I was reading through this study from Ecology yesterday, which tells the interesting story of how coastal dune ecology in northern California was invaded in the 19th century and subsequently disrupted. In order to stabilize the ever-shifting sand dunes, a grass called Ammophila arenaria, the European beachgrass, was planted along the coastline. A. arenaria grows from a strong, thick network of branching rhizomes, allowing it create a fast hold on loose soil and, as the coastal managers intended, create a framework that slowed erosion.
Of course, what was preferable to coastal managers wasn't for the native wildlife. A. arenaria has spread all the way up to British Columbia since then, supplanting the native populations and potentially pushing one particular species of plant to extinction in the near future.
Lupinus tidestromii (link to the researchers' project homepage with some great photographs) was flowering on the dunes of northern California long before the European beachgrass arrived. The beachgrass is a direct competitor with L. tidestromii for the basics - sunlight, water and territory - but according to the authors, there are two other ways in which A. arenaria threatens L. tidestromii.
First, A. arenaria has limited L. tidestromii's seed scarification and germination. By anchoring the sand dunes, the invasive beachgrass has greatly reduced the chance for strong winds to remove the top layer of soil and vegetation from the dunes, which can expose and disperse dormant seeds. The researchers believe that L. tidestromii probably thrived when this mode of scarification was more prevalent since its seedlings are usually the first to establish after such "blowouts", but are quickly overtaken by A. arenaria.
Second, there is not only direct competition between the invasive and the native, but also a type of indirect competition called "apparent" competition that is playing a greater role in L. tidestromii's decline. In general, this type of competition revolves around two producers and a shared predator. One producer's population changes, which leads to a change of the predator population and finally, a change in the second producer's population (this is not exclusive to invasive populations, this mechanism applies to native communities as well). In this case, the invader A. arenaria is providing housing for L. tidestromii's main pre-dispersal seed predator, the deer mouse (Peromyscus maniculatus), which is bolstering their numbers and increasing pre-dispersal predation within 100 meters of these refuges. The beachgrass covers large areas of coastal land, giving these animals plenty of places to hide and plenty of opportunities to snatch up seeds en masse before they can be dispersed.
In order to restore L. tidestromii's dwindling population, the beachgrass needs to be removed, which will reduce the area of refuge for the deer mouse, reduce their populations and alleviate some of the pressures on the lupine's seed dispersal. The researchers have already projected an increase in one population of L. tidestromii, from only a marginal reduction in seed predation.
(I tried to keep in mind while reading that this is only two species evaluated regarding an introduced species that affects many other organisms in a wider ecosystem; that the effects are so pervasive in so small an interaction is remarkable.)
The authors believe that apparent competition may be responsible for homogenization and certain cases of selective extinction dependent on predator preference:
When invasive plants compete strongly with native plant communities via apparent competition, native species preferred by consumers are selectively eliminated from the community. As a result, invaded communities will ultimately contain a more homogenous composition plant species that are not preferred by consumers.There are many examples where changes in the abundance of an herbivore or introduction in an exotic herbivore changes plant community composition towards less preferred species. Throughout eastern North America, white-tailed deer have increased in density due to habitat fragmentation, supplemental food sources and the eradication of large carnivores; this in turn causes a reduction in the relative abundance of their preferred plant species (Augustine and McNaughton 1998). In addition, the introduction of exotic cattle to American landscapes similarly shifts plant communities toward those species that are not preferred (Fleischner 1994).
Two of the three populations of L. tidestromii that were analyzed are on a projected decline to extinction. Unless measures are taken to reduce the omnipresent influence of European beachgrass, this unique little lupine may disappear for good.
Dangremond, E., Pardini, E., & Knight, T. (2010). Apparent competition with an invasive plant hastens the extinction of an endangered lupine Ecology DOI: 10.1890/09-0418