Some new concepts
The idea of 'Metapopulation' reflects the dynamic nature of many animal populations and includes the following factors:

Colonisation: A new area of habitat is colonised by reptiles or amphibians spreading out from an established breeding site nearby.

Establishment: The new arrivals breed and increase in numbers to the carrying capacity of the site   (i.e. the maximum numbers the site is able to sustain in terms of available food, cover, hibernation, sites etc).

Extinction: At the local level extinction might occur due to sheer bad luck - a harsh winter, accidental grass fire, a series of unfavorable summers, or an isolated pollution incident.

Recolonisation: The process starts again with new individuals from a nearby site.

Probably this is the manner in which most metapopulations operate over long or short time scales. But the process only continues to operate if there are surviving groups of animals which can colonise and recolonise breeding sites.

For instance, amphibians may persist as breeding populations around a single pond for decades but, inevitably, accidents happen which result in local extinction. This is not a problem if there is another colony nearby which can recolonise the old site. But what if the next colony is 5 kilometers away, or 100 metres away but cut off by a motorway? It is necessary for there to be corridors of suitable habitat linking one breeding site to the next, so that the dynamic ebb and flow of the metapopulation can operate.

Habitat fragmentation: A small colony without habitat corridors is isolated and is in the long term doomed to extinction, unless it can be connected to others. Habitat fragmentation is widely recognised as a major problem for small animals which are poor dispersers - they do not readily move across unsuitable habitat.

Most habitat fragmentation is due to human activity, by removal or interruption of habitat corridors or dispersal routes. The recent upsurge of interest in toad crossings is a classic example of this, where the animals are following age-old migration routes which now cross new, or busier, roads. Often, habitat fragmentation is more subtle, and might involve factors such as over-zealous 'tidying-up' of rough areas in urban parks, a change in storm water drainage as part of a development scheme, leading to wetlands drying out. Sometimes development acknowledges the need to retain wetlands for amphibians without taking account of the need for terrestrial habitat within easy reach of the pond.

How does this fit with current ideas of habitat loss? In fact, habitat fragmentation, when viewed from the metapopulation perspective, is probably a more insidious threat than obvious habitat destruction. In the short-term, hastily applied conservation measures to stabilise populations around good, but isolated breeding sites may only be stopgap measures. It may be that we now have numbers of sites for these species which look reasonably safe in 1996, but half may have disappeared due to chance by 2025.

Translocation: Artificial movement of animals between sites can circumvent the problems discussed above. In spite of the fact that multiple unrecorded translocations have already occurred via small children with jam jars, this is an approach to be pursued with caution. Translocation refers to any movement of animals (or plants) from one place to another. This includes activities that release animals into a site - termed introduction or re-introduction (depending on whether the animals were previously found at the site). The translocation of the great crested newt, natterjack toad, sand lizard and smooth snake is strictly controlled by the licenses issued through the Wildlife and Countryside Act 1981 and Conservation (Natural Habitats, etc) Regulations 1994.

(Provided by and reproduced with permission of: English Nature)