The Effects of Environmental Variation on the Persistence of Freshwater Invertebrates in California Streams

Freshwater streams are incredibly variable habitats where abiotic conditions change daily and seasonally. The species that inhabit streams must have the physiological capacity to deal with that variation. Variation that extends outside of the limits of an organism’s tolerance thresholds can cause sub-lethal or lethal stress and dictate which habitats are suitable to the organism. In addition to variation in abiotic conditions, organisms must fit into the ecological interaction web of their habitat and handle variation in biotic conditions. In my dissertation, I focus on two pathways for organisms to encounter new sources of environmental variation: invasion of novel habitats and climate change. Understanding how organisms deal with environmental variation is critical to predicting where they can live now and in the future.In Chapter 1, I aimed to identify the temperature and oxygen conditions that limit respiration and locomotion in the highly invasive New Zealand mud snail, Potamopyrgus antipodarum. To accomplish this, I generated thermal performance curves for resting respiration rate and voluntary locomotor behaviors under normoxia and hypoxia to find the conditions that limited each performance. I found that extremely high and low temperatures limited respiration and activity, but respiration rate was more sensitive to changes in oxygen at low temperatures. Hypoxia did not significantly decrease activity. Under hypoxic conditions, activity was less thermally sensitive, increased under high temperatures, and may be fueled by anaerobic metabolism. Relying on anaerobic energy is a time-limited survival strategy, so further warming and deoxygenation of freshwater systems may limit the spread of this invasive species.In Chapter 2, I focused on finding the abiotic and biotic conditions that explain variation in Potamopyrgus antipodarum population density in San Francisco Bay Area streams. P. antipodarum is a prolific invader of freshwater streams in California and worldwide, with the ability to form extremely dense populations that dominate the invaded habitat. To strengthen our ability to predict where this species can thrive, it is critical to understand which abiotic and biotic conditions explain variation in population density. I approached this need by measuring 25 conditions over one year in three streams with varying densities of P. antipodarum populations. The different streams could be distinguished by their water chemistry profiles. Conductivity and filamentous green algae cover were the factors most correlated to population density, but there was only marginal linear statistical support for the response to algae. Overall, P. antipodarum is robust to considerable variation in many ecological factors, and each factor may have a small effect in controlling their population sizes.In Chapter 3, I focused on the effect that past environmental variation, specifically thermal history, had on the response to different warming regimes in the larval caddisfly Dicosmoecus gilvipes. To accomplish this, I tested the molecular physiology responses of three populations of field-acclimatized D. gilvipes from different eco-regions (mountain, valley, coast) to current and future warming scenarios. I hypothesized that distinct thermal histories could differentiate populations at baseline levels of gene expression and gene expression changes in response to daily warming and heat shock. Population-specific responses were apparent under the control and daily warming conditions, while responses to heat shock were similar across populations. In addition, underlying gene expression patterns in the daily warming and heat shock treatments were different. These results highlight the importance and limitations of measuring the stress response of wild-caught organisms in their natural environment