Spatial refugia, invasion, and climate change: Species conservation in the Anthropocene
Widespread species invasions are resulting in landscapes that support various combinations of non-native and native species, with some areas completely transformed by invaders and others providing spatial refugia for native species. Maintaining native refugia in the face of global change is a key challenge for conserving species diversity in the Anthropocene. Here we examine how spatial dynamics interact with temporal dynamics to influence conservation of native biodiversity and decision-making regarding invasive species management. We first present a conceptual model about how variation in space can affect invasion dynamics over time, emphasizing how land use and environmental changes – key challenges presenting in the Anthropocene – may affect this framework. Then we apply this model to a specific case of California serpentine outcrops and invasion by non-native Mediterranean grasses. We use a 31-year dataset of plant composition in which we have observed a series of invasions and subsequent recessions by a non-native annual grass, Bromus hordeaceus. While these areas are important competitive refugia for native herbaceous species because they have low nutrient, freely draining soils, we ask whether climate change can provide resource pulses that shift competitive hierarchies and allow invasion.
Integrating spatial and temporal dynamics of invasion are essential to develop management strategies for species conservation in the Anthropocene. We find that climate change can influence the extent of spatial refugia for native species and alter how invasions occur over time. In our serpentine grassland case study, population models of Bromus and native forbs indicate that Bromus invasion cycles are driven by climate change, but that the invasions are transient due to periodic dry periods. In contrast, we find that native species populations are primarily driven by the effects of intraspecific density dependence and local disturbance, with muted responses to precipitation. We compare this result with previous findings that anthropogenic nitrogen deposition in serpentine grasslands completely transform the spatial refugia into non-native communities. This highlights that while serpentine grassland are an important spatial refugia for native plant species, different temporal dynamics may alter the degree to which this is true. We conclude by discussing management strategies – including targeted grazing and water manipulations – that may help maintain the conservation value of serpentine refugia in the face of increasing global change.