Ecology of the Black-faced sheathbill on Marion Island

McClelland, Gregory T. W. (2013-12)

Thesis (PhD)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: As the pace of climate change has begun to accelerate so too has it become clear that the direct impacts thereof are likely to have profound consequences for many island systems. Moreover, it has also been suggested that climate change will exacerbate the effects of many invasive species, so further impacting both diversity and ecosystem functioning. Forecasts for such interactions have been most pronounced for the Southern Ocean islands, which are home to a wide variety of endemic species. This thesis is about such interactions and their specific impacts on a key endemic, the black-faced sheathbill (Chionis minor) on the Prince Edward Islands. Of increasing concern is how invasive rodent populations in the Southern Ocean may be responding to global climate change, as ameliorating conditions on these islands are forecast to decrease thermal and resource restrictions on rodents. However, firm evidence for changing rodent populations in response to climate change, and demonstrations of associated impacts on the terrestrial environment, are entirely absent for the region. In Chapter 2 of this thesis, these relationships are explored for invasive house mice (Mus musculus) on Marion Island. Using spatially explicit capture-recapture modeling, it is determined that mouse populations across a range of habitats have increased over time. Owing to an extended breeding season, made possible by ameliorating conditions brought on by climate change, the total number of mice on the island at annual peak density more than doubled over the past decade. It is also demonstrated that mice directly reduce invertebrate densities, with biomass losses up to two orders of magnitude in some habitats. Because of the importance of invertebrates to nutrient cycling on the island, such changes are likely to have significant ecosystem-level impacts. In Chapter 3 the focus expands to examine how increasing mouse impacts and other outcomes of climate change are affecting the ecology of the black-faced sheathbill. It has been established that invasive house mice are capable of suppressing the populations of several seabird species in the Southern Ocean. However, mouse impacts on the region’s few island endemic land-birds remain largely unexplored. Further, a significant effect of climate change may be realized by altering interspecific interactions, specifically food webs. A significant portion of sheathbill diets is derived from rockhopper penguins, a species currently under a climate-change-driven decline, which may have significant effects on sheathbills. The study found that terrestrial invertebrates are no longer a significant prey resource for sheathbills on Marion Island, and that sheathbills have effectively been displaced from a formerly important winter food resource by mice. In response, the number of sheathbills foraging in king penguin colonies increased. Moreover, a reduced rockhopper penguin population lead to significant declines in both the number and proportion of sheathbills foraging in rockhopper penguin colonies. The sum result was a significant decline in the body condition of female sheathbills. Rather than decrease reproductive output, sheathbills responded by decreasing clutch size and producing significantly fewer male nestlings. While population estimates did not detect a reduction in the number of sheathbills, population projections suggest that the population is in decline, with the reproductive population declining faster than the absolute population. There is need for greater study of island species, as for even relatively well-studied taxa such as birds many aspects of ecology remain significantly less studied when compared to species occurring on continents. For example, basal metabolic rate (BMR) is a fundamental characteristic of all endotherms, yet only a handful of island birds have had their BMR measured, and fewer still to a level that allows intraspecific analysis. In Chapter 4 the BMR of black-faced sheathbills on Marion Island was measured to determine whether the unique phylogenetic position and ecology of sheathbills equate to a unique BMR when allometrically compared to other birds. It was found that the BMR of sheathbills is typical for a bird of its size. However, significant intraspecific variation was found to occur, with differences in habitat quality a likely driver. The results of the study show that the combined effects of climate change and invasive species can have significant consequences for terrestrial endemics in the Southern Ocean. Further, the long-term changes observed in sheathbills make clear the need for improved documentation and study of island species in general, as many of the responses observed in this study are significant but subtle and would not have been evident without detailed knowledge of species ecology and vital rates. Giving greater focus to insular biota is imperative to understanding their current status and ecology as well as establishing a barometer against which further global change can be measured and mitigation measures evaluated. Specific conservation responses for the black-faced sheathbill on Marion Island include the provision of nest boxes at king penguin colonies, and eradication of house mice. The latter would have long-term benefits for the species, invertebrates, ecosystem functioning generally, and likely also for important seabirds such as several species of albatrosses whose chicks are being increasingly preyed on by mice. Eradication would, however, be difficult and expensive, and with substantial potential non-target effects, including on sheathbills, that would have to be carefully managed. In the absence of local mouse eradication, and with ongoing climate change, specific management of the sheathbill population through the provision of supplementary nesting sites seems the most appropriate conservation action. It should therefore be examined in small-scale trials to ascertain the likelihood of unintended consequences. Importantly, the maintenance of Prince Edward Island as largely free of invasive species is key to the conservation of the local black-faced sheathbill subspecies, Chionis minor marionensis, endemic to the Prince Edward Island group.

AFRIKAANSE OPSOMMING: Soos die tempo van klimaatsverandering begin om te versnel, het dit ook duidelik geword dat die direkte impak daarvan waarskynlik ernstige gevolge vir baie eilande gaan hê. Verder word dit is ook voorgestel dat klimaatsverandering die gevolge van baie indringerspesies sal vererger, so ʼn verdere impak het op beide diversiteit en die funksionering van die ekosisteem. Voorspellings vir sulke interaksies is die meeste uitgespreek vir die Suidelike Oseaan-eilande, wat ook die tuiste van 'n wye verskeidenheid van endemiese spesies is. Hierdie tesis is oor sulke interaksies en hul spesifieke impak op 'n sleutel endemiese spesie is, die swart gesig skedebek (Chionis minor) op die Prince Edward-eilande. ʼn Groter bron van bekommernis is hoe uitheemse knaagdier bevolkings in die Suidelike Oseaan kan reageer teenoor globale klimaatsverandering, aangesien toestande op die eilande voorspel word om hitte en hulpbron beperkings vir knaagdiere te verminder. Maar, ferm bewyse vir die verandering van knaagdier bevolkings in reaksie op klimaatsverandering, en demonstrasies van gepaardgaande impakte op die terrestriële omgewing, is heeltemal afwesig vir die streek. In Hoofstuk 2 van hierdie tesis, word hierdie verhoudings ondersoek vir indringende huis muise (Mus musculus) op Marion-eiland. Ruimtelik vang-terugvang modelle word gebruik om vas te stel dat die muis bevolkings oor 'n verskeidenheid van habitatte mettertyd toegeneem het. As gevolg van 'n uitgebreide broeiseisoen as gevolg van die verligting van toestande gebring deur klimaatsverandering, het die totale aantal muise op die eiland by die jaarlikse hoogtepunt digtheid meer as verdubbel oor die afgelope dekade. Dit is ook getoon dat muise die digtheid van ongewerweldes direk verminder het, met biomassa verliese tot twee ordes in sommige habitatte. As gevolg van die belangrikheid van die ongewerweldes vir voedingstof sirkulering op die eiland, behoort sulke veranderinge waarskynlik 'n beduidende ekosisteem-vlak impak te hê. In Hoofstuk 3 word die fokus verbreed om te sien hoe die verhoging van die muis impakte en ander uitkomste van klimaatsverandering die ekologie van die swart gesig skedebek beïnvloed. Daar is vasgestel dat indringende huis muise in staat is om die bevolkings van verskeie spesies seevoëls te onderdruk in die Suidelike Oseaan. Maar die muis impak op die streek se paar eiland endemiese land voëls bly grootliks onverken. Verder kan 'n beduidende uitwerking van klimaatsverandering verwesenlik word deur die wysiging van interspesifieke interaksies, veral voedselwebbe. 'n Beduidende gedeelte van skedebek dieet word gekry van Geelkuifpikkewyne, 'n spesie wat tans onder 'n klimaat-veranderinggedrewe agteruitgang is, wat ook 'n beduidende uitwerking het op die skedebek. Die studie het gevind dat terrestriële ongewerweldes nie meer 'n beduidende prooi hulpbron vir die skedebek op Marion-eiland is nie, en dat die skedebek effektief is verplaas uit 'n voorheen belangrike winter kos hulpbron deur muise. In reaksie hierop het die aantal skedebekke wat kos soek in die koning pikkewyn kolonies toegeneem. Verder, 'n verlaagde Geelkuifpikkewyn bevolking lei tot 'n beduidende afname in beide die aantal en persentasie van skedebekke wat kos soek in Geelkuifpikkewyn kolonies. Die gevolg was 'n beduidende afname in die liggaamstoestand van die vroulike skedebekke. Eerder as ʼn afname van reproduksie, het skedebekke gereageer deur 'n vermindering in die aantal eiers en produseer aansienlik minder manlike kuikens. Terwyl bevolking skattings nie 'n afname in die aantal skedebekke kan vind nie, dui bevolking projeksies daarop dat die bevolking besig is om af te neem, met die voortplanting bevolking wat vinniger daal as die absolute bevolking. Daar is 'n behoefte vir 'n groter studie van eiland spesies, omdat selfs vir betreklik goed bestudeerde groepe soos voëls baie aspekte van die ekologie aansienlik minder bestudeer bly in vergelyking met spesies op die vastelande. Byvoorbeeld, basale metaboliese tempo (BMT) is 'n fundamentele kenmerk van alle endotermiese diere, maar net 'n handjievol van die eiland voëls het hul BMT laat meet, en nog minder tot 'n vlak wat dit moontlik maak intraspesifieke analise. In Hoofstuk 4 was die BMT van die swart gesig skedebek op Marion-eiland gemeet om te bepaal of die unieke filogenetiese posisie en ekologie van skedebekke gelyk aan 'n unieke BMT wanneer allometries vergelyk word met ander voëls. Daar is gevind dat die BMT van skedebekke tipies is vir 'n voël van sy grootte. Daar is egter belangrike intraspesifieke variasie gevind, met verskille in habitat kwaliteit as 'n waarskynlike verduideliking. Die resultate van die studie toon dat die gekombineerde effek van klimaatsverandering en indringerspesies beduidende gevolge vir terrestriele inheemse spesies in die Suidelike Oseaan kan hê. Verder maak die lang-termyn veranderinge waargeneem in skedebekke dit duidelik dat die behoefte aan verbeterde dokumentasie en studie van die eiland spesies in die algemeen, omdat baie van die reaksies waargeneem in hierdie studie betekenisvol is, maar subtiel en sou nie gewees het sonder gedetailleerde kennis van die spesies ekologie van die spesie nie. Om ʼn groter fokus op die insulêre biota te plaas is noodsaaklik om hul huidige status en die ekologie te begryp, sowel as om 'n barometer waarteen verdere globale verandering gemeet kan word en versagtende maatreëls geëvalueer. Spesifieke bewaring antwoorde vir die swart gesig skedebek op Marion-eiland sluit in die voorsiening van nes bokse by koning pikkewyne, en die uitwissing van huis muise. Laasgenoemde sou lang-termyn voordele vir die spesie en ongewerweldes hê, asook funksionering van die ekosisteem in die algemeen, en waarskynlik ook vir belangrike seevoëls soos verskeie spesies van albatrosse wie se kuikens toenemend geëet word deur muise. Uitwissing sou egter moeilik en duur wees, en het 'n aansienlike potensiaal vir nieteiken effekte, insluitend op skedebekke, wat sal versigtig moet bestuur word. In die afwesigheid van plaaslike muis uitwissing, en met voortdurende verandering van die klimaat, spesifieke bestuur van die skedebek bevolking deur die voorsiening van aanvullende broeiplekke blyk die mees geskikte bewaringsaksie. Dit moet dus ondersoek word in 'n kleinskaal proewe om die waarskynlikheid van onbedoelde gevolge te bepaal. Wat belangrik is die instandhouding van Prince Edward Eiland as grootliks vry van indringerspesies en is die sleutel tot die bewaring van die plaaslike swart gesig skedebek subspesie, Chionis minor marionensis, endemies aan die Prince Edward Eiland groep.

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