The predatorprey interaction

All predators show some degree of preference, feeding mainly on certain species of prey. Aspects of the predator that bias it toward consuming more of some prey than others include sensory capabilities, foraging mode, and behavioral mechanism of prey capture. For prey, many aspects of life style and body plan influence their vulnerability. These traits of predator and prey were introduced in Sections 8.2.4 and 8.3, and here we elaborate on the many ways that prey abundance, size, morphology, and behavior shape the interactions between individual predators and prey.

9.2.1.1 Vertebrate predators

Prey abundance and size are both strong predictors of prey consumption of fishes. The number of prey eaten increases with prey abundance for all types of predators, at a decelerating rate due to the time limitation imposed by the handling and ingestion of individual food items. This relationship is known as a functional response curve. Whenever more than one type of prey is present, gut analyses generally find that prey that are abundant in the environment are also common in the diet (Allan 1981). However, the correspondence often is not 1:1, indicating some degree of preference. Prey choice can be strongly influenced by contrast, motion, and size, all of which serve to make certain prey more conspicuous. Many studies have established that predation intensity increases with prey size (Metz 1974, Allan 1978). Larger prey items are expected to be preferred because they offer a greater energy reward and simply because they are more readily detected.

Feeding behavior often changes with experience and learning in vertebrate predators. Ringler (1979) found that brown trout preferred large prey (the mealworm Tenebrio molitor) over small (the brine shrimp Artemia salina); however, this preference developed gradually over 4-6 days and the least preferred prey never was completely excluded from the diet (Figure 9.6). Changes in fish predatory behavior due to experience result in higher rates of predation. Searching often improves via greater reactive distances, higher swimming speeds, and greater path efficiency, while attack latency may decrease and capture success may increase (Dill 1983). The result is a tendency to specialize on the prey that the predator has consumed most frequently in its recent feeding history, with an accompanying increase in foraging efficiency. Hunger can influence predation rate by modifying any of several aspects of predatory behavior. As hunger decreases, searching also decreases owing to changes in movement speed and reactive distance. In addition, the probability that an attack will follow an encounter declines, and handling time increases (Ware 1972). Capture rate consequently varies with hunger level.

Environmental variables, especially those that affect prey visibility, can significantly modify predation rates. Although visually dependent predators can feed under quite dim light, prey capture success declines with falling light levels. A light

FIGURE 9.6 The size preference for large prey of driftfeeding brown trout in a laboratory stream. Wild trout were maintained on a diet of brine shrimp. In this experiment, brine shrimp (dashed lines) only were provided on day 1, and the larger mealworms (solid lines) were added on day 2. Drift rates were 5 (open circles, low) and 10 (solid circles high) per minute. "Electivity" is a measure of preference based on prey consumed in relation to prey available. (Reproduced from Ringler 1979.)

FIGURE 9.6 The size preference for large prey of driftfeeding brown trout in a laboratory stream. Wild trout were maintained on a diet of brine shrimp. In this experiment, brine shrimp (dashed lines) only were provided on day 1, and the larger mealworms (solid lines) were added on day 2. Drift rates were 5 (open circles, low) and 10 (solid circles high) per minute. "Electivity" is a measure of preference based on prey consumed in relation to prey available. (Reproduced from Ringler 1979.)

intensity of 0.1 lux, corresponding to late dusk or a full moon, often is the lower threshold for effective visual location of prey (Hyatt 1979). Even within the range we consider daylight, however, gradation in light level can be influential. Wilzbach et al. (1986) compared the feeding of cutthroat trout in pools from forested sections of streams with pools from open (logged) sections. Prey were captured at higher rates in open pools, and artificial shading lowered the capture rate to that observed in shaded pools. Under varying light conditions corresponding to twilight, moonlight, and overcast night conditions, the foraging efficiency of young Atlantic salmon in the laboratory was unaffected by current velocity until light levels fell below 0.1 lux, at which point the fish were more efficient at prey capture in slower currents (Metcalfe et al. 1997). When provided a choice of foraging location, juvenile salmon shifted toward slower velocity positions as light level was reduced. Habitat characteristics can also influence prey availability and ease of prey capture. The capture rate of epibenthic prey declines with increasing complexity of the substrate; for example, sculpins feeding in the laboratory were able to capture prey more readily from a sandy bottom than from a more heterogeneous cobble habitat (Brus-ven and Rose 1981).

9.2.1.2 Invertebrate predators

Relative body size of species within a food web strongly influences trophic relationships, influencing resource partitioning, diet breadth, and predator-prey interactions (Warren and Lawton 1987, Chase 1999). Larger species may outgrow predation risk, entering a "size refuge'' at some stage of their life cycle, whereas smaller species may never reach a size where they escape predation. Although these statements may apply to all predator-prey interactions they are especially true within invertebrate systems, as is illustrated by the extensive mutual predation and cannibalism seen within the predator guild of Broadstone Stream, UK (Woodward and Hildrew 2002a). The six species (three predaceous midges, a caddisfly, an alderfly, and a dragonfly) exhibited marked size differences, but relative size relationships changed seasonally due to growth (Figure 9 7). Small predators had the narrowest diets, and niche overlap was greater when sizes overlapped strongly and was reduced as predator size diverged (Figure 9 8). The largest predator, Cordulegaster boltonii, was preyed upon only by larger conspecifics, and the smallest, Zavrelimyia barbatipes, was eaten by all five of the larger species and by conspecifics. The direction of intraguild predation could be reversed whenever early instars of large species

FIGURE 9 7 Relative abundance size-spectra of benthic macroinvertebrates in the Broadstone Stream, UK, on six sampling occasions in 1996-1997. The double-headed arrows indicate the size ranges of the six predator species. From largest to smallest the predators include the dragonfly Cordulegaster boltonii, the alderfly Sialis fuliginosa, the caddisfly Plectrocnemia conspersa, and three tanypod midges Macropelopia nebulosa, Trissopelopia longimana, and Zavrelimyia barbatipes. (Reproduced from Woodward and Hildrew 2002a.)

FIGURE 9 7 Relative abundance size-spectra of benthic macroinvertebrates in the Broadstone Stream, UK, on six sampling occasions in 1996-1997. The double-headed arrows indicate the size ranges of the six predator species. From largest to smallest the predators include the dragonfly Cordulegaster boltonii, the alderfly Sialis fuliginosa, the caddisfly Plectrocnemia conspersa, and three tanypod midges Macropelopia nebulosa, Trissopelopia longimana, and Zavrelimyia barbatipes. (Reproduced from Woodward and Hildrew 2002a.)

coexisted with late instars of small species. In this system, clearly, food web structure was influenced mainly by body size relationships, although encounter probabilities and foraging mode also were influential.

Predator foraging mode affects prey vulnerability, interacting with aspects of prey movement to influence localized encounter rates and departures. Mobile prey are likely to flee if able to detect the approach of large, actively searching predators, and so predator impact may be greatest with least mobile prey. This is a complication for cage experiments, which have the potential to overestimate predator impact when predator and prey are confined, and to underestimate whenever prey can escape or enter from the surrounding environment (Wooster and Sih 1995). For sit-and-wait predators, prey mobility may increase their mortality as a consequence of increased encounter rates. In the Broadstone Stream, predation by the dragonfly C. boltonii, a sit-and-wait predator, fell most heavily on mobile mayflies, which were not greatly depleted due to high prey exchange rates, but their losses were indeed attributable to consumption rather than flight (Woodward and Hildrew 2002b). In the same system, the net-spinning caddis Plec-trocnemia conspersa was also reported to have the greatest impact on mobile prey (Lancaster et al. 1991). Prey abundance, movement by crawling or drifting, and speed of prey movement and predator attack likely are additional variables affecting encounter rate and capture success with sit-and-wait predators.

The foraging behavior of predaceous invertebrates does not appear to be much influenced by

Plec Structure
FIGURE 9.8 Pair-wise dietary overlap among invertebrate predators as a function of differences in individual predator body size using mean log dry mass of pairs of predators among size classes within each species. See Figure 9.7 for species codes. (Reproduced from Woodward and Hildrew 2002a.)

prey availability or prior experience, although it has been suggested that predators aggregate in areas of high prey density (Townsend and Hildrew 1978, Malmqvist and Sjostrom 1980). However, Peckarsky and Dodson (1980) found that predaceous stoneflies were no more likely to colonize cages containing high prey densities than cages with few prey. Peckarsky (1985) argued that the absence of any aggregative behavior in these predators is explained by the ephemeral nature of prey patches, since highly mobile potential victims like Baetis can rapidly disperse. Hunger level did influence which prey were consumed by the stonefly Hesperoperla pacifica offered a choice between the soft-

bodied, agile-swimming mayfly Baetis bicauda-tus, and the slow and clumsy Ephemerella altana, which has a spiney and rigid exoskele-ton (Molles and Pietruszka 1983). Starved stone-flies ate mostly E. altana, while satiated stoneflies ate both prey in about equal numbers. When freshly killed prey were offered to starved predators, however, a preference for Baetis was evident. The proposed explanation was that starved predators attacked both prey equally, but with increasing satiation began to restrict their attack only to Baetis.

Habitat complexity and the availability of refuges can markedly alter predation rates. Refuges may be absolute, rendering the prey unavailable, but more commonly they serve to reduce the likelihood of encounter and capture. In laboratory trials with two invertebrate predators, four invertebrate prey, and various substrate conditions, Fuller and Rand (1990) showed that all variables affected prey capture rates. Baetis was more vulnerable than the other prey (an ephemerellid mayfly, a black fly larva, and several hydropsychids), probably because its mobility led to high encounter rates. The predators, a stonefly and an alderfly, differed in their predation rates on various substrates due to differences in their sensing of prey with their antennae and pursuit success. The substrates, which included sand, gravel mixed with pebbles, and artificial turf, resulted in differential capture success via its effects on encounter rates and by facilitating the construction of stronger retreats in some caddis. Although the particular outcomes may be influenced by specifics of the experimental design, such effects of habitat complexity on prey capture probably are common.

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