John Richardson

Riparian zones support a broad diversity of organisms and ecosystem functions, like decomposition, which are integral to both terrestrial and aquatic ecosystems. Riparian zones are globally threatened, and management agencies are increasingly focused on protecting the ecological functions and diversity of these systems. Identifying factors limiting decomposition and invertebrate diversity in riparian zones can improve our understanding of how these ecosystems operate. In this thesis, I identified mechanisms contributing to the diversity of terrestrial invertebrates in riparian zones: flooding and drying cycles, nutrient and water availability, microclimate gradients, vegetation and microhabitat diversity, and unique food resources. I experimentally determined whether water, nutrients, and distance from stream limit (1) early-stage mass loss of leaf litter from red alder (Alnus rubra) and western red cedar (Thuja plicata) trees, and (2) terrestrial invertebrate abundance and diversity in headwater riparian zones in southwestern British Columbia. My experiments revealed that moisture is a limiting factor to red alder leaf litter mass loss during the summer dry period: watered litter lost 4% more mass than un-watered litter in four red alder litterbag trials. Nutrient availability may be limiting to western red cedar mass loss at my sites: leaf litter with nutrient additions lost 5% more mass than litter without nutrients in one of three western red cedar trials, suggesting that nutrients may be limiting to western red cedar mass loss at my sites. The terrestrial invertebrate community appeared sensitive to nutrient additions: pitfall trap abundance and order richness were lower at stations with nutrients. Trap abundance, taxonomic richness, and community composition also differed based on month of capture. I also documented differences in microclimate variables with distance from stream. Temperature and moisture conditions within 1 m of the stream differed from conditions farther away during the summer dry period between July and August, but only temperature differed in winter between December and January. This small difference in microclimate with distance from stream did not appear to influence invertebrate abundance and diversity, or mass loss of either litter species. These results contribute to our understanding of which factors limit decomposition and diversity in headwater riparian zones.

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Disturbances affect ecosystem stability and the delivery of ecosystem services. Stability responses to disturbances involve multiple components, including changes in the levels (indicating resistance) and recovery time (resilience) of state variables. The stability of stream ecosystems is an active research area, however, empirical studies commonly measure single components of stability, and the basis of its spatio-temporal variability is often unclear. Bivariate and multi-dimensional frameworks were proposed to facilitate stability comparisons within and across ecosystems. I recommended adjusting two bivariate frameworks to better address diverse disturbance-response trajectories, shifting baselines, and broaden their applications for streams and other ecosystems. In my dissertation, I quantified multiple stability components of terrestrial-derived particulate organic matter (POM) availability and breakdown in small, temperate streams under forest harvesting. I focussed on stream POM dynamics, as they underpin the stability of consumer productivity (e.g., fish) and nutrient cycling, which are important aquatic ecosystem services. First, I investigated whether the natural variability of leaf litter breakdown – critical for detecting disturbance impacts – was affected by natural, weather-driven discharge variations across years, and decomposer preference of local versus exotic litter species. I showed that inter-annual hydrologic variations poorly explained litter breakdown, whose natural range of variation exceeded the benchmarks set by a popular bioassessment framework. Accordingly, I recalibrated reference conditions of litter breakdown to allow more robust bioassessments. Decomposer preference did not differ significantly between high-quality native and exotic litter, supporting their standardised use for disturbance studies across geographic regions. Secondly, I demonstrated that the resilience of litter breakdown to forest harvesting was likely greater in streams affected by thinning (with 50% basal area removal of riparian trees; 2-9 years post-harvest) than those affected by clear-cutting with or without riparian buffers (8-15 years). Thirdly, I modelled the resistance of stream POM quantity under variable, realistic harvesting impacts. Logging-induced changes in litterfall were more influential than peak discharge and stream temperature alterations in regulating POM quantity. Management strategies minimising riparian forest disturbances would more likely sustain detrital resource availability and productivity in small streams. My results indicated that riparian vegetation, through litterfall and shading, importantly controlled the stability of stream POM dynamics.

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Agriculture is the primary cause of sedimentation, nutrient enrichment, and insecticide contamination of freshwater ecosystems. Despite the widespread co-occurrence of these ecological stressors, little is known about their potential interactive effects. I conducted three experiments manipulating combinations of these stressors in order to evaluate their cumulative effects on freshwater ecosystems at different scales of biological organization (community, ecosystem, meta-ecosystem). First, I evaluated stream invertebrate community responses to sedimentation, nutrient enrichment, and the insecticide chlorpyrifos using laboratory microcosms with distinct microhabitats. I demonstrated that chlorpyrifos can interact non-additively with fine sediment (reversal) and nutrients (antagonism), with potentially deleterious impacts on small-sized invertebrates. Furthermore, invertebrates in gravel microhabitats were more severely affected than those in leaf packs. Second, I manipulated levels of nutrients, sediment, and the insecticide imidacloprid in experimental pond ecosystems. I demonstrated these stressors had antagonistic effects on pelagic and benthic invertebrate diversity. Moreover, the results suggested imidacloprid increased ecosystem metabolism indirectly, through negative effects on invertebrate consumers. Finally, I explored processes at the scale of the river network meta-ecosystem. Using a network of experimental channels, I investigated how multiple-stressor interactions within tributaries affected downstream ecosystems. My results indicated that complex nutrient-sediment interactions within tributaries could strongly alter the flux of organisms from tributaries to downstream ecosystems. Furthermore, I observed that at small spatial scales, these alterations of within-network migration patterns could be more influential than the transport of the stressors from headwaters to recipient ecosystems. My research contributes novel evidence suggesting that complex interactions among nutrient enrichment, sedimentation, and insecticide contamination are frequent in freshwater ecosystems, and have distinct mechanisms operating at different scales. In particular, these findings underscore the importance of considering multiple-stressor interactions in insecticide environmental risk assessments; even at low concentrations, interactions with other stressors may result in unexpected negative effects for aquatic biota and ecosystem processes.

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In tropical areas, deforestation and forest degradation are major threats to forests and the surrounding ecosystems, such as riparian forests and streams. These threats to riparian forests and streams in agricultural areas can be reduced by the implementation of riparian buffers. However, disturbances from cleared areas may still impact the forests due to the high edge-to-area ratio of buffers. Selective logging inside the buffers also has the potential to degrade riparian forests and streams. Forest regeneration has the potential to restore ecosystems in degraded lands, but disturbances from ongoing agriculture are expected to arrest or delay the regeneration process in the buffers. I evaluated the efficacy of two riparian buffer management strategies: (1) land abandonment for natural regeneration and (2) the maintenance of mature forest. I also sampled additional sites of different ages to evaluate where buffer treatments fit after riparian forest alterations. I hypothesized that the riparian buffers resulting from land abandonment would have less forest canopy, simpler stand structure, less large wood, higher primary production, and higher decomposition rates than a regenerated riparian forest that was surrounded by mature forest. I expected the same outcomes when comparing the riparian buffers of mature forest versus mature riparian reference sites. I found that forest structure did not differ significantly between riparian buffer management treatments, however, my ordination analysis revealed signs of forest degradation after selective logging in the riparian buffers of mature forest. I found no significant effect of riparian buffer management in any stream variable studied. Large wood was related to channel width and stem density. Stream respiration increased and primary production decreased as the regeneration process advances. Decomposition differed among species, apparently by differences on leaf structural compounds. My results show that both buffer management strategies can be effective for protection of riparian and stream ecosystem in agricultural landscapes. Land abandonment is a viable and inexpensive restoration action where ongoing disturbances are mild and the propagules for regeneration are available. While the implementation of riparian buffers of mature forests is an effective strategy, selective logging should be excluded from these areas as disturbances may intensify in the future.

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Untangling the mechanisms linking the physical world to ecological processes is paramount for effectively conserving and restoring habitat for threatened species. Pacific salmon rearing in small streams have been particularly well studied in this regard given that physical habitat features (e.g., velocity) strongly influence their performance and that habitat alteration is a major cause of their decline in many areas. However, despite strong evidence that stream salmonids are food limited, we lack commensurate understanding of how habitat influences their food supply - suspended invertebrates drifting downstream (invertebrate drift). Consequently, the mechanisms linking physical habitat structure to salmonid production remain unclear. My dissertation attempts to address this issue. First, in Chapter 2, I review and synthesize the mechanisms underlying invertebrate drift, discuss potential caveats in methodology, and identify key knowledge gaps. I particularly highlight how the physical and behavioural processes governing drift entry are highly dependent on context-specific abiotic and biotic attributes (e.g., hydraulics, individual condition). In Chapter 3, I use flow manipulation experiments to show that some of this context dependency can be explained by considering behavioural and morphological traits of invertebrate taxa, which underlie their tendencies to drift behaviourally or passively; for instance, body shape predicted the magnitude of responses to increased flows. In Chapter 4, I show that aggregate community-level drift rates vary spatially in streams over scales relevant to individual drift-feeding salmonids. Specifically, I measured spatially explicit rates of drift production, demonstrating that shallow high velocity riffles and deep low velocity pools form distinct sources and sinks of drift within stream networks. In Chapter 5 I build on this result to show that drift generation in riffles coupled with strong preferences for low velocity pools by salmonids leads to maximum fish production occurring in habitats with intermediate ratios of pool-riffle areas; in essence, a trade-off between increasing space but declining food as pool area increases. I extend these results with bioenergetic simulations to show that the shape of this trade-off is sensitive to alternative modes of prey delivery (e.g., aerial inputs of terrestrial invertebrates), which may be decoupled from in-stream habitat conditions.

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Freshwater ecosystems are important natural emitters of the greenhouse gas CO₂. The magnitude and direction of the exchange of CO₂ between freshwaters and the atmosphere, or flux, is influenced by the concentration of CO₂ in the water. Every organism within a freshwater ecosystem influences the net CO₂ balance of that ecosystem either through respiration, photosynthesis or both. Thus, large changes in populations due to natural or anthropogenic stressors and the underlying food web structure of the ecosystem have the potential to alter CO₂ fluxes of aquatic ecosystems. To evaluate the influences of species loss on food web structure and CO₂ fluxes of aquatic ecosystems, I experimentally manipulated species from different consumer trophic levels (predator, grazer, or detritivore) and tested the effects of these losses on CO₂ fluxes of experimental streams, ponds and bromeliads. In streams, I found that influences on CO₂ emissions were most sensitive to the loss of a predatory insect compared to other trophic levels, including a tadpole grazer and an insect detritivore. Similarly, the removal of a fish predator to ponds or an insect predator to bromeliads resulted in trophic cascades that significantly influenced the CO₂ flux of the ecosystem. Both the identity of the predator and interspecific competition among predatory insects influenced the strengths of cascading effects of predators on CO₂ emissions from bromeliads. However, across all three ecosystems (streams, ponds, and bromeliads) predators, via trophic cascades, had surprisingly consistent effects on the CO₂ flux of the ecosystem. Finally, as alterations to predator abundance often occurs in concert with increasing water temperatures and nutrient loading, I determined the individual and interactive effects of these stressors on pond communities. I found that nutrients often increased top-down control of predators on CO₂ fluxes, but the individual effect of warming and its combined effects with nutrients had negative effects on both consumers and primary producers making predictions about CO₂ fluxes complicated. My results provide novel insights into the influence of predators and food web structure on CO₂ fluxes and the potential for predator loss to markedly alter CO₂ fluxes of freshwaters.

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Freshwater ecosystems worldwide are degraded by habitat loss, fragmentation and conversion. The practice of ecological river restoration has developed to address degradation, but there has been limited monitoring and assessment of river restoration projects that could be used to improve the science of restoration ecology. I used meta-analysis and studied floodplain ponds restored for juvenile coho salmon (Oncorhynchus kisutch) in southwestern British Columbia, Canada to test ecological and conservation science hypotheses about how restoration projects are planned and assessed. I evaluated the efficacy of the umbrella species concept, which suggests that conservation strategies designed for one species may benefit co-occurring species, using meta-analysis. I empirically assessed the potential for coho to be an umbrella species in restored ponds. I studied the relationship between biodiversity and ecosystem function (i.e., standing biomass) and explicitly considered the role of habitat complexity in mediating that relationship. I evaluated the influence of habitat at different scales (watershed, pond and micro-habitat) on the abundance and biomass of juvenile coho and other aquatic vertebrates. I used standard meta-analytic techniques to assess the umbrella species concept and found conservation strategies designed for umbrella species generally benefit co-occurring species. For the empirical studies, I sampled vertebrates in 17 restored ponds in three watersheds three times over a year. I sampled benthic invertebrates and algae once and documented habitat (e.g., depth, cover) at the pond and trap scale. Coho abundance and biomass, as well as that of other aquatic species, varied across ponds indicating a gradient in response to restoration. There was a positive relationship between species diversity and standing biomass, although that relationship was not consistent across taxonomic groups or with respect to habitat complexity. There was a relationship between watershed-scale habitat features (e.g., landcover, elevation) and the relative abundance and biomass of species present, however, different species responded similarly to micro-habitat types suggesting that watershed scale factors acted as a filter for community composition. This study demonstrated that valuable insight into restoration can be gained by studying patterns from a broad study of restored systems and that restoration designed around a single species can benefit other species.

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