Colonisation, emigration and equilibrium of stream invertebrates in patchy habitats

Abstract

Understanding of the ecology of stream assemblages is enhanced by quantifying colonisation and dispersal rates, but these are difficult to measure at small scales. Using field-located artificial streams that mirrored natural patches of habitat, we investigated immigration, emigration and stability of invertebrates on leaf-litter packs over 38 days.
We fitted equilibrium and “drift-out” models to immigration and emigration data. Overall colonisation by individuals (plateau at ˜Day 12) and taxa (total 50; plateau ˜21 taxa at ˜Day 24) was rapid and curvilinear towards a stable asymptote, suggesting saturation and equilibrium.
Abundant early colonists, mainly several chironomid species, dictated the shape of the colonisation curve, but the patterns varied among species. Thus, the plateaux were due to a successional shift in composition rather than a simple equilibrium.
The equilibrium model fitted total numbers and taxa, and individual taxa that were gradual colonists, but not taxa lacking constant immigration.
High turnover was due mainly to rarer taxa. The assemblage was maintained by drift-mediated immigration, compensating for individuals lost through emigration, emergence or mortality.
The masking effect of a few taxa and substantial turnover rather than simple accumulation of individuals are important considerations regarding assemblage composition over time. Rapid turnover of individuals provides for rapid colonisation of denuded substrata downstream.