In Part 1 of this series, I showed how difficult it was for frogs to cross busy roads without getting run over. The second reason that frogs and roads don’t mix flows on from the first one – if frogs can’t cross roads safely, then they can’t move safely between different areas of habitat that are separated by roads. This leads to something ecologists call habitat isolation.
Habitat isolation – what’s so interesting about that?
Habitat isolation causes a number of problems for frogs and other living things. Here are three of them:
1. A group of frogs (a population) living in an isolated area of habitat can be separated from other populations of the same species. This means that friends from elsewhere rarely – if ever – come to visit. Over time this can lead to a loss of genetic diversity in the population, and something called inbreeding depression. Basically, without new frogs arriving in the population, the frogs that remain become too closely related to each other. This can reduce their fitness, making them less likely to survive and reproduce.
2. If a population living in an isolated habitat dies out, no new animals can move in to start a new one. So we are left with suitable – but unoccupied – habitat. This is a common problem in urban environments for small animals that move along the ground (such as frogs, reptiles and mammals), because cities often contain small patches of habitat for wildlife that are separated by busy roads. Animals that can fly (such as birds, bats and some insects) can cross busy roads more easily. In a study of pond-breeding frogs in Melbourne, Australia (Parris 2006), I found that isolation by roads reduced the number of species of frogs that lived at a pond. Statistical modelling (a nifty method of analysing field data) predicted that the most-isolated pond (that is, the one surrounded by the most roads) would have only about 1/5 of the species as the least-isolated pond.
3. Some animals (such as the growling grass frog Litoria raniformis) need a network of populations in a network of habitat patches to survive in the long term – this is known as a metapopulation (Heard et al. 2013). Different patches in the network are occupied at different times. For example, some patches might be particularly good for frogs in times of drought, and others might be occupied only when conditions are very wet. If too many habitat patches in the network become isolated by roads, animals can’t move freely between them when they need to and the metapopulation doesn’t work properly. In some cases, metapopulations collapse and the all the populations within them die out.
What can we do?
Keeping connections between habitat patches (such as habitat corridors) will allow frogs to keep moving between them when they need to do so. This is particularly important in places where new housing and industrial developments are being built – it’s much easier to keep existing connections in the landscape than to put them back later!
Creating new habitat corridors and building road overpasses and underpasses may also help frogs and other animals move around urban landscapes, but these measures can be very expensive. Also, a recent study of some Australian frogs found that they weren’t very keen to use underpasses (Hamer et al. 2014).
Where some habitat patches are going to become isolated from others in a network, it may be possible to create new wetlands to make up for the ones that the frogs will no longer be able to reach safely.