Abstract
Introduced non-native fishes can cause considerable adverse impacts on freshwater ecosystems. The pumpkinseed Lepomis gibbosus, a North American centrarchid, is one of the most widely distributed non-native fishes in Europe, having established self-sustaining populations in at least 28 countries, including the U.K. where it is predicted to become invasive under warmer climate conditions. To predict the consequences of increased invasiveness, a field experiment was completed over a summer period using a Control comprising of an assemblage of native fishes of known starting abundance and a Treatment using the same assemblage but with elevated L. gibbosus densities. The trophic consequences of L. gibbosus invasion were assessed with stable isotope analysis and associated metrics including the isotopic niche, measured as standard ellipse area. The isotopic niches of native gudgeon Gobio gobio and roach Rutilus rutilus overlapped substantially with that of non-native L. gibbosus, and were also substantially reduced in size compared to ponds where L. gibbosus were absent. This suggests these native fishes shifted to a more specialized diet in L. gibbosus presence. Both of these native fishes also demonstrated a concomitant and significant reduction in their trophic position in L. gibbosus presence, with a significant decrease also evident in the somatic growth rate and body condition of G. gobio. Thus, there were marked changes detected in the isotopic ecology and growth rates of the native fish in the presence of non-native L. gibbosus. The implications of these results for present and future invaded pond communities are discussed.
Highlights
Introduced non-native fishes can cause considerable adverse impacts on the structure and function of aquatic ecosystems, in freshwaters, the evidence is often circumstantial or speculative (e.g. Gozlan 2008; Gozlan et al 2010)
It was apparent that when comparing the isotopic niche sizes among experimental ponds where L. gibbosus were present and absent, both G. gobio and R. rutilus had significant reductions in their isotopic niche in L. gibbosus presence
In the latter study, which examined the potential impact of L. gibbosus presence on the growth of native Eurasian perch Perca fluviatilis in the same experimental ponds as the present study, no effect on growth was observed in either species
Summary
Introduced non-native fishes can cause considerable adverse impacts on the structure and function of aquatic ecosystems, in freshwaters, the evidence is often circumstantial or speculative (e.g. Gozlan 2008; Gozlan et al 2010). Where niche partitioning is not possible, the niche variation hypothesis predicts that the increased competitive interactions between the species will result in diet constriction, leading to increased diet specialisation post-invasion (Van Valen 1965; Thomson 2004; Olsson et al 2009). These outcomes were recently observed in invasive topmouth gudgeon Pseudorasbora parva populations in the U.K., where strong patterns of niche divergence and constriction were evident in invaded native fish communities (Jackson and Britton 2014; Tran et al 2015). The derivation of empirical data within robust experimental designs can assist risk assessment processes by providing increased insights into the risks posed by specific species, as well as concomitantly testing aspects of relevant ecological theory (Copp et al 2014)
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