Patterns of abundance and morphology as indicators of ecosystem status: A meta-analysis

A series of studies have suggested that abundance and morphology distributions approximate the lognormal in undisturbed communities and depart from the lognormal with disturbance. However, this proposed capability to indicate ecosystem status has been challenged on theoretical, methodological and statistical grounds. This paper quantifies the departure from the lognormal in natural communities, and the sensitivity of such departures to disturbance, species richness, sample size, temporal and spatial scale, taxa, methodological protocols and other confounding factors. We have conducted a rigorous test of the hypothesis that distance to the lognormal represents a powerful indicator of ecosystem status. We tested three measures of distance to the lognormal and their sensitivity by reviewing 38 case studies and simulated community patterns and examined the potential and pitfalls of the approach. The most robust parameter for measuring the departure from the lognormal was found to be the normalized distance to the lognormal (ΔL). ΔL proved to be a reliable and adaptable indicator of disturbance, which is effective over a broad range of biological systems (terrestrial and aquatic, most taxa, social and economic). We show that ΔL can be measured either by quantifying abundance or by organism size, a cheaper and easy to obtain metric. Abundance distributions provide an indication of system status on a shorter time scale than size distribution. Taken together, they provide clues to the direction in which the system is moving. The sensitivity analysis shows which methods will lead to consistent results across disciplines. Our simulations confirm that disturbance consistently pushes complex systems away from the lognormal pattern, as suggested by empirical data. We conclude that the departure from the lognormal can be used as an indicator of status of a dynamic ecosystem as long as appropriate procedures are followed. Systems approximating the lognormal (ΔL close to 0) can usually be considered self-organized and little disturbed by external influences.