Past and current drivers of species climatic niches and geographic distributions

Abstract

The distribution of species determines global patterns of biodiversity and ecosystem functions. Current species distributions are the result of the complex interplay of evolutionary, environmental, ecological and anthropogenic processes, which act across multiple spatial and temporal scales. Understanding the relative importance of these drivers for different taxa and regions is crucial for informing scientific and applied conservation debates on management of species under environmental change. This thesis investigates how past natural climate change, recent anthropogenic land cover change, and phylogenetic relationships may have contributed to shaping current climate niches and species distributions. This research focuses on two typical open-habitat plant genera in New Zealand for which the species-level geographic distributions are well known: the herbaceous genus Acaena (18 species) and the grass genus Chionochloa (34 species).

New Zealand has experienced a substantial and very recent (<1,000 years ago) land cover change from being a largely forest-covered land to being dominated by open non-woody habitats. Therefore, for open-habitat plant species, the areas with potentially suitable habitat have substantially increased. This thesis investigates to which degree species have colonised these recently opened areas. The relative importance of the following factors for high abundance of Acaena species in newly opened habitats was quantified; (i) geographical habitat features, (ii) species climatic niches, and (iii) species functional traits. The results show that geographical features of habitat and climatic factors appear to be more important than species functional traits for utilising newly available open habitat.

The geographic distribution of environmental conditions is a key driver of the distribution of species and ecosystems. Under environmental change, not only the environmental conditions at the location may change, but also the spatial arrangement and availability of those environmental conditions. Identifying the changes in the distribution and extent of such analogous environment conditions allows us to quantify and map environment change over time. In this thesis, an index quantifying the spatial availability of climate conditions was developed and applied to current and past climate conditions in New Zealand. Key locations in New Zealand with currently rare climates include the West Coast in the South Island and southeastern parts of the North Island; whereas regions with currently common climate conditions are located in low-lying areas on both islands. Locations in the central South Island have a current climate that is currently less widely available than it was at the Last Glacial Maximum (LGM); on the other hand, current climate conditions of the South Island’s west coast and large parts of the North Island are currently more widely available than at the LGM.

Past environmental change can contribute to shaping current species distribution, and geographical patterns of climate conditions are an important control of these distributions. Therefore, the importance of spatial availability of current and past (LGM) climate conditions for current species distributions was analysed. Current distributions of the open-habitat plant species investigated here are more correlated with patterns of current climate conditions than past climate conditions. Thus, for taxa investigated here, past climate change appears to be less important than current climate conditions for shaping their current distributions. Currently realised climatic niches and range patterns of species will reflect a legacy of past environmental and evolutionary processes. Two key historical drivers of species distributions were investigated here; evolutionary history expressed through phylogenetic relationships and environmental stability. Results indicated that currently realized niches of species investigated here are only poorly explained by phylogenetic relationships and long-term environmental stability. However, in areas where the magnitude of past climate change was larger than New Zealand, past climate change might play a stronger role in shaping the current distribution of plants.

This study has shown that, for the open-habitat plant species investigated here, past climate and land cover changes can help to explain current species distributions for some species but not for others. Differences in these responses between species are likely due to the spatial arrangement of the species climatic’ niche conditions. This study highlights the roles of current and past environmental drivers in shaping current species niches and distributions.