Evaluating and validating abundance monitoring methods in the absence of populations of known size: review and application to a passive tracking index

Abstract

Rarely is it possible to obtain absolute numbers in free-ranging populations and although various direct and indirect methods are used to estimate abundance, few are validated against populations of known size. In this paper, we apply grounding, calibration and verification methods, used to validate mathematical models, to methods of estimating relative abundance. To illustrate how this might be done, we consider and evaluate the widely applied passive tracking index (PTI) methodology. Using published data, we examine the rationality of PTI methodology, how conceptually animal activity and abundance are related and how alternative methods are subject to similar biases or produce similar abundance estimates and trends. We then attune the method against populations representing a range of densities likely to be encountered in the field. Finally, we compare PTI trends against a prediction that adjacent populations of the same species will have similar abundance values and trends in activity. We show that while PTI abundance estimates are subject to environmental and behavioural stochasticity peculiar to each species, the PTI method and associated variance estimate showed high probability of detection, high precision of abundance values and, generally, low variability between surveys, and suggest that the PTI method applied using this procedure and for these species provides a sensitive and credible index of abundance. This same or similar validation approach can and should be applied to alternative relative abundance methods in order to demonstrate their credibility and justify their use.

Evaluation of estimation quality of a general paradigm for indexing animal abundance when observations are counts

“…Thus, valid statistical comparison methods among index values are essential. Statistical theory has been developed for a broad class of indexing approaches (Engeman, 2005) and methods for validating indexing procedures have been delineated (Allen and Engeman, 2015). The missing piece in this picture has concerned the validity of applying Gaussian-based statistical inference when index values were obtained from count data (i.e., Poisson-distributed data).…”

Environmental effects are stronger than human effects on mammalian predator-prey relationships in arid Australian ecosystems

“…Engeman and Evangelista, 2006;Blaum et al, 2008;Claridge et al, 2010). Almost all studies addressing the ecological roles of dingoes in Australian ecosystems have used this fauna sampling technique (Allen et al, 2013b), for which a great deal of methodological development and validation has been undertaken (Engeman et al, 1998;Engeman, 2005;Allen and Engeman, 2014). Track intrusions (or footprints) for each species in each 24 h period were counted on 50 sand plot tracking stations spaced 1 km apart on each transect (100 sand plots in total).…”

“…Tracking plot surveys were conducted quarterly and at the same time each year, although excessive wind or rain prohibited undertaking some surveys. Sampling fauna in this way allows PTI calculations to be used as a reliable measure of activity or relative abundance when analysed appropriately (Wilson and Delahay, 2001;Engeman, 2005;Allen and Engeman, 2014; but see Hayward and Marlow, 2014and Hayward et al, 2015 for further discussion).…”

The effect of wildfire on population dynamics for two native small mammal species in a coastal heathland in Queensland, Australia

“…However, when population demographics are not necessary other indexing methods besides the number of unique individuals might also be available for population monitoring with statistical properties and methodologies developed for some (e.g., Engeman, 2005), and methods have been described for validating the procedures in the absence of known populations (Allen and Engeman, 2015). In particular, a variety of approaches have been developed for monitoring many rodent species (e.g., Engeman and Whisson, 2006;Whisson et al, 2005), with an index based on camera trap data most likely to provide the ability to simultaneously monitor multiple species of rodents (e.g., Baldwin et al, 2014).…”

“…In particular, a variety of approaches have been developed for monitoring many rodent species (e.g., Engeman and Whisson, 2006;Whisson et al, 2005), with an index based on camera trap data most likely to provide the ability to simultaneously monitor multiple species of rodents (e.g., Baldwin et al, 2014). In any case, a new approach would require testing and validation (Allen and Engeman, 2015). Yet, the mark-recapture approach provides a range of information pertaining to a population, and can be used in conjunction with simple estimates to gain a better understanding of population size and processes, even with difficulties like those we encountered for avoiding violation of assumptions (which were apparent even for Melomys, one of the best candidates for markrecapture estimation amongst Australian small mammals).…”