Ideal free distribution
11.5.1 Ideal free distribution
In the simplest version of the IFD, food is supplied to a foraging habitat or patch at a fixed rate, where it is consumed immediately. If predators are ‘free’ to select among patches, have perfect or ‘ideal’ knowledge of the rates of resource supply and do not differ in their competitive abilities, then we ex- pect the number of predators in a patch to be pro- portional to the total food input to that patch. This is called the ‘input-matching rule’. A number of experimental studies have tested the input- matching prediction, including studies by Milinski (1979, 1984) with sticklebacks (Gasterosteus aculeatus ), Godin and Keenleyside (1984) with cichlids (Aequidens curviceps) and Abrahams (1989) with guppies (Poecilia reticulata). These studies and others (see Parker and Sutherland 1986; Tregenza 1995 for reviews) show that the number of fish foraging in a patch is often pro- portional to the rate of resource input, supporting the input-matching prediction of IFD. However, the further prediction of IFD theory, that the feed- ing rates of individual foragers should be equal in all patches, is rarely true (Parker and Sutherland 1986; Hugie and Grand 1998; Tregenza and Thompson 1998). A number of factors can lead to unequal feeding rates among individuals, includ- ing unequal competitive abilities. Simple IFD mod- els have been modified to include the impact of unequal competitors (e.g. Parker and Sutherland 1986; Sutherland and Parker 1992), and in these models it is the sum of the competitive units that is predicted to match the input of resources to a patch. This is called the input-matching of com- petitive units. However, different combinations of numbers and types of competitors may satisfy this prediction. A recent study by Hugie and Grand (1998) suggests that even when foragers differ in their competitive abilities, the expected distribu- tion of foragers predicted by IFD models with unequal competitors will tend to match the predictions of simple IFD models. Hugie and Grand (1998) review a number of studies with fish that support their hypothesis.
Classical IFD models assume a continuous input of resources into a patch and immediate Classical IFD models assume a continuous input of resources into a patch and immediate
11.2 to predict the potential feeding rates of bluegill sunfish feeding in the open water, the vegetation and on the bare bottom of Michigan ponds and lakes. They found that large bluegill, which were not threatened by predators, foraged mostly in the habitat with the highest predicted energy gain. Thus, the fish behaved according to OFT.
The foraging model used by Mittelbach (1981) and Werner et al. (1983a) assumes that there is no interference between predators and that feeding rates are a function of prey density. In this case, all fish should feed initially in the habitat that pro- vides the highest foraging return. However, we expect relative habitat profitabilities to change as prey are depleted within a habitat, causing fish to switch habitats. In the studies of Mittelbach (1981) and Werner et al. (1983a), habitat profitabilities did change through time and large bluegill were able to track these changes, switching habitats to feed where there was the highest predicted energy gain. Theoretically, if foragers are of equal competitive ability, this system should settle into an equilib- rium where the distribution of foragers among habitats results in each habitat yielding the same feeding rate (Lessells 1995; Oksanen et al. 1995). Habitats that have higher prey productivity will have higher forager densities. However, equilib- rium is unlikely in a strongly seasonal environ- ment such as in the bluegill study, where predator and prey dynamics vary in response to changing environmental conditions. In a more constant
environment, equilibrium may occur. Power’s (1984) classic study of armoured catfish (Loricari- dae) feeding on periphyton (attached algae) in a Panamanian stream may be one such case. In Power’s (1984) study, algal productivities differed among stream pools due to differences in shading. The density of armoured catfish was directly pro- portional to the percentage of open canopy over the stream. However, algal standing crops and Lori- caridae growth rates, and estimates of their feeding rates, were similar in both sunny and dark pools. Power’s (1984) results therefore match the pre- dictions of ideal free habitat selection based on consumer–resource dynamics (Lessells 1995; Oksanen et al. 1995).
A number of studies have considered how inter- ference amongst predators may affect predicted habitat distributions in standing-stock IFD mod- els (e.g. Sutherland and Parker 1985, 1992; Parker and Sutherland 1986; Ruxton et al. 1992; Lessells 1995; Moody and Houston 1995; Stillman et al. 1997; van der Meer and Ens 1997). In these models, predator intake rate is assumed to decrease with increasing predator density as the result of inter- ference. Predicted habitat distributions in these models are sensitive to the way in which interfer- ence is incorporated into the predator’s functional response (Stillman et al. 1997; Weber 1998). How- ever, the results of Free et al. (1977), Lessells (1995) and Oksanen et al. (1995) suggest that patterns of habitat use derived from models of ideal free habitat selection and consumer–resource dy- namics may hold true unless interference is quite strong. For example, the armoured catfish studied by Power (1984) are known to exhibit feeding inter- ference, yet their habitat distribution closely matched the predictions of IFD theory.