Temporal and spatial patterns in the shifting of otter diet to invasive prey after river damming

Abstract

River damming promotes profound changes in aquatic biodiversity, including the facilitation of biological invasions, with subsequent impacts on native communities. The Eurasian otter (Lutra lutra) is a generalist aquatic predator that often incorporates new prey types when they become available. We studied the short‐ and long‐term effects of river damming on the trophic ecology of the otter across a riverine landscape gradient, from the flooded area to upstream tributaries, focusing on the largest artificial lake of Western Europe. Through the analysis of otter feces (spraints), we describe otter diet composition in 14 sites at different distances from the impounded area in three periods: (i) the pre‐filling phase (i.e., lotic conditions); (ii) the immediate filling phase; and (iii) 16 years after the dam closure. The diet of the otter drastically shifted in the short term, from being dominated by native cyprinids to being mainly composed of invasive species of fish and crayfish. In the long term, the number of invasive species consumed by the otter continued to increase while native fish completely disappeared as prey near the reservoir. Presumably, the construction of the dam generated clear spatial gradients of abundances of native and invasive fish species. These patterns arose in the short term but became accentuated in the long term and were mirrored in otter diet composition. Our results demonstrate the potential of otter diet analyses as a surrogate for the impacts of human alterations on aquatic ecosystems, including those of river damming and the associated facilitation of biological invasions.

A review and meta-analysis of the environmental biology of bleak Alburnus alburnus in its native and introduced ranges, with reflections on its invasiveness

“…Elevated abundances of A. alburnus in reservoirs and rivers do not seem to be controlled by the region's predators, such as Lutra lutra, which is known to be an opportunistic predator. However, despite the high abundance of introduced fish species, the diet of L. lutra has been found to contain limited occurence or no evidence (Bedmar et al 2022) of the invading species. With specific reference to A. alburnus in Iberia, no evidence was found of otter predation on A. alburnus at a large reservoir in the River Guadiana from spraint samples collected before 2001 and in 2003 as well as in 2018, despite the high A. alburnus abundance in this last period (Bedmar et al 2022).…”

“…However, despite the high abundance of introduced fish species, the diet of L. lutra has been found to contain limited occurence or no evidence (Bedmar et al 2022) of the invading species. With specific reference to A. alburnus in Iberia, no evidence was found of otter predation on A. alburnus at a large reservoir in the River Guadiana from spraint samples collected before 2001 and in 2003 as well as in 2018, despite the high A. alburnus abundance in this last period (Bedmar et al 2022). Similarly, the efficient nonnative piscivorous Sander lucioperca was not found to reduce high abundance of A. alburnus in a study of 14 different populations across Portuguese drainage basins (Ribeiro et al 2021).…”

Biological invasions in rivers and associated ecosystems: New insights, challenges, and methodological advancements

“…Five papers within this SI consider the environmental and ecological implications of INNS becoming established within waterbodies. Bedmar, Blanco‐Garrido, Delibes, and Clavero (2022) considers the ecological legacy of habitat transformation combined with biological invasions in rivers and their associated ecosystems. Their study on the Iberian Peninsula examined how the diet of the Eurasian Otter, Lutra lutra , has shifted from one dominated by native cyprinid fish to being mainly comprised of non‐native fish and crayfish.…”

“…Three key aspects are evident from the studies presented within the special issue. First, long term studies (e.g., Bedmar et al, 2022; Guareschi et al, 2022; Kjærstad et al, 2022), are imperative within the field of invasion biology. Without such studies, understanding the temporal trajectories of invasion dynamics and factors influencing establishment may dictate that monitoring efforts are too short‐sighted to be effective (Christie et al, 2019).…”