Patch use under predation hazard: effect of the red imported fire ant on deer mouse foraging behavior

Ecology, Jan, 1997 by Wendee N. Holtcamp, William E. Grant, S. Bradleigh Vinson

INTRODUCTION

Optimal foraging theory (OFT) is based on the premise that animals should choose behaviors in ecological time that tend to maximize fitness, because these behaviors have been selected for over evolutionary time (Stephens and Krebs 1986). OFT has received criticism in recent years because fitness typically has been measured in a currency of energy (Schoener 1971, Cowie 1977, Hubbard and Cook 1978, Schmid-Hempel et al. 1985). In other words, animals were assumed to behave in a manner that maximized net energy gain or energy intake rate. Although it is true that, for many species, fitness is correlated positively with energy gain (e.g., Jameson 1953, Dewsbury 1981), fitness ia dependent on other inputs as well. Obviously, if an animal is preyed upon, its future fitness is dramatically and irretrievably decreased. In cases with a conflict between maximizing energy intake and avoiding predation, an animal's behavior may not be in agreement with classic optimal foraging predictions (Sih 1980, Lima et al. 1985, Gilliam and Fraser 1987, Newman et al. 1988, Abrahams and Dill 1989, Gotceitas 1990, Nonacs and Dill 1990).

Brown (1988) developed a patch use model (Eq. 1) that uses the marginal rate of substitution (see Caraco 1979) to incorporate an animal's risk of predation (P), metabolic costs (C), and missed opportunity costs (MOC; costs associated with foraging at the expense of alternate fitness-enhancing activities) into a common currency:

H = P C MOC. (1)

The model's predictions are simple: an animal should leave a patch when the benefits of foraging, represented by its harvest rate (H), no longer exceed the sum of the costs, represented by P, C, and MOC. By measuring an animal's quitting-harvest rate in a patch, one can estimate its perception of costs and benefits. Brown (1988) advocates the use of giving-up densities (GUDs) in place of quitting-harvest rates. GUD is the final prey density in a patch once a forager has harvested that patch. As long as the forager experiences diminishing returns in its rate of resource harvest, GUDs should correlate directly with quitting-harvest rates in a patch. The use of GUDs in lieu of quitting-harvest rates has received considerable empirical support (Brown and Alkon 1990, Brown et al. 1992, Kotler et al. 1993)

In this study, we tested Brown's (1988) patch use model by comparing GUDs and in-patch harvest rates of deer mice (Peromyscus maniculatus) foraging in the presence vs. the absence of red imported fire ants (Solenopsis invicta). The model predicts that GUDs and in-patch harvest rates should be higher when predation risk is increased (Brown 1988, 1992), because foragers should demand higher feeding rates to balance increased costs. The assumption of diminishing returns on harvest rate, which allows GUDs to be used as surrogates for quitting-harvest rates, was tested using a modified version of Holling's disc equation (Holling 1959, Kotler and Brown 1990). In addition, harvest rate, attack rate (parameters of Holling's disc equation), and handling times were compared between deer mice foraging in the presence vs. the absence of fire ants. A second objective of this study was to identify tradeoffs made by deer mice foraging under predation risk by comparing 13 additional attributes of foraging behavior in the presence vs. the absence of fire ants.

METHODS

Study animals

The deer mouse is a common cricetine rodent whose geographical range and temporal activity overlap extensively with that of the red imported fire ant in the southeastern United States (Grzimek 1990). Red imported fire ants were introduced to the United States in the 1930s and have since colonized [greater than]50 million ha in the southeastern USA (Lofgren 1986, Callcott and Collins 1995). Red imported fire ants prefer proteinaceous foods at most times of the year (Stein et al. 1990), occur in nest densities as high as 680 [ or -] 475 mounds/ha (Porter et al. 1991), and are extremely efficient in locating food (Urbani and Kannowski 1974). Fire ant activity is maximal when soil temperatures are between 22 [degrees] and 36 [degrees] C, but ants will forage at soil temperatures of 15 [degrees]-43 [degrees] C (Porter and Tschinkel 1987). During the spring and fall, fire ants are crepuscular; during the summer, fire ant activity is highest at dusk and continues throughout the night (S. B. Vinson, unpublished data). For crepuscular to nocturnal mice such as the deer mouse (Schmidly 1983), we would expect foraging behavior to be most heavily influenced by fire ants during these seasonal periods of activity overlap.

Imported fire ants are omnivorous, preying primarily on invertebrates but also on a wide variety of native birds, reptiles, and mammals (e.g., Johnson 1961, Ashdown 1969, Hill 1969, 1972, Mount 1981, Mount et al. 1981, Ridlehuber 1982, Masser and Grant 1986, Sikes and Arnold 1986, Freed and Neitman 1988, Grant and Killion 1988, Wilson and Silvy 1988, Flickinger 1989, Drees 1992). Despite numerous reports of fire ant predation on native vertebrates, the ecological consequences of fire ant predation on native vertebrates remain speculative (Allen et al. 1994). Small ground-dwelling mammals, such as field mice, seem particularly susceptible to fire ant predation, and field research has shown that small mammals alter their patterns of habitat selection to avoid fire ants (Smith et al. 1990, Killion 1992, Stoker 1992, Killion and Grant 1993, Ferris 1994 a, b, Killion et al. 1995; W. N. Holtcamp and W. E. Grant, unpublished manuscript). Whether or not fire ant predation is extensive enough to cause population fluctuations, it is a real cost for an animal that must actively avoid ants while pursuing its usual activities.