Cat colonies reshape the abundance and body size of lizards.

Abstract

The cat (Felis catus) is one of the worst invasive species, with well-documented impacts on biodiversity, especially on islands. Despite this, local administrations commonly implement trap-neuter-return (TNR) programmes to manage free-roaming cat populations. Here, we assess the effects of cat colonies on the endemic Tenerife lizard (Gallotia galloti). We used fruit-baited pitfall traps placed near and away from cat colonies to estimate lizard abundance and collect biometric data. Lizard abundance was lower near cat colonies compared with nearby control areas. Moreover, the individuals captured close to the colonies were smaller. Measurements of lizard carcasses found near colonies indicate that cats preferentially prey on larger individuals, explaining both their reduced abundance and their smaller body size. Interestingly, despite their smaller size, lizards near cat colonies presented higher scaled mass index values. Stable isotope analyses (δ13C and δ15N) of faecal samples suggest that these lizards partially rely on dried cat food available within colonies. Overall, cat colonies significantly altered lizard abundance, size structure and body condition. Given the ecological importance of G. galloti as both a seed disperser and prey, our findings highlight the need to limit the spread of free-roaming cat colonies, particularly in protected areas, to mitigate the effects on native biodiversity.