Historical ecology with real numbers: past and present extent and biomass of an imperilled estuarine habitat

Abstract

Historic baselines are important in developing our understanding of ecosystems in the face of rapid global change. While a number of studies have sought to determine changes in extent of exploited habitats over historic timescales, few have quantified such changes prior to late twentieth century baselines. Here, we present, to our knowledge, the first ever large-scale quantitative assessment of the extent and biomass of marine habitat-forming species over a 100-year time frame. We examined records of wild native oyster abundance in the United States from a historic, yet already exploited, baseline between 1878 and 1935 (predominantly 1885-1915), and a current baseline between 1968 and 2010 (predominantly 2000-2010). We quantified the extent of oyster grounds in 39 estuaries historically and 51 estuaries from recent times. Data from 24 estuaries allowed comparison of historic to present extent and biomass. We found evidence for a 64 per cent decline in the spatial extent of oyster habitat and an 88 per cent decline in oyster biomass over time. The difference between these two numbers illustrates that current areal extent measures may be masking significant loss of habitat through degradation.

Figure 1.

Maps illustrating: oyster ground areal extent (a) historically and (b) presently in estuaries in the US and the percentage change in (c) oyster ground extent and (d) oyster biomass in estuaries for which comparable historic and modern data were available.

Figure 2.

A box-whisker plot of (a) the percentage of estuary area containing oyster grounds past and present by coast. Proportion of estuary area occupied was significantly higher historically along all coasts (quasi-binomial generalized linear model; Kruskal–Wallis χ²₁ = 5.1, p = 0.02; χ²₁ = 5.2, p = 0.02; χ²₁ = 8.3, p < 0.01 for the Atlantic, Gulf and West coasts, respectively). (b) The mean estuary wide density of market-sized eastern oysters historically and presently on subtidal oyster grounds in the US (Atlantic estuaries n = 6; Gulf estuaries n = 21; Kruskal–Wallis χ²₁ = 2.76, p = 0.05).