Trophic consequences of invasive mrigal carp for native mud carp by comparing the results of intra- and inter-specific competition

Abstract Predicting the ecological impacts of invasive species on analogous native species has great significance for native population conservation. We tested how the strength of interspecific competition from a coexisting invasive species varies with abundance and how this compares with intraspecific competition. The model species was the native mud carp, and the coexisting invasive species was mrigal carp. The replicated mesocosm used each species in an allopatric control (n = 10), in intraspecific competition treatments (n = 20, 30), in a sympatric control (n = 5 for both species) and in interspecific competition treatments (n = 10, 15, 20 for both species). The treatment effects were the specific growth rate (SGR), position and size of trophic (isotopic) niches. Compared to mrigal carp, mud carp had a significantly lower survival rate (median: 0.8) and SGR. The mud carp SGR significantly decreased when the con- and inter-specific densities increased, showing a density dependence, but there was no significant difference between intra- and inter-specific competition. Compared to intraspecific competition in which mud carp expanded their niche, the presence of mrigal carp drove the mud carp trophic niche toward broader sizes at lower trophic positions, but intensified interspecific competition drove the mud carp trophic niche toward constriction. Both mud carp and mrigal carp expanded their niches in invaded communities, and their niches overlapped with each other. Their niche expansions implied plasticity in resource utilization. These results also suggested a complexity in how ilyophagous invasive species integrate into food webs and alter their structure.