Any species that is likely to become endangered within the foreseeable future
A vulnerable species is one in which the species is susceptible to endangerment. The species is likely to become endangered when the population of that species declines. In order to prevent the eventual endangerment of the species, the circumstances causing its decline have to be acted upon to improve its survival and reproduction.
One of the factors leading to the vulnerability of a species is habitat loss or destruction. For instance, human activities (e.g. deforestation) have a huge negative impact on the forests. Trees serve as an important habitat of many animals and cutting them could mean the loss of not just a home but also their food source.
The International Union for Conservation of Nature uses a list of criteria when considering a species as vulnerable. Some of these criteria are as follows: (1) population reduction, (2) the extent of occurrence is approximately less than 20,000 km2, (3) population is estimated to number fewer than 10,000 mature individuals, (4) very small or restricted population, and (5) quantitative analysis showing the probability of extinction in the wild is at least 10% within 100 years.1
1 Vulnerable species. Retrieved from ://en.wikipedia.org/wiki/Vulnerable-species.