Food dispersion and foraging energetics; a mechanistic synthesis for field studies of avian benthivores

Ecology, March, 1996 by James R. Lovvorn, Michael P. Gillingham

INTRODUCTION

Considerable data and methodology have developed around modeling and measuring both the energy costs of free existence in animals (Nagy 1987, Costa 1988, Goldstein 1988, Birt-Friesen et al. 1989) and their foraging strategies (Tome 1988, Beauchamp et al. 1992, Houston and Carbone 1992, Ball 1994). Likewise, much effort has recently focused on refining concepts and measurements of patch structure, and identifying appropriate spatial scales for different ecological analyses (O'Neill et al. 1988, Kotliar and Wiens 1990, Malatesta et al. 1992). However, few studies have linked these disciplines mechanistically to analyze effects of resource dispersion on rates of energy expenditure and intake (Mason and Patrick 1993, Turner et al. 1993, Lovvorn 1994a). In this paper, we develop and explore an individual-based model that relates field measurements of the dispersion of benthic foods to search costs and foraging profitability (energy intake minus expenditure) of diving ducks.

From an autecological perspective, a number of studies have sought to quantify the energy cost of foraging in a given species (Croll 1993, Wilson and Culik 1993 and references therein). For example, the cost of diving (including pauses between dives) in Tufted Ducks (Aythya fuligula) in water at 7.4 [degrees] C has been reported as 18.9 W/kg or 1.7 times the cost of resting at the surface (Bevan and Butler 1992). However, this cost was measured at a single depth (0.6 m); and because of depth-dependent differences in mechanical power required for descent vs. bottom foraging, both depth and dive duration can appreciably affect the energy cost of a dive (Lovvorn et al. 1991, Lovvorn and Jones 1991, 1994, Lovvorn 1994a). Analyses of trade-offs among different foraging strategies must account for such variations in energy costs under different conditions (Beauchamp et al. 1992). However, it is difficult to measure oxygen consumption in chambers at the water surface for all combinations of dive depth and duration observed in different species in the field. A synthesis of biomechanics and respirometry, whereby values of aerobic efficiency (mechanical power output / aerobic power input) are applied to calculations of mechanical energy cost, offers the capability of estimating dive costs under different conditions as has been done for aerial flight (Pennycuick 1989, Lovvorn and Jones 1994).

From a resource management perspective, we often need to know how much habitat is required to support a population of animals, in order to set habitat protection priorities, acceptable levels of impact, and standards for restoration (Goss-Custard 1977, Korschgen et al. 1988). Past studies have calculated the average energy requirements of birds, and then compared these estimates to total food biomass present to infer sustainable population levels (Anderson and Low 1976, Cornelius 1977, Korschgen et al. 1988, Lovvorn and Baldwin 1996), impacts on the food base (Grant 1981, Howard and Lowe 1984, Baldwin and Lovvorn 1994), or competition with other species (Eadie and Keast 1982). However, food dispersion affects the biomass that can be fed upon profitably, and thus the fraction of food organisms subject to depletion (Lovvorn 1994a). Moreover, models using parameters averaged over entire populations might yield different results from individual-based models that simulate the foraging energetics of many individuals (Huston et al. 1988). The latter distinction is especially important to evaluating spatial effects, because foraging economics often vary widely among individuals depending on their specific locations in heterogeneous habitats (Roese et al. 1991). Individual-based models are needed to analyze how food requirements vary with food dispersion and consequent search costs, and how to sample food organisms in ways that reflect their economic availability to foragers.

From an ecosystem perspective, the role of vertebrates in structuring prey communities and in nutrient regeneration depends on spatial and temporal patterns of predation, grazing, and excretion. Such patterns depend in turn on foraging profitability relative to food dispersion, i.e., search effort and food densities for which energy costs exceed gains and foraging ceases. Estimates of food requirements that do not consider the spatial pattern of food intake have unclear ecological implications, especially for animals that forage over large areas. For example, nutrients excreted by eiders and gulls in the Gulf of St. Lawrence are unimportant to the Gulf's total nutrient budget, but input by birds at aggregation sites can be locally significant (Bedard et al. 1980; see also Ruess et al. 1989, Powell et al. 1991, Manny et al. 1994). Shorebirds switching prey as profitability changes with prey depletion can alter the structure of invertebrate communities (Schneider 1978); and patchy herbivory can affect plant dispersion quite differently from more continuous grazing (Andrew and Jones 1990, Hyman et al. 1990). Thus, linking foraging energetics to the patch structure of food organisms can allow "scaling up" of organismal physiology and biomechanics to effects at community and ecosystem levels (Huston et al. 1988, Ehleringer and Field 1993). However, most spatial foraging models have used simple constants for quite variable physiological values such as locomotor costs, without comparing the consequences of physiological variability to that of other parameters at larger scales (Hyman et al. 1990, Roese et al. 1991, Mason and Patrick 1993, Turner et al. 1993).