Loss of Biodiversity （生物多样性） Affects Human Society

If a species of bee disappears forever or a particular plant is extinct，what does it have to do with us humans？Well，according to a team of international scientists，biodiversity is dropping below levels （21）___ （consider） safe for the health and happiness of human societies．

The issue is that everything is inter-connected and ecosystems support our societies （22）___ they provide us with，for example，food，fibres and fuels．If species go on disappearing，this can disturb many vital processes （23）___ crop pollination （授粉） and the decomposition （分解） of waste．

A framework which defines the environmental limits within （24）___ humans can really operate--called planetary boundaries--says （25）___ （lose） more than 10% of the biodiversity in an area places the local ecosystem at risk．Ecosystems are all different but this percentage is considered as a good measure of safety．

A study published in the magazine Science suggests that 58% of the world's land surface （26）___ （reduce） below this level already．These areas house 71% of the global population．Professor Andy Purvis，from Imperial College London and the Natural History Museum，is one of the authors of the study．He says，"Once we're the wrong side of the boundary，（27）___ doesn't mean everything goes wrong immediately，but there is a remarkable higher risk （28）___ things will go badly wrong．"

The researchers found that grasslands，savannas and shrub lands were most affected by biodiversity loss on average．Purvis hopes this report （29）___ become a wake-up call to those who design policies．Here's his warning："Decision-makers worry a lot about economic recessions （衰退），but an ecological recession could have even （30）___ （bad） consequences-and the biodiversity damage we've had means we're at risk of that happening．Until and unless we can bring biodiversity back up，we're playing ecological roulette （轮盘赌）．"