Food availability and population parameters for squirrels differ even in neighbouring urban parks

Abstract

Some species occur in higher densities in cities than in natural habitats, despite the variety of urban stressors. Urban fragments can be extremely heterogeneous though, and species’ responses might vary between urban patches. However, only few studies examine mammalian populations from different fragments in the core of cities, where stressors are at their maximum. The Eurasian red squirrel (Sciurus vulgaris) is highly abundant in urban green spaces and readily makes use of supplemental food sources. We examined natural and supplemental food sources as well as squirrel populations in three small parks in an urban core. We found pronounced differences in natural and supplemental food availability between the parks. Squirrel population densities were generally higher at all three urban sites when compared to a geographically close located natural forest, but with variation between the parks. The squirrel density in the smallest park in the summer season was 42 times higher (index of minimum number alive: 8.8 individuals/ha) than in the forest, whereas it was only 20 or respectively 16 times higher in the two other parks. Individuals from the park with the highest population density had access to the highest amount of supplemental food sources. However, side effects from non-natural food items, e.g., related to nutrition, also need to be taken into consideration and might counteract the advantages of supplemental food sources. This study shows that urban effects can vary even at the smallest scale between nearby urban habitat patches and this needs to be considered when drawing conclusions about the effects of urbanisation and deciding about conservation actions.