Long-term ecological trends of flow-dependent ecosystems in a major regulated river basin

Abstract

The case for restoring water to the environment in the Murray–Darling Basin, Australia, is based mainly on condition assessments, although time series provide valuable information on trends. We assessed trends of 301 ecological time series (mean 23 years, range 1905–2013) in two categories: (1) ‘population’ (abundance, biomass, extent) and (2) ‘non-population’ (condition, occurrence, composition). We analysed trends using log-linear regression, accounting for observation error only, and a state–space model that accounts for observation error and environmental ‘noise’. Of the log-linear series (n=239), 50 (22%) showed statistically significant decline, but 180 (78%) showed no trend. For state–space series (n=197) one increased, but others were stable. Distribution of median exponential rates of increase (r) indicated a small but statistically significant declining trend, though 35–39% of the series were positive. Our analysis only partly supports, though does not refute, prevailing assumptions of recent ecological decline in the Murray–Darling Basin. The pattern is of fluctuating stability, with declines during droughts and recovery after flood. The overall trend from our meta-analysis is consistent with a pattern of historical decline to a hybrid ecosystem followed by slow, recent decline for some components and stability for others, with considerable variation in trends of specific ecological components: in short, there are ecological ‘winners’ and ‘losers’.