Search

Organism-environment interactions take place through a multitude of processes that generate patterns across scales in space and time, but our understanding of pattern and processes is traditionally constrained by observational limitations. Contemporary technological advances in remote sensing, explored in this thesis, are extending the power and capability of ecological investigation. Three-dimensional (3D) ecosystem structure can now be analysed across scales from millimetres to kilometres and from minutes to decades, providing insight into scale-dependent patterns and their driving processes in complex and dynamic systems like temperate reefs. Remote sensing technologies are available for 3D mapping and recent years have seen a rapid expansion in their use in field ecology. In chapter 2, I reviewed the current state of the art in high-resolution 3D ecosystem mapping technologies and their applications, highlighting the emerging era of 3D spatial ecology and identifying potential barriers to widespread uptake. I addressed a paucity of information on the accuracy and practicality of emerging optical remote sensing tools in ecological contexts by testing structure-from-motion photogrammetry and terrestrial laser scanning, in three coastal habitats, over three spatial scales. The accuracy of structure-from-motion photogrammetry, compared to terrestrial laser scanning models, was greatest at fine spatial scales (25 m², < 1 cm resolution) on more stable substrates like rock, with mean ± sd absolute difference of 4 mm ± 14 mm. Accuracy decreased with increasing spatial scale and in less stable vegetated scenes, with a maximum difference of 56 mm ± 111 mm in saltmarsh at a scale of 2500 m² extent and < 2 cm resolution. Structure-from-motion photogrammetry was more portable, faster, flexible and lower-cost than terrestrial laser scanning, but was more vulnerable to error propagation. Capturing sufficient ecologically relevant spatial and temporal variation in 3D structure is challenging in complex, dynamic habitats like intertidal temperate reefs. In chapter 3 I used the tools tested in chapter 2 to investigate spatial and temporal patterns in the structure of biogenic Sabellaria alveolata reef across scales. At a habitat scale (~35,000 m² extent, 10 cm horizontal resolution) most of the variation in reef structural change was explained by a combination of systematic trends with shore height and positive spatial autocorrelation up to the scale of colonies (1.5 m) or patches (4 m). Plot-scale mapping (2500 m² extent, 10 cm horizontal resolution) over five years (2014-2019, 6-month intervals) revealed previously undocumented temporal patterns in reef accretion and erosion. The system was highly dynamic at small spatial and temporal scales (< 4 m, 6 months), but reef accretion and erosion compensated each other, resulting in stable habitat structure over larger scales (>130 m, 5 years). This scale-dependent variability would have been impossible to capture with conventional methods like quadrat, transect or point-based survey using GPS or theodolite, demonstrating the value of modern 3D mapping technologies to enhance our understanding of ecosystem dynamics across scales. Subtidal temperate reefs hosting diverse communities are often found in high-energy waters, but these are understudied compared to lower energy seas, and knowledge of reef distribution is lacking. In chapter 4 I used multiscale 3D seafloor data and hydrodynamic information to predict the spatial distribution of geogenic reef and biogenic Sabellaria spinulosa reef habitats in a high tidal energy region. Random Forest models for reef substrate and S. spinulosa reef had balanced accuracy mean ± 95% CI of 80.7% ± 0.8% and 77% ± 1% respectively. Mean bed shear stress was the most important variable in both models, highlighting the importance of including measures of hydrodynamic energy in predictive mapping of high-energy temperate reef habitats. My research demonstrates the increased power and insight that can be gained with contemporary 3D mapping and monitoring tools in field ecology. I showed that habitat structure in complex systems can be simultaneously highly dynamic and remarkably stable depending on the scale of observation, and that multiscale structural metrics are central to cost-effective mapping of subtidal temperate reef ecosystems. The collective works highlight the need for multiscale and multidisciplinary analysis and the value of embracing technological solutions for ecology in the age of big data. The emerging field of 3D ecosystem mapping and high-resolution remote sensing will have far-reaching implications for research, management and public engagement.

Many freshwater ecosystems sustain several invasive species. Here I examine multiple invasions in two highly invaded and well catalogued catchments; Lake Naivasha, Kenya and River Thames, England. New metrics, derived from stable isotope analysis, are used to provide measures of trophic diversity and to examine dietary interactions among species. I test the hypothesis that functionally similar sympatric species will occupy a smaller niche than their allopatric counterparts. Additionally, I quantify the impact of multiple invasive species on ecosystem structure and functioning in order to address the question; do interactions among species amplify or mitigate one another's impact? In Lake Naivasha, the stable isotope metrics revealed serial replacement of invasive species due to dietary interactions. Invasive red swamp crayfish were eventually excluded from the lake due to niche restriction in the presence of a more recent invader, the common carp. Now, the crayfish have migrated into the catchment where they overlap with a species of native river crab. Here, I found a novel mechanism of invasion, whereby the crayfish restricted their niche at the invasion front in order to reduce competition with crabs. Crayfish also caused significant changes in invertebrate community structure and increased decomposition rates, which indirectly resulted in displacement of the crabs. In the Thames catchment, I catalogue the non-indigenous species and show how invasion rates have increased significantly since 1800 due to globalisation. Using the four species of invasive crayfish present (red swamp, signal, Turkish and virile), I demonstrate their extensive diet plasticity using novel measures of niche width and individual specialisation based on stable isotope data. Interactions among the crayfish were examined and this revealed that each species has varying and independent impacts on invertebrate community structure, algal standing stock and decomposition rates. Hence, interactions among invaders are not expected to amplify or mitigate one another's impact and instead, the combined impact will be the sum of their allopatric impacts.

Woodlands, characterised by a matrix of trees, shrubs and open interspaces, are important biomes on Earth. Woodlands support a number of important ecosystem functions such as primary production, carbon fixation and nutrient cycling, and provide multiple ecosystem services that are essential for human livelihoods. Woodland structure and function are regulated by both large-scale shifts in climate, and smaller-scale variation in species interactions, resource availability and land management. Predicted changes in climate are expected to increase dryness and intensify management activities (e.g., grazing, plant removal), imposing substantial challenges on the functioning of woodlands and their dependent biota. Exploring how woodlands change across a climatic (aridity) gradient and among different management practices is essential for understanding how they adapt to drier climates and intensified woodland management, and to predict the ecological consequences of increasing aridity on their functions. This thesis examines the response of woodland structure and function to increasing aridity and woody plant removal, and the impact of biotic (e.g., plant traits, competition, grazing) and abiotic (e.g., climate, soil) drivers at both microsite and sub-continental scales, based on meta-analysis and field survey. Chapter 1 provides an overview of woodland structure and function, and their biotic and abiotic driving factors, highlighting important findings on the impact of increasing dryness on woodland structure (e.g., trees, shrubs, and open interspaces) and function (e.g., fertile islands). Chapter 2 examines the allometric response of different Australian woody plant genera to increasing dryness. Chapters 3 and 4 describe variation in biocrust cover and the fertile island effect beneath perennial vegetation across different patch types at the microsite scale, and with increasing aridity at the sub-continental scale. Chapters 5 and 6 synthesise the ecosystem outcomes of removing woody plants and the impact of woody plant traits, climatic regimes, and removal practices on the effectiveness of woody plant removal across the globe. Chapter 7 provides a synopsis of previous chapters, highlighting the adaptation strategies of woodlands to predicted drier climates, suggesting alternative woodland management under changing climates, and providing direction for future work in this field.

Freshwater ecosystems have less surface area than the oceans but play an outsized role in the global carbon cycle. Organic matter produced in situ or entering as terrestrial detritus may accumulate in sediments, be respired and released as CO2 to the atmosphere, or exported to the ocean. Aquatic decomposition of terrestrial detritus, periphyton primary productivity, benthic invertebrates (decomposers and grazers), and invertebrates that emerge from rivers contribute to the transport of carbon both downstream and between the terrestrial and aquatic ecosystems. I asked how decomposition rate, primary productivity, benthic macroinvertebrate biomass, and emergent macroinvertebrate biomass change with stream order in Yosemite Valley and El Portal, California, USA. In Summer 2019, I collected data at 19 sampling sites in the Merced River and its tributaries (Tenaya Creek and Yosemite Creek). Along with my main variables – decomposition rate, periphyton growth rate, benthic invertebrate biomass, and emergent invertebrate biomass – I considered environmental factors – elevation, temperature, and waterflow velocity. I found faster leaf decomposition at lower elevation and higher temperatures. Also, decomposition declined marginally with increasing invertebrate biomass. Other fluxes and stocks showed no relationship to stream order. Low elevation, high order rivers may therefore become more heterotrophic and release more CO2 to the atmosphere as a result of faster mineralization of allochthonous organic material without apparent compensatory increases in photosynthetic uptake. How the balance of photosynthesis and respiration in ecosystems will respond to climate change is one of the major unknown feedbacks in forecasting the future course of carbon accumulation.

Broadleaf cattail, Typha latifolia is a species of cattail native to North America, while narrow-leaf cattail, Typha angustifolia is considered to be a non-native species originating in Europe. These two species hybridize to form the hybrid cattail, Typha x glauca. Narrow-leaf cattail and hybrid cattail are both considered invasive species and are managed throughout North America. This research project aims to understand the effects of Typha angustifolia and Typha x glauca on ecosystem structure and processes by comparing wetlands dominated by non-native narrow-leaf and hybrid cattail species to wetlands containing only native broadleaf cattail. Eight wetlands distributed throughout central and southeastern Ohio were sampled from June until October 2017, for this study. Ecosystem metrics assessed include, richness and diversity of benthic macroinvertebrate communities, plant community composition, plant biomass, litter decomposition, and soil microbial biomass determined by total lipid analysis. Plant species richness differed between wetland types, with mixed sites containing Typha latifolia, Typha angustifolia, and Typha x glauca having higher species richness. There was no significant difference in plant species diversity, macroinvertebrate diversity, organic matter decomposition or microbial biomass between wetland types.

Acid mine drainage (AMD) is a prevalent legacy of coal mining within Appalachia. Streams receiving AMD effluent are drastically altered both chemically and biologically. Hewett Fork, a stream in southeastern Ohio, is one such affected stream. Although treatment methods have reduced acidity considerably downstream, the ability of Hewett Fork to sustain a biological community compared to those found in reference conditions remains unclear. To assess this, tiles colonized with diatom assemblages from an unimpacted stream were transplanted into Hewett Fork along a stream health gradient, from poor to good, and sampled after one, three, and six weeks in the treated stream. Chlorophyll a concentrations and species diversity metrics were calculated to compare reference assemblages to transplanted assemblages. Results suggested that after an initial one week acclimation period, assemblages at the uppermost and lowermost sites along the reach were relatively similar to those found in reference conditions, while sites within the middle region continued to show signs of impairment, although the factor(s) causing this impairment remain unknown. These findings suggest that although treatment has been effective on a site-specific basis, the expected linear-response to treatment may not be achieved due to underlying factors that are inhibiting reference-like biological communities from reestablishing within the affected stream reach.