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A population usually does not uniformly distribute in space. Previous studies have found that when a population is getting larger, the variance of the population abundance over a space (i.e., aggregation degree) will also be increasing. This mean-variance relationship is the well-known Taylor’s power law (V=aMb). The exponent b in Taylor’s power law is an indicator of the “aggregation potential” of a species. That is, for a population has a higher b, it will be more aggregated than another population with the same abundance but a smaller b.

While theory has predicted that the aggregation potential of a population is related with life history traits, empirical evidence is lacking. Researchers from the Institute of Oceanography, National Taiwan University, now provided the first empirical evidence that supports the theory. Moreover, they found that species under fishing pressure tend to have higher “aggregation potential” than their non-targeted counterparts, probably because fishing has truncated their age/size-structure. The research was led by Professor Chih-hao Hsieh and his student Ting-Chun Kuo, and is published in Ecology in May, 2016.

The team used a 50-year spatiotemporal data from the California Current Ecosystem to examine fishing and life history effects on the Taylor’s exponent of 29 marine fishes. By comparing spatial distributions of exploited and unexploited fishes living in the same environment, they found that unexploited species with smaller size and generation time exhibit larger b, supporting theoretical prediction. In contrast, this relationship in exploited species is much weaker, as the exponents of large exploited species were higher than unexploited species with similar traits. They suggested that fishing increases spatial aggregation potential of a species, likely through degrading their size/age structure. Furthermore, the spatially aggregated populations might have lower recovery rate in spatial occupancy even when fishing stopped. The findings provide critical evidences for that fishery management strategy should concern the changes in age and spatial structure of exploited fishes.

Reference:
Kuo, T. C., S. Mandal, A. Yamauchi, and C. H. Hsieh (2016) Life history traits and exploitation affect the spatial mean-variance relationship in fish abundance. Ecology. 97: 1251-1259

Figure 1. (a-d) A population with larger average life history traits (e.g., body size) has lower aggregation potential, i.e., less aggregated distribution, comparing to a population with the same abundance but smaller traits. However, the size/age-structure of exploited species may be truncated, results in increases in aggregation. The relationship of life history traits versus Taylor’s exponent (aggregation potential index) for unexploited species is shown as the dashed line in (e). The slope of life history traits against Taylor’s exponent of exploited species (solid line) becomes flatter, because the larger species now behaves more like small unexploited species.