Assessing ecological responses to environmental change using statistical models.
Abstract
There is a clear need to improve our ability to assess the ecological consequences of environmental change. Because of the complexity of ecosystems and a need to disentangle the effects of multiple pressures, predictions are often reliant on models and expert opinion. These require validation with observed data; in this respect, long-term data sets are particularly valuable. Innovative statistical methods (nonparametric regression and additive models) are presented for identifying nonparametric ecological trends and changes in seasonality in response to environmental change. These are illustrated through the example of Loch Leven, a shallow freshwater lake. Monitoring data for 35 years are examined, spanning periods of enrichment, ecological recovery and changing climate. Models are developed for phosphorus and nitrogen; temperature and rainfall; Daphnia grazers; and chlorophyll a, with the ecological objectives of examining trends in water quality and the corresponding trends in nutrient availability, grazer abundance and climate. The analysis highlighted a generally decreasing availability of P over the study period, and generally increasing nonparametric trends in nitrate concentration and rainfall. Increasing spring temperatures were also evident, as were significant nonparametric changes in density of summer grazers. Significant reductions are highlighted in spring and summer chlorophyll a, related to the return of Daphnia to the loch. However, no response in chorophyll a to the later declining trends in P is apparent, but seasonality has changed. Synthesis and applications. The analysis highlights the value of nonparametric statistical models for assessing complex ecological responses to environmental change. The models outlined can examine key ecological impacts of climate change, particularly effects on the timing of seasonal events and processes. The models are illustrated using long-term water-quality data from Loch Leven to explore patterns in key environmental drivers and ecological responses affecting freshwater ecosystems. Analysis of chlorophyll a, in particular, highlighted the value of examining the seasonal trends separately, with different trends evident for winter and spring and a changing seasonal pattern.