Ontogenetic microhabitat shifts in stream invertebrates with different biological traits

摘要

We investigated the life history patterns and instar-specific microhabitat preferences of three selected stream invertebrates with different biological and ecological traits: Ephemera danica and Rhithrogena semicolorata (two mayflies), and Hydropsyche siltalai (a caddisfly). The samples were taken from two nearby Piedmont streams throughout a one-year period in Southwest France. Water depth, current velocity, substratum composition, and particulate organic matter were used to calculate proportional use values for each species, and to quantify instar-specific microhabitat preferences. The extent to which species exhibited ontogenetic niche shifts seemed to depend on their biological attributes, and particularly on their life history patterns. Ephemera danica had a two-year life cycle with individuals from all size-classes occurring throughout the year in the streams, but there was a spatial segregation of individuals at different developmental stages (i. e., young larvae in boulders with medium depth and current velocity, older individuals in shallow and sandy areas with weak current velocity). Rhithrogena semicolorata and Hydropsyche siltalai were univoltine, and showed weaker ontogenetic microhabitat shifts. The former species remained in stony and pebbly areas, and larvae were able to withstand the temporal fluctuations of depth and current velocity. The latter spent most of its larval life in boulders, whereas last instar larvae (which could occur throughout the year) shifted to pebbles. Most studies of resource use and population dynamics consider conspecific individuals as ecologically equivalent, although the microhabitat preferences of invertebrates could change at different developmental stages. Our results suggest that ontogenetic niche shifts may play a substantial role in the structure and dynamics of populations and communities, by adjusting species' requirements to the spatial and temporal dynamics of environmental conditions, including abiotic and biotic conditions.