Pond networks support high levels of biodiversity when compared to other freshwater ecosystems such as rivers, lakes and streams. The persistence of species in these small, sometimes ephemeral, aquatic habitats depends on the dispersal of individuals among ponds in the landscape. However, the number of ponds across the landscape is at a historical low as urbanisation and intensified agricultural practices have led to a substantial loss of ponds (nodes in the pond network) over more than a century. Here, we examine the extent and drivers of pond loss in a heavily urbanised landscape (Birmingham, UK) over 105 years and determine how pond loss influences key structural properties of the pond network using graph theoretic approaches. Specifically, we calculated minimum spanning trees (MST) and performed percolation analyses to determine changes in both the spatial configuration and resilience of the pond network through time. Pond numbers declined by 82% between ca1904 and 2009, such that pond density decreased from 7.1 km -2 to 1.3 km -2 . The MST analyses revealed increased distance between ponds in the network (i.e. edge length increased) by up to 49% over the 105-year period, indicating that ponds in the modern landscape (2009) were considerably more isolated, with fewer neighbours. This study demonstrates that graph theory has an excellent potential to inform the management of pond networks in order to support ecological communities that are less vulnerable to environmental change.
Marine ecosystems can be modified and shaped by irregular interannual variations in oceanic current patterns and temperatures, such as El Niño and La Niña. These large scale oceanic events have also been shown to influence environmental stressors such as invasive marine species (IMS). Our study indicates that there is a causative link between these climatic events, and atypical detections of native and IMS. Significant La Niña events between 1970 and 2014 were associated with sightings of tropical crab species in temperate waters following a lag period of 18–24 months from the onset of the event. We identified a total of 72 records of six tropical portunid crabs species (from both Charybdis and Scylla) in temperate waters of south-western Australia following these La Niña events, based on reports in published scientific literature, grey literature and museum records, as well as citizen science networks such as FishWatch and PestWatch apps. We suggest that La Niña conditions facilitated transportation and temporary establishment of crab larvae from their native tropical habitat to temperate regions. As the strength of La Niña events is likely to increase into the future due to the escalating effects of climate change, it is likely that there will be a growth in associated atypical establishment events of IMS. Consequently, biosecurity managers will need to reprioritise resources in order to accommodate the potential impacts of these large scale oceanic events as part of their surveillance programmes.
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