The latest news on the much-promoted Garden Bridge over the Thames has been thrown into doubt by the Hodge Report, which was commissioned by London Mayor Sadiq Khan. A more detailed financial summary can be found here: http://www.publicfinance.co.uk/news/2017/04/garden-bridge-should-be-scrapped-finds-hodge-report
But is there a biodiversity case for the bridge? In terms of value for money, I would say no. The River itself functions as a dynamic corridor for biota of all kinds. Like all rivers, the biosystem dynamics are well-understood, with the flow of nutrients and mineral material downstream to be deposited in the Thames Estuaries and the sea. Biota have developed strategies for dealing with the tidal nature of the Thames. While the balance of flow is towards the sea, there are twice daily movements in the other direction. Thus as a biotic corridor, the Thames does not need assistance from us, although there would be merit in naturalising the banks. Such naturalisation has occurred only in isolated areas, and the value of land along the banks means that it is difficult to get owners and developers to agree to sacrifice river frontage to habitats.
So would the bridge assist as a biological corridor over the Thames? This could help flightless fauna, and some terrestrial wildlife. Again, most fauna have developed strategies for crossing barriers, including hitchhiking with birds (in the case of insects) and utilising rafts of vegetation or detritus to cross the river itself. Thus while the garden bridge could provide some positive benefit in biological corridor terms, it is doubtful that it would create a detectable increase in wildlife crossing the Thames. The bridge is really more of an aesthetic feature, and developed a romantic following amongst some Londoners.
I am struck when in the upper stories of London buildings how much potential unused habitat exists. A real and much less costly biological benefit could be gained from simply creating pockets of habitat on the London skyline. Much like the naturalisation of the Thames bankside, there is policy aspirations to achieve this, but it is more evident in its absence. There is still much to be done for London to achieve real biodiversity enhancements in line with national policy. The Garden Bridge is not the answer.