Paper evaluates potential for sampling bias when detecting population trends in amphibians

Global decline in amphibian populations is a significant theme of the scientific literature. Population trends for abundant or intensely studied amphibian species are sometimes well understood. However, estimating population trends for rare, poorly studied, or cryptic species can be challenging.  Some investigators have sought to estimate population trends for such species by resurveying sites where the species were found in the past.

Recently, Dr. Craig Loehle and Philip Weatherford of NCASI developed a paper that evaluated the challenges associated with using historic data gathered over time to estimate population trends for rare species. The paper, which appeared in Ecological Modelling, focused on three factors that may influence population trend estimates: metapopulation (a population consisting of multiple discrete local populations or subpopulations) processes; detectability; and population volatility. Studies of pond-breeding amphibians were used to illustrate issues that may arise with retrospective studies; however, the principles discussed are applicable across taxonomic groups.

The study found that, for metapopulations, if only initially occupied sites are resurveyed, decline may be falsely inferred. Detectability issues and volatility may also lead to false conclusions. Thus, only longer-term and spatially extensive data can allow determination of the dynamics of potential metapopulations. To establish population trend estimates, the authors recommend that all original sites be resurveyed using a multi-year resurvey protocol with detectability correction. While accounting for detectability is encouraged, the authors caution that this cannot completely compensate for lack of information on metapopulation processes or population fluctuations, which act on longer time scales. They recommend long-term studies to decrease the potential for basing conservation policies on biased estimates.

The abstract for the paper follows.

“In retrospective studies, discrete population units such as ponds may be resurveyed at a later time using only the set of initially occupied sites. There are possible confoundings that affect estimates of occupancy change under these conditions. For most possible parameter values for a metapopulation, simulations and analytical results show that turnover leads to a tendency to observe a decline in the proportion of initially occupied sites that are occupied at a later time even when the overall metapopulation is stable or increasing. For a given time interval, the spurious decline will be greater when metapopulation turnover is higher. If site-level detectability d is <1, a single resurvey of only the initially occupied sites will show a decline of 1-d even if no change has taken place. Finally, volatile populations can be difficult to resurvey, especially if sample units are chosen based on having an abundance of the species at the earlier survey. All three issues can exist simultaneously and their influence on trend estimates can be difficult to distinguish based on samples at only two points in time. A sample of literature illustrates clear cases where these biases could exist, even though a variety of survey methods were used. Suggestions are made for improved sampling, including resampling the entire original set of sites and conducting multi-year resurveys.”  

Reference 

Loehle, C. and P. Weatherford. 2017. Detecting population trends with historical data: Contributions of volatility, low detectability, and metapopulation turnover to potential sampling bias. Ecological Modelling 362:13–18. https://dx.doi.org/10.1016/j.ecolmodel.2017.08.021