Macroinvertebrate response to management strategies and habitat condition at Kealia Pond NWR, Maui, Hawaii

Invertebrates are an essential food source for breeding and nonbreeding waterbirds and their abundance and distribution is dependent on vegetation and a diversity of environmental variables. To determine macroinvertebrate relationships within specific plant communities and habitat conditions, a 2-year study was conducted at Kealia Pond National Wildlife Refuge, Maui, Hawaii. Macroinvertebrate density, richness, and distribution were measured in response to roto-tilling treatments, microhabitat variables, and tilapia predation.; Monocultures of exotic vegetation were roto-tilled to increase plant diversity and open-water/vegetation interspersion for foraging and breeding Hawaiian Stilts (Himantopus mexicanus knudseni) and Hawaiian Coots (Fulica alai). Invertebrate samples and abiotic variables (water temperature, salinity, depth, and dissolved oxygen) were collected within specific plant communities in roto-tilled plots and controls (monoculture vegetation). Vegetation structure and the presence of freshwater were the most important variables regulating invertebrate presence, abundance, and richness. Corixidae and Chironomidae were the most abundant invertebrate families throughout the study site. Invertebrate richness and densities of isopods, amphipods, crawfish, odonates, and Chironomus spp. were highest in vegetated habitats with fresh ground water inputs. In contrast, corixids and Polypedilum nubifer were most common in habitats with patchy vegetation or bare substrate. Isopods and amphipods occurred in higher densities in shallowly flooded sites, but odonates, such as Coenagrionidae, Libellulidae, and Aeshnidae were most common within relatively deeper water habitat.; Enclosures were constructed to determine effects of tilapia on invertebrate densities and richness. Five tilapia (8-12 cm) were released within 3 replicate enclosures in bare substrate and sedge habitat. Invertebrate densities within enclosures stocked with tilapia indicated that tilapia negatively impacted invertebrates because invertebrate densities decreased 48%-96% after tilapia were introduced. Tilipia predation rates did not appear to be influenced by vegetation structure and tilapia were not strongly prey-size selective. However, food availability and prey behavior influenced tilipia predation rates because corixid densities decreased at faster rates than chironomid densities in enclosures. This information is the basis for providing guidance on management actions that affect food resources for waterbirds including invertebrate abundance, distribution, and diversity in relation to changes in plant communities and habitat condition.