Climate change and introduced species are among the top five threats to freshwater
systems face. Tropical regions are considered to be especially sensitive to the effects of
climate change, while island systems are more susceptible to species introductions.
Climate-driven changes in rainfall are predicted to decrease streamflow and increase flash
flooding in many tropical streams. In addition, guppies (Poecilia reticulata), an invasive
fish, have been introduced to many tropical freshwater ecosystems, either intentionally
for mosquito population control, or accidentally because of the aquarium trade. This
dissertation examines the effects of climate-driven change in rainfall and introduced
guppies on stream structure (resource and invertebrate biomass and composition) and
function (nutrient recycling) in Trinidad and Hawaii. In the first data chapter we used a
time series to examine how nutrient recycling of guppies changes in the first 6 years after introduction to a new habitat and to examine drivers of these changes. We found that
when guppy populations establish in a new environment, they show considerable
variation in nutrient recycling through time. This resulted from changes in guppy density
in the first two years of introductions, and changes in individual excretion in subsequent
stages. In the following chapter we utilized a rainfall gradient that mimics forecasted,
climate-driven changes in precipitation and resulting changes in streamflow to examine
the effects of climate change on stream food resources and macroinvertebrates. We found
that the drying of streams across the gradient was associated with a decrease in resource
quality and a 35-fold decline in macroinvertebrate biomass. Invertebrate composition also
switched to taxa with faster turnover rates. In the third data chapter we used this same
space-for-time substitution approach to determine if climate-driven changes in stream structure also affected stream function. We showed that population nutrient recycling
rates declined at the drier end of our rainfall gradient as a result of drops in population
densities. We also found that under the current climate scenario, community excretion
supplied up to 70% of the nutrient demand, which was ten-fold lower with projected
climate changes in streamflow. Lastly, since freshwater ecosystems often face multiple
human impacts, including climate change and invasive species, we wanted to understand
how climate-driven changes in flow might alter the impact of introduced guppies on
stream ecosystems. We selected several streams with guppies and several without
guppies along the Hawaii rainfall gradient to examine if the effect of guppies changed
with differences in streamflow. We found that the two stressors had synergistic effects on
macroinvertebrate biomass and nutrient recycling rates. We concluded that climate
change appeared to enhance effects of guppies, through direct and indirect effects. Overall, this dissertation shows that both climate change and species invasion can affect
stream ecosystems at multiple levels of organization. This dissertation demonstrates that
the effects of anthropogenic stressors are not static through time, and emphasizes the
need and utility of using several methodological approaches when measuring the
temporal effects of stressors. We also underline the significance of assessing multiple
stressor interactions, as more than one stressor often impacts ecosystems. / Graduate / 2019-09-01
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/11028 |
| Date | 01 August 2020 |
| Creators | Frauendorf, Therese |
| Contributors | El-Sabaawi, Rana |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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