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O Gato-Feral na Ilha Deserta (Ria Formosa) : população, adaptações ecológicas e impacto nos vertebrados nativos
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Os gatos-ferais são um dos predadores invasores mais comuns em ilhas, causando fortes impactos
negativos sobre a vida selvagem nativa. O primeiro passo para interpretar e quantificar estes impactos é
realizar estudos sobre sua dieta, sobre os fatores que condicionam os seus efetivos e que testem o seu
efeito nas populações de presas nativas. O presente trabalho realizado na ilha Deserta (Parque Natural
da Ria Formosa), pretende explorar o efeito da predação dos gatos-ferais em espécies nativas insulares,
e analisar os determinantes da ocorrência de gatos-ferais e de uma das suas presas - roedores. Para tal,
foram capturados roedores em vários pontos de armadilhagem e testados os fatores que determinam a
sua presença e abundância: características do habitat (p.e. vegetação), o grau de perturbação (p.e.
proximidade ao restaurante) e de predação por gatos ferais (número e RAI - Índice de Abundância
Relativa). A amostragem dos gatos-ferais foi efetuada com recurso a armadilhagem fotográfica para
determinar a sua distribuição, abundância relativa e número. Testou-se, ainda, o efeito das características
do habitat (p.e. proximidade a zonas de abrigo), e da disponibilidade de alimento (p.e. RAI de
passeriformes e roedores) no número de gatos-ferais. A dieta foi determinada através da análise de
dejetos recolhidos em transetos pedestres. Durante a amostragem foram apenas capturados Mus spretus,
cuja presença na ilha Deserta é principalmente determinada pela percentagem de cobertura arbustiva e
pela distância à ria e ao mar. A dieta dos gatos-ferais é constituída principalmente por roedores, aves,
répteis, e alimentos de origem antrópica. O número de gatos-ferais é positivamente influenciado pela
abundância relativa de presas, nomeadamente passeriformes, e pela proximidade à ria, a zonas de abrigo
e ao restaurante. Podemos também supor a existência de um efeito positivo entre a abundância de
roedores e o número de gatos. Este estudo forneceu novos dados sobre os determinantes de ocorrência
de Mus spretus (i.e. importância do substrato arbustivo) e os dados da dieta dos gatos-ferais parecem
indicar que este felino pode ter impactos negativos nas populações nativas, particularmente de aves
marinhas vulneráveis. As informações da dieta e do uso do habitat dos gatos-ferais contribuem para o
desenvolvimento de programas de monitorização e conservação mais eficazes.
Feral cats are one of the most common invasive predators on islands, causing strong negative impacts on native wildlife. The first step to understand and quantify these impacts is to conduct studies on their diet, the factors that constrain feral cats’ numbers, and test their effect on native prey populations. This work was carried out in Deserta island (Ria Formosa Natural Park) and aims to explore the effect of feral cat predation on native island species, and to analyze the factors that influence the space use of the feral cat population and of one of its main preys - rodents. To achieve this, rodents were captured at several trapping points and we estimated their presence and abundance, and tested the drivers of these metrics: the characteristics of the habitat (e.g., vegetation), the disturbance (e.g., distance to the restaurant), and predation levels (number and RAI (Relative Abundance Index) of cats). For feral cats’ sampling, camera trapping was used to determine their distribution, numbers, and relative abundance. We tested the effect of the habitat characteristics (e.g., distance to shelter areas) and food availability (e.g., RAI of passerines and rodents), on cat’s numbers. Cats’ diet was determined by analysis of scats collected along pedestrian transects. During the rodent sampling, only Mus spretus was captured, and their presence on Deserta island is mainly determined by habitat drivers, in particular, is positively influenced by shrub cover and distance to the lagoon and the sea. The diet of island feral cats consisted mainly of rodents, birds, reptiles, and food of anthropogenic origin. Cats’ population size is positively influenced by the relative abundance of prey, in particular passerines, and the proximity to the lagoon, shelter areas and to the restaurant. We can also assume the existence of a positive effect of rodent abundance on cat numbers. This study provided new data on Mus spretus occurrence drivers (i.e. importance of shrub coverage), and the feral cat’s diet seem to indicate the existence of a negative impacts on native populations, particularly vulnerable seabirds. Information on the diet and habitat use by feral cats contributes to the development of more effective monitoring and conservation programs.
Feral cats are one of the most common invasive predators on islands, causing strong negative impacts on native wildlife. The first step to understand and quantify these impacts is to conduct studies on their diet, the factors that constrain feral cats’ numbers, and test their effect on native prey populations. This work was carried out in Deserta island (Ria Formosa Natural Park) and aims to explore the effect of feral cat predation on native island species, and to analyze the factors that influence the space use of the feral cat population and of one of its main preys - rodents. To achieve this, rodents were captured at several trapping points and we estimated their presence and abundance, and tested the drivers of these metrics: the characteristics of the habitat (e.g., vegetation), the disturbance (e.g., distance to the restaurant), and predation levels (number and RAI (Relative Abundance Index) of cats). For feral cats’ sampling, camera trapping was used to determine their distribution, numbers, and relative abundance. We tested the effect of the habitat characteristics (e.g., distance to shelter areas) and food availability (e.g., RAI of passerines and rodents), on cat’s numbers. Cats’ diet was determined by analysis of scats collected along pedestrian transects. During the rodent sampling, only Mus spretus was captured, and their presence on Deserta island is mainly determined by habitat drivers, in particular, is positively influenced by shrub cover and distance to the lagoon and the sea. The diet of island feral cats consisted mainly of rodents, birds, reptiles, and food of anthropogenic origin. Cats’ population size is positively influenced by the relative abundance of prey, in particular passerines, and the proximity to the lagoon, shelter areas and to the restaurant. We can also assume the existence of a positive effect of rodent abundance on cat numbers. This study provided new data on Mus spretus occurrence drivers (i.e. importance of shrub coverage), and the feral cat’s diet seem to indicate the existence of a negative impacts on native populations, particularly vulnerable seabirds. Information on the diet and habitat use by feral cats contributes to the development of more effective monitoring and conservation programs.
Descrição
Tese de mestrado, Biologia da Conservação , 2022, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Espécies invasoras Ecossistemas insulares Felis catus Mus spretus Cadeias alimentares Teses de mestrado - 2022