<div>
<p><a>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. </a></p>
<p> 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 (<i>Ictalurus punctatus</i>) has drastically altered the fish
assemblage of the San Juan River, yet the impacts of riparian invasion by a
fruit-bearing tree, Russian olive (<i>Elaeaganus agustifolia</i>) 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. </p>
<p>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. </p>
<p>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. </p>
<p>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.</p>
</div>
<br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/11374218 |
| Date | 20 December 2019 |
| Creators | Christopher A Cheek (8130312) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Invasion_Meltdown_Investigating_Mutual_Facilitation_across_Ecosystem_Boundaries/11374218 |
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