Community assembly and connectivity along an urban river system

Background/Question/Methods

With the rapid increase of urban land throughout the world, understanding the function of urban ecosystems has emerged as one of the major intellectual challenges for science, requiring integration over numerous disciplines. Understanding the processes that contribute to the assembly of communities in urban areas requires examination of both natural and anthropogenic filters on species composition. We examined the stream bank vegetation and seed dispersal along three 2^nd order streams in the Rahway River watershed located in three counties of NJ. Along each stream we sampled four river zones (above a dam, directly below the dam, above a concrete embankment, below a concrete embankment). Once a month at each zone from March to October 2011, we randomly collected four leaf litter piles, cold stratified for two months, and germinated seeds on sterile soil in the greenhouse. We identified all individuals to species where possible. Vegetation surveys were performed at each zone. We compared plant community structure between litter piles and field vegetation surveys to access movement of seeds and community assembly along the river.

Results/Conclusions

We found 140 plant species in the litter. We found 153 plant species in the vegetation surveys of the river bank vegetation. Of these species, 55 species overlapped between the litter and the riverbank vegetation, 98 species were found only in the riverbank vegetation, and 85 species were found only in the litter. Non-native invasive plant species such as Artemisia vulgaris, Microstegium vimineum, and Polygonum cuspidatum are found most frequently in both the riverbank and litter vegetation. These results suggest that invasive plant species promote the greatest connectivity among sites along streams in this urban watershed. Additionally, anthropogenic species (D. ischaemum and D. sanguilnalis, crab grasses) not found commonly in the bank vegetation were common in the litter, indicating inputs of seeds from lawns and other urban habitats. Here we show that riparian vegetation in urban areas are assembled by both natural and anthropogenic processes. Management of both invasive species and anthropogenic habitats along the stream bank is important for the conservation and restoration of native riparian communities.