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This thesis focuses on the guppy, Poecilia reticulata, as an invasive species. Its non-native distribution, the biology behind its success and the reasons for its introduction are examined. A worldwide email survey revealed that the guppy is established in at least 73 countries outside of its native range and that mosquito control schemes and the release of unwanted aquarium fish are the two primary routes of introduction. Knowledge gaps were identified; primarily the scarcity of scientific evidence for negative impacts of guppy introductions and similarly for mosquito control efficacy. Replicated mesocosm experiments demonstrated that female guppies are capable of routinely establishing populations, and that these retain behavioural viability over several generations. The first mesocosm study suggested that founders with very different evolutionary histories were equally good at establishing populations. The second mesocosm study suggested that monandrous females were extremely successful at establishing behaviourally viable populations, with no decline in behavioural variation. The effectiveness of guppies as mosquito control agents was examined in two related foraging experiments. The first study found little evidence for the presence of ‘prey switching’ in guppies, questioning the validity of previous work advocating their introduction to stabilise prey populations. The second study revealed a preference for non-vector mosquito larvae in a two-prey system. However, both mosquito species were consumed equally readily when habitat complexity increased. The presence of conspecifics affected female foraging behaviour. The presence of males reduced the strength of prey preference in the first study, and the presence of conspecifics of either sex removed prey preference in the second. Both demonstrate that multi-prey systems have important implications for the efficacy of poeciliids in biological control. Despite severe demographic bottlenecks, their adaptability and ability to rapidly increase in numbers enable guppies to establish and persist when introduced. Such bottlenecks are typical of introduction scenarios, warning that particular caution should be exerted when introducing this species, or other livebearing fish, to natural water bodies.

Biodiverse tropical forests are declining rapidly, due in part to the appropriation of natural land for human use. There is an urgent need to better understand how biodiversity changes in response to this landscape transformation, and to find conservation solutions that support both people and nature. Cacao farming, a valuable source of agricultural revenue in many tropical countries, can lead to a mosaic of both active and abandoned agroforests. These abandoned agroforests undergo secondary succession and have the potential to be important reservoirs of biodiversity. To evaluate this potential, I assessed the biodiversity of active cacao agroforests, abandoned agroforests, and primary forest on the tropical island of Trinidad. I surveyed tree, ground vegetation, epiphyte, and bird assemblages, and computed taxonomic and functional diversity. I further asked whether there are generalisable patterns of biodiversity change over succession across taxa, and how my results fit within the wider narrative of ecological succession theory and research. I found that taxonomic and functional alpha-diversity in all assemblages were mostly either maintained throughout succession, or recovered quickly across taxa. There were, however, substantial changes in taxonomic and functional composition (beta- diversity) over succession, with patterns of biodiversity change largely uncorrelated across taxa. Overall, there was little congruence between expectations based on theory and existing empirical research, and the results of my study. This outcome emphasises the need to deepen our understanding of successional processes across biomes, starting points of succession, and taxa. My research also highlights that, while primary forests contain more specialist species, actively cultivated cacao agroforests and young secondary forests in Trinidad are remarkably biodiverse and jointly contribute to supporting regional gamma diversity. These results place cacao agroforests within a people and nature framework, and demonstrate that human-altered habitats can be important reservoirs of tropical biodiversity.