Climate change and invasion impacts in the sub-Antarctic

Treasure, Anne M. (Anne Margaret) (2012-03)

Thesis (PhD)-- Stellenbosch University, 2012.


ENGLISH ABSTRACT: Climate change and biological invasions are major threats to biodiversity. In particular, these threats are predicted to influence terrestrial systems in the sub-Antarctic, where significant ecosystem responses to both have already been seen. In this thesis, the sub-Antarctic Prince Edward Island group is used as a model system in which to investigate key questions relating to climate change and invasive species impacts. The island group comprises two islands, Marion (MI) and Prince Edward (PEI), both of which are experiencing rapid warming, yet have different invasive assemblages and in consequence are experiencing different impacts. Variation in the patterns of invasive species richness and abundance and their underlying causes are matters of considerable ecological and conservation significance. While an increase in thermal energy availability typically results in an increase in species richness, the mechanisms underlying these patterns are poorly understood. In Chapter 2 of this thesis, these relationships are explored for springtails, an important component of the soil fauna on Marion Island. Energy explains a large amount of the spatial variation in indigenous and invasive springtail species richness. Disturbance thresholds and stressful temperatures are more important than increased population sizes in determining this variation in species richness. As both indigenous and invasive springtail species richness and abundance are strongly related to temperature, a warming climate could have far-reaching consequences for these organisms. In particular, invasive species are predicted to be at an advantage relative to indigenous species under warming conditions. One species where this seems especially likely, given its physiological responses to experimental warming and drying, is the large invasive tomocerid, Pogonognathellus flavescens. Determining whether this will be the case depends on understanding the factors underlying its range limits and abundance structure. Moreover, few studies have sought to distinguish the causal basis of abundance structure and range limits, particularly for invasive species. Thus, in Chapter 3, local microclimate variables and physiological tolerances of the invasive springtail, P. flavescens (a habitat generalist), are examined. The results suggest that the species should be widely distributed across a range of habitats on MI. However, the springtail is restricted to indigenous Poa cookii tussock grassland habitats in the southeast. The current range limits are set by dispersal limitation (i.e. contingent absences) whilst abundance structure is a function of variation in soil substrate quality. However, over time, the widening distribution of P. cookii, as a consequence of a major management intervention (the eradication of feral cats), may enable P. flavescens to colonise all suitable areas. In Chapter 4, the focus changes to what has been considered the third major response to climate change, along with range and phenological responses - changing animal body sizes. Body size is one of the most significant and obvious features of animals and is of considerable ecological and physiological importance. A prediction of the temperature-size rule (TSR) is that with warming, body size of the weevil species on both MI and PEI should decline. However, predation by mice of the weevils on MI should fundamentally affect the pattern of such change, causing it to differ from neighbouring PEI, indicating synergistic impacts between climate change and invasions. Analysis of a 24-year data set indicates a decline in the body size of all weevil species on PEI with increasing temperature. However, on MI, a negative relationship between mean annual temperature and body size is found only for Palirhoeus eatoni, a species not eaten by mice. A possible explanation for the positive relationships found for the other species could be due to higher metabolic demands imposed on mice in colder years than in warmer ones. Any increase in predation coupled with a preference for larger sizes, which the mice clearly show, would lead to a decline in the mean size of the weevil species. Due to the relationship between body size and metabolic rate and the importance of the weevils in the islands’ food webs, changes to the body size of these organisms could have significant consequences for the island ecosystems’ functioning. The thermal environment experienced by organisms also has a direct effect on survival, growth and reproduction. The physiological response of organisms to rapidly changing climates is therefore a primary concern. Organisms may respond to variable environmental conditions through phenotypic plasticity as well as behaviour. Chapter 5 of this thesis shows that of the weevil species and populations investigated on MI, most display phenotypic plasticity, the form of which is in keeping with the ‘Hotter is Better’ hypothesis. This could be due to rare extreme temperature events and the advantage for the performance curves to incorporate high temperatures experienced in the environment. Mismatches between thermal optima and preferred temperatures displayed by all species could mean that these weevils are well equipped to cope with warming conditions on MI unless the prediction of an increase of rare extreme events such as extreme temperatures is realised. Rapidly changing climates and an increase in the introduction of non-indigenous species are issues of major conservation concern. This has increased the significance of studies on the impacts of these threats. However, this thesis shows that to understand such processes, it is essential that an integration of disciplines be undertaken. This thesis thus adopts a multidisciplinary approach and highlights key issues associated with both climate change and biological invasions. The patterns and predictions of species and community responses to these environmental changes are complex. Moreover, predicting such responses is likely to be problematic, especially as multiple factors will change concurrently and how these factors might change is unclear. This highlights the importance of long-term records for understanding organism responses to such changes. Furthermore, impacts on indigenous species are likely to be exacerbated by the predicted increase in the rate of introductions with climate change. This makes the case for preventing the dispersal of invasive species to new areas all the more important.

AFRIKAANSE OPSOMMING: Klimaatsverandering en indringer spesies is belangrike bedreigings vir biodiversiteit. In besonder word voorspel dat hierdie bedreigings terrestriële sisteme in die sub-Antarktiese sal beïnvloed, waar beduidende ekosisteem reaksie aan beide reeds gesien is. In hierdie tesis word die sub-Antarktiese Prince Edward eiland groep as a model sisteem gebruik om belangrike vrae met betrekking tot klimaatsverandering en die impak van indringer spesies te ondersoek. Die eiland groep bestaan uit twee eilande, Marion (ME) en Prince Edward (PEE), wat beide versnellende verwarming ervaar, maar tog verskillende indringer samestellings het en vervolgens verskillende impakte ervaar. Variasie in die patrone van indringerspesierykheid en vollopheid en hulle onderliggende oorsake is van aansienlike omgewings en bewarings betekenis. Terwyl 'n toename in die beskikbaarheid van energie tipies lei tot 'n toename in spesierykheid, word die onderliggend meganismes van hierdie patrone swak verstaan. In Hoofstuk 2 van hierdie tesis, word hierdie verhoudings vir springsterte ondersoek, 'n belangrike komponent van die grond fauna op ME. Energie verduidelik 'n groot hoeveelheid van die ruimtelike variasie in inheemse en indringende springstert spesierykheid. Versteuringsdrempels en stressvolle temperature is meer belangrik as die toename in bevolking groottes in die bepaling van hierdie variasie in spesierykheid. Aangesien beide inheemse en indringende springstert spesierykheid en vollopheid sterk verwant is aan temperatuur, kan 'n verwarmende klimaat verreikende gevolge vir hierdie organismes hê. In die besonder word voorspel dat indringerspesies bevoordeeld sal wees relatief tot inheemse spesies onder verwarmende toestande. Een spesie waar dit veral blyk om geneig te wees, gegewe sy fisiologiese reaksie tot eksperimentele verhitting en uitdroging, is die groot indringer tomocerid, Pogonognathellus flavescens. Om te bepaal of dit die geval sal wees, hang af van die begrip van die onderliggende faktore van sy voorkomsgrense en vollopheidstruktuur. Daarbenewens is daar min studies wat gepoog het om te onderskei tussen die veroorsakende basis van vollopheidstruktuur en voorkomsgrense, veral vir indringerspesies. Dus, in Hoofstuk 3, word plaaslike mikroklimaat veranderlikes en fisiologiese toleransies van die indringer springstert, P. flavescens ('n habitat generalis), ondersoek. Die resultate stel voor dat die spesie wyd verspreid moet wees oor 'n verskeidenheid van habitatte op ME. Maar, die springstert is beperk tot inheemse Poa cookii polle grasveldhabitatte in die suidooste. Die huidige voorkomsgrense word daar gestel deur verspreidingsbeperking (dws voorwaardelike afwesighede), terwyl vollopheidstruktuur 'n funksie is van die variasie in die grond substraat kwaliteit. Maar, die uitbreidende verspreiding van P. cookii as gevolg van 'n groot bestuursingryping (die uitwissing van wilde huiskatte), kan P. flavescens in staat stel om alle geskikte gebiede te koloniseer met verloop van tyd. In Hoofstuk 4 verander die fokus na wat as die derde groot reaksie op klimaatsverandering beskou word, saam met voorkoms en fenologiese reaksies - veranderende diere liggaamsgroottes. Liggaamsgrootte is een van die beduidendste en mees voor die hand liggende eienskappe van diere en is van aansienlike ekologiese en fisiologiese belang. 'n Voorspelling van die temperatuur-grootte-reël (TGR) is dat met verwarming, liggaamsgrootte van die snuitkewerspesies op beide ME en PEE sal afneem. Hoe ookal, predasie deur muise van die snuitkewers op ME moet fundamenteel die patroon van sodanige verandering op PEE beïnvloed, wat sinergistiese impakte tussen klimaatsverandering en indringings aandui. Die ontleding van 'n 24-jarige datastel dui aan op 'n afname in die liggaamsgrootte van alle snuitkewer spesies op PEI met ‘n toename in temperatuur. Maar, op ME is 'n negatiewe verhouding tussen die gemiddelde jaarlikse temperatuur en liggaamsgrootte net gevind vir Palirhoeus eatoni, 'n spesie wat nie deur die muise geëet word nie. 'n Moontlike verduideliking vir hierdie positiewe verhoudings wat gevind is vir die ander spesies kan wees as gevolg van hoër metaboliese eise op die muise in kouer jare as in warmer jare. Enige toename in predasie, tesame met 'n voorkeur vir groter mates, wat die muise duidelik wys, sou lei tot 'n afname in die gemiddelde grootte van die snuitkewer spesies. As gevolg van die verhouding tussen liggaamsgrootte en metaboliese tempo, sowel as die belangrikheid van die snuitkewers in die eilande se voedselwebbe, kan veranderinge in die liggaamsgrootte van hierdie organismes beduidende gevolge op die eiland ekosisteme se funksionering hê. Die termiese omgewing wat deur organismes ervaar word het ook 'n direkte invloed op oorlewing, groei en voortplanting. Die fisiologiese reaksie van organismes op vinnig veranderende klimate is dus 'n primêre bron van kommer. Organismes kan reageer op veranderlike omgewingstoestande deur fenotipiese plastisiteit sowel as gedrag. Hoofstuk 5 van hierdie tesis toon dat van die snuitkewerspesies en bevolkings wat ondersoek is op ME, die meeste fenotipiese plastisiteit vertoon, die vorm wat in ooreenstemming is met die ‘Warmer is Beter’ hipotese. Dit kan wees as gevolg van seldsame uiterste temperatuur gebeure en die voordeel vir die prestasie kurwes om hoë temperature wat ervaar word in die omgewing in te sluit. Mismatches tussen termiese optima en voorkeur temperature vertoon deur alle spesies kan beteken dat hierdie snuitkewers goed toegerus is om die verhitting op ME te hanteer, tensy die voorspelling van 'n toename van seldsame uiterste gebeure soos uiterste temperature gerealiseer word. Vinnig veranderende klimate en 'n toename in die bekendstelling van nie-inheemse spesies is kwessies van groot bewarings kommer. Dit het die betekenis van studies oor die impak van hierdie bedreigings verhoog. Hierdie tesis toon egter dat om sulke prosesse te verstaan, dit noodsaaklik is dat 'n integrasie van die dissiplines onderneem word. Hierdie tesis aanvaar dus 'n multi-dissiplinêre benadering en beklemtoon die belangrike kwessies wat verband hou met beide klimaatverandering en biologiese indringing. Die patrone en voorspellings van spesies en die gemeenskapsreaksies op hierdie omgewingsveranderinge is kompleks. Verder, die voorspelling van sodanige reaksies sal waarskynlik problematies wees, veral omdat verskeie faktore gelyktydig sal verander en hoe hierdie faktore kan verander is onduidelik. Dit beklemtoon die belangrikheid van lang termyn rekords vir die begrip van organisme reaksies op sulke veranderinge. Verder, die impak van inheemse spesies is geneig om te vererger deur die voorspelde toename in die tempo van bekendstellings met klimaatsverandering. Dit maak die taak vir die bestuur van die voorkoming dat indringerspesies nuwe gebiede bereik al hoe meer belangrik.

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