Diversity-disturbance relationships: frequency and intensity interact

Abstract

An influential ecological theory, the intermediate disturbance hypothesis (IDH), predicts that intermediate levels of disturbance will maximize species diversity. Empirical studies, however, have described a wide variety of diversity-disturbance relationships (DDRs). Using experimental populations of microbes, we show that the form of the DDR depends on an interaction between disturbance frequency and intensity. We find that diversity shows a monotonically increasing, unimodal or flat relationship with disturbance, depending on the values of the disturbance aspects considered. These results confirm recent theoretical predictions, and potentially reconcile the conflicting body of empirical evidence on DDRs.

Figure 1.

(a) Diversity, measured as the complement of Simpson's index (1−λ, where and p_i is the proportion of the ith morph), is shown for each of five disturbance frequencies (left to right in each series, 0, 1, 4, 8 or 16 disturbances over 16 days) at each of three disturbance intensities. Points show means ± s.e. for three replicate microcosms. (b) Frequencies of different morphotypes (WS, four subcategories, each a different shade of grey; SM, white bars; FS, black bars) at each combination of disturbance intensity and frequency. Bars show mean ± s.e. for three replicate microcosms at each disturbance frequency (from left to right in each group of five bars: 0, 1, 4, 8, 16 disturbances) and intensity.