Mature forest canopies worldwide sustain an enormous diversity of arthropods, many of which are specialist natural enemies. However, with the exception of species that exhibit massive outbreaks, host-specific canopy arthropods are thought to have relatively little influence on tree health and overall forest productivity. My thesis examines the role of one such arthropod in a temperate forest stand in central Ontario. The maple spindle gall mite Vasates aceriscrumena (Riley) (Acari: Eriophyoidae) is a host-specific canopy parasite that induces galls on leaves of sugar maple Acer saccharum Marsh. I examine three diverse topics related to this host-parasite system: 1) the seasonal phenology of this mite in mature sugar maple canopies, 2) the impacts of galling on host physiology and growth and 3) the distribution of this mite across host ontogeny and within the broader context of the local forest community. With respect to phenology, I document a previously unobserved interaction between the gall-inducer and a gall-invading mite (ubiquitous in the canopy but new to science) and consequences of this on the bionomics of galling mite populations. This work is also the first to examine differential physiological responses to galling across two distinct stages in the ontogeny of the host. Infected leaves in mature trees show drastic reductions in gas-exchange processes (photosynthesis, stomatal conductance and water use efficiency) while infected sapling leaves show no such detectible responses. Further, I find a significant negative correlation between radial increment growth in mature trees and levels of mite galling, as well as significantly increased galling frequency in mature trees compared with understory saplings. Finally, I explore the relationship between galling abundance and abiotic and biotic variables within a large mapped forest plot and show that gall densities are most strongly correlated with local species diversity and less so with host densities. Overall, my research provides a new perspective on the influence of host-specific, dispersal-limited canopy arthropods as major drivers of ‘age-dependent’ reductions in physiological performance and growth of older trees and as natural enemies that are strongly associated with local forest community distribution patterns.

Mature forest canopies worldwide sustain an enormous diversity of arthropods, many of which are specialist natural enemies. However, with the exception of species that exhibit massive outbreaks, host-specific canopy arthropods are thought to have relatively little influence on tree health and overall forest productivity. My thesis examines the role of one such arthropod in a temperate forest stand in central Ontario. The maple spindle gall mite Vasates aceriscrumena (Riley) (Acari: Eriophyoidae) is a host-specific canopy parasite that induces galls on leaves of sugar maple Acer saccharum Marsh. I examine three diverse topics related to this host-parasite system: 1) the seasonal phenology of this mite in mature sugar maple canopies, 2) the impacts of galling on host physiology and growth and 3) the distribution of this mite across host ontogeny and within the broader context of the local forest community. With respect to phenology, I document a previously unobserved interaction between the gall-inducer and a gall-invading mite (ubiquitous in the canopy but new to science) and consequences of this on the bionomics of galling mite populations. This work is also the first to examine differential physiological responses to galling across two distinct stages in the ontogeny of the host. Infected leaves in mature trees show drastic reductions in gas-exchange processes (photosynthesis, stomatal conductance and water use efficiency) while infected sapling leaves show no such detectible responses. Further, I find a significant negative correlation between radial increment growth in mature trees and levels of mite galling, as well as significantly increased galling frequency in mature trees compared with understory saplings. Finally, I explore the relationship between galling abundance and abiotic and biotic variables within a large mapped forest plot and show that gall densities are most strongly correlated with local species diversity and less so with host densities. Overall, my research provides a new perspective on the influence of host-specific, dispersal-limited canopy arthropods as major drivers of ‘age-dependent’ reductions in physiological performance and growth of older trees and as natural enemies that are strongly associated with local forest community distribution patterns.

Doctor of Philosophy / Department of Biology / Jesse B. Nippert / Ecological thresholds are breakpoints where small increases in external pressure can generate rapid and difficult to reverse ecological transitions. Often, ecological thresholds are not recognized until they are crossed at a large-scale, leading to unintended and lasting externalities. In tallgrass prairie, we identified ecological thresholds of 3-year fire returns and ~60% grass cover, based on mechanistic field studies and long-term fire and grazing experiments. When tallgrass prairie is pushed passed these thresholds, it makes an abrupt transition to a self-reinforcing shrubland state. Demographic bottlenecks, niche partitioning, and altered fire feedback mechanisms account for both the non-linear nature of grassland-shrubland transitions and the resistance of established shrublands to fire and drought. In the last decade, only ~27% of Central Great Plains tallgrass prairie was burned every 1-2 years, and therefore ~73% of this region is susceptible to shrubland and woodland transitions in the next two to three decades. If transitions to shrublands and woodlands do occur, we expect a multi-trophic loss of grassland biodiversity, decreased cattle production, and the potential for damaging woodland fires in close proximity to human development. However, knowledge of fire thresholds, adaptive management tools, and bottom-up citizen action campaigns are creating a rare window of opportunity to avoid transformation of the remaining tallgrass prairies.

My work focuses on the community ecology of symbioses, specifically of fungal endophytes and their hosts. This thesis describes how plant defense compounds and a seed endophyte influence community structure of maize fungal endophytes. Maize produces benzoxazinoids (BXs), compounds toxic to microbes and insects. I assessed the influence of three factors on endophyte communities: host BX production, host neighbor identity and presence of a BX-detoxifying endophyte, Fusarium verticillioides (FV). To determine the influence of BXs on communities, two BX-producing (BX+) and one BX-nonproducing (BX–) genotype were planted in Ridgetown and Harrow, Ontario (triculture). Fungi were isolated and tested for tolerance to 2-benzoxazolinone (BOA), a toxic BX byproduct. Species and functional diversity (community distribution of BOA tolerance levels) was calculated. In seedling roots and mature leaves, the community proportion with low BOA tolerance was greater in BX– than BX+ plants. Fusarium abundance was up to 35 times higher in mature leaves of BX+ than BX– plants. Next, to assess the effect of host neighbor identity on communities, BX– monocultures were planted, and communities from BX– plants in monoculture and triculture compared. Monoculture root communities had higher species diversity than those in triculture. In vitro experiments were conducted to evaluate the influence of BOA on endophyte species interactions. FV facilitated species with lower BOA tolerance in the presence of BOA. Finally, fields were planted with a BX+ and BX– genotype in Ontario, Canada and Georgia, USA. Seed was inoculated with FV (FV+) or sterilized (FV–). FV abundance was highest in BX+FV+ plants, and Fusarium abundance was greater in BX+ than BX– plants in mature leaves. In Georgia, BX+FV+ communities in below ground tissue had lower abundance of BOA sensitive species than BX+FV–. Overall, results suggest that BXs are a habitat filter that increased colonization by horizontally transmitted and seed-born Fusarium species. This invokes the hypothesis that selective breeding for enhanced BX concentrations increased abundance of Fusarium species in maize. The in vitro study indicated that FV could facilitate other species. In contrast, field results suggest that FV interacts competitively with community members, a trait enhanced in the presence of BXs.

Iron mining began in Wawa, Ontario in the late 1800s and ceased in 1998. The sintering process of iron pyrite produced sulfur dioxide which led to the acidification of nearby lakes. Benthic macroinvertebrate samples were collected from lakes along a gradient of historical impairment in Wawa to examine the extent to which the benthos of the lakes would separate along the historical impairment gradient. The results show that the lakes are not separated along a gradient of impairment, and acid-intolerant taxa were collected in previously acidified lakes. There was no ameliorative intervention to combat the historical acidification and the observed recovery of water chemistry and benthos is entirely due to natural ecosystem processes. The two sampling methods (littoral kick sampling using a D-net and Hester-Dendy substrates) used to sample the benthos in these lakes can lead to substantial differences in the taxa collected.

This thesis will focus on the use of agonistic behaviour in both direct and indirect competition in two arboreal species: the eastern chipmunk (Tamias striatus) and Cyphoderris monstrosa, a primitive orthopteran insect. Chipmunks are an example of indirect competitors as they are competing for finite resources for dealing with abiotic stresses to ensure their survival. Chapter 2 of this project investigates the behavioural time budget for above ground activity at a time of critical importance for overwinter survival. The chipmunks in this study displayed a focus of their time budget on eating and collecting food in preparation of winter survival while they were above ground. Chapter 3 of this research project studies the physiological differences between males of Cyphoderris monstrosa which engage in aggressive territorial contests. The data suggest that metabolic scope is correlated with RHP. Males’ ability to mobilize energy reserves may be an important factor in contest outcomes.

This thesis will focus on the use of agonistic behaviour in both direct and indirect competition in two arboreal species: the eastern chipmunk (Tamias striatus) and Cyphoderris monstrosa, a primitive orthopteran insect. Chipmunks are an example of indirect competitors as they are competing for finite resources for dealing with abiotic stresses to ensure their survival. Chapter 2 of this project investigates the behavioural time budget for above ground activity at a time of critical importance for overwinter survival. The chipmunks in this study displayed a focus of their time budget on eating and collecting food in preparation of winter survival while they were above ground. Chapter 3 of this research project studies the physiological differences between males of Cyphoderris monstrosa which engage in aggressive territorial contests. The data suggest that metabolic scope is correlated with RHP. Males’ ability to mobilize energy reserves may be an important factor in contest outcomes.

The aims of this thesis were to characterize the molecular ecology of the hyporheic zone – between dissolved organic matter (DOM) and microbes – and to test whether seasonal and spatial patterns existed in correlation with seasonal ecosystem processes. The hyporheic zone is an area of vertical integration between groundwater and surface water, and lateral integration between terrestrial and stream ecosystems. Colonization corers were used to collect in situ DOM and bacterial communities from the hyporheic sediments of two streams that varied in hydroperiod (i.e., permanent vs. intermittent). DOM was collected using passive samplers and analyzed using 1H NMR and fluorescence spectroscopy; bacteria were characterized using terminal-restriction fragment length polymorphism. At the permanent site, bacteria correlated significantly with seasonal environmental factors including: fall communities with DOM concentration; spring and winter communities with nitrate concentrations; and summer communities with temperature. Bacterial communities at the intermittent site were significantly correlated with flooding as a function of hydrologic connectivity. Sediment communities were discriminated between hyporheic sediments and interstitial porewaters, and shared several operational taxonomic units (OTUs). Sediment communities were more distinct when hydrologic connectivity was low, and porewater communities changed dramatically upon flooding. Fifteen out of 259 OTUs were shared across aquatic sediments, interstitial porewater and watershed soil samples. DOM was spatially and seasonally dynamic in both sites. Five key DOM groups described using 1H NMR spectroscopy revealed spatial differences between the permanent and intermittent sites. EEM-PARAFAC models confirmed that despite significantly different molecular components, the relative sources of DOM at both sites were similar, including humic-like terrestrial sources and tyrosine (microbial) sources. This study provides new knowledge on both organic matter dynamics and bacterial communities in a dynamic aquatic ecotone, and also confirmed the hypothesis that bacterial communities correlated significantly with ecosystem processes within a watershed.

Iron mining began in Wawa, Ontario in the late 1800s and ceased in 1998. The sintering process of iron pyrite produced sulfur dioxide which led to the acidification of nearby lakes. Benthic macroinvertebrate samples were collected from lakes along a gradient of historical impairment in Wawa to examine the extent to which the benthos of the lakes would separate along the historical impairment gradient. The results show that the lakes are not separated along a gradient of impairment, and acid-intolerant taxa were collected in previously acidified lakes. There was no ameliorative intervention to combat the historical acidification and the observed recovery of water chemistry and benthos is entirely due to natural ecosystem processes. The two sampling methods (littoral kick sampling using a D-net and Hester-Dendy substrates) used to sample the benthos in these lakes can lead to substantial differences in the taxa collected.

My work focuses on the community ecology of symbioses, specifically of fungal endophytes and their hosts. This thesis describes how plant defense compounds and a seed endophyte influence community structure of maize fungal endophytes. Maize produces benzoxazinoids (BXs), compounds toxic to microbes and insects. I assessed the influence of three factors on endophyte communities: host BX production, host neighbor identity and presence of a BX-detoxifying endophyte, Fusarium verticillioides (FV). To determine the influence of BXs on communities, two BX-producing (BX+) and one BX-nonproducing (BX–) genotype were planted in Ridgetown and Harrow, Ontario (triculture). Fungi were isolated and tested for tolerance to 2-benzoxazolinone (BOA), a toxic BX byproduct. Species and functional diversity (community distribution of BOA tolerance levels) was calculated. In seedling roots and mature leaves, the community proportion with low BOA tolerance was greater in BX– than BX+ plants. Fusarium abundance was up to 35 times higher in mature leaves of BX+ than BX– plants. Next, to assess the effect of host neighbor identity on communities, BX– monocultures were planted, and communities from BX– plants in monoculture and triculture compared. Monoculture root communities had higher species diversity than those in triculture. In vitro experiments were conducted to evaluate the influence of BOA on endophyte species interactions. FV facilitated species with lower BOA tolerance in the presence of BOA. Finally, fields were planted with a BX+ and BX– genotype in Ontario, Canada and Georgia, USA. Seed was inoculated with FV (FV+) or sterilized (FV–). FV abundance was highest in BX+FV+ plants, and Fusarium abundance was greater in BX+ than BX– plants in mature leaves. In Georgia, BX+FV+ communities in below ground tissue had lower abundance of BOA sensitive species than BX+FV–. Overall, results suggest that BXs are a habitat filter that increased colonization by horizontally transmitted and seed-born Fusarium species. This invokes the hypothesis that selective breeding for enhanced BX concentrations increased abundance of Fusarium species in maize. The in vitro study indicated that FV could facilitate other species. In contrast, field results suggest that FV interacts competitively with community members, a trait enhanced in the presence of BXs.