Update (this has now been published):
McCarthy, M.A., Moore, J.L., Morris, W.K., Parris, K.M., Garrard, G.E., Vesk, P.A., Rumpff, L., Giljohann, K.M., Camac, J.S., Bau, S.S., Friend, T., Harrison, B., and Yue, B. (2013). The influence of abundance on detectabiliy. Oikos 122: 717–726.
How hard do we need to look to be sure a species is absent when it is not detected? This question is fundamental in ecology. It is relevant when determining the appropriate level of survey effort, when compiling lists of species, when determining the extinction or absence of species, and when developing surveillance strategies for invasive species.
Without sufficient survey effort, species are not detected perfectly. Imperfect detection arises because species may be temporarily absent, hidden from view, or simply require extra effort to find. The detectability of species can be defined by the rate at which individuals of a species (or groups of those individuals) are encountered.
Detectability of species will increase with abundance, all else being equal. But what is the nature of that relationship? We present a model of this relationship, with the rate of detection being a power function of abundance (Fig. 1). The exponent for this function (b) will equal 1 if individuals are encountered independently of one another. When clustering of individuals increases with abundance, we expect this exponent to be less than 1, but greater than 0.
As values for the scaling exponent approach 0, the detection rate becomes less sensitive to abundance (Fig. 1). Knowing how detection rate scales with abundance can assist when determining detection rates of rare species. This is important because detecting rare species is often important, yet estimates of detection rate are often most uncertain for these species. A scaling relationship would allow extrapolation of detection rates to cases when species are rare.
Our paper describes the development of our model of how detectability scales with abundance, and we used three field trials to estimate the scaling exponent. The results were consistent with our expectation that the scaling exponent would lie between 0 and 1. And as expected, a value close to 1 was obtained in a study that was designed to conform to the assumption of a random distribution of individuals.
The field trials were conducted in a remnant of eucalypt woodland in Royal Park near The University of Melbourne where searchers looked for plants and coins (Figs 2 & 3), in an exotic grassland in Royal Park (Fig. 4) searching for planted Australian native species (Figs 5 and 6), and in eastern Australian forests searching for frogs (Fig. 7).
Want to know more? Please read the paper. Email me for a copy.