Human impacts on connectivity in marine and freshwater ecosystems assessed using graph theory: a review

Human activities are altering the processes that connect organisms within and amongst habitats in aquatic ecosystems. Little is known about whether impacts to connectivity are similar across marine and freshwater systems, and... [ view full abstract ]

Human activities are altering the processes that connect organisms within and amongst habitats in aquatic ecosystems. Little is known about whether impacts to connectivity are similar across marine and freshwater systems, and across different species and ecosystems. To identify generalities in human impacts on connectivity, we conducted a review of studies which modelled functional connectivity using graph theory. Graph theory is a general framework that allows cross-system comparisons. In graph theory habitats or populations are represented by “nodes” and dispersal is represented by “links.” We assessed the 42 studies and where human impacts to connectivity were assessed they were grouped into four categories: (1) habitat alteration, (2) human movements causing species introductions, (3) overharvesting, and (4) climate change and ocean acidification. Habitat alteration was the most commonly studied stressor in freshwater systems, whereas, overharvesting, in terms of larval dispersal among protected areas, was most commonly addressed in marine systems. Few studies have directly assessed effects of climate change, suggesting an important area of future research. Graph representations of connectivity revealed similarities across different impacts and systems, for instance climate change, habitat loss and overharvesting can all remove nodes, suggesting they may have similar effects on functional connectivity. Comparable impacts of different human activities on connectivity suggest common conservation strategies for the conservation of aquatic species.