The ability of herbivores to influence growth in marine plant communities is well documented. However, the role of nonherbivorous fauna in directly affecting growth and productivity in marine plants has only recently begun to emerge. The boring isopod Sphaeroma peruvianum and encrusting barnacle Balanus spp. are closely associated with prop roots of the red mangrove Rhizophora mangle on the Pacific coast of Costa Rica. This study first examines the impact, both spatially and temporally, of these two invertebrates on the growth and productivity of mangrove prop roots. Second, the ability of benthic or water column predators to mediate the extent of the faunal impacts on growth and production of prop roots is assessed.Roots were tagged and monitored seasonally for isopod and barnacle inhabitation and settlement. Experimental caging techniques established that the presence of burrowing isopods on prop roots can cause a 50%, and of encrusting barnacles, a 30%, decrease in root growth rate and a 62% (with isopods) and 52% (with barnacles) decrease in net root production. The magnitude and spatial extent of these effects are mediated indirectly by predation. Aerial roots not yet in contact with the substrate are susceptible to isopod and barnacle recruitment. Aerial roots in contact with the substrate (ground roots) are free from encrusting barnacles primarily because of the foraging activity of a benthic predator, the hermit crab Clibanarius panamensis, which climbs these roots during high tide to forage indiscriminately on newly settled barnacles. Isopod absence from lateral roots that originate from ground roots (lateral-G roots) is explained best by predation rather than dispersal barriers. Isopods burrowed and survived for extended periods in lower lateral-G roots when predators were excluded. Isopods are probably excluded from older roots that have entered the substrate because they cannot burrow into the tough, woody tissues of these roots.
Male Dendrobates pumilio return home if displaced, suggesting that their vocal behaviors maintain territories. Many females are also site specific, perhaps because involved in parental care. Observations concerning mating behavior suggest that some D. pumilio are at times polygynous. RECENT studies relating ecology to social behavior and mating systems in anuran amphibians reveal an impressive diversity of social systems (Emlen, 1976; Wells, 1977a, b). Careful characterization of such systems are not available for most species, however. In species with vocal, hyperdispersed males, behaviors have been labeled as individual avoidance or territoriality when data to distinguish between these two superficially similar systems are lacking (Wells, 1977a). This study of Dendrobates pumilio was therefore designed to 1) test for territoriality and 2) provide a preliminary description of some breeding behaviors. D. pumilio, a "poison dart" frog (Meyers and Daly, 1976), is common in the Atlantic lowland tropical forests of Central America. It is diurnal and forages for small insects in leaf litter (Limerick, 1976). Both sexes are bright red with purple-blue legs and contain alkaloids in the skin (Albuquerque et al., 1971; Meyers and Daly, 1976). Males call from logs and tree bases. When such sites are evenly distributed on the forest floor, males appear to be hyperdispersed (Bunnell, 1973). Females lay eggs in moist leaf litter or under logs (Savage, 1968). Adults carry tadpoles from the ground to water-filled bromeliads (Starrett, 1960). Both males and females are reported to carry tadpoles (Kitasako, 1967; Silverstone, 1975; Wells, 1977b). Individual males remain in the same spot for many days and occasionally attack approaching intruders (Bunnell, 1973). Aggressive behaviors can be provoked by playing male calls through a speaker 1-2 m from a calling male (Bunnell, 1973), but such provocation does not
The tenets of optimal foraging theory are used to contrast the behavior of the predatory snail Acantina spirata when feeding on the barnacles Balanus glandula and Chthamalus fissus under conditions of satiation and starvation. As predicted in optimal diet models, A. spirata is less selective (ratio of attack frequency on a prey species to number of individuals available) when the higher ranking prey has low abundance. When given a choice, starved snails attack both barnacle species equally, whereas satiated individuals preferentially attack B. glandula, the more profitable prey (ash-free dry weight of barnacles ingested per unit handling time). Under starvation conditions, equal attack frequency does not result in equal prey species consumption because Acanthina spirata is more successful at attacking C. fissus than B. glandula.The assumption of constant prey encounter rates in optimal diet models is not met when A. spirata goes from a state of satiation to starvation. The encounter rate on B. glandula is lowered due to a decrease in attack success. A loss of feeding skills in starved A. spirata is responsible for the greater difficulty snails have in gaining access through the opercular plates of B. glandula.Behavioral changes in A. spirata as snails pass from satiation to hunger translate into an energetic disadvantage during feeding for hungry snails for two reasons. First, higher prey handling times result in a decreased rate of biomass intake. Second, alteration in the relative attack frequency between barnacle species, combined with a decrease in attack success on the more profitable prey leads to more frequent ingestion of the less profitable prey.
Aggregation patterns in Emerita analoga (in southern California) are delineated with respect to their spatial, daily, and seasonal components. Both abiotic and biotic factors are found to be associated with patterns of aggregation. Spatially, E. analoga aggregates from March through September to a significantly greater degree in the upper one-third area of the wash zone where exposure to wave shock and fish predation are probably decreased. Sand crabs are more aggregated on a daily basis during low tides than at high tides. This may be due to differential rates of migration caused by a decrease in the beach slope angle. Two seasonal peak periods of aggregation are present, one in the early spring, and one in the late summer. These periods occur during the times of highest reproductive female abundance. High seasonal intensities of aggregation probably function to facilitate mating through the maintenance of close proximity between males and females. Visual methods and/or quantitative sampling based on visual observations do not adequately reflect patterns of aggregation in E. analoga.
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