2 Introduction
2.1 What is connectivity?
Stream connectivity represents the amount of connected habitat between upstream and downstream segments within a river network Park et al. (2008). Stream networks become fragmented, restricting species movement, due to both natural (e.g., beaver dams, waterfalls) and anthropogenic disturbances (e.g., culverts, dams, stream removal). The creation of barriers can have positive biological impacts, such as protecting genetically pure populations of Westslope Cutthroat Trout The Alberta Westslope Cutthroat Trout Recovery Team (2013), but these are purposefully created in order to achieve a management objective. This purposeful consideration is also true for large infrastructure projects such as dams and stream removal which are known to have negative impacts on stream connectivity. However, the dominate type of potential barrier on the landscape is culverts. As culverts age, they become susceptible to blockages due to sedimentation and garbage, or become hanging due to increased erosion. These barriers are accidental and have large impacts on stream connectivity.
2.2 How can it be measured?
There are two types of connectivity:
- Structural connectivity represents how habitat patches (i.e., stream segments) are spatial distributed with respect to one another across the network.
- Functional connectivity represents not only how habitat patches are distributed, but recognizes 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.