Abstract
Monitoring flying-foxes is challenging as their extreme mobility produces highly dynamic population processes, considerable logistic difficulty, and variability in estimated population size. We report on methods for inferring population trend for the population of the spectacled flying-fox (Pteropus conspicillatus) in Australia. Monthly monitoring is conducted at all known roost sites across the species’ range in the Wet Tropics Region. The proportion of animals in camps varies seasonally and stochastic environmental events appear to be influential. We develop a state-space model that incorporates these processes and enables inference on total population trends and uses early warning analysis to identify the causes of population dynamics. The model suggests that population growth rate is stable in the absence of cyclones, however, cyclones appear to impact on both survival and reproduction. The population recovered after two cyclones but declined after a third. The modelling estimates a population decline over 15 years of c. 75% (mean r = − 0.12yr−1 and belief of negative trend is c. 83%) suggesting that conservation action is warranted. Our work shows that a state-space modelling approach is a significant improvement on inference from raw counts from surveys and demonstrates that this approach is a workable alternative to other methods.
Highlights
Monitoring is critical to threatened species management as it provides baseline data about a species’ abundance, distribution, and, over time, trends in these[1,2]
The total estimated P. conspicillatus population shows very strong annual dynamics with low numbers of animals counted in camps during the cooler months (April to September) and population peaks during the warmer months (November to February) (Fig. 1)
Monitoring conducted over a period of 154 months (12.8 years) and at every known P. conspicillatus camp in the species’ Wet Tropics range showed two distinct trends
Summary
Monitoring is critical to threatened species management as it provides baseline data about a species’ abundance, distribution, and, over time, trends in these[1,2]. Monitoring results are essentially inferences about the outcomes of processes operating in two realms; the focal species’ population dynamics, and, the process of observing and recording those dynamics. The nature of these processes can dramatically influence the reliability of estimates and the reliability of inferences drawn from them. As a consequence, separating population signal from biological (process) noise and observation error is critical. The black dots are observed counts of spectacled flying-foxes in camps, with dashed lines the 95% credibility intervals. A method to satisfactorily estimate the abundance and trends of flying-foxes must attempt to accommodate all of these complex issues
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