Department of Biology Graduate Theses

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Using Environmental DNA to Map a Contact Zone between Deeply Diverged Frog Mitochondrial Lineages in Southern Ontario
(2024-08-09) Robitaille, Madeleine Claire; Biology; Lougheed, Stephen
Contact zones are natural laboratories that facilitate the study of dynamics between diverging evolutionary lineages in primary (evolved in situ) or secondary (evolved in allopatry) contact; outcomes of contact zone dynamics have implications for understanding species range limits, can clarify questions on species delineation and mechanisms of speciation itself, and may be important in conservation prioritization. Chorus frog populations within Ontario and Quebec, Canada (currently collectively classified as Western Chorus Frogs, Pseudacris triseriata) comprise distinct mitochondrial DNA (mtDNA) lineages (> 10% sequence divergence in cytochrome b) representing Boreal (P. maculata) and Western (P. triseriata) Chorus Frogs. Within Ontario and Quebec, there are two designatable units (DUs), with the Great Lakes-St. Lawrence-Canadian Shield DU classified as Threatened in Canada. This DU approximately encompasses the Boreal Chorus Frog mitotype population. These diverging mitochondrial lineages are in probable secondary contact in Southern Ontario, yet the exact location of contact is uncertain, lying somewhere within a large region spanning ~150km (between Toronto, which is north of Lake Ontario, and the southern shores of Georgian Bay, part of Lake Huron). I used environmental DNA (eDNA) from water sampled from 50 chorus frog breeding ponds to better delineate this contact zone. eDNA facilitates intensive geographic sampling, allows us to identify single wetlands containing breeding assemblages with both haplotypes, and is less invasive than tissue sampling (e.g., toe clipping, buccal swabbing). I surveyed for the presence of each mtDNA lineage using droplet digital PCR with custom primer-probe mitochondrial cytochrome b sets to target each lineage. I used landscape layers to determine the habitat composition surrounding each observation. Ten wetlands in Southern Ontario had both mitochondrial lineages with evidence to support a broader, more interdigitated contact zone than previously thought. Landscape composition and bioclimatic variables did not significantly correlate with mitotype detection, at least at the scale I used. My study sets the stage for future research on implications of hybridization and speciation between these lineages; these findings may also help guide classification and conservation of these populations and refine conservation designations.
A Paleolimnological Assessment of Changes in Algal Assemblages on the St. Lawrence River at Cornwall, Ontario in Response to Local and Regional Stressors
(2024-08-07) Moir, Katherine; Biology; Cumming, Brian; Ridal, Jeffrey
The St. Lawrence River (SLR) is a large, complex fluvial system that connects the Laurentian Great Lakes to the Atlantic Ocean. A section of the SLR near Cornwall, Ontario has experienced numerous anthropogenic stressors since the mid-19th century, including pollution from waterfront industry and agricultural activity, which have impacted higher-food-web organisms. However, impacts on algal assemblages are less well understood. Recently, there has been evidence to suggest that climate-change impacts are further altering algal assemblage structure in lentic temperate, freshwater ecosystems, and it remains unclear whether similar changes are occurring large, fluvial systems. Although nutrient concentrations emanating from highly agricultural local watersheds are elevated, recent increases in cyanobacterial abundance in this section of the SLR may be attributable to a regional stressor such as the direct or indirect impacts of climate change. Additionally, other local stressors, including legacy sedimentary contamination by mercury and variations in light availability, may be influencing benthic algal community structure and need to be disentangled from potential climate signals. This thesis uses paleolimnological techniques (sediment cores, surface sediments) and spatial analyses to evaluate the impacts of sedimentary contamination, light availability, and elevated nutrient concentrations on paleolimnological proxies of algal production and assemblage structure in the SLR at Cornwall over the past century. Subfossil remains of diatoms (Class: Bacillariophyceae) were examined alongside sedimentary photosynthetic pigments and various measures of sediment geochemistry (bulk carbon and nitrogen, stable isotopes δ13C and δ15N, organic and carbonate content, and grain size) at sites along the Cornwall waterfront and in Lake St. Francis (LSF), a downstream fluvial lake. Results suggest that benthic algae on the SLR respond most strongly to changes in nutrient concentrations, although impacts of sedimentary contamination by metals are also important; climate-change impacts were not evident, though subtle site-specific effects may exist. When enough light was available to support benthic photosynthesis, variations in light availability had minimal impacts on benthic algal assemblage structure. Historical changes to industrial practices and hydrological modifications to the SLR were well recorded in the sediment cores. Overall, recent increases in cyanobacteria in LSF are likely attributable to elevated nutrient loads rather than climate-change impacts.
Evidence that metapopulation dynamics contribute to the maintenance of a species range limit
(2024-07-31) Gillies, Graydon; Biology; Eckert, Christopher
Species range limits may be enforced by various evolutionary and ecological processes. It is thought that many range limits are imposed by low fitness beyond a species’ range or the inability to disperse to suitable habitat beyond the range. However, transplant experiments and geographic surveys often do not support these hypotheses, requiring the use of an alternative framework. The metapopulation-hypothesis proposes that declines in habitat patch colonization rates or increasing patch extinction rates may cause the metapopulation of a species to collapse, thereby generating an abrupt range limit. Using coastal dune plant Camissoniopsis cheiranthifolia, I evaluated the metapopulation hypothesis by collecting data on the availability of suitable habitat, the abundance of C. cheiranthifolia and the plant community species composition at 7,031 randomly distributed 5m x 5m plots in two separate generations. Habitat structure changed towards the range limit, including a decline in the frequency of patches containing suitable habitat, the size of habitat patches, and the quality of patches, as well as an increase in patch isolation, but no clear trend in patch turnover. Occupancy by C. cheiranthifolia was more likely in larger patches that were less isolated from other suitable patches, as well as younger patches. Patches were more likely to be colonized by C. cheiranthifolia if they were larger and surrounded by a greater abundance of C. cheiranthifolia, while the species was most likely to go extinct from patches where the population was small. Patches were less likely to be colonized towards the range limit, but extinction did not vary geographically. Lastly, using a classic metapopulation model, measured rates of colonization and extinction closely predicted the decline in patch occupancy towards the range edge. This is the first study to estimate metapopulation parameters towards a species’ geographic range edge and demonstrate that metapopulation dynamics may contribute to the maintenance of species range limits.
Phenological trade-offs under biotic and abiotic selection in introduced populations of Lythrum salicaria
(2024-07-31) Akbar, Mia; Biology; Colautti, Robert
Evolutionary constraints imposed by a life history trade-off between age and size at maturity are common to a wide range of taxa. However, studies measuring phenotypic selection in natural populations often observe directional selection for both early reproduction and large size rather than predicted stabilizing selection around a local optimum. A further theoretical contradiction appears when this directional selection is accompanied by a missing evolutionary response. For example, directional selection for early flowering in plants is contrasted by variable phenological responses to climate change that do not demonstrate a unilateral shift to earlier flowering time. Focusing on North American populations of the wetland plant Lythrum salicaria, I used common garden experiments to investigate why variation for flowering time and size persists in the presence of strong genetic correlations. The first field experiment characterized full flowering schedules of 369 individuals representing 13 populations and revealed heritable genetic changes in phenology. Aspects of flowering schedule shape that are potentially adaptive such as mean, skew and kurtosis may weaken the trade-off between flowering time and size. The second field experiment measured six years of growth and reproduction in 221 seed families representing 20 populations in the presence and absence of insect herbivory. We found that insect herbivory results in plants that flower at a later time and smaller size than their predicted source population optimum. We also learned that patchy and variable selection from herbivores across years alters the strength of natural selection on flowering time and size, resulting in persistent phenotypic variation in both traits. Together, these results provide evidence that rapid phenological evolution can emerge along environmental gradients but is limited when strong biotic interactions reduce the efficiency of natural selection.
In the Weeds: Multiple stressors drive long-term invertebrate dynamics in two Great Lakes Areas of Concern (Cornwall and the Bay of Quinte)
(2024-07-29) Armstrong, Isaac Arthur; Biology; Cumming, Brian
The Great Lakes experienced severe twentieth-century anthropogenic impacts whose legacies are now interacting with invasive species and climate warming. Long-term data are needed to characterize the environmental effects of multiple interacting stressors and inform management objectives in designated Areas of Concern. Aquatic invertebrates are established water quality indicators and important to ecosystem function, however there are few data available pre-1970s on invertebrate dynamics in response to historic anthropogenic impacts within the Great Lakes. I used paleolimnological techniques to assess long-term change in invertebrate assemblages from two Great Lakes Areas of Concern. First, I analyzed spatial and temporal trends in chironomid assemblages in relation to historic industrial discharges in the St. Lawrence River at Cornwall, Ontario. Species distribution in surface-sediment samples from the Cornwall waterfront was significantly related to sedimentary Zn, potentially reflecting a legacy effect of textile mill effluent. Sediment cores showed a near absence of chironomid head capsules during the period of sedimentary contamination. As contaminant concentrations declined, there was a corresponding increase in chironomid abundance, diversity, and the relative abundance of pollution-sensitive taxa. I also investigated long-term effects of nutrient enrichment and dreissenid invasion on cladoceran assemblages in four sediment cores representing morphometrically distinct basins of the Bay of Quinte, Lake Ontario. Prior to the acceleration of eutrophication in the early 1900s, assemblages indicated a mesotrophic, piscivore-dominated system with diverse shoreline habitat. Cultural eutrophication was marked by increases in the relative abundance of Bosmina longirostris, Chydorus sphaericus, and Eubosmina. coregoni, while Daphnia longispina complex declined to very low relative abundances (0-5%). The dreissenid invasion c. 1994 and associated water clarity increase led to varied trophic responses across the Bay related to environmental legacies and subsequent biological invasions. Since ~2010, sediment cores show a widespread increase in the relative abundance and sedimentary flux of large littoral taxa and small generalist taxa, while large pelagic grazers remain impaired. Overall, my results suggest that remedial actions in Cornwall have allowed ecological recovery of benthic invertebrates, while pelagic zooplankton in the Bay of Quinte are being adversely impacted by multiple stressors.

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