In a world of shrinking opportunities, species conservation requires the ability to distinguish locations able to sustain viable populations from those that are not. We investigated the value of collecting additional years of data for two types of observer-based surveys to predict persistence of species twenty years into the future. We used data from the eastern North American forests of the Breeding Bird Survey to calculate 1) log abundance from time series of point-count data, and 2) permanence, the proportion of presences from time series of presence-absence data (consistent occupancy over time) on a species-specific basis. For non-rare birds we calculated both measures over 2-, 4-, 6-, 8, 10-, and 12-year intervals (all intervals ending in 1981) to investigate the sensitivity of log abundance and permanence to the number of years of survey information included. We used survey data from a contemporary interval twenty years later (2001-2006) to calculate persistence as 1) permanence, or 2) log abundance. For each species we regressed these persistence quantities against the earlier permanence/abundance measures.
Results/Conclusions
Permanence and log abundance were similarly successful predictors of future permanence when more than two years of data were included in their calculations. Among the five longest intervals, log abundance and permanence significantly predicted future permanence for 83-86% and 75-81% of species respectively (p<0.05). For the shortest interval (1980-1981) performance of these measures was less strong; log abundance and permanence significantly predicted future permanence for 79% and 63% of species respectively (p<0.05).
The performance gap between log abundance and permanence as predictors of future persistence widened for all time intervals when future log abundance defined persistence, although both measures remained quite consistent over longer intervals. Among the five longest intervals, log abundance and permanence significantly predicted future log abundance for 78-79% and 64-69% of species respectively (p<0.05). For the shortest interval the effectiveness margin increased considerably; log abundance and permanence significantly predicted future log abundance for 75% and 49% of species respectively (p<0.05).
Interestingly, the abilities of abundance or permanence to predict future species persistence did not appreciably improve when using between 4-12 years of survey data. Moreover, even very short intervals of abundance predicted persistence better than much longer intervals of permanence, particularly when future abundance defined persistence. Considering the tradeoff between effectiveness and the need for urgent action, we recommend collecting a short interval of temporally replicated point-counts to estimate abundance for applied species conservation.
