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Humanização da paisagem e moluscos terrestres: efeitos opostos sobre comunidades de espécies endémicas e não endémicas na ilha de São Tomé
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As atividades humanas são responsáveis pela atual crise de biodiversidade, a que está associada a extinção de espécies a um ritmo cada vez mais acelerado. As alterações do uso do solo estão entre as principais ameaças que têm levado ao severo declínio da biodiversidade que se tem acentuado nas últimas décadas. Os moluscos terrestres são um grupo muito diverso e que desempenha importantes funções nos ecossistemas. No entanto, também são muito sensíveis a alterações ambientais, pelo que têm o maior número de extinções antropogénicas registadas no reino animal e têm servido como modelo de estudo para compreender o impacto das atividades humanas na biodiversidade. Este trabalho teve como principal objetivo compreender os impactos das alterações do uso do solo na comunidade de moluscos terrestres da ilha de São Tomé. Especificamente foi avaliado: (1) como é que a riqueza e abundância de moluscos terrestres é afetada pela alteração do uso do solo; (2) quais as variáveis ambientais locais que melhor explicam a riqueza e abundância, e (3) quais as diferenças na composição e estrutura das comunidades entre os diferentes tipos de uso do solo. Foram amostrados 132 pontos, espalhados por quatro tipos de uso do solo: floresta nativa, floresta secundária, plantações de sombra e zonas não florestadas. No total foram observadas 33 espécies de moluscos terrestres, das quais 20 são endémicas. Ao nível dos pontos de amostragem, a riqueza e abundância foram maiores nos tipos de uso do solo com níveis intermédios de perturbação antropogénica: floresta secundária e plantação de sombra. No entanto, a elevada riqueza da floresta secundária é explicada pela elevada proporção de espécies endémicas, enquanto que nas plantações de sombra dominam as espécies não endémicas. A intensificação do uso do solo levou a uma diminuição da riqueza e abundância das espécies endémicas e a um aumento das espécies não endémicas. As espécies endémicas estão em geral associadas a ambientes florestados, preferindo locais com uso do solo menos intenso, bem como menor temperatura do solo, maior precipitação e mais manta morta. Estas condições aumentam os níveis de humidade, impedem a secação e proporcionam alimento e abrigo mais diversificado. As espécies endémicas também se encontraram associadas à vegetação típica de zonas com degradação intermédia, sugerindo que baixos níveis de perturbação podem favorecer algumas destas espécies. As espécies não endémicas foram raras na floresta nativa, mostrando uma forte associação a zonas não florestais e a vegetação típica de zonas mais degradadas, possivelmente devido a muitas delas serem espécies introduzidas adaptadas a ambientes antropogénicos. A estrutura das comunidades de moluscos terrestres na floresta nativa foi diferente da dos restantes tipos de uso do solo, sendo apenas diferente da das plantações de sombra quando se analisaram as comunidades de espécies endémicas e não endémicas separadamente. Estas diferenças devem-se por um lado à presença de diferentes espécies endémicas nos dois habitats e, por outro a muitas espécies não endémicas estarem restritas às plantações de sombra. Para compreender estas diferenças seria importante que estudos futuros se foquem na ecologia de cada uma das espécies de moluscos terrestres que ocorrem em São Tomé. Estes resultados reforçam a importância das alterações do uso do solo e das espécies introduzidas como fatores de ameaça para a biodiversidade única de São Tomé, apelando à urgência de adotar práticas agroflorestais e de gestão territorial que se coadunem com a sua conservação. Este é o primeiro estudo que avalia de forma sistemática o impacto das atividades humanas num grupo de invertebrados nas ilhas oceânicas do Golfo da Guiné.
Human activities are responsible for the current biodiversity crisis, which is associated to the increasingly accelerated rate of species extinction. Land use changes are among the main threats that have led to the severe decline in biodiversity that has steepen in recent decades. Land snails are a very diverse group that plays important roles in ecosystems. However, they are also very sensitive to environmental changes, thus have the highest number of anthropogenic extinctions recorded for the animal kingdom, and have served as models to understand the impact of human activities on biodiversity. The main objective of this work was to understand the impacts of land use changes in the land snail assemblage of São Tomé Island. Specifically, it was evaluated: (1) how the richness and abundance of land snails is affected by land use change; (2) which local environmental variables best explain richness and abundance, and (3) what are the differences in assemblage composition and structure between different land use types. We sampled land snail assemblages in 132 sampling points, stratified across four land-uses types: native forest, secondary forest, shade plantation and non-forested areas. 33 land snail species were observed, of which 20 are endemic. At the level of sampling points, richness and abundance were higher in land use types with intermediate levels of anthropogenic disturbance: secondary forest and shade plantation. However, the high richness of secondary forest is explained by the high proportion of endemic species, while non endemic species are dominant in shade plantations. Land use intensification led to a decrease in the richness and abundance of endemic species and to an increase of non-endemic species. Endemic species are generally associated with forested environments, preferring sites in less intensive land uses, as well as lower soil temperature, higher precipitation and more soil litter. These conditions increase humidity levels, prevent dehydration, and provide diversified food and shelter. Endemic species were also associated to vegetation typical from areas with intermediate degradation, suggesting that low levels of disturbance may favor some of these species. Non-endemic species were rare in native forest, showing a strong association with non-forest areas, and to vegetation typical from more degraded areas, possibly due to many of them being introduced species adapted to anthropogenic environments. The structure of land snail assemblages in native forest was different from that of other land use types, being distinct solely from that of shade plantations when endemic and non-endemic species assemblages were analyzed separately. These differences are due to the presence of different endemic species in the two habitats, and also to many non-endemic species being restricted to plantations. To understand these differences, it would be important that future studies focus on the ecology of each land snail species that occurs in São Tomé. The results reinforce the importance of land use changes in land use and introduced species as threats to the unique biodiversity of São Tomé, appealing to the urgent need to adopt agroforestry and land management practices that are in line with conservation. This is the first study doing a systematic assessment of the impact of human activities on a group of invertebrates in the oceanic islands of the Gulf of Guinea
Human activities are responsible for the current biodiversity crisis, which is associated to the increasingly accelerated rate of species extinction. Land use changes are among the main threats that have led to the severe decline in biodiversity that has steepen in recent decades. Land snails are a very diverse group that plays important roles in ecosystems. However, they are also very sensitive to environmental changes, thus have the highest number of anthropogenic extinctions recorded for the animal kingdom, and have served as models to understand the impact of human activities on biodiversity. The main objective of this work was to understand the impacts of land use changes in the land snail assemblage of São Tomé Island. Specifically, it was evaluated: (1) how the richness and abundance of land snails is affected by land use change; (2) which local environmental variables best explain richness and abundance, and (3) what are the differences in assemblage composition and structure between different land use types. We sampled land snail assemblages in 132 sampling points, stratified across four land-uses types: native forest, secondary forest, shade plantation and non-forested areas. 33 land snail species were observed, of which 20 are endemic. At the level of sampling points, richness and abundance were higher in land use types with intermediate levels of anthropogenic disturbance: secondary forest and shade plantation. However, the high richness of secondary forest is explained by the high proportion of endemic species, while non endemic species are dominant in shade plantations. Land use intensification led to a decrease in the richness and abundance of endemic species and to an increase of non-endemic species. Endemic species are generally associated with forested environments, preferring sites in less intensive land uses, as well as lower soil temperature, higher precipitation and more soil litter. These conditions increase humidity levels, prevent dehydration, and provide diversified food and shelter. Endemic species were also associated to vegetation typical from areas with intermediate degradation, suggesting that low levels of disturbance may favor some of these species. Non-endemic species were rare in native forest, showing a strong association with non-forest areas, and to vegetation typical from more degraded areas, possibly due to many of them being introduced species adapted to anthropogenic environments. The structure of land snail assemblages in native forest was different from that of other land use types, being distinct solely from that of shade plantations when endemic and non-endemic species assemblages were analyzed separately. These differences are due to the presence of different endemic species in the two habitats, and also to many non-endemic species being restricted to plantations. To understand these differences, it would be important that future studies focus on the ecology of each land snail species that occurs in São Tomé. The results reinforce the importance of land use changes in land use and introduced species as threats to the unique biodiversity of São Tomé, appealing to the urgent need to adopt agroforestry and land management practices that are in line with conservation. This is the first study doing a systematic assessment of the impact of human activities on a group of invertebrates in the oceanic islands of the Gulf of Guinea
Descrição
Tese de mestrado em Biologia da Conservação, Universidade de Lisboa, Faculdade de Ciências, 2021
Palavras-chave
Abundância Análise de comunidades Espécies endémicas Gastrópodes terrestres Riqueza específica Teses de mestrado - 2021