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 flters all play a role. Here we make
use of replicated elevational transects and the insights ofered by comparing indigenous and invasive
species to test four proximal mechanisms that have been ofered 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 briefy
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. Diferences 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 subAntarctic our fndings 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 feld experiments.
