Insects, like all animals, interact with other species in a variety of ways, as competitors, predators, prey, and mutualists. Interactions among species will be discussed in greater detail in Chapter 8. These interactions require varying degrees of energy or nutrient expenditure, or both. Contests among individuals for resources occasionally involve combat. Subduing prey and defending against predators also involve strenuous activity. Mutualism requires reciprocal exchange of resources or services. Obviously, these activities affect the energy and nutrient budgets of individual organisms.
1. Competitive Behavior Competition occurs among individuals using the same limiting resources at the same site. Energy expended, or injury suffered, defending resources or searching for uncontested resources affects fitness. Competition often is mediated by mechanisms that determine a dominance hierarchy. Establishment of dominant and subordinate status among individuals limits the need for physical combat to determine access to resources and ensures that dominant individuals get more resources than do subordinate individuals.
Visual determination of dominance status is relatively rare among insects, largely because of their small size; the complexity of the environment, which restricts visual range; and the limitations of fixed-focus compound eyes for longdistance vision (Matthews and Matthews 1978). Dragonflies have well-developed eyes and exhibit ritualized aggressive displays that maintain spacing among individuals. For example, male Plathemis lydia have abdomens that are bright silvery-white above. Intrusion of a male into another male's territory initiates a sequence of pursuit and retreat, covering a distance of 8-16 m. The two dragonflies alternate roles and directions, with the abdomens raised during pursuit and lowered during retreat, until the intruder moves to another site (P. Corbet 1962).
Mediation of competition by pheromones has been documented for several groups of insects. Adult flour beetles, Tribolium, switch from aggregated distribution at low densities to random distribution at intermediate densities, to uniform distribution at high densities. This spacing is mediated by secretion of quinones, repellent above a certain concentration, from thoracic and abdominal glands (Matthews and Matthews 1978). Larvae of the flour moth, Anagasta kunniella, secrete compounds, from the mandibular glands, that increase dispersal propensity, lengthen generation time, and reduce the fecundity of females that were crowded as larvae (Matthews and Matthews 1978). Bark beetles use repellent pheromones, as well as acoustic signals, to maintain minimum distances between individuals boring through the bark of colonized trees (Raffa et al. 1993, Rudinsky and Ryker 1976). Ant colonies also maintain spacing through marking of foraging trails with chemical signals (see earlier in this chapter and Chapter 3).
Acoustic signals are used by many Orthoptera and some Coleoptera to deter competitors. Bark beetles stridulate to deter other colonizing beetles from the vicinity of their gallery entrances (Rudinsky and Ryker 1976). Subsequently, excavating adults and larvae respond to the sounds of approaching excavators by mining in a different direction, thus preventing intersection of galleries. Some male crickets and grasshoppers produce a distinctive rivalry song when approaching each other (Matthews and Matthews 1978,Schowalter and Whitford 1979). The winner (continued occupant) usually is the male that produces more of this aggressive stridulation.
When resources are relatively patchy, males may increase their access to females by marking and defending territories that contain resources attractive to females. Territorial behavior is less adaptive (i.e., costs of defending resources exceeds benefits) when resources are highly concentrated and competition is severe or when resources are uniformly distributed and female distribution is less predictable (Baker 1972, Schowalter and Whitford 1979).
Marking territorial boundaries takes a variety of forms among animal taxa. Male birds mark territories by calling from perches along the perimeter. Male deer rub scent glands and scrape trees with their antlers to advertise their territory. Social insects, including ants, bees, and termites, mark nest sites and foraging areas with trail pheromones that advertise their presence. These trail markers can be perceived by other insects at minute concentrations (see Chapter 3). Many orthopterans and some beetles advertise their territories by stridulating.
However, many insects advertise their presence simply to maintain spacing and do not actively defend territories. Similarly, males of many species, including insects, fight over receptive females. E.Wilson (1975) considered defense of occupied areas to be the defining criterion for territoriality. Territorial defense is best known among vertebrates, but a variety of insects representing at least eight orders defend territories against competitors (Matthews and Matthews 1978, Price 1997). Because territorial defense represents an energetic cost, an animal must gain more of the resource by defending it against competitors than by searching for new resources. Nonaggressive males often "cheat" by nonadver-tisement and quiet interception of resources or of females attracted to the territory of the advertising male (Schowalter and Whitford 1979).
The type of territory differs among insect taxa, but usually it is associated with competition for food or mates (Matthews and Matthews 1978, Price 1997). Male crickets defend the area around their dens and mate with females attracted to their stridulation. Male eastern woodroaches, Cryptocercus punctulatus, defend mating chambers in rotten wood (Ritter 1964). Some insects that form leks defend small territories within the lek. Presumably, more females are attracted to this concentration of males, increasing mating success, than to isolated males (Price 1997). Such mating territories apparently are not related to food or ovipo-sition sites but may maximize attraction of females.
Two grasshopper species, Ligurotettix coquilletti and Bootettix argentatus, that feed on creosote bush, Larrea tridentata, in the deserts of the southwestern United States are perhaps the only territorial acridoids (Otte and Joern 1975, Schowalter and Whitford 1979). These grasshoppers defend individual creosote bushes. The larger bushes are more likely to harbor females, and opportunities for mating are increased by defending larger shrubs, especially at low grasshopper population densities. Schowalter and Whitford (1979) reported that male movement from small shrubs was greater than movement from larger shrubs, and contests for larger shrubs occurred more frequently. However, fewer males defended territories at high population densities, apparently because interception of females by nonstridulating males and more frequent combat decreased mating success of territorial defenders.
Males of the speckled wood butterfly, Pararge aegeria (Satyridae), defend sunspots on the forest floor, apparently because females are attracted to resources that occur in sunspots (Price 1997). Only 60% of the males held such territories, but these encountered many more females than did the nonterritorial males that searched for mates in the forest canopy. Defense of an oviposition site may be advantageous where sperm competition cannot be avoided by anatomical or physiological means, such as with mating plugs that prevent subsequent mating. Another butterfly, Inachis io, defends territories at the approach to oviposition sites, perhaps because of selective pressure from strong competition at the oviposition sites (Baker 1972). Other insects, especially the social Hymenoptera, defend nests, foraging trails, or food (Price 1997).
The benefits of defending food resources or mates must be weighed against the costs of fighting, in terms of time, energy, and risk of injury. Territorial insects may abandon territorial defense at high population densities when time spent fighting detracts from feeding or mating success (Schowalter and Whitford 1979).
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