To rate how much a species or an ecosystem is endangered, it is not enough just to take a look at numbers. It is necessary also to take the vulnerability to changes into consideration. Principally one can say: the more adaptable and self-regenerative, the less vulnerable. Stenoecious organisms are more vulnerable than euryoecious ones. Also endemics are especially vulnerable.
A species that only lives under very special circumstances is more vulnerable than one that can survive in very different biotopes.
The same concept is used in economic geography to describe how well an economy can cope with changes like natural catastrophes or exogene economic changes. An economy based mainly on one agricultural crop is much more vulnerable than one based on many different crops and domestic animals.