Modeling colonization and extinction dynamics gives insights into invasion success of Hydrilla verticillata in riverine rock pools

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Joshua T. Armstrong, Lesley P. Bulluck, C. Ryland Stunkle and James R. Vonesh, Center for Environmental Studies, Virginia Commonwealth University, Richmond, VA, Andy Davidson, Integrative Life Sciences, Virginia Commonwealth University, Richmond, VA

Background/Question/Methods
To fully understand invasion success, it is imperative to elucidate potential drivers of colonization and extinction through dynamic modeling. Hydrilla verticillata, a submerged aquatic plant, is native to Asia and has been an invasive to the United States since 1960. Dominating lakes and streams, Hydrilla has had negative impacts on local economies. It has been hypothesized that environmental factors could influence Hydrilla colonization success, but little is known of temporal and spatial influences of both colonization and extinction.
Leveraging a four-year data set collected from hundreds (N=745) of riverine rock pools along the James River fall line in Richmond, VA, we dynamically modeled occupancy, colonization, and extinction probabilities of Hydrilla using the “unmarked” package in R. Using a multi-year occupancy dataset, we modeled occupancy, colonization, and extinction as a function of environmental (e.g. depth, volume, and river height at which the pool floods), spatial (e.g. distance to river channel and nearest Hydrilla occupied pool) and temporal (e.g. year and Julian day) predictors.
Results/Conclusions
We found that environmental, spatial, and temporal factors all influenced occupancy, colonization, and extinction. Hydrilla occupancy was significantly more likely in larger pools with higher flood heights. Hydrilla colonization probability increased across years, and was significantly higher in pools with higher flood heights and that were closer to other Hydrilla pools. Hydrilla extinction probability decreased across years and was significantly higher in pools further away from other Hydrilla pools while significantly lower in more voluminous pools that were further away from the river channel and flooded at higher levels. Although we found environmental, temporal, and spatial factors to influence occupancy dynamics in general, the height at which a pool floods was the strongest predictor in all three processes. This indicates that flooding has a negative impact on Hydrilla in the rock pool system and more broadly suggests that disturbance is an important factor in limiting invasion success. It has been thought that most invaders take advantage of disturbances to invade a community; however, we found that a disturbance increased the likelihood of extinction in invaded pools and decreased colonization in uninvaded pools.