Interspecific Competition and Niche Changes in Tits (Parus spp.): Evaluation of Nonexperimental Data

Publisher Website
Google Scholar
Abstract
The role of nonexperimental observations for testing causal processes, such as interspecific competition, has been questioned during the present "competition controversy." Initially, niche changes between areas of allopatry and sympatry were considered to be strong evidence for interspecific competition. However, any single change may be due to alternative causes, in particular, to differences in the environment irrespective of the presence or absence of the putative competitor. We evaluated niche changes in European tits (Parus spp.). In all seven cases studied, one or two species showed a clear, divergent ecological shift in the area of sympatry. Likewise, Alatalo (1981) found a general pattern of divergence in foraging sites in mixed-species tit flocks compared with monospecific flocks. These significant tendencies toward divergent rather than convergent shifts in the presence of putative competitors cannot be explained by differences in the environment alone. The strongest evidence that interspecific competition causes niche changes comes from the observation that, within the guild, changes were found only in the species that morphologically most closely resembled the species that were absent in allopatry (weight ratio .ltoreq. 1.2). In general, we believe that well-done experiments, if feasible, are the best way to evaluate alternative hypotheses. However, it is also possible to make inferences about causal processes without experiments if alternative explanations of the observed patterns can be refuted. The nonexperimental approach is tedious and requires good knowledge of the animals and the question under consideration. Inclusion of several independent comparisons allowing general tests is of great value.