Morisita's index of patchiness and edge effects: A response to Butturi-Gomes and Petrere (2020)

Abstract

Abstract In a recent paper in Ecological Indicators, Butturi-Gomes and Petrere (2020) investigated Morisita's index of patchiness in relation to edge effects and concluded: that edge avoidance by a species drives its patchiness estimation … which might be the reason behind so many claims of population aggregation' and that 'the presence of edge avoidance leads to false claims of population aggregation, based solely on Morisita's patchiness index. One of the patchiness 'claims' that they specifically criticised was that of Barnes and Laurie (2018) for intertidal seagrass macrobenthos, Butturi-Gomes and Petrere stating that these authors: disregarded possible edges, which are indeed hard to define in marine environments, but are still of importance, particularly in seagrass substrates, as Smith et al. (2008) pointed out. What Butturi-Gomes and Petrere failed to mention, however, was (a) that Barnes and Laurie did not use Morisita's patchiness index, (b) that the index they did use was not the sole basis for their finding of patchiness, and (c) that they cited extensive previous work at their site that had investigated seagrass edge effects — indeed, rather than ignoring such edges they specifically sited their study area within a non-edge zone (sensu Smith et al., 2008). The patchiness they observed was not 'false' and it was not driven by active aggregation away from edges but most likely by the effect of predation by mobile nekton on prey densities.