Final answer:
A species is considered endangered if its population is so small that it is at risk of imminent extinction. This status is determined through criteria that include habitat loss, population trends, and threats like invasive species, with habitat protection being a key strategy for conservation.
Step-by-step explanation:
A species is considered endangered if it has so few individual survivors that the species could soon become extinct (option d). This definition is tied to criteria developed by conservation organizations such as the International Union for Conservation of Nature (IUCN) and legislation like the Endangered Species Act (ESA).
To identify if a species is endangered, biologists and conservationists analyze various factors such as habitat loss, population size, trends in population size, and the threats posed by invasive species. These criteria help in the identification, protection, and recovery of endangered species, which is a multistep process that includes assessing species' status, establishing protective measures to prevent further decline, and taking action towards recovery.
With habitat destruction being the leading cause of endangerment, the conservation focus often shifts to habitat protection, which is a cornerstone of conserving biodiversity. This suggests a species-by-species approach to conservation may overlook broader ecological issues such as habitat preservation and management of natural disturbances. However, recognising and protecting endangered species remains a crucial element in the broader efforts to sustain the Earth's natural diversity.