Invasive crayfish: drivers or passengers of degradation in freshwater ecosystems?

Invasive species, e.g. the freshwater crayfish Procambarus clarkii,
reportedly negatively influence the abundance of various aquatic species.
Moreover, these invaders are increasingly linked to ecological degradation
of aquatic ecosystems, since invaded habitats show increased levels of
turbidity, nitrogen and organic matter concentration. P. clarkii has,
among other impacts, been associated with eutrophication in invaded
habitats. However, observations indicate that the presence of P. clarkii
is often not accompanied by ecosystem degradation, begging the question
whether they are drivers of degradation, or function as passive passengers
along for the ride driven by other stressors. To test these contrasting
hypotheses, we conducted a full factorial experiment in 24 mesocosms with
P. clarkii and nutrient pollution (specifically N, P, K), a ubiquitous
stressor on aquatic ecosystems. Here we assessed the effects on community
compositions of morphologically identified macrophytes and chironomids,
and of compositions of bacteria, phytoplankton and diatoms identified
using environmental DNA (eDNA) metabarcoding. Nutrient pollution induced
significant shifts in macrophyte biomass, and in the composition of the
bacterial, diatom and phytoplankton communities. All microbial communities
treated with nutrient pollution diverged from the control initially, after
which the bacterial and phytoplankton communities converged back to the
control in the final weeks. In contrast, we found only marginal effects of
P. clarkii, rendering it unlikely as a significant short- to medium-term
driver of the tested biodiversity. Since microbial communities respond
quickly to changes in the environmental conditions, these results signify
that the utilized mesocosms were relatively stable in spite of P.
clarkii's presence. The studied crayfish density and timeframe may be
leveraged as threshold values in the design and execution of freshwater
management strategies that aim to avert potential negative impacts of P.
clarkii on ecosystem structure. Ultimately, the importance of nutrient
pollution is reinforced as a driver of environmental change in aquatic
ecosystems.