Linking phenology to population dynamics and distribution change in a changing climate

The fingerprints of anthropogenic climate change are increasingly visible in myriad ecological events and processes, from physiology to population dynamics and species’ distributions. In particular, the timing of life cycle events (phenology) is widely regarded as a sensitive indicator of ecological responses to climate change. Phenological responses to climate change, such as advanced timing of reproduction or lengthened activity periods, are widespread but vary markedly within and between species. These changes can alter the abiotic conditions and biotic interactions to which organisms are exposed, and determine the time available for population growth, which ultimately underpins species’ distributions. Consequently, there are many means by which phenological change may influence population dynamics and distribution. However, such relationships are rarely explicitly tested, and their underlying mechanisms are relatively unexplored. In this thesis, I therefore aimed to investigate the relationships between phenology, population dynamics and distribution. I also sought to examine how these may be affected by climate and mediated by biotic interactions, and to highlight potential community-level consequences of these relationships. I use butterflies as a model system, in part due to their sensitivity to environmental change and their status as indicators of insects and the state of terrestrial biodiversity. Here, I develop modelling approaches which can detect relationships between phenology, population dynamics and environmental drivers, and that are adaptable to other study systems. I further demonstrate how the model outputs can be used to investigate drivers of species’ range dynamics. I also identify specific biotic interactions that may underpin changes to species’ population dynamics following climate-driven phenological change. I first investigate the relationships between climate, phenology and population dynamics for a community of butterfly species. I demonstrate community-wide phenological advances in response to warm spring-summer temperatures, which may contribute to temporal synchrony in community dynamics under climate change. However, relative community composition is likely to vary between years as species’ population dynamics respond idiosyncratically to environmental variation. Critically, I highlight that early emergence is associated with population growth in some species over an elevational gradient, and suggest that a robust understanding of population dynamic responses to climate change may require knowledge of the relationship between phenology and population dynamics, and how it is affected by climate change. In the following chapters I address the potential role of phenological synchrony between a butterfly and its larval host plants in driving changes in population dynamics and range expansion. In Chapters 3 and 4 I present the results of field-based observational and experimental research on the brown argus butterfly Aricia agestis in the UK, particularly in populations where the species uses annual host plants of the family Geraniaceae. I first assay environment-driven variation in butterfly egg-laying behaviour on its ephemeral annual host plants. I show that climatic effects on dietary specialist species will depend on the interaction between direct effects of climate on behaviours (such as egg-laying), and indirect effects mediated by changes in the availability of preferred high-quality plant resources. I then investigate the climatic drivers of quality and phenology in these host plant resources. I demonstrate that climatic variation and change may generate a narrow phenological window of opportunity for exploitation by the consumer, with a potential for trophic asynchrony. In Chapter 5 I apply a novel modelling approach to investigate the scope for phenological change and climatic conditions to alter consumer population dynamics and distribution, focusing on the recent dynamics of the brown argus in the UK. I identify relationships between phenology and population dynamics that are consistent with the occurrence of population bottlenecks resulting from climate-driven asynchrony with ephemeral resources. I then demonstrate that patterns of phenology which improve consumer:resource synchrony are associated with consumer range expansion over a 25-year period. These findings provide a valuable insight into the relations between phenology, population dynamics and distribution, and the mechanisms that underpin these relationships. They suggest that a more complete understanding of the ecological effects of environmental change will require simultaneous consideration of the direct and indirect effects of climate on a diverse suite of interrelated processes that act from the individual to the (meta-)population level