Amphibians can be found in a wide variety of different aquatic habitats. Species that are known to breed in specific types of pond may still be found in other, perhaps very different, waterbodies. All waterbodies within which an amphibian has been recorded can be described as occupied. I have used occupied as the defining term for the recent series of blog entries that examine the impacts of fish, newts and pond density on common frogs. Some of the relationships revealed in the graphs don't appear quite as strong as we would perhaps expect. Whilst others appear a little surprising. For example, the number of fish ponds that are occupied by great crested newt in areas of high pond density appears to be higher than what our understanding of gcn ecology suggests. However, it is important to realise that just because a pond is occupied does not necessarily mean that the species of interest actually breeds there.
Defining what constitutes a breeding pond can actually be quite tricky. It is a pond in which amphibians breed (obviously), but how do we define breeding.
If breeding is defined only by the act of successful reproduction, we would need to follow eggs and tadpoles through their development and demonstrate that metamorphosis has actually occurred. Some may even argue that we also need to show that those metamorphs successfully survive to adulthood and return to the pond to breed once again.
A pond that usually desiccates before tadpoles can metamorphose would not therefore be defined as a breeding pond, even though amphibians have obviously displayed breeding behaviour (i.e.. courtship behaviour, laying eggs etc). Yet in very wet years, the same pond may retain water long enough to allow tadpoles to successfully metamorphose. Metapopulation theory describes the process by which species may breed successfully in source populations whilst also displaying non-successful breeding in sink populations. Sources and sinks may even switch back and forth over time.
When examining how habitat factors may affect species distribution, it is important to understand that some habitats such as ponds are often quite dynamic and their relative suitability may change over time. Think of a normally highly ephemeral pond in a very wet year, or a fish pond that fully desiccates during a drought.
An occupied pond may not necessarily be a successful breeding pond this year, but it does represent a potential breeding pond for future years.