Invasion Meltdown: Investigating Mutual Facilitation across Ecosystem Boundaries

Abstract

Biotic interactions play key roles in determining invasive\nspecies’ establishment success in receiving ecosystems (Tilman 2004). The invasional meltdown hypothesis suggests\nthat initial invaders can facilitate subsequent invasions through direct (e.g.,\ncommensalism, mutualism) and indirect (e.g., changes in habitat and energy\nflow) pathways (Simberloff & Holle 1999; Simberloff 2006). Such positive interactions among invaders can\nalter community-level processes, but little research on this has been done in\naquatic-terrestrial landscapes. My\ndissertation explores the links between reciprocal facilitation of invasive\nspecies and ecosystem change in a desert river system in the southwest USA. Like most rivers in the southwestern United\nStates, the San Juan River has been altered by hydrologic regulation and\nbiological invasions that affect ecosystem function and act synergistically to\ninduce substantial ecosystem change. Invasion\nof channel catfish (Ictalurus punctatus) has drastically altered the fish\nassemblage of the San Juan River, yet the impacts of riparian invasion by a\nfruit-bearing tree, Russian olive (Elaeaganus agustifolia) have largely\nbeen ignored. Channel catfish have been\nobserved consuming Russian olive fruits, but the level of facilitation between\nspecies and corresponding impact on the ecosystem is unclear. \n\nChannel catfish may benefit directly\nfrom Russian olive invasion by feeding on fallen fruits and/or indirectly from\nhabitat alteration and invertebrate prey production from Russian olive\ndetritus. Additionally, channel catfish\nmay facilitate germination, growth, and seed dispersal of Russian olive. Mutualism between these invaders could\nincrease the fitness of each species, thereby facilitating invasion success. Plant-animal mutualism is the most common\nform of facilitation among invaders, but no studies, to our knowledge, have\nevaluated facilitation between invasive riparian plants and aquatic invaders\nand their combined impact on ecosystem function. My goal preparing this dissertation is to\ndetermine whether mutual facilitation between riparian and aquatic invasive\nspecies influences ecosystem change through biotic interactions. \n\nTo test for mutual facilitation, I\nfirst determined the contribution of Russian olive fruits to channel catfish\ngrowth by evaluating seasonal diet composition across four sites and six time\nperiods. I then used replicated growth\nexperiments to determine assimilation rates of Russian olive fruits consumed by\nchannel catfish. Using bioenergetics models,\nI then determined how Russian olive subsidies in San Juan River contribute to channel\ncatfish biomass. To determine whether\nchannel catfish benefit Russian olive reproduction, I compared germination\nrates of seeds consumed by channel catfish to seeds consumed by terrestrial\nmammals and control seeds that had not been eaten. \n\nRussian olive fruits were the most\nimportant diet item for channel catfish during the fall and spring, comprising\nup to 57 and 70% of stomach contents by mass, respectively, and were consumed\nthroughout the year. Feeding trials revealed\nthat Russian olive fruits contributed little to growth or lipid deposition, but\nthey did provide metabolic energy allowing channel catfish fed exclusively\nRussian olive fruits to maintain weight. \nIn addition, Russian olive trees received a reproductive benefit through\nincreased germination success of seeds consumed by channel catfish over those\ntransported by water. Using bioenergetic\nmodels, I showed that Russian olive fruits subsidized 46% of San Juan River channel\ncatfish biomass, indicating that the subsidy from Russian olive fruits had a\npopulation-level impact. This dissertation\nthus establishes mutual facilitation by non-native species across ecosystem\nboundaries, a phenomenon that few studies heretofore have demonstrated in the\necology or invasion biology literature.