Are ecological concerns supported explicitly in Buddhism? Many practising Buddhists that I know also have strong ecological concerns. Is this concern for 'green' issues part of Buddhism or do the two things just attract the same kind of people i.e. people that like Buddhism also like green issues. Specifically does concern for the environment have any basis in canonical texts or commentaries or is it a modern development?
AFAIK there aren't any explicit references in the scriptures to ecological concerns and sustainability. I guess that in the days of the Buddha, there weren't many problems in that area. The ideas behind sustainability aren't new, but it wasn't until the second half of the 20th century (the energy crises in the 70s) that they became more prominent and that the growth of the entire human population and increased resource consumption were seen as a potential major problem for mankind.

I think what makes environmentalism and sustainability so interesting for Buddhists (or the other way around?) is that several main ideas are rather similar, or at least in line with each other. 
For example, the Buddha taught that everything, all life, is connected (dependent origination). Also there is no self, the self only exists in dependence upon causes and conditions (anatman). This means that if you damage the Earth then you indirectly are also damaging a part of yourself. 

Buddhism also teaches us that we should take the middle way, lead a simple life, don't engage in addictive behavior or excessive consumption, be kind to others. These ideas are also prominent in sustainability. We shouldn't waste resources and should try to improve the living conditions for everyone, not just the rich people living in the Western world.

So to answer your question; I think Buddhism and environmentalism/sustainability just attract the same kind of people. People that really care for the well-being of others.

BTW, being a protem moderator of the Sustainable Living SE site, I can't resist making at least one reference to that site here. ;-)