Detecting ecological thresholds and tipping points in the natural capital assets of a protected coastal ecosystem

Abstract

Concern about abrupt and potentially irreversible ecosystem thresholds and tipping points is increasing, as they may have significant implications for natural capital and human wellbeing. Although well established in theory, there are few empirical studies that provide evidence for these phenomena in coastal and estuarine ecosystems, despite their high value for provision of ecosystem services. To determine the likelihood of such events, we tested two statistical methods; sequential T-test analysis (STARS) and generalised additive models (GAMs) in a harbour ecosystem. These methods were applied to time series data spanning up to 25 years coupled with analysis of the relationships between drivers and natural capital asset flows. Results of the STARS analysis identified nonlinear thresholds in three of the natural capital assets of Poole harbour; mudflat area, Manila clam stocks and wader/wildfowl numbers, as well as an increase in several drivers affecting the harbour. The most prominent threshold was recorded in the Manila clam fisheries of the harbour, with stocks in two locations of the harbour declining by 73–78% between 2006 and 2008. We suggest that the historic decline in the Manila clam stocks of the harbour were partly attributable to illegal fishing pressure although other factors such as disease and lease bed holders switching to other species were also likely to have contributed. More recently (2015-onwards) wild clam stocks of the harbour have increased thanks to improved management measures by local authorities. Generalised additive models also identified the contribution of macroalgal mats, sediment shoaling and river flows to historic changes in mudflat area, saltmarsh area and wader/wildfowl numbers. We conclude that information on thresholds and tipping points obtained using these approaches can potentially be of value in a management context, by focusing attention on the interactions and positive feedbacks between drivers that may cause abrupt change in coastal ecosystems.