a. Very large animals:
"Generally, the larger the body size of an animal, the longer it lives and the fewer offspring it produces each year. Relatively large animals also tend to have relatively low population densities; thus, a viable population of, say, elephants occupies considerably more space than an equal-sized population of rabbits. Large predators such as tigers (Panthera tigris) have lower population densities than the herbivores on which they feed. A tiger has a home range that may occupy 100 square km (40 square miles), while a rabbit may survive in 1 hectare (0.004 square mile).
Large animals, by virtue of their low population densities, are at increased risk of extinction. Moreover, an animal species that produces few offspring each year and that suffers a major loss in numbers from human activity will need more time to recover than a species with high reproductive rates."
Taken from britannica.com
Link:
http://www.britannica.com/EBchecked/topic/133385/conservation/272691/Which-species-are-most-vulnerable-to-extinction