2 Introduction
2.1 What is Stream Connectivity?
Connectivity refers to the connections between ecosystem components across spatial and temporal scales. In riverine and stream systems, connectivity refers to water flow, water-mediated energy transfer and movement of aquatic organisms. Stream connectivity is important to support healthy populations and life histories of aquatic organisms at all trophic levels. Within-channel connectivity is particularly important for aquatic organisms, as many require connections between habitats to complete their life cycles. Different species and taxa have different requirements for connectivity and respond differently to reduced connectivity. For the purposes of this indicator, stream connectivity focuses on the amount of connected habitat between upstream and downstream segments within a river network (Park and Scrimgeour 2008).
Connectivity can be described along a continuum, ranging from highly connected habitats to highly isolated ones (Ward 1989). Research on ecological connectivity has increased since the 1990’s, however, most studies have focused on terrestrial systems (Fullerton and Sanderson 2010). Rivers branch and flow differently than land habitats, therefore methods that have been developed to measure connectivity on land do not always work for aquatic systems (Fullerton and Sanderson 2010). In response, researchers have recently created new approaches for measuring connectivity in rivers, either by adapting existing land-based methods or developing techniques for lotic systems.
2.2 How can it be measured?
There are two types of connectivity:
- Structural connectivity represents how habitat patches (i.e., stream segments) are spatially distributed with respect to one another across a network.
- Functional connectivity represents not only how habitat patches are distributed but recognizes that species will have different responses to barriers based on their dispersal abilities.
There are a wide breath of both structural and functional stream connectivity indicators available within the literature. These indicators are typically focused on either the hydrological (e.g., least flow cost, river fragmentation, topographic wetness, etc) or biological (hanging culvert density, integral index of connectivity, probability of connectivity, etc) aspects of connectivity. There is no single index that can answer every question, so choosing an indicator that meets the desired objectives, and understanding the limitations of the index are essential.