a12 = (K1 - N1 )N2 = 3.67 and a21 = (K2 - N2)N = -0.16 (8.2)

Conifer Aphids

| Predation: syrphid larva preying on a conifer aphid, Cinara sp., on

Douglas fir.

| Predation: syrphid larva preying on a conifer aphid, Cinara sp., on

Douglas fir.

has been demonstrated widely through biological control programs and experimental studies (e.g., Price 1997, D. Strong et al. 1984, van den Bosch et al. 1982, Van Driesche and Bellows 1996). However, many arthropods prey on vertebrates as well. Predaceous aquatic dragonfly larvae, water bugs, and beetles include fish and amphibians as prey. Terrestrial ants, spiders, and centipedes often kill and consume amphibians, reptiles, and nestling birds (e.g., C. Allen et al. 2004, Reagan et al. 1996).

Insects also represent important predators of plants or seeds. Some bark beetles might be considered to be predators to the extent that they kill multiple trees. Seed bugs (Heteroptera), weevils (Coleoptera), and ants (Hymenoptera) are effective seed predators, often kill seedlings, and may be capable of preventing plant reproduction under some conditions (e.g., Davidson et al. 1984,Turgeon et al. 1994, see Chapter 13).

Insects are an important food source for a variety of other organisms. Carnivorous plants generally are associated with nitrogen-poor habitats and depend on insects for adequate nitrogen (Juniper et al. 1989, Krafft and Handel 1991). A variety of mechanisms for entrapment of insects has evolved among carnivorous plants, including water-filled pitchers (pitcher plants), triggered changes in turgor pressure that alter the shape of capture organs (flytraps and bladderworts), and sticky hairs (e.g., sundews). Some carnivorous plants show conspicuous ultraviolet (UV) patterns that attract insect prey (Joel et al. 1985), similar to floral attrac tion of some pollinators (see Chapter 13). Insects also are prey for other arthropods (e.g., predaceous insects, spiders, mites) and vertebrates. Many fish, amphibian, reptile, bird, and mammal taxa feed largely or exclusively on insects (e.g., Dial and Roughgarden 1995, Gardner and Thompson 1998, Tinbergen 1960). Aquatic and terrestrial insects provide the food resource for major freshwater fisheries, including salmonids (Cloe and Garman 1996, Wipfli 1997).

Predation has been widely viewed as a primary means of controlling prey population density. Appreciation for this lies at the heart of predator-control policies designed to increase abundances of commercial or game species by alleviating population control by predators. However, mass starvation and declining genetic quality of populations protected from nonhuman predators have demonstrated the importance of predation to maintenance of prey population vigor, or genetic structure, through selective predation on old, injured, or diseased individuals. As a result of these changing perceptions, predator reintroduction programs are being implemented in some regions. At the same time, recognition of the important role of entomophagous species in controlling populations of insect pests has justified augmentation of predator abundances, often through introduction of exotic species, for biological control purposes (van den Bosch et al. 1982, Van Driesche and Bellows 1996). As discussed in Chapter 6, the relative importance of predation to population regulation, compared to other regulatory factors, has been a topic of considerable discussion.

Just as co-evolution between competing species has favored niche partitioning for more efficient resource use, co-evolution between predator and prey has produced a variety of defensive strategies balanced against predator foraging strategies. Selection favors prey that can avoid or defend against predators and favors predators that can efficiently acquire suitable prey. Prey defenses include speed; predator detection and alarm mechanisms; spines or horns; chemical defenses; cryptic, aposematic, disruptive, or deceptive coloration; and behaviors (such as aggregation or warning displays) that enhance these defenses (e.g., Conner et al. 2000, Jablonski 1999, Sillen-Tullberg 1985; see Chapter 4). Prey attributes that increase the energy cost of capture will restrict the number of predators able to exploit that prey.

Predators exhibit a number of attributes that increase their efficiency in immobilizing and acquiring prey, including larger size; detection of cues that indicate vulnerable prey; speed; claws or sharp mouthparts; venoms; and behaviors (such as ambush, flushing, or attacking the most vulnerable body parts) that compensate for or circumvent prey defenses (Jablonski 1999, Galatowitsch and Mumme 2004, Mumme 2002), and reduce the effort necessary to capture the prey. For example, a carabid beetle, Promecognathus laevissimus, straddles its prey, poly-desmid millipedes, and quickly moves toward the head. It then pierces the neck and severs the ventral nerve cord with its mandibles, thereby paralyzing its prey and circumventing its cyanide spray defense (G. Parsons et al. 1991).

Predators are relatively opportunistic with respect to prey taxa, compared to parasites, although prey frequently are selected on the basis of factors determining foraging efficiency. For example, chemical defenses of prey affect attractiveness to nonadapted predators (e.g., Bowers and Puttick 1988, Stamp et al.

1997, Traugott and Stamp 1996). Prey size affects the resource gained per foraging effort expended. Predators generally should select prey sizes within a range that provides sufficient energy and nutrient rewards to balance the cost of capture (Ernsting and van der Werf 1988, Iwasaki 1990,1991, Richter 1990, Streams 1994, Tinbergen 1960). Within these constraints, foraging predators should attack suitable prey species in proportion to their probability of encounter (i.e., more abundant prey types are encountered more frequently than are less abundant prey types; e.g., Tinbergen 1960).

Predators exhibit both functional (behavioral) and numeric responses to prey density. The functional response reflects predator hunger, handling time required for individual prey, ability to discover prey, handling efficiency resulting from learning, etc. (Holling 1959,1965, Tinbergen 1960). For many invertebrate predators, the percentage of prey captured is a negative binomial function of prey density, Holling's (1959) type 2 functional response. The ability of type 2 predators to respond individually to increased prey density is limited by their ability to capture and consume individual prey. Vertebrates, and some invertebrates, are capable of increasing their efficiency of prey discovery (e.g., through development of a search image that enhances recognition of appropriate prey; Tinbergen 1960) and prey processing time through learning, up to a point. The percentage of prey captured initially increases as the predator learns to find and handle prey more quickly but eventually approaches a peak and subsequently declines as discovery and handling time reach maximum efficiency, Holling's (1959) type 3 functional response. The type 3 functional response is better able, than the type 2 response, to regulate prey population size(s) because of its capacity to increase the percentage of prey captured as prey density increases, at least initially.

Various factors affect the relationship between prey density and proportion of prey captured. The rate of prey capture tends to decline as a result of learned avoidance of distasteful prey, and the maximum rate of prey capture depends on how quickly predators become satiated and on the relative abundances of palatable and unpalatable prey (Holling 1965). Some insect species, such as the periodical cicadas, apparently exploit the functional responses of their major predators by appearing en masse for brief periods following long periods of inaccessibility. Predator satiation maximizes the success of such mass emergence and mating aggregations (K.Williams and Simon 1995). Palatable species experience greater predation when associated with less palatable species than when associated with equally or more palatable species (Holling 1965).

In addition to these functional responses, predator growth rate and density tend to increase with prey density. Fox and Murdoch (1978) reported that growth rate and size at molt of the predaceous heteropteran, Notonecta hoffmanni, increased with prey density in laboratory aquaria. Numeric response reflects predator orientation toward, and longer residence in, areas of high prey density and subsequent reproduction in response to food availability. However, increased predator density also may increase competition, and conflict, among predators. The combination of type 3 functional response and numeric response (total response) makes predators effective in cropping abundant prey and maintaining relatively stable populations of various prey species. However, the tendency to become satiated and to reproduce more slowly than their prey limits the ability of predators to regulate irruptive prey populations released from other controlling factors.

The importance of predator-prey interactions to population and community dynamics has generated considerable interest in modeling this interaction. The effect of a predator on a prey population was first incorporated into the logistic model by Lotka (1925) and Volterra (1926). As described in equation 6.11, their model for prey population growth was as follows:

N1(t+1) = N1t + r^t - p1N1tN2t where N2 is the population density of the predator and p1 is a predation constant. Lotka and Volterra modeled the corresponding predator population as follows:

where p2 is a predation constant and d2 is per capita mortality of the predator population. The Lotka-Volterra equations describe prey and predator populations oscillating cyclically and out of phase over time. Small changes in parameter values lead to extinction of one or both populations after several oscillations of increasing amplitude.

Pianka (1974) proposed modifications of the Lotka-Volterra competition and predator-prey models to incorporate competition among prey and among predators for prey. Equation 6.12 represents the prey population:

where a12 is the per capita effect of the predator on the prey population. The corresponding model for the predator population is as follows

where a21 is the negative effect of predation on the prey population and P 2 incorporates the predator's carrying capacity as a function of prey density (Pianka 1974). This refinement provides for competitive inhibition of the predator population as a function of the relative densities of predator and prey. The predator-prey equations have been modified further to account for variable predator and prey densities (Berlow et al. 1999), predator and prey distributions (see Begon and Mortimer 1981), and functional responses and competition among predators for individual prey (Holling 1959, 1966). Other models have been developed primarily for parasitoid-prey interactions (see later in this chapter).

Current modeling approaches have focused on paired predator and prey. Real communities are composed of multiple predator species exploiting multiple prey species, resulting in complex interactions (Fig. 8.4). Furthermore, predator effects on prey are more complex than mortality to prey. Predators also affect the distribution and behavior of prey populations. For example, Cronin et al. (2004) found that web-building spiders, at high densities, were more likely to affect planthoppers, Prokelisia crocea, through induced emigration than through direct

Eotetranychus Typhlodromus

Julian Date

Densities of three phytophagous mites, Aculus schlechtendali, Bryobia rubrioculus, and Eotetranychus sp. (prey), and three predaceous mites, Amblyseius andersoni, Typhlodromus pyri, and Zetzellia mali, in untreated apple plots (N = 2) during 1994 and 1995. Data from Croft and Slone (1997).

mortality. Johansson (1993) reported that immature damselflies, Coenagrion hastulatum, increased avoidance behavior and reduced foraging behavior when immature dragonfly, Aeshna juncea, predators were introduced into experimental aquaria.

C. Symbiosis

Symbiosis involves an intimate association between two unrelated species. Three types of interactions are considered symbiotic, although the term often has been used as a synonym for only one of these, mutualism. Parasitism describes interactions in which the symbiont derives a benefit at the expense of the host, as in predation. Commensalism occurs when the symbiont derives a benefit without significantly affecting its partner. Mutualism involves both partners benefiting from the interaction. Insects have provided some of the most interesting examples of symbiosis.

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