, pers comm.). This may give the false impression that the species is rapidly expanding its range entirely through its own dispersal when it may in fact be assisted, and may explain why it is expanding faster than M. glareolus , a species not associated with humans.…”

“…However, C. russula did not competitively exclude another similarly sized shrew ( Sorex coronatus ) in the same region [15]. Regional coexistence of S. coronatus and C. russula was maintained by a degree of habitat specialization, with local coexistence facilitated by dispersal from these source habitats [15]. It is unclear what impacts C. russula has on S. minutus in continental Europe, as S. minutus is generally uncommon [25]–[27].…”

“…The situation mirrors that of the cane toad invasion in Australia where some species are similarly heavily impacted by the invader because of the speed of the invasion and large numbers of the invader [71]. Increased habitat specialisation of S. minutus as a response to the new invader [12], [15] may not be possible because of the relatively homogenous nature of the Irish landscape. Habitat types that may act as refugia (such as peatland and woodland) are of a fragmentary nature in Ireland.…”

Spatial distribution and basic ecology of Heterodera schachtii and H. betae wild populations developing on sea beet, Beta vulgaris ssp. maritima

“…The overall degree of differentiation or exclusion of the two nematode species was evaluated with the kappa statistic k (Cohen, 1960), estimated as described in Guelat et al (2008). This statistic is an analogue of the correlation coefficient for categorical variables.…”