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

Abstract

Abstract. 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 (ʻ opaeʻula), 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 (dD, d13 C, and d 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.

Grazing by an endemic atyid shrimp controls microbial communities in the Hawaiian anchialine ecosystem

“…Our work and previous studies suggest complex "top-down" and "bottom-up" processes influence overall community composition in the Hawaiian anchialine ecosystem (Bailey-Brock and Brock 1993;Capps et al 2009;Carey et al 2011;Dalton et al 2013;Sakihara et al 2015;Seidel et al 2016;Dudley et al 2017). Our microcosm grazing experiment and that of Sakihara et al (2015) provide clear evidence that grazing by H. rubra alters microbial communities, while Dalton et al (2013) found ponds with higher nutrient inputs supported more microbial and shrimp biomass.…”

“…However, detailed microbial surveys from both environmental communities and shrimp guts are lacking. Thus, while microbial ecology in Hawaii's anchialine ecosystem is subject to both “top‐down” and “bottom‐up” processes (Dalton et al 2013; Sakihara et al 2015; Seidel et al 2016; Dudley et al 2017), many questions remain.…”

Assessing the spatial–temporal response of groundwater‐fed anchialine ecosystems to sea‐level rise for coastal zone management

“…Infilling rates are also increased by the accumulation of organic matter from native and introduced plants that grow around the edges of pools situated on older lava flows (Brock & Kam, 1997). Additionally, leaf litter subsidies from both native and introduced tree species appear to alter food‐web dynamics and, in some cases, increase dissolved inorganic nutrient levels (Dudley et al, 2017; Nelson‐Kaula et al, 2016). Studies have not examined whether vegetation around anchialine pools may grow more rapidly in areas with sewage‐enriched groundwater.…”

“…Surveys of over 400 pools in Hawai'i show that introduced predatory fishes (poeciliids and tilapia) have a strong negative effect on the occurrence of endemic species in anchialine pools (Marrack et al, 2015). In pools with introduced poeciliids, the dominant grazing shrimp Halocaridina rubra and predatory shrimp Metabetaeus lohena changed their behaviour and retreated out of pools during the day into the subsurface aquifer (Capps et al, 2009; Carey et al, 2011; Dudley et al, 2017). In exclusion experiments, decreased grazing pressure from H. rubra , such as might occur when introduced fish are present, led to increased epilithon biomass (Sakihara et al, 2015), which could increase the rates of pond infilling.…”

Anchialine pool shrimp (Halocaridina rubra) as an indicator of sewage in coastal groundwater ecosystems on the island of Hawaiʻi

“…The goal of this study was to test the potential use of the dominant anchialine pool grazer, the atyid shrimp Halocaridina rubra, as a bioindicator for sewage in groundwater flowing through coastal anchialine habitats. Adult H. rubra are usually less than 1.5 cm in length and may be found in densities as high as 175 per 0.25 m 2 feeding on biofilms that cover the rocky substrate [40,41]. Healthy anchialine pools typically do not contain visible macroalgae or significant amounts of epilithon [42,43], and therefore an alternate bioindicator organism would be useful.…”