Design of occupancy studies with imperfect detection

1. Occupancy is an important concept in ecology. To obtain an unbiased estimator of occupancy it

is necessary to address the issue of imperfect detection, which requires conducting replicate surveys

at the sites being sampled. As the allocation of total effort can be done in different ways, occupancy

studies should be designed carefully to ensure an efficient use of available resources.

2. In this paper we address the design of single-season single-species occupancy studies with a focus

on: (1) issues relating to small sample sizes and (2) the potential relevance of including the precision

of the detectability estimator as a criterion for design. We explore analytically the model with con-

stant probabilities and examine how bias and precision are affected by the numbers of sites and rep-

licates used.

3. We show how, for small sample sizes, the estimator properties depart from those predicted by

large sample approximations, emphasize the need to use simulations when designing for small sam-

ple sizes and provide a new software tool that can assist in this process.

4. We offer advice on the amount of replication needed when the probability of detection is a quan-

tity of interest and show that, in this case, it is more efficient to reduce the number of sites and

increase the amount of replication per site compared with situations where only occupancy is of

concern.

5. Synthesis and applications. It is essential to have clearly stated objectives before starting a study

and to design the sampling accordingly. As the allocation of effort into replication and sites can be

done in different ways, occupancy studies should be designed carefully to ensure an efficient use of

available resources. To avoid waste, it is crucial to anticipate the quality of the estimates that can be

expected from a particular study design. The discussion and guidance provided here is of special

interest for those designing occupancy studies with small sample sizes, something not uncommon in

the context of ecology and conservation.