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Can time-to-detection models with fewer survey replicates provide a robust alternative to traditional site-occupancy models?

Henry, Dominic1; Lee, Alan1; Altwegg, Res1

Published Feb 18, 2020 on Dryad. https://doi.org/10.5061/dryad.msbcc2fv7

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Feb 18, 2020 version files 513.24 KB

Abstract

  1. Occupancy models are widely used in ecology because they explicitly separate the observation and state processes and hence account for imperfect species detection. Traditional occupancy models that record detection/non-detection (DND) of a species typically rely on either spatial or temporal  survey replication to estimate model parameters. Recording the time until a species is first encountered after starting a survey is often possible with little extra effort and such time-to-detection (TTD) surveys may be more efficient than pure DND surveys. Using continuous time data, TTD occupancy models can in theory estimate occupancy and detection parameters using a single TTD survey. These models therefore have the potential to drastically reduce the logistical effort and costs associated with traditional occupancy survey designs. However, the robustness and general applicability of TTD models has not been widely addressed and their effectiveness in different study systems remains unknown.
  2. We use simulations and bird data of 63 species from a field study in the Karoo region of South Africa to explicitly compare estimates of occupancy, detection and species richness between DND and TTD models under various levels of survey replication and for species with different occupancy and detection characteristics.
  3. Simulations revealed that for inconspicuous species (low detection probability) single survey TTD models can perform better or equally as well as DND models with a higher number of replicates. This effect was attenuated in widespread species (high occupancy probability). The benefits of TTD models were more pronounced at low survey replicates and performance of the two methods converged quickly as the number of survey replicates increased. The difference in model performance related to precision around estimates while the bias in parameter means was fairly low. However, results from the field data showed that a single TTD survey was not adequate to reliably estimate occupancy, detection and species richness; especially in rare and inconspicuous species. Increasing the number of TTD surveys to two replicates improved the models substantially.
  4. Our results demonstrate the general utility of TTD surveys depends on the characteristics of the species considered in the study. A single TTD survey may be sufficient in some study designs but is unlikely to be sufficient in most multi-species field scenarios where communities are made up of species that have a wide range of detection and occupancy probabilities. TTD surveys do provide benefits however in that data can be used to construct detection curves which can be used to guide survey effort in the design of future studies.