The science of connected ecosystems: What is the role of catchment-scale connectivity for healthy river ecology?

Riverine biological communities are highly resilient to extreme flood and/or drying disturbance regimes that would otherwise be destructive because these organisms can recolonise from upstream, floodplain, or hyporheic refugia when suitable conditions return. Healthy rivers require a high degree of connectivity to support complex life cycles of many lotic organisms and associated ecosystem functioning. Similarly, connectivity is required for appropriate geophysical functioning; permitting flux of water and sediment that drives channel-forming and ecological processes. Ecological and geophysical processes have operated in this temporal and spatial patchwork of disturbance and recovery pre-Anthropocene. Human impacts are increasing constraints on river and floodplain connectivity, severing many natural pathways, and degrading river ecosystem functioning. River restoration seeks to re-establish some of those biological and physical connections to enhance some level of system health. However, increasing sediment connectivity may be detrimental to river health in some instances. Strongly connected catchments can transmit excessive quantities of sediment from inappropriate land management, detrimental invasive species can spread more widely, and many ecosystem processes can exceed positive feedback control. Simply restoring connectivity will not necessarily lead to healthy river ecosystems. River management requires a greater understanding of how and when connectivity can and should be restored. Although current thinking is often that greater connectivity is better, we illustrate with examples from New Zealand rivers that this is not always the case. The benefits and costs of maintaining or restoring river connectivity need to be given as much attention as the restoration and maintenance of river systems per se.