Invasion Meltdown: Investigating Mutual Facilitation across Ecosystem Boundaries
Biotic interactions play key roles in determining invasive species’ establishment success in receiving ecosystems (Tilman 2004). The invasional meltdown hypothesis suggests that initial invaders can facilitate subsequent invasions through direct (e.g., commensalism, mutualism) and indirect (e.g., changes in habitat and energy flow) pathways (Simberloff & Holle 1999; Simberloff 2006). Such positive interactions among invaders can alter community-level processes, but little research on this has been done in aquatic-terrestrial landscapes. My dissertation explores the links between reciprocal facilitation of invasive species and ecosystem change in a desert river system in the southwest USA.
Like most rivers in the southwestern United States, the San Juan River has been altered by hydrologic regulation and biological invasions that affect ecosystem function and act synergistically to induce substantial ecosystem change. Invasion of channel catfish (Ictalurus punctatus) has drastically altered the fish assemblage of the San Juan River, yet the impacts of riparian invasion by a fruit-bearing tree, Russian olive (Elaeaganus agustifolia) have largely been ignored. Channel catfish have been observed consuming Russian olive fruits, but the level of facilitation between species and corresponding impact on the ecosystem is unclear.
Channel catfish may benefit directly from Russian olive invasion by feeding on fallen fruits and/or indirectly from habitat alteration and invertebrate prey production from Russian olive detritus. Additionally, channel catfish may facilitate germination, growth, and seed dispersal of Russian olive. Mutualism between these invaders could increase the fitness of each species, thereby facilitating invasion success. Plant-animal mutualism is the most common form of facilitation among invaders, but no studies, to our knowledge, have evaluated facilitation between invasive riparian plants and aquatic invaders and their combined impact on ecosystem function. My goal preparing this dissertation is to determine whether mutual facilitation between riparian and aquatic invasive species influences ecosystem change through biotic interactions.
To test for mutual facilitation, I first determined the contribution of Russian olive fruits to channel catfish growth by evaluating seasonal diet composition across four sites and six time periods. I then used replicated growth experiments to determine assimilation rates of Russian olive fruits consumed by channel catfish. Using bioenergetics models, I then determined how Russian olive subsidies in San Juan River contribute to channel catfish biomass. To determine whether channel catfish benefit Russian olive reproduction, I compared germination rates of seeds consumed by channel catfish to seeds consumed by terrestrial mammals and control seeds that had not been eaten.
Russian olive fruits were the most important diet item for channel catfish during the fall and spring, comprising up to 57 and 70% of stomach contents by mass, respectively, and were consumed throughout the year. Feeding trials revealed that Russian olive fruits contributed little to growth or lipid deposition, but they did provide metabolic energy allowing channel catfish fed exclusively Russian olive fruits to maintain weight. In addition, Russian olive trees received a reproductive benefit through increased germination success of seeds consumed by channel catfish over those transported by water. Using bioenergetic models, I showed that Russian olive fruits subsidized 46% of San Juan River channel catfish biomass, indicating that the subsidy from Russian olive fruits had a population-level impact. This dissertation thus establishes mutual facilitation by non-native species across ecosystem boundaries, a phenomenon that few studies heretofore have demonstrated in the ecology or invasion biology literature.