AIM : Beta diversity describes the variation in species composition between sites
and can be used to infer why different species occupy different parts of the
globe. It can be viewed in a number of ways. First, it can be partitioned into
two distinct patterns: turnover and nestedness. Second, it can be investigated
from either a species identity or a functional-trait point of view. We aim to
document for the first time how these two aspects of beta diversity vary in
response to a large environmental gradient.
LOCATION : Maloti-Drakensberg Mountains, southern Africa.
METHODS : We sampled ant assemblages along an extensive elevational gradient
(900–3000 m a.s.l.) twice yearly for 7 years, and collected functional-trait information
related to the species’ dietary and habitat-structure preferences. We
used recently developed methods to partition species and functional beta diversity
into their turnover and nestedness components. A series of null models
were used to test whether the observed beta diversity patterns differed from
RESULTS : Species beta diversity was driven by turnover, but functional beta
diversity was composed of both turnover and nestedness patterns at different
parts of the gradient. Null models revealed that deterministic processes were
likely to be responsible for the species patterns but that the functional changes
were indistinguishable from stochasticity.
MAIN CONCLUSIONS : Different ant species are found with increasing elevation,
but they tend to represent an increasingly nested subset of the available functional
strategies. This finding is unique and narrows down the list of possible
factors that control ant existence across elevation. We conclude that diet and
habitat preferences have little role in structuring ant assemblages in montane
environments and that some other factor must be driving the non-random patterns
of species turnover. This finding also highlights the importance of distinguishing
between different kinds of beta diversity.