Waterways are key to maintaining connectivity for many species. This is especially true in urban areas, where they may represent the only remaining continuous green space. While they often connect native species that might otherwise disappear, they can just as easily act as highways for invasive species that take advantage of the inherent disturbance in waterways and higher nutrient levels from pollution.
In order to manage invasive plants in an urban environment, it’s important to understand how they’re using the limited amount of waterways that are left. While there have been a lot of studies on how animals move along urban stream corridors, and how plants move along stream corridors in general, less is known about how individual plant species and plant assemblages move along urban streams.
A new study looks at connectivity in urban riparian networks and their surrounding landscape, and asks whether native and non-native species use these networks in the same way. It focuses on three sections of the Rahway River in northeastern New Jersey, a watershed that was one of the first urbanized areas in the United States and currently carries toxic substances and discharge from wastewater treatment plants. Packs of leaf litter were collected and vegetation surveys were conducted along the river banks and then analyzed for composition.
The good news is that riparian systems are effective corridors for plants in urban areas. The Rahway River was effective in dispersing seeds for numerous species, both native and non-native. However, spatial and temporal connectivity was different between native species and non-native invasives. Propogules from non-native species were more common overall, especially earlier in the spring, meaning that they are likely dominating native species by preemption.
The most common species in both litter and riverbank vegetation were non-native, invasive species: Artemisia vulgaris, Microstegium vimineum, and Polygonum cuspidatum. Non-native species comprised around half of both the riverbank vegetation and the litter vegetation.
The results suggest that managing for invasive plant species along urban waterways would be most effective at the watershed scale. Municipalities that lie downstream would benefit from coordinating with upstream municipalities to prevent further spread of invasives. Unfortunately, urban management is more often conducted at much more local scales, and usually focuses on recreation rather than conservation. Greater cooperation among all stakeholders across the watershed region could make a large difference.
With proper management, urban streams can limit the spread of non-native species, and act as more effective corridors for native ones. That makes green spaces in cities and other densely populated areas more useful for humans and plant species alike.
Aronson, M. F. J., M. V. Patel, K. M. O’Neill, and J. G. Ehrenfeld. 2017. Urban riparian systems function as corridors for both native and invasive plant species. Biological Invasions 19(12): 3645-3657.