GEOGRAPHIC VARIATION IN ASYMMETRIC COMPETITION: A CASE STUDY WITH TWO LARVAL ANURAN SPECIES

Abstract

The outcome of interspecific competition may be modulated by a large number of factors, both biotic and abiotic. In this paper, we examined experimentally the effects of geographic variation, population density, and abiotic stress on the outcome of interspecific competition between two species of larval anurans. Three different populations of natterjack toad (Bufo calamita) were set to compete with common toad (Bufo bufo) in replicated artificial ponds under different combinations of osmotic stress (freshwater or brackish water) and intraspecific density (high and low). Among the B. calamita populations used, only one of them is sympatric to B. bufo and inhabits a freshwater environment. The two other populations, allopatric to B. bufo, occupy freshwater and brackish water environments, respectively. Survival, length of the larval period, and size at metamorphosis were analyzed at both the specific and the populational interaction levels. Intra- and interspecific competition did occur and generally resulted in a decrease in survival rates, an increase in the duration of the larval period, and a decrease in size at metamorphosis. Water salinity also prolonged the larval period and interacted with intra- and interspecific density, intensifying its effects. Interspecific competition was asymmetric, since B. bufo was competitively superior to B. calamita, causing larger effects on B. calamita than those experienced by B. bufo. However, the three populations of B. calamita showed different competitive effects and responses when they were set to compete with B. bufo. The population sympatric to B. bufo had the stronger competitive effect and response, causing a strong impact on survival, growth, and developmental rate of B. bufo. The allopatric population inhabiting the brackish environment also had a marked effect on B. bufo survival, although only at high density. The other allopatric population only had a significant effect on the size at metamorphosis of B. bufo. The impact of B. bufo on B. calamita was greater for the allopatric populations than for the sympatric one. This pattern suggests that the nature and outcome of ecological interactions are a function of environmental conditions and population history.