Niche opportunities and introduced birds: temporal variation in resource abundance.

Abstract

The niche-opportunity hypothesis predicts that some introduced species establish and spread successfully because their new environment provides expanded niche opportunities (better resources or physical environment) compared with their native environments. We tested the prediction that introduced birds in New Zealand experience increased niche opportunities because (1) winter seed resources were more abundant and (2) there was less temporal variation in resource availability during the breeding season in New Zealand. Our results suggest that in New Zealand an increase in winter resource abundance and a reduction in predation risk may have resulted in more available foraging habitat for introduced birds. However, although the local climatic conditions were less variable during the breeding season in New Zealand, we found no evidence that temporal variation in invertebrate food resources through the breeding season was reduced. This suggests that enhanced niche opportunities, if they exist, are not due to an increased abundance or temporal reliability of invertebrates, but rather enhanced niche opportunities may exist because of a warmer, more stable climate during the breeding season and lower energetic costs associated with foraging activities.