Marginal habitat can provide a refuge from competing species

As long as islands of marginal habitat can remain (or be) connected, threatened or introduced plants can coexist with their competitors.   Ecology based agriculture is based in part on biodiversity and this is the type of insight that can help one maintain it. Not only can survival of natives be insured, but also the survival of introduced species.  Who would have thought that marginal habitat could be a haven for species persistence?   The important part is that these islands of habitat must remain CONNECTED or they become vulnerable to local extinction events.

http://phys.org/news/2013-01-invading-species-extinguish-native.html