Do population indicators work? Investigating correlated responses of bird populations in relation to predator management

Using population indicators to evaluate conservation achievements is widely practised, yet seldom empirically tested. If populations are consistently correlated in response to a shared ecological driver, the indicator species approach can be used as a cost-effective, ecologically-based shortcut to measuring the effects of conservation management. Long-term monitoring of forest bird populations associated with mammalian pest control programmes in New Zealand provides a useful framework for testing the population indicator species concept. We evaluated population trends in 21 bird species vulnerable to predation by introduced mammals (primarily mustelids and rodents) at managed and unmanaged beech (Nothofagus) forest sites. Correlated population trends between species pairs were detected at individual sites. However, neither positive nor negative correlations in species trends could be predicted by life history traits and predator management did not produce consistent, correlated population trends among sites. Our results do not support the use of a population indicator approach to management and reporting for forest birds in New Zealand. Relationships between purported indicator taxa and other species need to be understood for various management scenarios before population indicators can be confidently applied to measuring conservation achievement.