Experimental Evidence of Disrupted Dispersal Causing Decline of an Australian Passerine in Fragmented Habitat

Abstract

Abstract: We evaluated two hypothetical explanations for the decline of the cooperatively breeding Brown Treecreeper ( Climacteris picumnus ) in fragmented habitat: habitat degradation and isolation. We monitored the reproductive performance of approximately 50 breeding groups in Eucalyptus woodlands in the New England tablelands of northeastern New South Wales during 1996–1998. In addition, we translocated females to contiguous woodland and woodland fragments with unpaired males. We predicted that experimentally relocated females would reject territories with unpaired males in fragments if the habitat was degraded but not if males were unpaired due to isolation. We moved 11 females to fragments with unpaired males and 5 females to contiguous habitat with unpaired males. Nine females remained with males in fragments. Five of these females attempted nesting and 3 produced fledglings. No female relocated to contiguous habitat with an unpaired male remained or paired. We rejected habitat degradation as an explanation for the current decline of Brown Treecreepers within remnants, although degradation likely played a role in the past at a regional scale. Exceedingly low female recruitment within fragments and a lack of female dispersal between fragments provide additional evidence that female dispersal into territories in fragments rarely occurs naturally. We conclude that patch isolation is responsible for the high proportion of unpaired males in fragmented habitat.