Population closure and the bias‐precision trade‐off in spatial capture–recapture

Abstract

1. Spatial capture-recapture (SCR) is an increasingly popular method for estimating ecological parameters. SCR often relies on data collected over relatively long sampling periods. While longer sampling periods can yield larger sample sizes and thus increase the precision of estimates, they also increase the risk of violating the closure assumption, thereby potentially introducing bias. The sampling period characteristics are therefore likely to play an important role in this bias-precision trade-off.Yet few studies have studied this trade-off and none has done so for SCR models.2. In this study, we explored the influence of the length and timing of the sampling period on the bias-precision trade-off of SCR population size estimators. Using a continuous time-to-event approach, we simulated populations with a wide range of life histories and sampling periods before quantifying the bias and precision of population size estimates returned by SCR models.3. While longer sampling periods benefit the study of slow-living species (increased precision and lower bias), they lead to pronounced overestimation of population size for fast-living species. In addition, we show that both bias and uncertainty increase when the sampling period overlaps the reproductive season of the study species. 4. Based on our findings, we encourage investigators to carefully consider the life history of their study species when contemplating the length and the timing of the sampling period. We argue that both spatial and non-spatial capture-recapture studies can safely extend the sampling period to increase precision, as long as it is timed to avoid peak recruitment periods. The simulation framework we propose here can be used to guide decisions regarding the sampling period for a specific situation.

Estimating Deer Populations Using Camera Traps and Natural Marks

“…This suggests that the model would estimate a decreased density in the face of mortality. Additionally, researchers have reported that bias in population size estimates should be minimal if the sampling period does not coincide with the peak of the reproductive season (Dupont et al 2019). Because our study sampling began in late May, at the end of the fawning period at the site, we do not think the bias from fawn mortality significantly affected results.…”

Evaluation of Cottontail Pellets Collected in Suboptimal Conditions for DNA Analysis

“…Older pellets may be from rabbits that are no longer occupying an area, and thus the sample represents what rabbits have been present in the area but are not necessarily present at the time of sampling. This potentially is most important in abundance estimations based on multilocus genotyping where a closed population is assumed (Dupont et al 2019), but because we only recommend the highest quality pellets for genotyping, these are also likely the most recently deposited and representative of the cottontails present. Clearing a transect of pellets a few days prior to sampling would ensure collected pellets are only a few days old, but this necessitates visiting each site twice per collection and could be logistically prohibitive on long transects, such as those favored by intensive monitoring programs.…”

Investigating effects of soil chemicals on density of small mammal bioindicators using spatial capture-recapture models

“…Accuracy of density estimated by SCR models is often dependent on the capture-recapture survey duration [ 32 , 93 ]. Most small mammal bioindicators, including Peromyscus , are r -selected species that have high reproductive and mortality rates and generally short lifespans compared with K -selected large mammals [ 94 , 95 ].…”

“…Most small mammal bioindicators, including Peromyscus , are r -selected species that have high reproductive and mortality rates and generally short lifespans compared with K -selected large mammals [ 94 , 95 ]. Therefore, short survey periods should be used for r -selected species to mitigate bias that can result from violating the demographic closure assumption of single-session capture-recapture models [ 32 , 37 ]; however, the potentially optimal survey period length for r -selected species that can produce density estimates with both high precision and accuracy is approximately 14–30 days [ 93 ]. Relative to the life history characteristics of Peromyscus , our five-day survey may have been too short to obtain sufficient data for precisely estimating density-chemical relationships.…”

“…For example, Linzey and Grant [ 16 ] captured an average of 16 individual Peromyscus during each of their three-day survey periods, Phelps and McBee [ 18 ] captured an average of 34 individual Peromyscus during each of their five-day survey periods, and Gerber and Parmenter [ 78 ] captured an average of 37 individual Peromyscus during each of their five-day survey periods. Nevertheless, recent studies have identified survey durations being overly brief as a widespread and common problem with small mammal capture-recapture studies in general [ 93 , 96 – 98 ].…”