Actually, sentence 2 is arbitrarily correct in terms of grammar, but native Chinese will never say like that due to the following reason:
数量 is a compound word (from 数, number, and 量, quantity) which is linked by the relationship "and". Combining these words with another one-word clause to become one word (i.e. without any indication to show that there are 2 words, such as the particle 的) will be unnatural.
Using the character 的 more than once in a clause is unnatural and will be avoided whenever possible. So you can't have 野生大熊猫的少的数量是很多原因造成的.
So sentence 2 is considered wrong and most Chinese will choose sentence 1.
P.s. Modern Chinese is closely related to classic Chinese. So basically words which were not used in classical Chinese can be treated as the words in point 1.
Correct me if I am wrong.