Improving studies of species population trends based on historical records of occurrence

Concern over a decline in detections of some species such as pond-breeding amphibians has led to many investigators revisiting old survey data to establish a baseline for evaluating population trends over time. For example, ponds are often considered to be a survey unit, and fewer occupied ponds over time may be interpreted as an indicator of decline. However, this approach for estimating population trends presents unique challenges for inference.  

Recently, in a paper published in Ecological Modelling, Dr. Craig Loehle and Philip R. Weatherford of NCASI, evaluated the nature and implications of factors that can influence studies that rely on use of historic data gathered over time, such as in museum records or multiple literature records. The authors found that three factors—metapopulation processes, detectability, and population volatility—can confound studies of population trends for species such as pond-breeding amphibians.  

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.R. 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://doi.org/10.1016/j.ecolmodel.2017.08.021