In this thesis I assessed the causes of long-term changes in two large, commercially important decapod crustacean populations, American lobster (Homarus americanus) and snow crab (Chionoecetes opilio), in the northwest (NW) Atlantic Ocean. By combining available time-series data, including commercial landings, research surveys, and local ecological knowledge (LEK), I explored the causes of an observed ecosystem shift in the NW Atlantic (~1950–2009) which entailed a region-wide decline of groundfish and an increase in benthic invertebrates, including these decapods. Three hypotheses were examined to explain the increase in decapod abundance: (1) the predation hypothesis, whereby a decrease in predatory groundfish led to an increase in their decapod prey (top-down effects); (2) the climate hypothesis, whereby changes in temperature or other climatic variables helped to increase decapod numbers (bottom-up effects); and (3) the anthropogenic hypothesis, whereby changes in fishing pressure drove decapod population dynamics. I explored these hypotheses separately for lobster and snow crab, which may experience different ecological and commercial pressures. First, I investigated the interactions between predatory groundfish and lobster in the inshore region of southwest Nova Scotia. Long-term fisheries-independent abundance indices for lobsters and their predators are available for Gulf of Maine (GOM) waters in the USA, but not in Canada. To address research gaps I designed and executed a survey to collect the LEK of lobster fishermen fishing in the Canadian GOM. Forty-two fishermen were interviewed. Corresponding survey results from the USA were compared to the LEK results. Both sources provided evidence for a top-down effect (predation release), contributing to observed increases in GOM lobster abundance and landings. Second, I explored relationships between lobster abundance and landings in the NW Atlantic as they may relate to temporal changes in predators, temperature, climate (North Atlantic Oscillation Index, NAOI), and fishing. Available landings data and fisheries-independent abundance estimates were collated to investigate trends in lobster abundance and catch. Links between lobster, groundfish, temperature and climate indices were explored using mixed effects models. Results offered partial support for the predation hypothesis, namely in the waters off Newfoundland, Nova Scotia, and southern New England as well as broad support for a climate effect on early life stages. This effect appeared related to a region-wide climate signal, the NAOI, but was independent of changes in water temperature. Fishing effort appeared to be following lobster abundance, rather than regulating abundance in a consistent way. Third, variation in snow crab abundance was examined through meta-analysis of time-series data of cod and crab abundance and temperature. Temperature had opposing effects on the two species: snow crab abundance was negatively correlated with temperature whereas cod and temperature were positively related. Controlling for the effect of temperature, the analysis revealed significant negative interactions between snow crab and cod abundance, with cod leading snow crab up to a five-year lag. Results indicate that snow crab is largely influenced by temperature during early post-settlement years and becomes increasingly regulated by top-down mechanisms as they approach fishery recruitment. Overall, I conclude that both climate and predation can act as population controls on large decapod populations, but these variables affect decapods at different life stages.

Increasingly unpredictable, extreme and erratic rainfall with higher temperatures threatens to undermine the adaptive capacity of food systems and ecological resilience of smallholder landscapes. Despite growing concern, land managers still lack quantitative techniques to collect empirical data about the potential impact of climatic variability and change. This thesis aims to assess how ecosystem services and function and how this links with biodiversity and human wellbeing in smallholder agro-ecosystems in a changing climate. To this end, rather than relying on scenarios or probabilistic modelling, space was used as a proxy for time to compare states in disparate climatic conditions. Furthermore, an integrated methodological framework to assess ecosystem services at the field and landscape level was developed and operationalised, the results of which can be modelled with measures of wellbeing. Various multidisciplinary analytical tools were utilised, including ecological and socio-economic surveys, biological assessments, participatory open enquiry, and documenting ethnobotanical knowledge. The study was located within monsoon rice farms in the Terai Plains of Nepal, and dry season vegetable farms in Northern Ghana. Sites were selected that are climatically and culturally diverse to enable comparative analysis, with application to broad areas of adaptive planning. The linkages that bring about biophysical and human changes are complex and operate through social, political, economic and demographic drivers, making attribution extremely challenging. Nevertheless, it was demonstrated that within hotter and drier conditions in Ghana long-tongued pollinators and granivores, important for decomposition processes and pollination services, are more abundant in farms. Results further indicated that in cooler and drier conditions in Nepal, the taxonomic diversity of indigenous and close relative plant species growing in and around farms, important for the provisioning of ecosystem services, decreases. All other things equal, in both Nepal and Ghana findings indicate that overall human wellbeing may be adversely effected in hotter conditions, with a potentially significantly lower yields, fewer months of the year in which food is available, higher exposure to natural hazards and crop loss, unemployment, and psychological anxiety. Yet, surveys indicate smallholders continue to maintain a fair diversity of species in and around farms, which may allow them to secure basic necessities from provisioning ecosystem services. Moreover, farmers may employ adaptive strategies such as pooling labour and food sharing more frequently, and may have greater access to communication, technology, and infrastructure. Novel methodological and empirical contributions of this research offer predictive insights that could inform innovations in climate-smart agricultural practice and planning.