Abstract
Although the relationship between species richness and available energy is well established for a range of spatial scales, exploration of the plausible underlying explanations for this relationship is less common. Speciation, extinction, dispersal and environmental filters all play a role. Here we make use of replicated elevational transects and the insights offered by comparing indigenous and invasive species to test four proximal mechanisms that have been offered to explain relationships between energy availability, abundance and species richness: the sampling mechanism (a null expectation), and the more individuals, dynamic equilibrium and range limitation mechanisms. We also briefly consider the time for speciation mechanism. We do so for springtails on sub-Antarctic Marion Island. Relationships between energy availability and species richness are stronger for invasive than indigenous species, with geometric constraints and area variation playing minor roles. We reject the sampling and more individuals mechanisms, but show that dynamic equilibrium and range limitation are plausible mechanisms underlying these gradients, especially for invasive species. Time for speciation cannot be ruled out as contributing to richness variation in the indigenous species. Differences between the indigenous and invasive species highlight the ways in which deconstruction of richness gradients may usefully inform investigations of the mechanisms underlying them. They also point to the importance of population size-related mechanisms in accounting for such variation. In the context of the sub-Antarctic our findings suggest that warming climates may favour invasive over indigenous species in the context of changes to elevational distributions, a situation found for vascular plants, and predicted for springtails on the basis of smaller-scale manipulative field experiments.
Highlights
Identifying the factors which explain spatial variation in the species richness, abundance and biomass of organisms is a primary goal of ecology
Eight indigenous and four invasive springtail species were sampled on the west transect, and seven indigenous and four invasive species were found on the east
When summer and winter data were pooled, rarefaction curves indicated that sampling had reached an asymptote for most sites (Fig. S2), and was considered adequate for the summed data, which was used for all further analyses (Table S3)
Summary
Identifying the factors which explain spatial variation in the species richness, abundance and biomass of organisms is a primary goal of ecology. Much focus has been given to the need for investigations which consider simultaneously the multiple mechanisms that may underlie richness variation across spatial extents (Table 1), and several authors have provided proposals for how this can be done[4,11,12,13]. Among these proposals lies the idea that the investigation of patterns in alien species may be insightful, because biological invasions offer natural experiments that can www.nature.com/scientificreports/. Increased energy enables additional trophic levels to occur that are occupied by new consumer species so increasing richness
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