Will networks allow invasive species and disease to spread?

If a habitat network increases the connectivity of a landscape for species which aren't particularly good at dispersal, there are concerns that it will do the same for less desirable species which are better at colonising new areas.  That could mean that the network allows invasive species like the grey squirrel to move through a landscape or it may allow diseases to spread unchecked from one area to another. 

However, species which are invasive are likely to be able to move through a landscape relatively easily, network or no network.  That's part of what makes them so invasive - they are capable of dispersing across large distances, and can often travel through a relatively hostile matrix, which might prevent other species from moving.  This may be of concern to farmers, who need to consider bio-security risks, although the additional risks from habitat networks need to be compared with those that already arise from inter-farm movements of livestock and materials.

Unfortunately, it is possible that a connection between one habitat area and another could allow or enhance the spread of other undesirable species and that's partly why some people are sceptical about the use of corridors and other structural connections.  If we can't guarantee that a structural connection will be used by the species we want to protect, and if there's a possibility that it could be used by an undesirable species, it's questionable whether creating the connection is a good investment of resources.

As a result, many of the existing habitat network projects have focused on functional connections rather than structural - designing a network to be as suitable as possible for the species in question, rather than trusting to chance. By targeting habitat creation and land management changes, networks can be 'fine-tuned' so they provide increased connectivity for the species that need it most, without undesired side-effects.  It's also important to identify potential invasive species when a network is being planned, so that the risks can be assessed and a suitable management strategy incorporated within the network.