Effects of invasion at two trophic levels on diet, body condition, and population size structure of Hawaiian red shrimp

We examined the degree to which invasion by non‐native, nitrogen (N)‐fixing riparian trees and poeciliid fish alters diet of a common grazer/detritivore in brackish ponds on the dry coast of Hawai‛i Island. Because this shrimp, Halocaridina rubra (ʻōpaeʻula), displays a preference for autotrophic components of epilithon, we hypothesized that tree canopy would reduce their body condition and abundance, but that this would be moderated by nutrient quality of leaf litter (high‐quality, non‐native, nitrogen‐fixing tree vs. low‐quality, endemic non‐nitrogen‐fixing tree). We hypothesized that poeciliid invasion would reduce population size and body condition of H. rubra by direct predation and by inducing reductions in feeding rate. Analysis of stable isotopes (δD, δ 13 C, and δ 15 N) showed that direct consumption of leaf litter detritus supplemented H. rubra diet in ponds with canopy cover. In addition, epilithon in ponds with canopy cover was isotopically more similar to leaf litter than epilithon in ponds without canopy cover, and our evidence suggests this may be due to contributions of allochthonous nutrient sources to epilithon assemblages. However, canopy cover, whether native or invasive, did not decrease epilithon growth rates and had little effect on shrimp body condition or population size. In contrast, poeciliid invasion reduced daylight grazing on the benthos, and caused a reduction in body condition among larger shrimp. Our results indicate that primary succession by invasive trees may shift food web linkages in these ponds from autotrophic to heterotrophic support, without negative impacts on native grazer/detritivore communities. We suggest that poeciliid invasions have sub‐lethal effects on H. rubra as well as their direct predation effects, with potential long‐term implications for the ecosystem function of anchialine ponds and other coastal systems.