Rozprawy doktorskie na temat „Introduced birds”

I investigated parasitic infection and immunocompetence in populations of introduced bird species in New Zealand (NZ) that had experienced a range of population bottlenecks (11-808 individuals), and compared these parameters to non-bottlenecked conspecifics in the United Kingdom (UK). My aims were two-fold; firstly to assess if population bottlenecks are linked to increased parasite loads and/or decreased immunocompetence, and secondly, to assess at what severity of bottleneck these effects become evident. I found that ectoparasite load (chewing lice, Order: Phthiraptera, Sub-Orders: Amblycera & Ischnocera) was significantly higher in the more severely bottlenecked species in NZ than in the UK, whilst this difference became non-significant at more moderate bottlenecks. The difference was mainly driven by the Sub-Order Amblycera. The prevalence of avian malaria (Plasmodium spp.) was significantly negatively correlated to bottleneck size within NZ, after controlling for body mass. Total leucocyte and differential lymphocyte counts were elevated in the less bottlenecked species that were infected with malaria, whilst the populations at the more severe end of the bottleneck spectrum did not exhibit such a response. Furthermore, heterophil/lymphocyte (HL) ratio (a parameter used as an indicator of environmental and/or immunological stress), was significantly raised in the more bottlenecked species when compared to their UK counterparts, and this difference was correlated with the size of the bottleneck. Immunocompetence was further assessed by the experimental challenge of six introduced birds species in NZ with the mitogen phytohaemagglutinin (PHA). Immune response to PHA was significantly correlated to bottleneck size, but in the opposite direction to that predicted; immune response was greater in the more bottlenecked species. However, this may be an indication of increased investment in immunity, due to increased parasite and pathogen pressure or differential investment in varying components of the immune system. Finally, the immune response to PHA was compared in nestlings of two species that had experienced very different bottlenecks (70 vs. 653). After controlling for ectoparasitic infestation, I found no difference between the two species; however, this finding may be confounded by interspecific competition. Overall, my findings suggest that more severe population bottlenecks may result in increased susceptibility to pathogens, and impact on the immune system. This has a number of implications for the development of conservation protocols, and future avenues of research are suggested.

Humankind has redistributed a large number of species outside their native geographic ranges. Although the majority of introduction attempts fail to establish populations, the cumulative negative effect of successful non-native species has been and will continue to be large. Historical records of land bird introductions provide one of the richest sources of data for testing hypotheses regarding the factors that affect the successful establishment of non-native populations. However, despite comprehensive summaries of global avian introductions dating back two decades only very recent studies have examined the successful establishment of non-native bird species worldwide. It is clear that a non-random pattern exists in the types of land bird species that have been chosen by humans to be introduced outside their native range. Out of the 44 avian families from which species have been chosen for introduction almost 70% of introduction attempts have been from just five families (Phasianidae, Passeridae, Fringillidae, Columbidae, Psittacidae). Notably, these families include game species, insectivorous song birds, and species from the pet trade. It has been hypothesised that the fate of introduced species may be determined in part by heritable characteristics that are shared by closely related taxa. In my analyses, I have used current comparative methods to demonstrate that intrinsic eco-physiological characteristics are significant predictors of the worldwide success of introduced land bird species. The results of my analyses contribute to a greater ecological understanding of the traits that correlate with the successful establishment of non-native species. Notably, the three major conclusions that I have drawn from this thesis are: 1. Non-random patterns of successful establishment exist for introduced land bird taxa that have experienced a repeated number of introduction attempts. This result supports the idea that introduced species have an inherent likelihood of either succeeding or failing to establish non-native populations. 2. Eco-physiological traits are important correlates for determining the variability in introduction outcome for non-native land bird species. With reliable information on introduction attempts and taxa-specific traits predictive models are possible that quantify the outcome of repeated introduction attempts across non-native species. 3. Islands are not universally less resistant than mainland regions to the successful establishment of non-native species. This perception is a reflection of the greater number of introduction attempts to islands rather than an effect of biotic resistance. Any differences in the success of introduction attempts can be attributed largely to differences in the proportion of introductions that have been made across biogeographic regions. I have highlighted that data are accessible for global analyses of the variability in the successful establishment of non-native species. Although establishment success is not a deterministic process, the characteristics of an introduced species can influence the probability of its succeeding. I have shown that with adequate eco-physiological information, and for introduced land bird species at least, this probability can be predicted. These results refute previous suggestions that the stochastic component of species introductions will always overshadow any emerging patterns of successful establishment among non-native populations.

Cervids are a widespread family of ungulates, native to the Americas, Europe and Asia and introduced to Australia. Densities of cervids have increased in North America, Europe and Japan during the last couple of decades, due to changes in land use, reduced hunting, lack of large carnivores and changes in management practices. Where they occur at high densities, native and introduced cervids may have profound effects on vegetation, causing knock-on effects on other taxa. Cervids are in some instances managed towards changing the local distribution through supplemental feeding. These supplemental feeding stations are causing localized areas of high cervid abundance in the landscape, and may have effects on plants and animal species. In this thesis I investigate how high cervid densities and the practice of supplemental feeding impact birds and small mammals through effects on; 1) food resources, and 2) habitat structure. Utilizing a simulated moose (Alces alces) browsing experiment, I tested how varying moose densities and varying site productivity affects vole preference for bilberry. I found that voles preferred unbrowsed bilberry at low productivity sites, while they preferred lightly browsed bilberry at high productivity sites, I was however unable to explain the preference pattern with the chemical composition of the bilberry. However, moose do impact vole food preferences and this could, depending on the productivity of the site, potentially affect food selection and population dynamics over large geographical areas. Moose browsing pressure and nutrient input gradually decline with distance from supplemental winter feeding stations. I utilized this gradient to test how moose activity affects reproduction in pied flycatchers (Ficedula hypoleuca) and great tits (Parus major). The two species showed contrasting responses to moose feeding stations, great tits preferred to nest away from feeding stations where they showed higher feeding frequency and also produced more fledglings compared to close to feeding stations. Pied flycatchers on the other hand preferred to nest close to feeding stations where they had higher feeding frequency and fledgling mass was higher compared to further away. Hence, for both species moose browsing is affecting food availability that subsequently affects reproductive performance. Moose supplemental feeding stations also changed the habitat for small mammals from dwarf shrub dominated to grass and forb dominated. This did not have any effect on reproductive performance of the small mammals, but it did affect species composition along a gradient from the feeding stations. There was higher abundance of Microtus spp. and common shrews (Sorex araneus) close to feeding stations, while bank voles (Myodes glareoleus) had higher abundance away from feeding stations, the latter however was not statistically significant. Small mammals in Australian coastal heath were generally negatively associated with introduced rusa deer (Cervus timorensis), however one species responded positively to historical densities of rusa deer. It was however not possible to confirm these effects as being due to changes in habitat structure for the small mammals. However they may be explained by qualitative changes in the vegetation or alternatively by direct effects of deer through trampling or predation. Cervids and their management have an impact on other fauna, however the effects vary depending on small mammal and bird species. The effects of cervids also depend on the habitat requirements and foraging preferences of the small mammals and birds. Effects may be mediated through both food resources and habitat structure and are dependent on site productivity and cervid density.

It is generally accepted that populations which experience severe bottlenecks have a reduction in fitness. One of the most frequently reported fitness costs is increased hatching failure in bottlenecked populations of birds. The mechanism responsible for increased hatching failure is unknown. Research on other animals suggest that reduced population numbers cause unavoidable inbreeding that in turn leads to abnormalities in the gametes. In this thesis I examine some of the possible causes for increased hatching failure in severely bottlenecked populations of introduced birds in New Zealand. I look at three traits identified as a cause for infertility or hatching failure previously and determine whether there is a link with the size of a population s bottleneck. It is possible that reduced numbers of sperm reaching the site of fertilisation is a primary cause of hatching failure. I examined the perivitelline membrane of various species of introduced birds and counted the total number of sperm present to compare to how many would be expected in non-bottlenecked species. Although there was no relationship between the size of the bottleneck and the number of sperm present, all species had lower than expected sperm counts. In many species of mammals, a reduction in the quality of sperm is attributed to inbreeding depression bought about by genetic bottlenecks. I next compared the level of sperm abnormalities, variation in midpiece size sperm, and sperm motility with the size of the bottleneck each species passed through when introduced to New Zealand. There was no significant correlation between either the variation in midpiece size or sperm motility with bottleneck size. However, there was a trend for species that passed through more severe bottlenecks to have a slightly higher level of midpiece size and lower motility. Finally, I examined whether there was a link between abnormalities in the eggshell and the size of the respective bottleneck. There was no significant change in eggshell thickness or any change in the number of pores associated bottleneck size. However, there was a decreased number of round pores in severely bottlenecked species, although the consequences of this are unknown. My findings do not directly link a single cause for increased hatching failure in bottlenecked species of birds, but they do highlight the need for monitoring of reproductive traits in endangered species that have experienced a population bottleneck.

The introduction of exotic bird species to New Zealand (NZ) from the United Kingdom (UK) over 100 years ago unintentionally created an ideal study system to examine potential changes in developmental stability due to bottleneck effects. In this study I measured fluctuating asymmetry (FA; random deviations from symmetry between bilaterally symmetrical traits) in 13 species of introduced birds in NZ. FA has been used for conservation purposes as an early warning system of increased developmental instability (DI; the inability to cope with random genetic or environmental perturbations during development). I evaluated DI using FA in several anatomical external and internal morphological traits, and compared differences in body morphology between introduced and source populations in relation to the bottleneck size. I also examined FA in nestlings in two closely related introduced species that passed through two different-sized population bottlenecks. Differences in FA in relation to bottleneck severity were only observed in external traits. FA in external traits in some NZ populations differed from their UK counterparts, but it was in the opposite direction than predicted. FA in external traits varied significantly across NZ populations of introduced species - the most severe bottlenecks species exhibited higher levels of FA than species that passed through larger bottlenecks. There were no patterns in FA and bottleneck size for skeletal traits, most likely due to differences in environmental and genetic stressors resulting in species- and characterspecific FA relationships. Nestling FA was the same for both species, despite the large difference in bottleneck size. FA did decrease over the nestling period, although not at the same rate for each trait, most likely due to the differing costs of development, functional importance, and other environmental stressors that might influence FA in each trait differently. Overall changes in body morphology occurred in four species introduced to NZ, and all species exhibited some changes in trait morphology but were not related to bottleneck size. Finally, the proportion of deformities (deviations from normal phenotype) was higher in NZ than in UK suggesting passing through a bottleneck increased the probability of abnormalities. Although the associations between FA, body morphology and bottleneck severity are complex, my results confirm that measures of morphology have the potential of being useful indicators of DI in the management of endangered species.

Species translocations result in demographic bottlenecks that may produce inbreeding depression and reduce genetic variation through random sampling and drift, an outcome that could decrease long-term fitness and adaptive potential of many New Zealand species. Despite considerable evidence for costs associated with inbreeding and reduced genetic variation, some species have recovered from a small number of individuals and are thriving, perhaps via high growth rates, differential survival of heterozygous individuals or inbreeding avoidance. I examined the genetic consequences of species translocations in saddlebacks (Philesturnus carunculatus) with additional data provided for robins (Petroica australis) where possible. I first assessed whether contemporary genetic variation represented historical levels or a decline following demographic bottlenecks. I then examined whether sequential demographic bottlenecks caused sequential genetic bottlenecks and reviewed whether populations founded with a small number of birds were likely to go extinct. This analysis was followed by an investigation of two mechanisms that may maintain or reduce fitness costs, differential survival of heterozygous individuals and mate choice to avoid genetically similar individuals. Evidence from museum specimens suggests that low levels of genetic variation in contemporary saddlebacks is no different to historical genetic variation in the only source population, Big South Cape Island. An ancient founding event to Big South Cape Island is probably the cause of severe genetic bottlenecking rather than the demographic bottleneck caused by rats in the 1960s. In robins, genetic variation decreased slightly between museum and contemporary samples suggesting that recent population declines and habitat fragmentation have caused reductions in current levels of genetic variation. Serial demographic bottlenecks caused by sequential translocations of saddlebacks did not appear to decrease genetic variation. Loss of genetic variation due to random sampling was probably minimized because the low level of genetic variation remaining in the species was probably represented in the number of birds translocated to new islands. Models assessing future loss of genetic variation via drift showed that high growth rates combined with high carrying capacity on large islands would probably maintain existing genetic variation. In contrast, low carrying capacity on small islands would probably result in considerable loss of genetic variation over time. Saddleback populations on small islands may require occasional immigrants to maintain long-term genetic variation. Saddleback and robin populations established with a small number of founders did not have an increased risk of failure, suggesting that inbreeding was not substantial enough to prevent populations from growing and recovering. However, modelling showed that translocated saddleback and robin populations grow exponentially even when egg failure rates (a measure of inbreeding depression) are extremely high. Although inbreeding depression may be considerable, populations may be judged healthy simply because they show strong growth rates. Discounting the problem of inbreeding depression may be premature especially under novel circumstances such as environmental change or disease. Finally, two mechanisms proposed to avoid or delay the costs of inbreeding depression and loss of genetic variation do not appear to be important in saddlebacks or robins. Heterozygosity was not related to survivorship in saddlebacks that successfully founded new populations, and neither saddlebacks nor robins chose genetically dissimilar mates to avoid inbreeding. In conclusion, most saddleback populations should not require genetic management, although populations on small islands will probably need occasional immigrants. In robins, large, unfragmented populations should be protected where possible.

The depiction of birds by artists such as Don Binney, Bill Hammond, Michael Parekowhai and Grant Whibley has served as metaphors in the conceptual systems of post-settler New Zealanders’ expression of identity. This project investigated unease in New Zealand post-settler identity and its dislocation from the past by considering works depicting native birds. Is depicting native - rather than introduced birds, an incongruous and romantic settler iconography in identity, leading to a re-telling of our place in this land at the expense of not only the rightful indigenous place of Maori, but of our own cultural becoming? By exploring the painting of birds as metaphors of New Zealand post-settler identity, the project aimed to contribute to the complex issues surrounding the entwined and entangled post-settler relationships of both the past and present. This painting project investigated these issues through the medium of oil paint, culminating in a body of artwork presented in an exhibition with an accompanying exegesis representing 20% of the work.

The choice of a place to live and reproduce is crucial for species� survival in providing them with adequate resources and shelter from predators or climatic conditions. Determining habitat suitability in endangered species is important for the success of translocation as a conservation tool. In addition, understanding mechanisms (source/sink system versus ecological traps) that drive habitat selection in translocated animals may be critical to population viability. In New Zealand, where ecosystems are highly vulnerable to extinction, habitat restoration on predator-free off-shore islands is an important recovery tool. Therefore, there is a need to understand the relationship between the establishment of the translocated populations and the characteristics of their new environment. Previous research indicated that re-introduced populations of Stewart Island robin (Petroica australis rakiura - Toutouwai) and South Island saddleback (Philesternus carunculatus carunculatus - Tieke) on Ulva Island (Stewart Island), New Zealand, showed preferences for coastal habitats that were characterized by low-lying dense vegetation and open ground cover. In this study, we further investigated territorial establishment in these two populations since re-introduction and looked at how birds utilised the landscape. I hypothesised that sites colonised soon after re-introduction were of high quality and later on, birds moved into unsuitable habitats. I defined habitat quality at a micro-scale in terms of vegetation structure, nest characteristics and food availability. I modeled bird presence and nesting success in relation to habitat components to determine factors in the environment that influenced breeding site selection and contributed to successful nesting in these two species. I discussed results in comparison to similar bird-habitat models developed for the South Island saddleback population on Motuara Island (Marlborough Sounds) and examined explanatory variables in each model. Translocated birds in the three studied populations first established territories in coastal scrub, and in the following years moved into larger coastal forest stands. Although vegetation structure was the primary variable explaining site selection in these populations, vegetation composition should still be considered important as it dictated the suitability of nesting substrate and the availability of food items. There was no evidence that first-colonised areas were more suitable habitats, and I concluded that these cases could not be used as examples of ecological traps. Instead, results suggested that with increased density robins and saddlebacks on Ulva have more recently settled in sites less suitable to nesting and foraging, thus underlying a source/sink structure. However, the sparse distribution of food items on Motuara contributed to a lack of territorial behavior and environmental effect on breeding success; therefore a source/sink system could not be confirmed in this population. I recommended that future translocation sites give preference to mixed-size stands with broadleaved species that are characterised by dense canopy below 4 m height and with suitable cavities in live trees. Lastly, due to robins� and saddlebacks� attraction to conspecifics and their territorial behavior, resources evenly distributed across the landscape could also increase their survival and reproductive success.

Includes bibliographical references (leaves 78-94).
Acacia cyclops A. Cunn ex G. Don, or rooikrans (Fabaceae), of Australian origin, is an invasive environmental weed in South Africa where it threatens the unique vegetation of the Cape Floral Kingdom. The invasiveness of the plant in South Africa is, in part, due to its high annual seed-yields, together with the suitable climatic and edaphic factors found here. This study investigated the role of a seed-feeding weevil, Melanterius servulus, which was first introduced into South Africa during 1991, as a biological control agent of A. cyclops seeds, together with the combined effects of indigenous seed-sucking bugs and birds. The weevils utilise filled green pods of A. cyclops for adult feeding, oviposition and larval development and completely destroy seeds during these activities. Examination of the reproductive phenology of A. cyclops showed that seeds are produced annually, during summer and that the presence of filled green pods on the plants coincides with the period of reproductive activity of the weevils. Melanterius servulus populations have established readily at all the original release sites, and within four years of release, damage levels to seeds commonly reached 90%. The weevils have dispersed at an average rate of approximately 2 km per year. Since becoming established in South Africa. the seeds of A. cyclops have been utilised by a variety of generalist vertebrate and invertebrate organisms. Indigenous polyphagous alydid bugs feed on the mature seeds, rendering a portion of the crop inviable when damage levels are high (more than five feeding-punctures per seed). Several bird species are attracted to and feed on the fleshy aril that surrounds theseeds of A. cyclops. Passage of the seeds through the gut of birds enhances germination rates. There is a synergetic relationship between alydids and birds because seeds with low levels of alydid feeding (one to four punctures per seed) germinated more readily after passage through birds than seeds that were not eaten by birds.

Widespread anthropogenic invasions have prompted concerns that naturalized organisms could threaten biodiversity. In particular, invasive weeds can negatively affect native biota through a variety of means, including disrupting mutualisms. This thesis was designed to observe and test dispersal mutualisms in a native forest during autumn when the majority of plant species are fruiting. In this thesis I examined whether the invasive plant barberry (Berberis glaucocarpa) was influencing the behaviour of a native frugivore bellbird (Anthornis melanura) and a range of dispersal related services in a native forest, Kowhai Bush near Kaikoura. To test these 18 banded bellbirds were followed through autumn 2011. These observe bellbirds were split between control and test bird. Barberry fruit was removed from the test bird territories. I recorded whether bellbirds changed their territory sizes, foraging and daily behaviours. During 52 hours of observations, bellbirds were never observed feeding on barberry fruit. No significant changes to bellbird behaviour or territories were observed after the removal of barberry fruit. Bellbird diet overall was dominated by invertebrates (83% of foraging observations), with smaller contributions from fruit (16%, nearly all on Coprosma robusta), nectar and honeydew. Since bellbirds did not eat barberry fruit, removal of this weed is unlikely to negatively affect bellbirds during autumn. Which other bird species were dispersing barberry was recorded. I recorded 242 hours of videotape footage on 24 fruiting plants. A total of 101 foraging events were recorded of 4 different bird species: silvereyes (Zosterops lateralis) 42 visits, blackbirds (Turdus merula) 27 visits, song thrush (Turdus philomelos) 29, and starlings (Sturnus vulgaris) 3 visits. The species differed in the mean length of time they spent in plants, so the overall contribution to barberry fruit removal was 32.6% silvereyes, 24.3% blackbirds, 42.9% song thrush and 0.1% starlings. To find out the relative contribution of exotic and native birds to dispersal of fruits in Kowhai Bush, I mist-netted 221 birds of 10 species and identified any seeds in the 183 faeces they deposited. A total of 21 plant species were observed fruiting in Kowhai Bush during this time. A total of 11 different plant species were identified from 1092 seeds. Birds were further observed feeding on 3 other plant species which were not observed in faecal samples. This left 7 plants with unobserved dispersal vectors. There were likely four main dispersers, bellbirds, silvereyes, song thrush and blackbirds and five minor, brown creeper (Mohoua novaeseelandiae), tui (Prosthemadera novaeseelandiae), fantails (Rhipidura fuliginosa), dunnock (Prunella modularis) and starlings. However there was considerable variability between these bird species dispersal abilities. Introduced birds’ song thrush and blackbirds were observed dispersing naturalized plant seeds at higher than expected rates in comparison to native frugivores bellbirds and silvereyes. I also measured the gape sizes on mist netted birds and on samples of fruit from Kowhai Bush. Both silvereyes and bellbirds were found to be eating fruit larger than their gape, but despite this two native (Hedycarya arborea and Ripogonum scandens) and three exotic plants (Vitis vinifera, Taxus baccata and Crataegus monogyna) had large fruit that were probably mainly dispersed by song thrush and blackbirds. Hence, introduced birds were important seed dispersers for large fleshy fruited seeds in Kowhai Bush. Demonstrating that interactions among native and exotic flesh fruited plants and frugivores is important within forest communities.

The aim of the thesis was to document patterns in breeding and migration in Swedish Canada geese Branta canadensis, to explain these against the genetic and historical background of the population, and to test predictions of hypotheses pertaining to parental investment. The Canada goose population in Sweden was founded by the introduction of a few individuals in the 1930's. DNA fingerprint similarity between geese breeding in Sweden was on average at the same level as between inbred close relatives in other wild bird species. The genetic variability of the population appeared to be considerably reduced in comparison to that of Canada geese breeding in North America. Dispersal and migration patterns were studied using plastic neck-bands that could be identified at long distance. Most Canada goose females nested at the lake where they grew up. Males were more prone to disperse than females, although most of them still returned to breed close to their area of origin. Geese from three breeding areas in Sweden had different winter distributions, although wintering areas overlapped considerably. Individual geese tended to return to the same wintering area as they had used in previous years. The females' investment in the egg clutch was related to the migration distance from spring foraging areas to the nesting area, suggesting an energetic cost of migration for egg production. Within breeding seasons, clutch size decreased with later initiation of nesting, but only in years with early breeding. A probable reason for this decrease was that body reserves available for egg production were larger in early layers. In years with late breeding, clutch size did not decrease, most likely because late-nesting females could supplement their body reserves by foraging on fresh vegetation. Nest defence intensity was studied by recording the behaviour of the female geese when a human approached the nest. The results largely confirmed predictions for nest defence intensity extracted from parental investment theory.

Diss. (sammanfattning) Umeå : Umeå universitet, 1993, härtill 6 uppsatser

digitalisering@umu

The song thrush (Turdus philomelos) was introduced to New Zealand from Britain during the mid 19th century and has become one of the most common terrestrial bird species in New Zealand. In this study, I surveyed a range of life-history traits in New Zealand song thrushes for comparison with traits of British thrushes. Clutch size, egg size and nest size have decreased, while the nestling period is shorter and the incubation period longer. This combination of changes suggests birds are investing less energy into each reproductive bout. Birds also appear unable to raise large broods, as nestling starvation is common in New Zealand, which suggests that food is limiting. I experimentally tested the ability of song thrushes to incubate enlarged clutches and broods, but productivity was not higher for enlarged broods and natural 3- and 4-egg clutches produced similar numbers of fledglings. Thus reduced clutch size may be an adaptation to the local environment. Differences in female incubation behaviour, with 3- and 4-egg clutches receiving higher levels of incubation and more visits per hour than 5-egg clutches, also suggest New Zealand thrushes have difficulty coping with clutches as large as those in Britain. The decrease in clutch size between New Zealand and Britain is in the direction and magnitude expected based on the change in latitude, which supports the hypothesis that factors affecting foraging time and food availability, such as daylength, temperature and rainfall, may be selecting for smaller clutches. Egg size was also found to have decreased in New Zealand, though this may be the result of smaller adult size. Hatchling mass was related to egg volume, but I found no effect of egg volume or clutch size on hatching success. However, nests containing more pointed eggs (i.e., abnormally-shaped eggs), had lower survival and hatching/fledgling success. Data from the national nest record database and my study both suggest that differences in song thrush productivity are the result of differential survival of nestlings. Nestling mortality due to starvation was common at Kowhai Bush, but rare in Britain, so either adult condition or food availability may be lowering reproductive success in New Zealand. High rates of nest failure (>65%) could also affect clutch size, but the strong directional selection imposed by food limitation during the nestling period suggests that increases in food supply would result in increased reproductive success even with the same levels of nest failure. When comparing clutch size throughout New Zealand, I found a significant, positive relationship with rainfall, which further suggests that food limitation may be the main factor driving changes in life-history traits of song thrushes in New Zealand.

Tese de mestrado, Biologia da Conservação, Universidade de Lisboa, Faculdade de Ciências, 2016
Com a população mundial a aumentar, os impactos antropogénicos nos ecossistemas naturais vão sendo cada vez mais evidentes. Assistimos actualmente a uma elevada taxa de extinção de espécies, e com elas se perdem as funções e os serviços ecossistémicos que prestam, deixando os ecossistemas mais vulneráveis às pressões humanas. Uma dessas funções, essencial à manutenção da dinâmica florestal, é a dispersão de sementes, que nos trópicos é feita essencialmente através de relações mutualistas entre plantas e animais. A eficácia da dispersão pode variar substancialmente em função da espécie dispersora e depende não só da quantidade de sementes que os animais transportam, mas também da qualidade do tratamento que estes proporcionam às sementes. A perda de um dispersor chave num ecossistema pode portanto comprometer a viabilidade das populações de plantas e alterar a dinâmica da vegetação. Os efeitos da perda da biodiversidade são particularmente graves nas ilhas, onde se concentra grande parte das espécies, sobretudo endémicas. Aqui, as espécies evoluíram em isolamento, existindo naturalmente uma menor diversidade de espécies e possivelmente uma menor redundância funcional entre espécies. Estas características fazem com que estes ecossistemas sejam mais frágeis, e é nas ilhas que se tem verificado a maioria das extinções de espécies. Uma das principais ameaças identificadas é a introdução de espécies invasoras, que competem com as espécies nativas, acabando por as excluír. São Tomé é uma pequena ilha oceânica de clima tropical, situada no sistema insular do Golfo da Guiné. Com uma das maiores concentrações de endemismos de todo o mundo, São Tomé está a sofrer as consequências dos impactos humanos, estando a sua biodiversidade bastante ameaçada. Este trabalho tem então como objectivo perceber de que forma as aves estão a contribuir para a dispersão de sementes, uma função ecossistémica essencial para as florestas ricas em endemismos de São Tomé. Para tal, foram definidos três objectivos específicos: (1) avaliar a rede de dispersão de sementes por aves do sub-bosque em floresta de montanha, através da análise dos excrementos de aves capturadas com redes verticais; (2) avaliar a contribuição específica dos pombos como potenciais dispersores de sementes em São Tomé, através da análise de conteúdos de papos de pombos caçados; (3) caracterizar a dispersão de sementes na floresta de montanha, unindo os dados provenientes dos pontos anteriores, ponderados com a abundância relativa das aves. As redes de dispersão de sementes foram construídas analisando as matrizes de interação com o ‘package’ bipartite do programa R. A frequência de interação foi quantificada como o número de excrementos ou papos de cada espécie de ave contendo pelo menos uma semente intacta de cada espécie de planta. Foram capturadas 15 espécies de aves com redes verticais no sub-bosque em floresta de montanha, todas elas endémicas. Confirmámos dispersão de sementes por seis destas espécies, de destacar a contribuição do Olho-grosso (Speirops lugubris), responsável por 88% da frequência de interação. As espécies de plantas nativas dominam esta rede (14 espécies, 70% das espécies identificadas), incluindo algumas espécies endémicas ameaçadas, enquanto que poucas são as espécies introduzidas incorporadas na rede de dispersão (seis espécies, 20% das espécies identificadas). Através de curvas de acumulação de espécies, é possível afirmar que o esforço de captura foi bastante elevado, tendo-se conseguido capturar 71% das espécies de aves que ocorrem na área de estudo e 65% das plantas a serem dispersas. A comparação entre as redes de dispersão de sementes da floresta primária (obô) e da floresta secundária (capoeira) revelou poucas diferenças, o que pode estar relacionado com a proximidade geográfica das áreas de amostragem. Estes resultados sugerem ainda que os dispersores se movem entre os dois tipos de floresta, contribuindo para a disseminação das sementes entre habitats, o que vai facilitar tanto a invasão por espécies introduzidas, como a regeneração de espécies nativas em zonas de floresta degradada. A análise da rede de dispersão de sementes por pombos em São Tomé permitiu verificar a importância das três espécies endémicas caçadas como potenciais dispersores de sementes. Em especial a Rola (Columba malherbii) e o Cécia (Treron sanctithomae), tanto por serem as mais abundantes, como por serem as que mais frequentemente continham sementes no papo. Esta rede é dominada por sementes nativas (53 % da frequência de ocorrência) e apenas uma pequena percentagem de introduzidas (21% da frequência de ocorrência), sendo que uma destas (Cecropia peltata) está incluída na lista das 100 piores espécies invasoras do mundo. Já o Pombo-do-mato (Columba thomensis) é uma espécie mais rara, tendo a sua população vindo a decrescer maioritariamente devido à pressão de caça. É necessária alguma cautela ao analisar estes resultados, uma vez que as sementes nos papos dos pombos teriam ainda de passar por todo o processo de digestão, podendo levar à sua destruição antes de serem depositadas. No entanto, mesmo que poucas sementes resistam à passagem pelo trato digestivo da ave, a probabilidade destas serem dispersas a longa distância é elevada, dada a grande capacidade de voo dos pombos quando comparada com as das restantes aves existentes na ilha. A dispersão de sementes nas florestas de montanha de São Tomé é suportada exclusivamente por aves endémicas, um resultado surpreendente mesmo no contexto de ilhas oceânicas. A grande importância do Olho-grosso como dispersor de sementes resulta não só da frequência com que as dispersa, mas também da sua elevada abundância nestas florestas. Apesar do método utilizado ser dirigido a aves do sub-bosque, foi possível capturar também alguns indivíduos de espécies mais associadas à copa das árvores, como o Neto-de-olho-grosso (Zosterops feae). Foi também possível comprovar que o Pardal (Serinus rufobrunneus) é sobretudo um predador de sementes, já que apenas uma reduzida proporção dos dejectos continha sementes intactas. Os pombos parecem ser quantitativamente pouco relevantes, sobretudo devido à sua abundância reduzida, mas podem constituir importantes dispersores a longa distância e de sementes de maiores dimensões. Foi ainda detetada alguma complementariedade entre os vários dispersores, já que apenas o Tordo (Turdus olivaceofuscus) e o Papafigos (Oriolus crassirostris) foram encontrados a dispersar as sementes maiores (8-10mm). Estes dois dispersores, de maiores dimensões, possuem uma garganta mais larga que lhes permite engolir sementes maiores. A elevada ‘species strength’ destes dispersores reflete isso mesmo: uma grande dependência deles por parte das plantas com sementes maiores, já que estas aves são as únicas capazes de dispersar as suas sementes. O Tchin-tchin-txoló (Ploceus sanctithomae), a Camussela (Ploceus grandis) e o Pastro (Onycognathus fulgidus) já foram observados a consumir frutos e sementes, mas permanecem indicados apenas como potenciais dispersores, pois não foi possível comprovar que desempenhem esta função. Dos nove dispersores de sementes encontrados neste estudo, seis estão ameaçados de extinção, segundo os critérios da IUCN. Este é um facto preocupante, uma vez que a extinção de qualquer um destes dispersores irá ter um impacto negativo na dispersão de sementes, com consequências desconhecidas para a estabilidade do ecossistema. Apesar da importância da dispersão de sementes para a dinâmica florestal, muito pouco se sabe sobre esta função do ecossistema em São Tomé. Este é um trabalho pioneiro nesta área, na medida em que foram dados os primeiros passos para compreender como funciona esta importante função do ecossistema nesta ilha. No entanto ainda há muito por perceber. É preciso uniformizar a metodologia de amostragem e aplicá-la a todos os dispersores e também potenciais dispersores, estendendo a amostragem no espaço e no tempo. É ainda necessário compreender a qualidade do tratamento que cada dispersor dá a cada espécie de planta, inibindo ou aumentando a sua taxa e velocidade de germinação. Para além das aves, focadas neste trabalho, outros grupos de animais terão certamente um papel relevante na dispersão de sementes, nomeadamente os invertebrados, os morcegos frugívoros e alguns dos mamíferos introduzidos, como a Lagaia (Civettictis civetta) e o Macaco (Cercopithecus mona). É também necessário avaliar até que ponto as espécies de plantas introduzidas se comportam como invasoras e de que forma competem com as nativas, diminuindo as suas probabilidades de recrutamento. Finalmente, o transporte das sementes, mesmo que viáveis, não se traduz necessariamente no recrutamento de novos indivíduos: a qualidade do local de deposição é um factor crucial no contributo das aves para a dinâmica populacional dessa planta, e no conjunto, para a dinâmica florestal. Em resumo, esta tese fornece a primeira caracterização da rede de dispersão de sementes de São Tomé, abrindo as portas para um conhecimento mais detalhado e abrangente das interacções biológicas que sustêm as comunidades biológicas nestes ecossistemas.
The island of São Tomé, in the Gulf of Guinea, is a biodiversity hotspot with a remarkable number of endemic species and unique forest ecosystems. Much of its biodiversity is currently threatened by the increasing human population and associated habitat change, which is simultaneously threatening ecosystem functions and services. Seed dispersal is one of such key ecosystem services, essential for forest regeneration. The goal of this work is to understand how forest birds are contributing to the dispersal of both native and introduced seeds in São Tomé, providing the first community-level assessment of species interactions in maintaining ecosystem functions in the island. We collected droppings from mist netted birds and analysed the crop contents of hunted pigeons. All 18 sampled bird species were endemic and nine of them were dispersing seeds. The São Tomé Speirops played a central role as seed disperser. The seed dispersal network is dominated by native plant species, with only a few introduced species being dispersed in the forest. We corrected the frequency of occurrence of seeds in the droppings with independent estimates of bird abundance, and showed that this correction is key to understand disperser contribution to the overall network. Birds are therefore playing a double role, facilitating the dispersal of introduced species but also spreading the native flora. These results highlight a difficult conservation dilemma, since birds might be simultaneously contributing to forest regeneration and to biological invasion.