cases as well as a sensitivity analysis are presented. The model is found to predict the correct order of magnitude of the urban temperature excess. The heat island effect is found to be the net result of several competing physical processes. In general, reduced evaporation in the city center and the thermal properties of the city building and paving materials are the dominant parameters. It is suggested that such a model could be used in engineering calculations to improve the climate of existing and future cities. Impact on animals Ant colonies in urban heat islands have an increased heat tolerance at no cost to cold tolerance. Species that are good at colonizing can utilize conditions provided by urban heat islands to thrive in regions outside of their normal range. Examples of this include the grey-headed flying fox (Pteropus poliocephalus) and the common house gecko (Hemidactylus frenatus). Grey-headed flying foxes, found in Melbourne, Australia, colonized urban habitats following the increase in temperatures there. Increased temperatures, causing warmer winter conditions, made the city more similar in climate to the more northerly wildland habitat of the species. With attempts to mitigate and manage urban heat islands, temperature changes and the availability of food and water are reduced. With temperate climates, urban heat islands will extend the growing season, therefore altering breeding strategies of inhabiting species. This can be best observed in the effects that urban heat islands have on water temperature. With the temperature of the nearby buildings sometimes reaching an over difference from the near-surface air temperature, precipitation will warm rapidly, causing run-off into nearby streams, lakes and rivers (or other bodies of water) to provide excessive thermal pollution. The increase in thermal pollution has the potential to increase water temperature by . This increase will cause the fish species inhabiting the body of water to undergo thermal stress and shock due to the rapid change in temperature of their habitat. Urban heat islands caused by cities have altered the natural selection process. Selective pressures like temporal variation in food, predation and water are relaxed causing a new set of selective forces to roll out. For example, within urban habitats, insects are more abundant than in rural areas. Insects are ectotherms. This means that they depend on the temperature of the environment to control their body temperature, making the warmer climates of the city perfect for their ability to thrive. A study