Evidence of alternative trophic pathways for fish consumers in a large river system in the face of invasion

Background/Question/Methods

Large rivers are susceptible to anthropogenic alteration which can result in drastic and unexpected changes to their functional ecology. The ability of such functional patterns to respond to change is a central aspect of ecosystem resilience. We evaluated spatial-temporal changes in the fish community of the Upper Mississippi River System (UMRS) using data from six study reaches. Species were classified into one of 14 feeding guilds, and individual mass was calculated using length-mass relationships. Mass per unit effort (MPUE) was then calculated for each feeding guild annually (1994-2019) per gear type and standardized using the multigear mean standardization method (MGMS). Both Analysis of Similarity (ANOSIM) and Chi-square tests were used to determine differences in the MPUE among reaches. We then investigated functional diversity by estimating the total functional feeding groups present (N), Margalef’s d, Pielou’s J`, Shannon’s Diversity (H), and Simpson’s Diversity Index. An AR(1) time series model was then applied to determine any proportional changes in each feeding guild class over 25 years. To investigate the impact of invasive carp species in the southern reaches, a Chow test was applied to observations between 2000 and 2005 to detect changes in the proportion of each guild class.

Results/Conclusions

ANOSIM and Chi-square revealed differences in feeding guild classes among reaches. Time series analysis determined that Invertivore/detritivores have been consistently declining in all six study reaches since 1994, with few groups showing a positive or negative change. Simpson’s Index revealed lower feeding guild diversity in southern, invaded reaches; however, we found no differences in the Shannon Index between invaded and non-invaded reaches. Chow tests revealed six abrupt changes following the introduction of invasive carp species. For example, herbivore/detritivores were not previously present prior to 2001 and demonstrated an increase in their proportion following invasion. There was little evidence that invasive carp species impacted the proportional mass of any feeding guild class. Higher diversity in the northern reaches may be acting as a barrier to the future range expansion of carp species in the UMRS. Diversity metrics also support other studies that southern reaches are less diverse and shifting to carp-dominated communities. Evaluating the spatial-temporal patterns of functionally defined ecological communities is beneficial to understanding the resilience of a system, particularly to biological invasion. Such studies may also provide further insight into the trophic needs of a system when considering future restoration initiatives.