Proportionate spatial sampling and equal-time sampling of mobile animals: A dilemma for inferring areal dependence

Patch or island area is one of the most frequently used variables for inference in conservation biology and biogeography, and is often used in ecological applications. Given that all of these disciplines deal with large spatial scales, exhaustive censusing is not often possible, especially when there are large numbers of patches (e.g. for replication and control purposes). Therefore, data for patches or islands are usually collected by sampling. We argue that if area is to be used as an inferential factor, then the objects under study (i.e. the patches) must be characterized on an areal basis. This necessarily means that fixed-area sampling is inadequate (e.g. a single standard quadrat or transect set within patches irrespective of the patch area) and that some form of area-proportionate sampling is needed (e.g. a fixed areal proportion of each patch is surveyed by random allocation of standard quadrats across each patch). However, use of area-proportionate sampling is not usually dissociated from the increased temporal intensity of sampling that arises from using this approach. The dilemma we see is deciding how much of the area-specificity of variables such as species richness, rare-species indices or probabilities of occurrence of individual species is related to the area-proportionate survey protocol and how much is due to the temporal intensity of surveys.
We undertook a study in which we balanced temporal and spatial effects by increasing the time spent surveying smaller patches of vegetation to account for the area-ratio difference. The estimated species richness of birds of the
box–ironbark system of central Victoria, Australia, was found to depend strongly upon area when area-proportionate sampling alone was performed. When time-balancing was imposed upon area-proportionate sampling, the differences between smaller (10-ha) and larger (40-ha) areas were much reduced or effectively disappeared. We show that species found in the additional surveys used to conduct the time-balancing were significantly less abundant than
species recorded in area-proportionate sampling. This effect is probably most severe for mobile animals, but may emerge in other forms of sampling.