The abundance structure of Azorella selago Hook. f. on sub-Antarctic Marion Island: testing the peak and tail hypothesis

Understanding the spatial distribution of organisms and the factors underlying it are key questions in ecology. Two competing hypotheses exist about the form of spatial variation in abundance. The abundant centre hypothesis suggests that abundance is highest in the centre of a species' range and declines towards the range margins. By contrast, the peak and tail spatial pattern in abundance posits that several high abundance areas exist across a species range. Here, we test these competing hypotheses by surveying the abundance of the keystone plant species Azorella selago Hook. f. (Apiaceae) across sub-Antarctic Marion Island on a regular spatial grid. We also examine several factors that might explain variation in abundance. Azorella selago occurs between ca. 30 and 850 m above sea level, with sharp discontinuities in abundance at ca. 30 m and at 667 m a.s.l. The survey and analyses revealed a complex abundance structure with patches of high abundance alternating with areas of low abundance or absence, providing support for the peak and tail hypothesis, but with some support for the abundant centre idea too. Variation in abundance was best explained by a model including the negative effects of elevation and of closed vegetation. Our work provides support for the peak and tail pattern of spatial variation in abundance, which has profound importance for understanding the mechanisms underlying the spatial distribution of abundance and other macroecological regularities.