Helping frogs in cities

Posted on July 25, 2012

It’s been estimated that the loss and degradation of aquatic habitats caused by urbanisation threatens around 40% of amphibian species worldwide.  How significant are the impacts of urbanisation at different spatial scales?  For example,  compared with changes to the landscape as a whole, how important are the local effects of urban development on the habitat and water quality of particular ponds?  To explore these issues, surveys of frog tadpoles in 65 ponds in Greater Melbourne,  Australia, were carried out over two breeding seasons.  Surveys were based on trapping, netting and visual observation, and data on a range of environmental variables were collected.  Regression modelling was used to assess the effects of landscape and local habitat variables on the numbers of frog species.  Of the six species found, one (Litoria sp.) occurred in 40% of ponds, four occurred in 6-18% of ponds, and one (Geocrinia victoriana) occurred in a single pond.  One landscape indicator, the proportion of green open space within a 1000-m radius of a pond, had an overwhelmingly positive influence on frog species, probably because of its beneficial associations with habitat connectivity and the dispersal, foraging and shelter of amphibian larvae.  The amount of aquatic vegetation in a pond also had a beneficial, though more modest, effect.  Human population density and water conductivity both showed strong negative relationships with the number of frog species, while weaker negative effects  linked to shading and the density of predatory fish (mainly Gambusia) could be observed.  Based on these findings, urban frog communities should benefit from management measures that include (a) city planning designs that minimise shading by tall buildings; (b) the use of stormwater retention ponds to protect water quality and limit conductivity; and (c) the control of predatory fish – for example, by draining ponds outside the frog breeding season.

Reference:  Hamer, A.J. & Parris, K.M.  2011.  Local and landscape determinants of amphibian communities in urban ponds.  Ecological Applications 21(2), 378–390.