Recent GGE/JDPE graduate Max Castorani will be presenting this work in the Competition I session at ESA. Come see him on Tuesday at 3:40 PM in Hyatt Regency Ballroom D.
Among the most universal observations in invasion ecology is the tendency for invasive species to transform ecological communities by outcompeting natives. Many of the world’s most notorious invaders—kudzu, fire ants, zebra mussels, lionfish, and many others—strongly outcompete their native counterparts. However, recent theoretical models predict that invading prey species may also interact with their native competitors indirectly through shared native predators. Nevertheless, indirect effects are largely overlooked in the science and management of invasive species, even though they appear to be common and significant mechanisms of impact and can complicate eradication or restoration efforts. My latest work, in collaboration with one of my Ph.D. advisors, Kevin Hovel at San Diego State University, provides a unique empirical test of these largely unexplored theoretical predictions in two invaded estuaries of southern California.
We carried out a series of field experiments to understand how an invasive mussel (Arcuatula senhousia)—which was introduced to California from east Asia—impacts a native bivalve assemblage. We tested the strength and mechanisms of direct competition, as well as whether native predators might indirectly mediate these interactions. Our experiments revealed that invasive mussels compete with both juvenile and adult stages of native bivalves: Arcuatula reduce the abundance and size of native bivalve recruits by preemptively exploiting sediment surface space and reduce the survival of adult natives through overgrowth competition.
However, we also discovered that introduced mussels attract native predators, indirectly increasing predation of native clams, especially for poorly-defended native species. Therefore, invasive prey can indirectly increase predation rates on native competitors by changing the abundance or behavior of shared predators, but the magnitude of this apparent competition strongly depends on the vulnerability of natives to predation.
Interestingly, our results illustrate that the susceptibility of invasive prey to predation can greatly exacerbate impacts on native competitors, a counterintuitive conclusion unless indirect interactions are considered. This is significant because without knowledge of indirect effects, the introduction of a palatable invasive species may be seen as a low concern for management. Our study is also among the few to clearly illustrate invader-driven apparent competition in marine and estuarine habitats, which are among the most heavily invaded of all ecosystems.