Interference Competition in a Tropical Ripple Bug (Hemiptera: Veliidae)

Abstract

A population of the ripple bug Rhagovelia scabra (Bacon) (Hemiptera: Veliidae) in the Rio Guacimal, Costa Rica, is structured by interference competition into three components: (1) optimal habitat users, who occupy fast-flowing areas such as the heads of pools; (2) suboptimal habitat users, who occupy stiller water; and (3) non-users, who form dense lethargic clusters by the margins of the stream. The distributions are enforced by the dominant animals and lead to a priority of prey capture. Dominance is determined mostly by age, sex, and disability. ECOLOGISTS RECOGNIZE TWO FORMS of competition (Miller 1967): Exploitation, in which an organism interacts only with its resources, and interference, in which an organism interacts directly with its competitors, excluding them from resources and thereby obtaining more for itself. The theoretical consequences of exploitation are fairly well worked out, and in fact form the foundation of the Lotka-Volterra equations on which most competition models are based. However, the theoretical consequences of interference are less well understood, and, moreover, seem to lead to a greater variety of outcomes (Schoener 1976). Many questions present themselves. Does interference simply space animals out over the available habitat (Huxley 1934) or does it generate a have/have-not situation in which one segment of the population lives amidst relative plenty while another starves, disperses, or lives in areas that will not support offspring (Brown 1969) ? If the latter, what determines the proportions of the population falling into these classes and how do they change with fluctuations in food supply? Does interference constitute a form of population regulation (see Rogers and Hassel 1974. Rogers and Hubbard 1974, Wynne-Edwards 1962, Wilson 1977 for different approaches), and, if so, is regulation the essence of, or coincidental to, its evolution? How are subordinate animals maintained in the population if they are so severely selected against by interference? This paper describes a very simple field study on interference competition and some of its consequences in a population of the tropical ripple bug Rhagovelia scabra Bacon (Hemiptera: Veliidae). The data are of interest for two reasons. First, they show how a surprisingly 'vertebrate' form of population structure can exist in an insect using simple behavior patterns. The ripple bug's population structure closely resembles that which Brown (1969) describes for birds. Second, working with insects allows manipulations that are difficult, if not impossible, for vertebrates, on which most studies of this type are conducted. The research site was on the Rio Guacimal, a high mountain stream near Monte Verde, Guanacaste Province, Costa Rica. The work was conducted during June-July 1976. The methods are very simple and are described along with the results.