Abstract:
1. Ecological theory attempts to predict how impacts for native species arise from biological
invasions. A fundamental question centres on the feeding interactions of invasive and native
species: whether invasion will result in increased interspecific competition, which would result
in negative consequences for the competing species, or trophic niche divergence, which would
facilitate the invader’s integration into the community and their coexistence with native species.
2. Here, the feeding interactions of a highly invasive fish, topmouth gudgeon Pseudorasbora
parva, with three native and functionally similar fishes were studied to determine whether
patterns of either niche overlap or divergence detected in mesocosm experiments were apparent
between the species at larger spatial scales. Using stable isotope analysis, their feeding
relationships were assessed initially in the mesocosms (1000 L) and then in small ponds
(<400 m2) and large ponds (>600 m2).
3. In the mesocosms, a consistent pattern of trophic niche divergence was evident between
the sympatric fishes, with niches shifting further apart in isotopic space than suggested in
allopatry, revealing that sharing of food resources was limited. Sympatric P. parva also had a
smaller niche than their allopatric populations.
4. In eight small ponds where P. parva had coexisted for several years with at least one of
the fish species used in the mesocosms, strong patterns of niche differentiation were also
apparent, with P. parva always at a lower trophic position than the other fishes, as also
occurred in the mesocosms. Where these fishes were sympatric within more complex fish communities
in the large ponds, similar patterns were also apparent, with strong evidence of trophic
niche differentiation.
5. Aspects of the ecological impacts of P. parva invasion for native communities in larger
ponds were consistent with those in the mesocosm experiments. Their invasion resulted in
divergence in trophic niches, partly due to their reduced niche widths when in sympatry with
other species, facilitating their coexistence in invaded ecosystems. Our study highlights the
utility of controlled mesocosm studies for predicting the trophic relationships that can
develop from introductions of non-native species into more complex ecosystems and at larger
spatial scales.
