- Biological invasions and climate change represent two preeminent threats to ecological communities and biodiversity, altering the distribution and abundance of species, disrupting existing species interactions and forming unprecedented ones, and creating novel ecological communities. Many of the most successful invasive species are also ecosystem engineers, species that physically modify the abiotic state of the ecosystem, and consequently have broad impacts on community structure, ecosystem processes, and ecosystem services. As ecosystems face dual hazards from biological invasions and climate change, it is imperative to understand what factors influence invasion success, how invasive species alter physical and biological processes, and how a changing climate alters the course of invasion.
In this dissertation, I investigate the interactions of two invasive, dune-forming beachgrasses within the U.S Pacific Northwest coastal dune ecosystem and their influence on dune geomorphology and ecosystem services. Two species of non-native beachgrasses, Ammophila arenaria (L.) Link and A. breviligulata (Fernald), were intentionally introduced to the Pacific Northwest in the early 20th century for the purpose of sand stabilization. Since their introductions, they have displaced numerous endemic plants and animals, and facilitated the formation of tall, stable, and well-vegetated shore-parallel dune ridges throughout the region. However, the two Ammophila species differ in their distributions and their impacts on dunes: biogeographically, A. arenaria dominates coastal dunes in central Oregon and southward, whereas A. breviligulata dominates dunes in northwest Oregon and southwest Washington; geomorphologically, A. arenaria-dominated dunes are taller and narrower than A. breviligulata-dominated dunes. Ammophila breviligulata-dominated regions also experience higher sediment supply and rates of shoreline change, typically creating wider beaches and shorter dunes. Although Ammophila biogeography in part reflects historical planting patterns, invasion success might also relate to latitudinal variations in sand supply. Similarly, while some differences in dune shape may relate to species-specific differences in grass density, morphology, and growth form, spatially correlated sand supply and beach characteristics might also explain the differences.
Using a combination of observational surveys, experimental manipulations, and statistical and numerical modeling, I examine how species identity, species characteristics, and sediment supply influence dune geomorphology, invasion success, species interactions, and ecosystem services. In Chapter 2, I investigate the relative role of beachgrass density, species identity, beach characteristics, and sediment supply on dune geomorphology and rates of sand accretion using Gaussian Bayesian networks. I found that dune shape was primarily influenced by beach sediment supply and backshore slope, whereas beachgrass density was a smaller, albeit important determinant of foredune shape. Still, A. arenaria-dominated dunes were taller and narrower than those of A. breviligulata, even after accounting for sediment supply and beach characteristics. These differences in dune shape likely arose because of the functional difference in sand capture of the two Ammophila species as a result of differences in growth form.
In Chapter 3, I use reciprocal transplant experiments to examine how species invasiveness and community invasibility contribute to the survival and growth of the two Ammophila species across a sand supply gradient. I determined that A. breviligulata and A. arenaria exhibit higher survival and productivity than the native dune grass Elymus mollis. Further, A. breviligulata and A. arenaria-dominated foredune communities were less invasible than E. mollis communities, primarily because E. mollis communities facilitated the survival and productivity of non-native species. Thus, the invasion success of the two Ammophila species results from both their high invasiveness and the high invasibility of the native E. mollis community.
In Chapter 4, I explore how rising temperatures and variations in sand burial rates associated with climate change might impact the growth, morphology, and species interactions of A. arenaria and A. breviligulata using a common garden experiment. I showed that A. breviligulata is competitively dominant to A. arenaria under a range of sand burial and temperature conditions by exhibiting higher productivity and exerting a negative density-dependent effect on A. arenaria tiller and biomass production. Although A. breviligulata was predicted to exclude A. arenaria across all experimental conditions, elevated temperatures and high rates of sand burial also increased the likelihood of coexistence between the two grass species. These results suggest that a warmer climate may limit the spread of A. breviligulata or promote coexistence between the two Ammophila species.
Finally, in Chapter 5, I assess how the removal of invasive beachgrass for the conservation of the threatened western snowy plover (Charadrius nivosus nivosus) creates context-dependent synergies and tradeoffs among coastal dune ecosystem services. I found that coastal protection, western snowy plover conservation, and endemic plant conservation varied among seven plover habitat restoration areas. Western snowy plover conservation services negatively covaried with coastal protection due to management actions. While dune shortening from beachgrass removal was the proximate cause of this tradeoff, the magnitude of the tradeoff was influenced by nearshore geomorphology in that dune shortening had a muted effect on coastal hazard exposure when it occurred on shallow-sloping, dissipative beaches. Thus, consideration of nearshore and beach geomorphology is important when managing for western snowy plover conservation because it may lessen tradeoffs among competing Pacific Northwest dune ecosystem services.
Overall, the findings from this dissertation research demonstrates the complex and interdependent connection between biological invasions and climate change on community structure, ecological processes, and ecosystem services of the U.S. Pacific Northwest coastal dunes. For management of coastal dunes, this research examines multiple factors that may contribute to the present-day distribution and abundance of the two Ammophila species, and their potential to spread to new areas in the coming decades. Moreover, I demonstrated why the two Ammophila species produce distinct dune shapes and resultantly exhibit differing amounts of protection against flooding and erosion hazards. Finally, although I found that Ammophila removal for plover conservation created tradeoffs in ecosystem service supply, strategic management can minimize tradeoff severity and alleviate potential conflicts among stakeholders. For Pacific Northwest coastal management, these findings can help to inform coastal managers of the potential for changes in the distribution of the two Ammophila species, their impacts on dune ecosystems and the services that they provide, and some possible strategies possible strategies for establishing new habitat restoration areas for endangered species while managing resources for multiple stakeholders.