Causes and Consequences of Territoriality in a Grazing Stream Insect

The use and defense of food and space in the territorial larval caddisfly Leucotrichia pictipes (Trichoptera: Hydroptilidae) were evaluated through field observations and experiments in southwestern Michigan. These grazing stream insects live in silk cases attached to hard substrates and aggressively expel intruders (both conspecifics and other species) that enter their algal—covered foraging areas. Defended areas contained significantly greater algal standing crops, which were a benefit resulting from resource defense. This territorial behavior often led to patterns of uniform spacing between larvae in high—density patches. Exceptions to these patterns tended to p rove the rule; when Leucotrichia cases were much closer than normal, at least one of the individuals had entered a nonterritorial life history stage. Larval territory size was determined by food requirements and food availability. For example, territory size is related to the resident's body mass according to the equation: Area = 2.39 Mass 0.84 . The exponent for this power function is similar to the exponent relating metabolic rate to mass in many insects, further supporting the view that territoriality in Leucotrichia is related to the defense of a limiting food resource. Testing the effect of food availability on territory size provided support for several optimal feeding—territory—size models; when availability was experimentally reduced, residents expanded the size of their territories relative to control larvae. Ordinarily, other competing grazer species (e.g., the mayfly nymph Baetis flavistriga) spent less time per unit area foraging within Leucotrichia's territory than they spent foraging elsewhere. However, when resident caddisflies were experimentally removed from their territories, these mayfly nymphs spent more time inside than outside the territory. The territory of Leucotrichia often provided a resource o asis containing significantly higher algal standing crops than unprotected areas, which were heavily utilized by a diverse guild of grazers. The mayfly nymphs seem to have adopted a territory—parasite strategy similar to that of some nectar—feeding birds, in that they regularly robbed resources from Leucotrichia territories by a combination of stealth and speed. Other members of the grazer guild were less able to gain access to these limited resources, so that Leucotrichia acted as a dominant competitor in this community.