Association is not causation: Treatment effects cannot be estimated from observational data in heart failure

Estuarine ecosystems are increasingly threatened by coastal development and climate change. The large number of estuaries globally necessitates risk assessment to prioritise conservation efforts. Schemes for assessing risk of ecosystem collapse have been designed around terrestrial ecosystems, often defined by a single characteristic vegetation type, with their applicability to estuaries unclear. Here we consider the causes and symptoms of estuarine ecosystem collapse and assess, using a case study of the Chesapeake Bay, the applicability of ecosystem-level risk assessments to estuarine ecosystems, typified by mosaics of habitats. Functional estuaries are characterised by habitat heterogeneity and connectivity, maintenance of constituent habitats through recruitment, and a complex trophic structure including apex predators. Additionally, primary production and biomass are dominated by benthic, as opposed to pelagic, species. Hence, homogenisation of habitat types, decreased connectivity, recruitment failure, loss of apex predators and a decreased ratio of benthic to pelagic biomass may be symptoms of a trajectory towards collapse. In terrestrial ecosystems, criteria used for assessing risk of ecosystem collapse include declining or restricted distribution of ecosystems, degradation of the abiotic environment, changes in species composition and declining ecological function. As the boundaries of estuaries are typically defined by topography, rarely do significant changes in the area of the ecosystem occur. Furthermore, because the extent of estuaries is typically small, assessments based on area of occupancy may over-inflate risk. Instead, criteria based on abiotic and biotic changes, many of which are documented through monitoring programs, may be most useful for risk assessments of estuarine ecosystems.