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Harmonia axyridis is a ladybird native to Asia, but introduced widely as a biocontrol agent. It is invasive and detrimental to native species in North America, which meant its arrival in Britain was met with concern. Establishment was seen as an opportunity to track the spread of an invasive alien species (IAS) whilst also monitoring impacts on native species. The aims of this thesis were to examine the responses of native British ladybirds to the arrival of H. axyridis, to establish the effect of the IAS on native ladybirds when compared to other drivers, and to investigate the possible facilitation of the H. axyridis invasion by natural enemy release. Modelling ladybird distributions with life-history and resource-use traits found that species predatory on a wide range of prey families had larger range sizes than those which ate fewer prey types. This suggests that the wide diet breadth of the IAS is likely to have played a critical role in the species’ rapid spread. Dietary niche overlap between H. axyridis and native ladybirds showed positive correlation with declines of native ladybirds. This indicates that the IAS is playing an important role, but the significance of urbanisation suggests habitat destruction is also significant. Abundance of H. axyridis was influenced by habitat type and aphid abundance, but not by the native ladybird community, suggesting the spread of the IAS will not be slowed by biotic resistance. Harmonia axyridis is attacked by native parasitoids, but at a much lower rate than is the native Coccinella septempunctata, in line with natural-enemy release theory. There was no evidence of attack rate increasing with time since arrival in an area. Overall, H. axyridis is an extremely successful IAS, with detrimental effects on native ladybirds which are likely to continue.

Intraguild competition and predation have been recognised as important ecological factors influencing the population dynamics of carnivores. The effects of these interactions are often asymmetrical due to a size-related dominancy hierarchy. However, it has been suggested that competitively subordinate carnivores can minimise the costs of predation and competition through spatial and temporal avoidance. Here I investigate the ecological and behavioural mechanisms by which cheetahs (Acinonyx jubatus) coexist with competitively stronger lions (Panthera leo) and spotted hyaenas (Crocuta crocuta). Fieldwork was carried out in the Okavango Delta, northern Botswana, between October 2008 and August 2011. A total of 20 Global Positioning System (GPS) radio-collars were fitted on all known cheetahs (n=6), lion prides (n=5) and spotted hyaena clans (n=6) in the study area (approx. 3 000 km2). Pre-programmed radio-collars recorded locations and activity continuously for each individual and these data were complemented with direct behavioural observations. Cheetah data were analysed with respect to the temporal and spatial likelihood of encountering lions and spotted hyaenas. Results suggest that the response to the risks posed by other predators is species-specific, habitat-specific and dependent on the immediacy of the risk. Resource partitioning was not the main mechanism for coexistence as cheetahs overlapped extensively with lions and spotted hyaenas in time, space and habitat use. Instead, cheetahs adjusted their spatial distribution in response to immediate risks or adapted their habitat use depending on their vulnerability (e.g. behaviours such as feeding or with differing levels of moonlight at night). In general, cheetah temporal and spatial distribution is a hierarchal process, firstly driven by resource acquisition and thereafter fine-tuned by predator avoidance. In addition, habitat heterogeneity seemed to be key in facilitating coexistence. Understanding the behavioural mechanisms that interacting apex predators adopt to regulate these negative interactions could be crucial to carnivore conservation, especially as human-related habitat loss is forcing species into ever smaller areas.

The coexistence of similar species may be related to a variety of resource utilization differences. Dietary resource utilization variation may be the most important difference allowing for the coexistence of sympatric snake species. Many watersnakes (Nerodia spp.) live in sympatry and use similar aquatic habitats feeding on similar prey. While these sympatric watersnakes may have different general foraging patterns, snake diet may be affected by a variety of factors. Therefore, I initiated an investigation to understand the coexistence of sympatric plain-bellied (N. erythrogaster), diamondback (N. rhombifer) and northern (N. sipedon) watersnakes by addressing their dietary resource utilization patterns. Results indicated that northern watersnakes ate fish families according to their availability except for the avoidance of Aphredoderidae. I also determined that northern watersnakes had smaller head sizes and a diet closer to the piscivorous diamondback watersnake. There were sex differences in snake head size and all three species had different head shapes relating to diet. In addition, gut contents were determined with plain-bellieds feeding mainly on anurans, diamondbacks on fishes, and northern watersnakes feeding mostly on fishes but with a higher anuran component than diamondback watersnakes. Stable isotope analyses provided long-term dietary information with diamondback watersnakes feeding at higher trophic levels while plain-bellied watersnakes fed more from terrestrial prey sources. The application of stable isotope techniques helped to demonstrate shifts in dietary resource utilization. This research has allowed me to reveal a complex foraging system affected by a variety of factors allowing for the coexistence of sympatric watersnakes.

The biological diversity on planet Earth is astounding. Understanding the origins of this diversity, and how it is maintained, are the twin goals of ecology and evolutionary biology. An early and oft-repeated insight in this investigation is that that similar organisms cannot coexist indefinitely. Theory predicts that individuals and species will compete for limited resources and whichever has even a slight advantage will drive all others extinct in a process known as ‘competitive exclusion’. By diversifying, species avoid competition, thereby ‘stabilizing’ their coexistence. Yet natural systems often display levels of diversity that are surprisingly high, given this theory and investigations of how the similarity of coexisting species is maintained have received much less attention. Using a combination of field studies and experiments I demonstrate that highly similar species of freshwater amphipods may compete for resources without resulting in competitive exclusion. These findings suggest that there exist a range of interactions among Hyalella amphipods, ranging from strong stabilizing effects due to ecological trade-offs, to weakly stabilizing effects, to a total lack of stabilizing effects among various pairs of species in this system. These findings demonstrate how the relative strength of stabilizing forces may vary among coexisting species. Although much effort has been dedicated to enumerating and classifying the ways in which ecological and evolutionary forces promote diversity among species, there has been far less attention paid to mechanisms such as convergent evolution, habitat filtering, competition for non-substitutable resources, and non-ecological speciation, among others. I surveyed current theory that may explain the high levels of similarity among species often found in natural systems. I describe how several ecological and evolutionary mechanisms may operate to promote the coexistence of similar species and present results from new theoretical combinations of mechanisms to demonstrate how they may further act in concert with one another.