Competition is the struggle for use of shared, limiting resources. Resources can be limiting at various amounts and for various reasons. For example, water or nutrient resources may be largely unavailable and support only small populations or a few species in certain habitats (e.g., desert and oligotrophic lakes) but be abundant and support larger populations or more species in other habitats (e.g., rainforest and eutrophic lakes). Newly available resources may be relatively unlimited until sufficient colonization has occurred to reduce per capita availability. Any resource can be an object of interspecific competition (e.g., basking or oviposition sites, food resources, etc.).

Although competition for limited resources has been a major foundation for evolutionary theory (Malthus 1789, Darwin 1859), its role in natural communities has been controversial (e.g., Connell 1983, Lawton 1982, Lawton and Strong 1981, Schoener 1982, D. Strong et al. 1984). Denno et al. (1995) and Price (1997) attributed the controversy over the importance of interspecific competition to three major criticisms that arose during the 1980s. First, early studies were primarily laboratory experiments or field observations. Few experimental field studies were conducted prior to the late 1970s. Second, Hairston et al. (1960) argued that food must rarely be limiting to herbivores because so little plant material is consumed under normal circumstances (see also Chapter 3). As a result, most field experiments during the late 1970s and early 1980s focused on effects of predators, parasites, and pathogens on herbivore populations. Third, many species assumed to compete for the same resource(s) co-occur and appear not to be resource limited. In addition, many communities apparently were unsaturated (i.e., many niches were vacant; e.g., Kozar 1992b, D. Strong et al. 1984). The controversy during this period led to more experimental approaches to studying competition. Some (but not all) experiments in which one competitor was removed have demonstrated increased abundance or resource use by the remaining competitor(s) indicative of competition (Denno et al. 1995, Istock 1973,1977, Pianka 1981). However, many factors affect interspecific competition (Colegrave 1997), and Denno et al. (1995) and Pianka (1981) suggested that competition may operate over a gradient of intensities, depending on the degree of niche partitioning (see later in this section).

Denno et al. (1995) reviewed studies involving 193 pairs of phytophagous insect species. They found that 76% of these interactions demonstrated competition, whereas only 18% indicated no competition, although they acknowledged that published studies might be biased in favor of species expected to compete. The strength and frequency of competitive interactions varied considerably. Generally, interspecific competition was more prevalent, frequent, and symmetrical among haustellate (sap-sucking) species than among mandibulate (chewing) species or between sap-sucking and chewing species. Competition was more prevalent among species feeding internally (e.g., miners and seed-, stem-, and wood-borers; Fig. 8.1) than among species feeding externally. Competition was observed least often among free-living, chewing species (i.e., those generally emphasized in earlier studies that challenged the importance of competition).

Competition Between Species

I Competition: evidence of interference between southern pine beetle, Dendroctonus frontalis, larvae (small mines) and co-occurring cerambycid, Monochamus titillator, larvae (larger mines) preserved in bark from a dead pine tree. The larger cerambycid larvae often remove phloem resources in advance of bark beetle larvae, consume bark beetle larvae in their path, or both.

I Competition: evidence of interference between southern pine beetle, Dendroctonus frontalis, larvae (small mines) and co-occurring cerambycid, Monochamus titillator, larvae (larger mines) preserved in bark from a dead pine tree. The larger cerambycid larvae often remove phloem resources in advance of bark beetle larvae, consume bark beetle larvae in their path, or both.

Most competitive interactions (84%) were asymmetrical (i.e., one species was a superior competitor and suppressed the other) (Denno et al. 1995). Root feeders were consistently out-competed by folivores, although this, and other, competitive interactions may be mediated by host plant factors (see later in this chapter). Istock (1973) demonstrated experimentally that competition between two waterboatmen species was asymmetrical (Fig. 8.2). Population size of Hes-perocorixa lobata was significantly reduced when Sigara macropala was present, but population size of S. macropala was not significantly affected by the presence of H. lobata.

Competition generally is assumed to have only negative effects on both (all) competing species (but see the following text). As discussed in Chapter 6, competition among individuals of a given population represents a major negative feedback mechanism for regulation of population size. Similarly, competition among species represents a major mechanism for regulation of the total abundance of multiple-species populations. As the total density of all individuals of competing species increases, each individual has access to a decreasing share of the resource(s). If the competition is asymmetrical, the superior species may com-

Stocked alone

Not stocked

Stocked alone

With H. lobata

Not stocked

With S. macropala

H. lobata S. macropala

Results of competition between two waterboatmen species, Hesperocorixa lobata and Sigara macropala, in 1.46 m2 enclosures in a 1.2-ha pond. Enclosures were stocked in June with adult H. lobata or S. macropala, or both, and final abundance was measured after 2 months. Waterboatmen in unstocked enclosures provided a measure of colonization. Vertical bars represent 1 S. D. N = 4-8. Data from Istock (1973).

petitively suppress other species, leading over sufficient time to competitive exclusion (Denno et al. 1995, Park 1948, D. Strong et al. 1984). However, Denno et al.

(1995) found evidence of competitive exclusion in <10% of the competitive interactions they reviewed. Competitive exclusion normally may be prevented by various factors that limit complete preemption of resources by any species. For example, predators that curb population growth of the most abundant competing species can reduce its ability to competitively exclude other species (R. Paine 1966,1969a, b).

Interspecific competition can take different forms and have different possible outcomes. Exploitation competition occurs when all individuals of the competing species have equal access to the resource. A species that can find or exploit a resource more quickly, develop or reproduce more rapidly, or increase its efficiency of resource utilization will be favored under such circumstances. Interference competition involves preemptive use, and often defense of, a resource that allows a more aggressive species to increase its access to, and share of, the resource, to the detriment of other species.

Many species avoid resources that have been marked or exploited previously, thereby losing access. It is interesting that males of territorial species usually compete with conspecific males for mates and often do not attack males of other species that also compete for food resources. Foraging ants may attack other predators and preempt prey resources. For example, Halaj et al. (1997) reported that exclusion of foraging ants in young conifer plantations increased abundances of arboreal spiders >1.5-fold. Gordon and Kulig (1996) reported that foragers of the harvester ant, Pogonomyrmex barbatus, often encounter foragers from neighboring colonies, but relatively few encounters (about 10%) involved fighting, and fewer (21% of fights) resulted in death of any of the participants. Nevertheless, colonies were spaced at distances that indicated competition. Gordon and Kulig

(1996) suggested that exploitative competition among ants foraging for resources in the same area may be more costly than is interference competition. Because competition can be costly, in terms of lost resources, time, or energy expended in defending resources (see Chapter 4), evolution should favor strategies that reduce competition. Hence, species competing for a resource might be expected to minimize their use of the contested portion and maximize use of the noncon-tested portions. This results in partitioning of resource use, a strategy referred to as niche partitioning. Over evolutionary time, sufficiently consistent partitioning might become fixed as part of the species' adaptive strategies, and the species would no longer respond to changes in the abundance of the former competi-tor(s). In such cases, competition is not evident, although niche partitioning may be evidence of competition in the past (Connell 1980). Congeners also usually partition a niche as a result of specialization and divergence into unexploited niches or portions of niches, not necessarily as a result of interspecific competition (Fox and Morrow 1981).

Niche partitioning is observed commonly in natural communities. Species competing for habitat, food resources, or oviposition sites tend to partition thermal gradients, time of day, host species, host size classes, etc. Several examples are noteworthy.

Granivorous ants and rodents frequently partition available seed resources. Ants specialize on smaller seeds and rodents specialize on larger seeds when the two compete. J. Brown et al. (1979) reported that both ants and rodents increased in abundance in the short term when the other taxon was removed experimentally. However, Davidson et al. (1984) found that ant populations in rodent-removal plots declined gradually but significantly after about 2 years. Rodent populations did not decline over time in ant-removal plots. These results reflected a gradual displacement of small-seeded plants (on which ants specialize) by large-seeded plants (on which rodents specialize) in the absence of rodents. Ant removal led to higher densities of small-seeded species, but these species could not displace large-seeded plants.

Predators frequently partition resources on the basis of prey size. Predators must balance the higher resource gain against the greater energy expenditure (for capture and processing) of larger prey (e.g., Ernsting and van der Werf 1988). Generally, predators should select the largest prey that can be handled efficiently (Holling 1965, Mark and Olesen 1996), but prey size preference also depends on hunger level and prey abundance (Ernsting and van der Werf 1988) (see later in this chapter).

Most bark beetle (Scolytidae) species can colonize extensive portions of dead or dying trees when other species are absent. However, given the relative scarcity of dead or dying trees and the narrow window of opportunity for colonization (the first year after tree death), these insects are adapted to finding such trees rapidly (see Chapter 3) and usually several species co-occur in suitable trees. Under these circumstances, the beetle species tend to partition the subcortical resource on the basis of beetle size because each species shows the highest survival in phloem that is thick enough to accommodate growing larvae and because larger species are capable of repulsing smaller species (e.g., Flamm et al. 1993). Therefore, the largest species usually occur around the base of the tree, and progressively smaller species occupy successively higher portions of the bole, with the smallest species colonizing the upper bole and branches. However, other competitors, such as wood-boring cerambycids and buprestids, often excavate through bark beetle mines, feeding on bark beetle larvae and reducing bark beetle survival (see Fig. 8.1) (Coulson et al. 1980, Dodds et al. 2001).

Many competing species partition resource use in time. Partitioning may be by time of day (e.g., nocturnal versus diurnal Lepidoptera [Schultz 1983] and nocturnal bat and amphibian versus diurnal bird and lizard predators [Reagan et al. 1996]) or by season (e.g., asynchronous occurrence of 12 species of water-boatmen [Heteroptera: Corixidae], which breed at different times [Istock 1973]). However, temporal partitioning does not preclude competition through preemptive use of resources or induced host defenses (see later in this chapter).

In addition to niche partitioning, other factors also may obscure or prevent competition. Resource turnover in frequently disturbed ecosystems may prevent species saturation on available resources and prevent competition. Similarly, spatial patchiness in resource availability may hinder resource discovery and prevent species from reaching abundances at which they would compete. Finally, other interactions, such as predation, can maintain populations below sizes at which competition would occur (R. Paine 1966, 1969a, b; see later in this chapter).

Competition has proved to be rather easily modeled (see Chapter 6). The Lotka-Volterra equation generalized for n competitors is as follows:

where Ni and Nj are species abundances, and a represents the per capita effect of Nj on the growth of Ni and varies for different species. For instance, species j might have a greater negative effect on species i than species i has on species j (i.e., asymmetrical competition).

Istock (1977) evaluated the validity of the Lotka-Volterra equations for co-occurring species of waterboatmen, H. lobata (species 1) and S. macropala (species 2), in experimental exclosures (see Fig. 8.2). He calculated the competition coefficients, a12 and a21, as follows:

The intercepts of the zero isocline (dN/dt = 0) for H. lobata were K1 = 88 and K1/a12 = 24; the intercepts for S. macropala were K2 = 6 and K2/a21 = -38. The negative K2/a21 and position of the zero isocline for S. macropala indicate that the competition is asymmetrical, consistent with the observation that S. macropala population growth was not affected significantly by the interaction (see Fig. 8.2). Although niche partitioning by these two species was not clearly identified, the equations correctly predicted the observed coexistence.

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