Impacts of an invader omnivore predator on detrital food webs in streams

Background/Question/Methods

The crayfish Procambarus clarkii is listed among the 10 worst invasive species in Europe because it may severely affect biodiversity and ecosystem functions. In forest streams, plant-litter decomposition is a key process driven by microbes and invertebrate shredders.

The presence of the omnivore predator P. clarkii is expected to i) have a top-down control on decomposition directly by consuming leaf-litter or indirectly by consuming shredders, but effects may depend on shredder abundance; and ii) change the shredder feeding behavior, but responses may depend on shredder naivety.

Firstly, we manipulated shredder abundance (6-12 individuals, Sericostoma sp.) in the presence/absence of crayfish in aquariums containing leaves. Secondly, we assessed shredder avoidance behavior to P. clarkiiby placing 5-mm meshes in aquariums that separated crayfish and leaves from shredders. After 21d, we quantified leaf decomposition and fine particulate organic matter (FPOM) production.

Then, a manipulative experiment, under natural conditions, was carried out at two stream sites, separated by a waterfall that prevents the upstream dispersal of the crayfish, using baskets containing leaves in the presence/absence of P. clarkii. Baskets were covered with 500-µm or 5-mm mesh-sizes to prevent or not invertebrate access. After 21d, we determined leaf decomposition, and invertebrate abundance and diversity.

Results/Conclusions

In laboratory experiments, leaf decomposition and FPOM production were higher in the presence of crayfish than of shredders, regardless shredder abundance. Invertebrates did not appear to recognize P. clarkii as a predator, at least in the first stages after its introduction.

At the upstream site, where P. clarkii was not reported, results were consistent with those in the laboratory. The lack of differences in treatments with the crayfish in the presence or absence of invertebrates suggests that leaf decomposition was mainly driven by the crayfish at the upstream site. Results from the field support the idea that in streams with low diversity and abundance of invertebrates, as the upstream site, P. clarkiiplays an important role in leaf decomposition.

At the downstream site, where P. clarkii was already introduced, leaf decomposition by the crayfish and invertebrates alone did not differ, and was highest when crayfish and invertebrates were together. Results suggest that invertebrates are able to adjust their behavior to the presence of P. clarkii.

Overall results indicated that the invader omnivore P. clarkii has the potential to affect detrital food webs by direct consumption of basal resources (leaf litter) and indirectly by decreasing the abundance of invertebrate shredders.