Sea lamprey nests promote the diversity of benthic macroinvertebrate assemblages

Abstract

The habitat heterogeneity hypothesis states that increased habitat heterogeneity promotes species diversity through increased availability of ecological niches. We aimed at describing the local-scale effects of nests of the sea lamprey (Petromyzon marinus L.) as ecosystem engineer on macroinvertebrate assemblages. We hypothesized that increased streambed physical heterogeneity caused by sea lamprey spawning would modify invertebrate assemblages and specific biologic traits and promote reach-scale diversity. We sampled thirty lamprey nests of the Nive River in three zones: the unmodified riverbed (control) and zones corresponding to the nest: the area excavated (pit) and the downstream accumulation of pebbles and cobbles (mound). The increased habitat heterogeneity created by lamprey was traduced by biological heterogeneity with a reduced density of invertebrates (1160 to 6540 individuals per m2 in control, 680 to 6460 individuals per m2 in pit and 1980 to 6240 individuals per m2 in mound) and number of taxa (23.5 {+/-} 3.9 taxa for control, 18.6 {+/-} 3.9 taxa in pit and 21.2 {+/-} 4.5 taxa for mound) in the pit compared to other zones. However the overall taxa diversity in nest increased with 82 {+/-} 14 taxa compared to the 69 {+/-} 8 taxa estimated in control zone. Diversity indices were consistent with the previous results indicating a loss of diversity in pit but a higher {beta} diversity between a pit and a mound than between two control zones, especially considering Morisita index accounting for taxa abundance. Trait analysis showed high functional diversity within zones with a reduced proportion of collectors, scrapers, shredders, litter/mud preference and small invertebrates in mound, while the proportion of "slabs, blocks, stones and pebbles" preference and largest invertebrates increased. Pit presented the opposite trend, while control had globally intermediate trait proportions. Our results highlight important effects on species and functional diversity due to habitat heterogeneity created by a nest-building species, what can ultimately influence food webs and nutrient processes in river ecosystems.