Habitat choice in a

11.5.3 Habitat choice in a

0 three-trophic-level system

A forager’s risk of mortality within a habitat Predicted

(b)

1.0 switch may also change due to the movement of their own predators between habitats. In a three-trophic- level system, predators at the top trophic level and

Preference for more hazardous site

foragers at the middle trophic level are both free to

0.5 select between habitats that differ in resources at +

+ the bottom trophic level. In this situation, habitat choice by the predators and foragers can be mod- elled as a spatial game. Hugie and Dill (1994) mod-

0 0.2 0.4 0.6 0.8 1.0 elled the situation where there are two habitats Tubifex density in more hazardous site (cm –2 )

that differ in their inherent riskiness, indepen- dently of predator densities, and that differ in the

Fig. 11.3 Predicted and observed habitat selection by rate of resource production. Foragers and predators juvenile creek chubs: (a) habitat preference in

experiments where habitats contained either one or are free to select between habitats and forager feed-

three predators; (b) habitat preference in experiments ing rates follow the assumption of IFD theory, with with habitats containing one or two predators. The

resource production within a habitat being divided x -axis shows the density of resources (Tubifex worms)

equally among foragers. Predator feeding rates are in the habitat with the higher mortality risk; the y-axis

based on their functional response to prey density, shows the observed habitat use (measured as preference

which also determines the prey’s mortality rate, for the riskier habitat). The vertical arrows labelled

and predators, being at the top trophic level, are ‘Predicted switch’ indicate the density of resources in

the riskier habitat, where the fish are predicted to switch assumed to have a constant mortality rate among

from feeding in the safer habitat to feeding in the riskier habitats. Both predator and prey are assumed to habitat. Points represent the results of individual feeding

choose habitats so as to maximize their net repro- trials. Crosses represent treatment means; horizontal

ductive rate (R 0 ), which is an appropriate measure bars represent pooled means for orthogonal contrasts.

of fitness only when population size is constant (Source: modified from Gilliam and Fraser 1987;

over time (Stearns 1992; see Hutchings, Chaper reproduced by permission of the Ecological Society of

7, this volume for alternative fitness measures). America.) Maximizing R 0 is the same fitness criterion used

by Gilliam (1982), and Hugie and Dill’s (1994) cri- Grand and Dill (1999) recently modelled this terion of optimal habitat choice by both predator case for foragers of equal and unequal competitive and prey is equivalent to Gilliam and Fraser’s ability. In general, these models predict that the criterion of minimizing m/f. dilution of risk with group size results in habitat

Hugie and Dill (1994) found that when habitats distributions that differ from the IFD input- differ in their inherent riskiness, independently matching rule. Specific predictions, however, of predator density, foragers should prefer the less depend on the relative risk of using alternative risky habitat, and the ratio of forager densities in habitats, the strength of risk dilution and the rela- the two habitats should be the inverse of the ratio

Fish Foraging and Habitat Choice

No interference

2.5 Fig. 11.4 Results of the habitat selection game between predator and prey. Habitats i and j differ in inherent riskiness

i /R j

(R) and productivity of the prey’s food resource (P). All panels show the ratio of prey (d i /d j ) and predator (d i ¢ /d j ¢ ) densities as a function of the ratios of habitat riskiness (R i /R j ) and resource productivity (P i /P j ). (a,b) Results for the basic model (no interference between predators); (c,d) results when predators interfere with each other’s foraging ability. Note that in (a) and (b), predator density responds to differences in habitat productivities but prey density does not. In (c) and (d), both predator and prey densities respond to habitat productivities. (Source: modified from Hugie and Dill 1994.)

of the habitat risks (Fig. 11.4a). For example, if one of foragers (Fig. 11.4a). Predator distributions, habitat is twice as risky as another, it should have however, are dependent on both relative habitat half the density of prey. There is of course consider- riskiness and relative habitat productivity, and able empirical support for the prediction that fish more predators will be found in the more produc- and other foragers should prefer the inherently less tive habitats (Fig. 11.4b). risky habitat (see reviews in Lima and Dill 1990;

The results of Hugie and Dill’s (1994) behav- Lima 1998a,b). The more surprising prediction of ioural game between predator and prey closely Hugie and Dill’s (1994) model is that habitat pro- parallel the predictions of ecological food web ductivity should have no effect on the distribution theory (Hairston et al. 1960; Oksanen et al. 1981)

Chapter 11

based on trophic-level responses to changes in en- vironmental productivity. In simple food-chain models with three trophic levels, the biomasses of the top and bottom trophic levels are predicted to respond positively to an increase in the productiv- ity of the environment, whereas the biomass of the middle trophic level remains constant (Oksanen et al. 1981). In these models, changes in biomass among trophic levels are due to numerical res- ponses brought about through births and deaths and not to movement of individuals between habitats of differing productivity (but see Wootton and Power 1993). The results of Hugie and Dill’s (1994) analysis show that the same pattern of response to an increase in productivity within a habitat (i.e. an increase in the abundance of the top trophic level and no change in the abundance of the middle trophic level) can result from behavioural decisions of predators and foragers, with no change in global population size by the consumers. Wootton and Power (1993) suggest the same result, although their theoretical development of a behav- ioural model is limited to a short appendix to their paper. Hugie and Dill (1994) also note that if pre- dators exhibit interference, then the abundance of both predators and foragers will depend on habitat productivity (Fig. 11.4c,d), which again is analo- gous to the findings of food-chain population mod- els with interference among the top carnivores (Wollkind 1976; Mittelbach et al. 1988).

Hugie and Dill (1994) conclude their paper with the statement that ‘Perhaps the single most impor- tant new insight arising from our modeling of habitat selection as a game is the prediction that prey distributions may not respond to experimental changes in food abundance, whenever their preda- tors are free to respond in a dynamic way’. I would suggest that an equally interesting and exciting result is that behavioural models and population dynamic models predict the same pattern of trophic-level response to environmental produc- tivity, albeit for different reasons. Schwinning and Rosenzweig (1990) examined a model system in which all three trophic levels were free to choose among habitats based on IFD criteria. They found that in general the system oscillated through time, with individuals shifting back and forth between

not achieve a simultaneous IFD. Schwinning and Rosenzweig (1990) present some of the factors that might stabilize such oscillations.

Adaptive responses by fish to predation risk and foraging gain may also lead to cascading effects in aquatic food webs. A number of studies have shown that a change in the abundance of piscivo- rous fish can have effects that cascade down the food chain to lower trophic levels, although these may be buffered by redundancy in marine food webs (see Polunin and Pinnegar, Chapter 14 and Persson, Chapter 15, this volume; and Kaiser and Jennings, Chapter 16, Volume 2). Such top-down effects are typically assumed to be transmitted by direct consumption of lower trophic levels by higher trophic levels. However, behavioural re- sponses by fish and other foragers may produce the same type of trophic cascade, if an increase in the abundance of a higher trophic level causes foragers at the next lower trophic level to reduce feeding ac- tivity, shift habitats or spend more time in a refuge (e.g. Turner and Mittelbach 1990; Brabrand and Faafeng 1993).