What keeps freshwater food webs together?
Freshwater habitats are often structured into physically separated open water, bottom and vegetated inshore mesohabitats. Each of these mesohabitats can host a different local community and rely on different sources of energy. Interestingly, the open water communities are typically dominated by small-bodied taxa and rapid turnover in biomass, while the bottom fauna is usually larger and has a slower turnover in biomass. Previous research proposed that their coexistence is mediated by mobile apex predators that link these communities in so-called multichannel food webs, but mechanistic explanations of this phenomenon were lacking.
Figure: Pelagic mesohabitats (blue symbols) are characterised by high productivity and small-bodied consumers, while the bottom habitats (brown symbols) are less productive with larger consumers. These two levels of asymmetry between habitats compensate each other and result in a balanced energy flow when reaching the top predator, promoting stability of the entire multichannel food web.
A new study published in Ecology Letters provides robust explanations for these empirical phenomena. The research lead by Samuel Dijoux, a PhD student from the Laboratory of Biodiversity and Ecology of Aquatic Insects at the Institute of Entomology and Department of Ecosystem Biology at the Faculty of Science, used a numerical model to assess how asymmetries between the mesohabitats influence the structure and stability of multichannel food webs. The study shows that an interplay between asymmetric habitat productivities and consumer body sizes can either promote species coexistence or lead to cascading extinctions in the community. A ‘symmetry in asymmetries’ maintains multichannel food webs through the two compensatory levels of asymmetry that modulate the energy flows in the different food web channels. The results also demonstrate how anthropogenic disturbances such as the widespread habitat eutrophication can have profound effects on freshwater communities.
Dijoux S., Boukal D. S. (2021) Community structure and collapses in multichannel food webs: Role of consumer body sizes and mesohabitat productivities. Ecology Letters in press: DOI: 10.1111/ele.13772