Ecologia comparada de quatro espécies de aranhas errantes do gênero Ctenus (Waickenaer) (Araneae, Ctenidae) em urna floresta na Amazônia central: bases para um modelo integrado de coexistência.

Abstract

1) Ctenus amphora, C.crulsi, C. manauara and C. villasboasi are nocturnal wandering spiders, mediurn to large sized, that forage on the ground, and are very abundant in "Reserva Florestal Adolfo Ducke" in central Amazonia. 2) During 14 months, I coliected data of about 3000 spiders, among censuses, behavioral - observations and experiments. I compared the species on annual seasonality of the abundance and reproduction, spacial patterns of abundance, diet, aspects of the behavior and reproduction, and anaiyzed how those habitat characteristics and interspecific interactions may iduence the spatial patterns of abundance. 3) The methodologies were: a) censuses and measurements of spiders in 70 short transects (about 50m of length) along the year, to evaluate seasonality and distribution in a srnaller scale; b) censuses along 15 km of trails, to determinate spacial patterns of abundance and to compare abundance of spiders with habitat variables; c) pitfall traps to estimate availability of food and frequency of passage of army ants; d) censuses at difYerent hour of the night and direct observations to compare behavior of the spiders; e) experiments with cages to compare intraspecific predation with cannibalism, and to evaluate the efficiency of capture of termites among spiders. 4) The abundance of Ctenus started high i . the middle of the hrst raining season, diminished still during this season, reached the lowest values during the dry season, and increased to high values until the rniddle of the rainy season. This pattern was stronger in C. amphora e C. villasboasi, but was similar for the four species. I found evidences of reproductive seasonality based on the structure of sizes in the populations, only in C. villasboasi. 5) In the short transect census, I did not detect tendencies of aggregation reveaiing different use of microhabitat by species. In the census along the trails I found three spacial patterns of abundance, long sections where C. crulsi and C. manauara dominated, other sections where C. amphora dominated, and a more homogeny distribution in C. villasboasi. The pattems repeatd thernselves in the three censuses, in Máy and October o f 1 994, and in February o f 1 995, indicating that local characteristics favor differentially the species. 6) The diet of the four species consisted mainly of abundant arthropods in the leaf iitter, but the proportion of termites of the genus Syntermes was much higher in the diets of C. crulsi and C. manauara. The same resuh was obtained in a second analysis, restricting the diet data to areas where a11 the species and termites coexisted. The activities in different hours of the night, and the rate of movements were similar arnong species. There are differences among species in the number of eggs per egg-sac, but the relationship arnong size of the spider and nurnber of eggs seems to be the same. 7) I compared the spacial patterns of abundance per species along the 15 krn of trails with data on local envkonmental characteristics, in search of causal relationships to explain those pattem. Initially, I made evaluations treating the variables independently, however, most partem of abundance of spiders and the characteristics of habitat are related to types of soil. Ctenus crulsi e C. manauara were most abundant in clay-soil places, where the superficial root layer is almost absent, the volume of leaflitter is low, Syntermes are abundant, and the pressure of predation by army ants is apparently high; C. amphora in sandy soil areas, where there is a thick superficial root layer, leaf litter is more abundant, there is less Syntermes, and predation by army ants is apparently low; and C. villasboasi close to creeks, where the soils are hidromorphic, the superficial root layer is thick, and Syntermes are alrnost absent. 8) Considering that Syntermes was the main prey for two species, and were rare in the diet of the other two, I rnade one experiment comparing the efficiency of capture of those termites by C. amphora e C. crulsi, selected because they have similar sizes and different diets. Ctenus crulsi captured the termites with a fiequency significantly higher than C. amphora. 9) The four species are quite abundant, and it is possible that they are themselves arnong the main predators of Ctenus, however, intrageneric predation was low compared with other studies with arachnids. In the experiment with cages, the rate of interspeciiic predation was not higher than the cannibalism rate. This result indicates that interspecific predation did not evolve as a mechanism to diminish the interspecific competition in this system. 10) Competition may be an important interaction among these species, they ail prey on arthropods of the leaf litter, and tlie strong spatial and temporal variation in sizes indicates that growth and reproduction are frequently under optimal conditions. These spiders are among the main predators of leaf litter, and potentially have an impact on tbe abundance of arthropods. However, there is no evidence that competition caused the spatial partem of distribution found. For exarnple, absence of C. amphora in areas with more Syntermes would be an evidence of exclusion by competition, because orher species of the same genus are more efficient to use this irrgortant resource, but they coexist in these areas. The hypothesis that competition produced the pattems observed cannot be rejected, but there are better alternative hypotheses to explain those pattems. I present a rnodel for the coexistence of these species that includes the importance of Syntermes, and illustrate how the resuits obtained can be used to the constniction of realistic and testable hypotheses for the csexistence of Ctenus.