Abundance, occupancy and aggregation

Predicting the change in abundance is pivotal for evaluating speciesí current conservation status and population viability. Empirical works have suggested that species with an increasing abundance have a more aggregated distribution than those with a declining abundance (namely, the change-aggregation hypothesis, CAH).

Journal cover: Basic and Applied Ecology v13

Front cover of the issue in Basic and Applied Ecology, signifying the transition from field observations to the presence-absence data used in many models and analyses.

C·I·B researcher, Cang Hui and colleagues designed an improved negative binomial distribution model of the occupancy-abundance relationship (OAR) to estimate the change in abundance from changes in occupancy or aggregation. Analysis of the model suggests that (i) in general the change in abundance is synchronized with the change in occupancy when the level of environmental heterogeneity remains constant, and (ii) there could exist a threshold of the population density above which the CAH is no longer valid.

Tests using data of epigaeic ants in Fynbos of South Africa collected from different seasons and macro-invertebrates from different localities in streams of central Spain verified these model propositions and thus support the use of this model as a monitoring method for assessing species persistence. Results also suggest that the change in abundance can be estimated from the change in occupancy often obtained from cost-efficient presence-absence records, and a revision of the traditional CAH is necessary to capture the threshold phenomenon in the change-aggregation relationship. This work thus signifies the use of the three distinct but related concepts of population structure (i.e. occupancy, abundance and aggregation) in conservation biology.

This work further sheds light on mechanisms behind the often two-phase range expansion in many invasive species. For instance, the existence of the CAH threshold could reflect a percolation process of species distribution where the aggregated structure strengthens with the increase of population size but slowly declines once the population size passes beyond the percolation threshold. The existence of this threshold thus implies a two-phase increase of occupancy, as in the range expansion of many invasive species: from an initially low spreading rate to a high rate after a certain lag phase. This jump in spreading rate could happen when the population size reaches the percolation threshold, consistent with the revised CAH presented in this paper. Because the two empirical datasets are not ideal for analyzing range expansion, the potential of using the revised CAH for explaining the two-phase range expansion in invasive species warrants further investigation. Even though long-term data are clearly more appropriate for testing the performance of the improved NBD model and the revised CAH, the current empirical evaluations support the use of these models as an alternative, cost-efficient technique for assessing species change in conservation status from monitoring their aggregation structure.

Read the paper:

Hui, C., Boonzaaier, C. & Boyero, L. 2012. Estimating changes in species abundance from occupancy and aggregation. Basic and Applied Ecology, 13: 169-177.

For more information, contact Dr Hui at