Can environmental fluctuation prevent competitive exclusion in sympatric flycatchers?

Ecology has been characterized by a central controversy for decades: namely , whether the distribution and abundance of organisms are determined by spe cies interactions, such as competitive exclusion, or by environmental condi tions. In part, this is because competitive exclusion has not been convinci ngly demonstrated in open, natural systems. In addition, traditional theore tical models cannot predict the outcome of competitive Interactions in the presence of environmental variability. In this paper we document the limiti ng influence of strong interspecific competition on population dynamics and nestling mortality in a mixed population of pied flycatchers (Ficedula hyp oleuca) and collared flycatchers (F. albicollis) in a narrow zone of sympat ry. Whereas the former species was limited mainly by interspecific competit ion, the latter species was limited by the concerted influences of intraspe cific competition and climate. The analysis suggests a march towards compet itive exclusion of the pied flycatcher during warm periods. However, compet itive exclusion is apparently prohibited on a local scale because intraspec ific competition among individual collared flycatchers intensifies when the y are forced to cope with severe environmental conditions, promoting the te mporary and local presence of pied flycatchers.