Predicting the consequences of species loss is critically important, given present threats to biological diversity such as habitat destruction, overharvesting and climate change. Several empirical studies have reported decreased ecosystem performance (for example, primary productivity) coincident with decreased biodiversity, although the relative influence of biotic effects and confounding abiotic factors has been vigorously debated. Whereas several investigations focused on single trophic levels (for example, grassland plants), studies of whole systems have revealed multiple layers of feedbacks, hidden drivers and emergent properties, making the consequences of species loss more difficult to predict. Here we report functionally important organisms and considerable biocomplexity in a sedimentary seafloor habitat, one of Earth's most widespread ecosystems. Experimental field measurements demonstrate how the abundance of spatangoid urchins--infaunal (in seafloor sediment) grazers/deposit feeders--is positively related to primary production, as their activities change nutrient fluxes and improve conditions for production by microphytobenthos (sedimentatry microbes and unicellular algae). Declines of spatangoid urchins after trawling are well documented, and our research linking these bioturbators to important benthic-pelagic fluxes highlights potential ramifications for productivity in coastal oceans.

Despite the increasing evidence of drastic and profound changes in many ecosystems, often referred to as regime shifts, we have little ability to understand the processes that provide insurance against such change (resilience). Modelling studies have suggested that increased variance may foreshadow a regime shift, but this requires long-term data and knowledge of the functional links between key processes. Field-based research and ground-truthing is an essential part of the heuristic that marries theoretical and empirical research, but experimental studies of resilience are lagging behind theory, management and policy requirements. Empirically, ecological resilience must be understood in terms of community dynamics and the potential for small shifts in environmental forcing to break the feedbacks that support resilience. Here, we integrate recent theory and empirical data to identify ways we might define and understand potential thresholds in the resilience of nature, and thus the potential for regime shifts, by focusing on the roles of strong and weak interactions, linkages in meta-communities, and positive feedbacks between these and environmental drivers. The challenge to theoretical and field ecologists is to make the shift from hindsight to a more predictive science that is able to assist in the implementation of ecosystem-based management.

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Although many studies have documented the impact of invasive species on indigenous flora and fauna, few have rigorously examined interactions among invaders and the potential for one exotic species to replace another. European green crabs (Carcinus maenas), once common in rocky intertidal habitats of southern New England, have recently declined in abundance coincident with the invasion of the Asian shore crab (Hemigrapsus sanguineus). Over a four-year period in the late 1990s we documented a significant (40-90%) decline in green crab abundance and a sharp (10-fold) increase in H. sanguineus at three sites in southern New England. Small, newly recruited green crabs had a significant risk of predation when paired with larger H. sanguineus in the laboratory, and recruitment of 0-yr C. maenas was reduced by H. sanguineus as well as by larger conspecifics in fielddeployed cages (via predation and cannibalism, respectively). In contrast, recruitment of 0-yr H. sanguineus was not affected by larger individuals of either crab species during the same experiments. The differential susceptibility of C. maenas and H. sanguineus recruits to predation and cannibalism likely contributed to the observed decrease in C. maenas abundance and the almost exponential increase in H. sanguineus abundance during the period of study.While the Asian shore crab is primarily restricted to rocky intertidal habitats, C. maenas is found intertidally, subtidally, and in a range of substrate types in New England. Thus, the apparent replacement of C. maenas by H. sanguineus in rocky intertidal habitats of southern New England may not ameliorate the economic and ecological impacts attributed to green crab populations in other habitats of this region. For example, field experiments indicate that predation pressure on a native bivalve species (Mytilus edulis) has not necessarily decreased with the declines of C. maenas. While H. sanguineus has weaker per capita effects than C. maenas, its densities greatly exceed those of C. maenas at present and its population-level effects are likely comparable to the past effects of C. maenas. The Carcinus-Hemigrapsus interactions documented here are relevant in other parts of the world where green crabs and grapsid crabs interact, particularly on the west coast of North America where C. maenas has recently invaded and co-occurs with two native Hemigrapsus species.

When changes in the frequency and extent of disturbance outstrip the recovery potential of resident communities, the selective removal of species contributes to habitat loss and fragmentation across landscapes. The degree to which habitat change is likely to influence community resilience will depend on metacommunity structure and connectivity. Thus ecological connectivity is central to understanding the potential for cumulative effects to impact upon diversity. The importance of these issues to coastal marine communities, where the prevailing concept of open communities composed of highly dispersive species is being challenged, indicates that these systems may be more sensitive to cumulative impacts than previously thought. We conducted a disturbance-recovery experiment across gradients of community type and environmental conditions to assess the roles of ecological connectivity and regional variations in community structure on the recovery of species richness, total abundance, and community composition in Mahurangi Harbour, New Zealand. After 394 days, significant differences in recovery between sites were apparent. Statistical models explaining a high proportion of the variability (R2 > 0.92) suggested that community recovery rates were controlled by a combination of physical and ecological features operating across spatial scales, affecting successional processes. The dynamic and complex interplay of ecological and environmental processes we observed driving patch recovery across the estuarine landscape are integral to recovery from disturbances in heterogeneous environments. This link between succession/recovery, disturbance, and heterogeneity confirms the utility of disturbance-recovery experiments as assays for cumulative change due to fragmentation and habitat change in estuaries.