Climate change and vascular plant species interactions on sub-Antarctic Marion Island by Peter Christiaan le Roux Submitted in partial fulfilment for the degree Philosophiae Doctor (Conservation Ecology) at Stellenbosch University Department of Conservation Ecology and Entomology Faculty of AgriSciences Supervisor: Prof. Melodie A. McGeoch Date: 15 April 2008 i Declaration By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the owner of the copyright thereof (unless to the extent explicitly otherwise stated) and that I have not previously in its entirety or in part submitted it for obtaining any qualification. Date: 4 July 2008 Copyright © 2008 Stellenbosch University All rights reserved ii Azorella selago on Marion Island iii Acknowledgements I thank and praise the Lord, through whom this work has been possible. “The LORD alone is God; God alone is our defense. He is the God who makes me strong, who makes my pathway safe. He makes me sure-footed as a deer; he keeps me safe on the mountains.” (Psalm 18: 31 - 33) I thank my supervisor, Melodie McGeoch, for support and encouragement. You’ve always been professional and personable, exceptional and humble, a friend and a mentor. It has been a pleasure working with you. Marion Burger, Zipho Buwa, Jacques Deere, Natalie Haussmann, Lizel Hugo, Mawethu Nyakatya, Asanda Phiri, Ethel Phiri, Tshililo Ramaswiela, Tess Rautenbach, Quintin Schutte and Jenna Snyman all helped with fieldwork, often in difficult conditions. Your help made the work go faster and your company made the weather seem much better. Jan Boelhouwers, Steven Chown, Niek Gremmen, Lizel Hugo, Dave Hedding, Jen Lee, Ian Meiklejohn, Erika Nortje, Mawethu Nyakatya, Ethel Phiri, Tshililo Ramaswiela, Brent Sinclair, Sarette Slabber and Valdon Smith are thanked for sharing data and experience, and for insights and comments on many manuscript drafts. One of the best things about working on Marion Island is the research community. The members of the Gogga, Bird and Mammal labs were always a source of ideas, encouragement and humour. It’s been a privilege to spend time with all of you. I thank all my colleagues at Stellenbosch University, especially the members of the SPACE (Spatial, physiological and conservation ecology) group and the CIA (Centre for the Investigation of Azorella). Life was rarely boring at the office! Thank you for your advice and your willingness to read through countless manuscript drafts. Climate data used in Chapter 2 was kindly provided by the South African Weather Service. Joe Perry, Kelvin Conrad and Peter Haase are thanked for making available the SADIE and SPPA software that was used in Chapter 4. Financial support was provided by the National Research Foundation, the South African National iv Antarctic Program (Grant number 2069543 and SNA 2004070900002) and Stellenbosch University. Finally, I thank all my friends and family. I cannot fully express what your love, support and understanding has meant to me. To all my housemates and hiking companions over the last years – you’re a crazy bunch and it’s been fun! To my family, thank you for your complete support since the beginning. I especially thank Brigitte Bruyns who made almost as many sacrifices as I have for this research. You’ve been my balance through this project, lifting me up when I’ve been down and motivating me when I’ve lost focus. Thank you. v Abstract Shifts in species ranges are an important consequence of climate change, and can affect the composition, structure and functioning of ecosystems. Generally, in response to rising temperatures, species have increased their altitudinal and latitudinal distributions along their cooler boundaries, although there has been large variation between species. However, in addition to climatic factors, species range limits are also sensitive to biotic interactions. Therefore, interspecific interactions may be able to influence the impact of climate change on species distributions. In this thesis climate change, range shifts and spatial variation in plant-plant interactions are documented to examine the potential for biotic interactions to mediate climate-driven altitudinal range 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 average climatic conditions, variability in climate parameters and extreme weather events. Combining these trends showed that while environmental conditions have ameliorated for some species, they have become more abiotically stressful for others (e.g. hygrophilous species). Concurrent with changes in the island’s climate, there have been rapid changes in the elevational distribution of Marion Island’s native vascular flora. On average species ranges expanded upslope (as predicted in response to the warming experienced on the island), although individual range expansion rates varied greatly. As a result of the idiosyncratic expansion rates, altitudinal patterns of species richness and community composition changed considerably. Therefore, both species- and community-level changes occurred in the flora of Marion Island, demonstrating the community’s sensitivity to climate change. To determine the potential for interspecific interactions to have mediated these changes in species elevational ranges, spatial variation in the balance of positive and negative plant-plant interactions were examined. Increasing spatial association between four dominant species along an altitudinal severity gradient suggested that the intensity of facilitation, relative to the intensity of competition, is greater under more severe conditions. While, interaction strength varied between species pairs, the nature of relationship was consistent across the species. At a broader spatial scale, the performance of the grass Agrostis magellanica was compared in the presence and in vi the absence of the cushion plant Azorella selago along the entire altitudinal range of both species. The influence of the cushion plant on A. magellanica switched from negative to positive with increasing environmental severity, reaching a positive asymptote under extreme environmental conditions. Therefore, on Marion Island the spatial variation in the nature of biotic interactions is related to environmental severity, and facilitative interactions can be strong under extreme environmental conditions. These results show that positive biotic interactions are important at higher elevations on Marion Island, and could thus accelerate upslope range expansions in response to climate change. Nonetheless, competitive inhibition of upslope species movement, especially in areas of dense vegetation (i.e. mid or low altitudes), could have the opposite result, acting antagonistically with the effect of rising temperatures. This research highlights the importance of considering interspecific interactions when examining the biotic implications of climate change, both for affecting the rate at which species ranges change and the extent of species distributions. vii Opsomming Die verskuiwing van spesies se verspreidingspatrone is ‘n belangrike gevolg van klimaatsverandering en kan die samestelling, struktuur en funksionering van ekosisteme affekteer. Oor die algemeen, in reaksie op verwarming, kom spesies op hoër hoogtes bo seespieël en hoër breedtegrade voor, alhoewel daar groot verskille tussen spesies in hierdie aspek is. Behalwe vir klimatologiese faktore, is spesies se verspreidingslimiete ook sensitief vir biotiese interaksies met ander spesies. Daarom kan interaksies tussen spesies die effek van klimaatsverandering op spesies- verspreiding verder beïnvloed. In hierdie verhandeling word klimaatsverandering, verandering in verspreiding van spesies en geografiese variasie in plant-plant interaksies ondersoek. Dit word gedoen ten einde die moontlikheid te toets dat biotiese interaksies die verandering in verspreidingspatrone van klimaat gedrewe spesies op sub-Antarktiese Marion Eiland beïnvloed. Marion Eiland se klimaat het aansienlik tussen 1949 en 2003 verander, met betekenisvolle neigings in biologies-relevante parameters, insluitend gemiddelde klimaatstoestande, variasie in klimaat en uiterste weerstoestande. As die veranderinge saam beskou word, wys dit dat, terwyl omgewingstoestande vir sommige spesies verbeter het, toestande vir ander spesies abioties meer ongunstig geword het (bv. akwatiese plantsoorte). Tesame met die veranderinge in die eiland se klimaat, het die hoogte bo seespieël verspreiding van Marion Eiland se inheemse vaatplante vinnig verander. Gemiddeld het spesies-verspreiding se hoogte bo seespieël na hoër hoogtes verskuif (soos voorspel in reaksie op verwarming), alhoewel die tempo van verskuiwing na hoër hoogtes tussen spesies verskil het. As gevolg van hierdie idiosinkratiese reaksies, het die aantal en samestelling van plant spesies op verskillende hoogtes op die eiland aansienlik verander. Die sensitiwiteit van Marion Eiland se flora ten opsigte van klimaatsverandering word deur hierdie verandering gedemonstreer. Om te bepaal of die interaksies tussen spesies verspreidingspatrone kan beïnvloed, is die geografiese variasie in die balans van positiewe en negatiewe interaksies ondersoek. Vier dominante vaatplante het meer geassosieerd geraak hoe hoër hulle bo seespieël voorgekom het. Dit is ‘n aanduiding dat die sterkte van die viii positiewe interaksies (fasilitering), relatief tot die sterkte van die negatiewe reaksie (kompetisie), groter onder uiterste omgewingstoestande is. Die sterkte van die interaksie het verskil tussen spesies pare, maar die verhouding tussen omgewingstoestande en die sterkte van die interaksie was dieselfde vir alle spesies. Op ‘n breër geografiese vlak is die opbrengs van die gras Agrostis magellanica in die teenwoordigheid en afwesigheid van die polsterplant Azorella selago op alle hoogtes waar die plante saam voorkom vergelyk. Die invloed van A. selago op A. magellanica was negatief op lae hoogtes bo seespieël, maar het meer positief geraak met ‘n toename in hoogte bo seespieël, tesame met omgewingshardheid. Die invloed van die polsterplant op die gras het ‘n positiewe asimptoot onder uiterste omgewingstoestande bereik. Dus, op Marion Eiland is die geografiese variasie in biotiese interaksies verwant aan omgewingstoestande, en positiewe interaksies kan selfs onder die uiterste omgewingstoestande sterk wees. Hierdie resultate wys dat positiewe biotiese interaksies tussen plante belangrik is by hoër hoogtes bo seespieël op Marion Eiland. Die interaksies kan dus die opwaartse verspreiding van spesies in reaksie op klimaatsverandering versnel. Nogtans kan negatiewe interaksies die teenoorgestelde effek hê aangesien kompetisie tussen plante, veral in plekke met digte plantegroei (d.w.s. lae of middel hoogtes bo seespieël) opwaartse verskuiwing van spesies kan verhinder. Hierdie navorsing dui aan hoe belangrik dit is om interaksies tussen spesies in ag te neem, aangesien die interaksies die tempo en omvang van veranderinge in verspreiding kan beïnvloed. ix Table of Contents Declaration ii Acknowledgements iv Abstract vi Opsomming viii Table of Contents x Chapter 1 – General introduction 1 Ecological consequences of climate change 1 Impacts of climate change on species distributions 2 Role of biotic interactions in determining species ranges 3 Spatial variation in the balance of positive and negative interactions 4 Thesis outline: Climate change, range shifts and species interactions 5 Study site: Marion Island 6 References 8 Chapter 2 - Changes in climate extremes, variability and signature on sub-Antarctic Marion Island 15 Introduction 15 Methods 18 Results 21 Discussion 24 Increasing duration between rainfall events 24 Changes in wind speed 26 Changes in the extremes and variability in temperature 27 Aggregate changes in climate 29 Conclusion 30 References 30 Chapter 3 - Rapid range expansion and community reorganization in response to warming 47 Introduction 47 Methods 49 Study site 49 Data collection 51 Analyses 53 Results 54 Range expansion 54 Individual species change 54 Community change 56 Discussion 57 Conclusion 60 References 61 x