Climate change and vascular plant species interactions on sub-Antarctic Marion island

[摘要] ENGLISH ABSTRACT: Shifts in species ranges are an important consequence of climate change, andcan affect the composition, structure and functioning of ecosystems. Generally, inresponse to rising temperatures, species have increased their altitudinal and latitudinaldistributions along their cooler boundaries, although there has been large variationbetween species. However, in addition to climatic factors, species range limits are alsosensitive to biotic interactions. Therefore, interspecific interactions may be able toinfluence the impact of climate change on species distributions. In this thesis climatechange, range shifts and spatial variation in plant-plant interactions are documented toexamine the potential for biotic interactions to mediate climate-driven altitudinalrange shifts on sub-Antarctic Marion Island.The climate on Marion Island changed considerably between 1949 and 2003,with significant trends in biologically-relevant parameters encompassing averageclimatic conditions, variability in climate parameters and extreme weather events.Combining these trends showed that while environmental conditions have amelioratedfor some species, they have become more abiotically stressful for others (e.g.hygrophilous species). Concurrent with changes in the island's climate, there havebeen rapid changes in the elevational distribution of Marion Island's native vascularflora. On average species ranges expanded upslope (as predicted in response to thewarming experienced on the island), although individual range expansion rates variedgreatly. As a result of the idiosyncratic expansion rates, altitudinal patterns of speciesrichness and community composition changed considerably. Therefore, both speciesandcommunity-level changes occurred in the flora of Marion Island, demonstratingthe community's sensitivity to climate change.To determine the potential for interspecific interactions to have mediated thesechanges in species elevational ranges, spatial variation in the balance of positive andnegative plant-plant interactions were examined. Increasing spatial associationbetween four dominant species along an altitudinal severity gradient suggested thatthe intensity of facilitation, relative to the intensity of competition, is greater undermore severe conditions. While, interaction strength varied between species pairs, thenature of relationship was consistent across the species. At a broader spatial scale, theperformance of the grass Agrostis magellanica was compared in the presence and in the absence of the cushion plant Azorella selago along the entire altitudinal range ofboth species. The influence of the cushion plant on A. magellanica switched fromnegative to positive with increasing environmental severity, reaching a positiveasymptote under extreme environmental conditions. Therefore, on Marion Island thespatial variation in the nature of biotic interactions is related to environmentalseverity, and facilitative interactions can be strong under extreme environmentalconditions.These results show that positive biotic interactions are important at higherelevations on Marion Island, and could thus accelerate upslope range expansions inresponse to climate change. Nonetheless, competitive inhibition of upslope speciesmovement, especially in areas of dense vegetation (i.e. mid or low altitudes), couldhave the opposite result, acting antagonistically with the effect of rising temperatures.This research highlights the importance of considering interspecific interactions whenexamining the biotic implications of climate change, both for affecting the rate at which species ranges change and the extent of species distributions.

[发布机构] Stellenbosch University