Patterns of Limiting Similarity in Tropical Forest Tiger Beetles (Coleoptera: Cicindelidae)

Sympatric tiger beetles in tropical forests comprise one of the few insect assemblages to exhibit patterns of limiting similarity, principally known for vertebrates (Hutchinsonian ratios). On 10 sites in the Old World (Borneo, New Guinea, Gabon) and New World tropics (Costa Rica, Panama, Ecuador, Peru', Bolivia), the most similar-sized sympatric tiger beetle species had a median mandible length (chord) ratio of 1.35. This ratio was the smallest on sites with more than three ground or undergrowth species (i = 1.26) and largest on sites with only two such species (i = 1.36). It was also small among species on the same site that foraged for insect prey on different substrates (ground, undergrowth leaves, tree trunks, etc.). The observed mandible ratios on sites with only two common species that were both ground-foragers were significantly greater than expected from random matches of tiger beetle species pairs. Populations of species that occurred on more than one site all varied geographically in mandible length but showed consistent mandible ratios (e 1.35) with the most similar-sized congener on each site. Preliminary base resource sampling by sticky traps showed a generally similar abundance and size distribution among sites during the initial part of the rainy season. However, marked seasonal differences in insect abundance and size range from site to site likely influenced the number of tiger beetle species per plot. The number of dry months (<150 mm x monthly rainfall) was shown to be inversely correlated with the number of ground and low foliage foraging tiger beetle species on each plot. A CONSIDERABLE PROPORTION of community structure theory is based on studies of vertebrates. The studies of species diversity, species packing, and subdivision of resources, initiated largely by MacArthur (1958) and Hutchinson (1959) using birds, have led to an abundance of research that has been reviewed several times (Pianka 1966, 1978; Ricklefs 1979; Schoener 1974; and many others). In general, the work based on bird communities leads to the conclusion that competition is the principal cause of observed patterns of community structure. The potential for these evolutionary consequences of competition appears to be especially high in the species-rich communities of tropical lowland forests. Because of the high number of species, chances for biotic interactions such as competition are also high. In several tropical vertebrate assemblages (Pearson 1975, Emmons 1980, and many others), two specific patterns that have received considerable attention are character divergence and limiting similarity. Divergence in characters important for use in obtaining a limiting resource, usually food, makes possible a subdivision of resources. This result is interpreted to reduce competition so that co-occurrence is possible (Lack 1947). Hutchinson (1959) first attempted to quantify the limiting similarity for potentially competing sympatric congeners by measuring the ratio of food-gathering morphological characters. Using birds (bill length) and mammals (skull length), he found this ratio to average 1.28. May and MacArthur (1972) later theorized that environmental variability could modify this degree of limiting similarity, but only if the variability was relatively severe. Evidence concerning the role of competition in shaping insect assemblages is, however, inconsistent. Many studies of insects result in data that are interpreted to indicate that interspecific competition in these assemblages is not a significant factor. Physical factors such as temperature and humidity (Ross 1957, Andrewartha 1961, Schaefer 1972, and many others) or natural predators (DeBach 1966) are seen by some as so important that interspecific competition rarely affects abundance or distribution of insects. Other investigators find interspecific competition among insects to be present, but suggest that it has resulted primarily in spatial (Gilotra et al. 1967, Moore and Fisher 1969, Fluker and Beardsley 1970, Chestnut and Douglas 1971, Budnik and Brncic 1974) or temporal (Caplan 1966; Istock 1966, 1967, 1973; Gerson 1967; Ueckert and Hansen 1971; Kirk 1971; Shapiro 1974) exclusion. Only a few examples of character divergence among sympatric insect species have been interpreted to be the result of interspecific competition. Some of these examples concern competition for oviposition site (Heatwole and Davis 1965, Force 1970, Price 1972) and mates (Waage 1975), and some concern competition for food (Brian 1956, Moore 1964, Stahl 1966, Ricklefs et al. 1969, Hamilton 1971, Swift and Fedorenko 1975, Van Zant et al. 1978). Although BIOTROPICA 12(3): 195-204 1980 195 This content downloaded from 157.55. 39.104 on Mon, 20 Jun 2016 06:05:47 UTC All use subject to http://about.jstor. org/terms Hutchinsonian ratios have seldom been found to apply to insects, Price (1975) showed such ratios for sympatric wasp ovipositor lengths. Pearson and Mury (1979) showed a ratio of 1.35 or more for mandible lengths in an assemblage of grassland tiger beetle species from southeastern Arizona (U.S.A.). Here I present evidence of character divergence and patterns of limiting similarity in tropical lowland forest tiger beetles. This evidence will consist of: 1) comparisons of patterns in mandible length and to some extent body length in seven genera of tiger beetles (fig. 1) from forest plots around the world, and 2) limited comparisons with patterns found in vertebrate communities, especially tropical forest birds. Tropical forest tiger beetles, as is typical of the entire family, are principally carnivorous. They are visual predators on live arthropods and, except for noxious ants and other arthropods with effective deterrents such as chemicals or thick cuticle, make foraging choices on the basis of prey size. Pearson and Mury (1979) established that mandible length was positively correlated with median prey size for tiger beetles. Mandible tips and palpi, however, enabled even the largest species to forage on most small prey items, and thus the greatest differences were found in the upper prey sizes. In tropical forests, most tiger beetle species forage on the forest floor, but some species forage only on living tree trunks and large branches, while others use leaves and small twigs as the principal foraging substrate. The last two species groups can be found from low undergrowth foliage to the canopy of the forest. The ground-foraging species are often seen on undergrowth leaves but only fly there to escape predation or for copulation. They are not known to use the foliage as a foraging substrate (Zikan 1929, Mandl and Pearson