Evaluating if Energy and Protein Limit Abundance of Hawaiian Moorhen

Food abundance can affect a species' distribution. In many studies of potential food limitation, researchers focus on carrying capacity estimates during the nonbreeding season for temperate species consuming a fixed food source. Estimates of energetic carrying capacity for year-round breeders feeding on a replenishing resource would be more difficult and require much data. To determine whether gathering detailed information on year-round carrying capacity would be an important investment, we conducted an assessment to determine whether there was evidence that energy or protein might limit numbers of the tropical, endangered Hawaiian moorhen (Gallinula chloropus sandvicensis). We compared observed numbers of moorhen at 15 Oahu, Hawaii, USA, wetlands with predicted numbers based on measured energy and protein in food plants and abundance of these food plants in each wetland and on estimates of energy expenditure of moorhen. We made comparisons assuming moorhen are limited by their ability to metabolize food plants, by competition for food, and by estimated costs associated with reproduction. We also compared ranked moorhen abundance and density with ranked energy and protein under different wetland management regimes. Energy values consistently overestimated expected numbers of Hawaiian moorhen at wetlands except for one wetland location (predicted, 3803 ± 4856; observed, 6.2 ± 10.8). In addition, we detected no significant relationship between moorhen abundance and measures of energy (all r2 = 0.02–0.73, all P > 0.1) or protein abundance (all r2 = 0.08–0.50, all P > 0.3). This lack of relationship held once we controlled for wetland area or when we considered whether wetlands were managed for waterbirds. Hawaiian moorhen on Oahu did not appear to be limited by energy, nor did they appear to select sites based on energy or protein, in contrast to many studies relating animal numbers to energy in nonbreeding situations. Consequently, we suggest that researchers and managers explore other potentially limiting factors for Hawaiian moorhen.