Critical Habitat

All species have particular requirements for their ecological habitat. These specific needs are known as critical habitat, and they must be satisfied if the species is to survive. Critical habitat can involve specific types of food, a habitat required for breeding (as is the case of species that nest in tree cavities), or some other crucial environmental requirement.

Some critical habitat features are obvious, and they affect many species. For example, although some species that live in desert regions are remarkably tolerant of drought, most have a need for regular access to water. As a result, the few moist habitats that occur in desert landscapes sustain a relatively great richness of species, all of which are dependent on this critical habitat feature. Such moist habitats in the desert are called oases, and in contrast to more typical, drier landscapes, they are renowned for the numbers of species that can be supported at relatively large population densities.

Salt licks are another critical habitat feature for many large species of mammalian herbivores. Herbivores consume large quantities of plant biomass, but this food source is generally lacking in sodium, resulting in a significant nutritional deficiency and intense craving for salt. Salt licks are mineral-rich places to which large herbivores may gravitate from a very wide area, sometimes undertaking long-distance movements of hundreds of miles to satisfy their need for sodium. Because of their importance in alleviating the scarcity of minerals, salt licks represent a critical habitat feature for these animals.

Many species of shorebirds, such as sandpipers and plovers, migrate over very long distances between their wintering and breeding habitats. Many species of the Americas, for example, breed in the arctic tundra of North America but winter in South America, some as far south as Patagonia. For some of these species, there are a few places along their lengthy migration route that provide critical staging habitats where the animals stop for a short time to feed voraciously. These critical habitats allow the animals to re-fuel after an arduous, energetically demanding part of their migration and to prepare for the next, similarly formidable stage.

For example, parts of Chesapeake Bay and the Bay of Fundy are famous critical habitats for migrating shorebirds on the east coast of North America. Chesapeake Bay is most important for birds migrating north to their breeding grounds, because at that time horseshoe crabs (Limulus polyphemus) spawn in the bay, and there is an enormous, predictable abundance of their nutritious eggs available for the shorebirds to eat. The Bay of Fundy is most important during the post-breeding, southern migration, because at that time its tidal mudflats support a great abundance of small crustaceans that can be eaten by these birds, which can occur there in flocks of hundreds of thousands of individuals. There are additional places on the east coast of North America that provide important staging habitat for these birds during their migrations, but none support such large populations as Chesapeake Bay and the Bay of Fundy. Therefore, these represent critical habitats that must be preserved if the shorebirds are to be sustained in large populations.

Another critical habitat feature for many species that live in forests is large dimension, dead wood, either lying on the ground as logs or as standing snags. Many species of birds, mammals, and plants rely on dead wood as a critical habitat feature that may provide cavities for nesting or resting, a feeding substrate, places from which to sing or survey the surroundings for food and enemies, or in the case of some plants, suitable places for seedlings to establish. Woodpeckers, for example, have an absolute need for snags or living trees with heart-rotted interiors in which they excavate cavities that they use for breeding or roosting. Many other animals are secondary users of woodpecker cavities.

Sometimes, different species develop highly specific relationships to the degree that they cannot survive without their obligate partners, which therefore represent critical, biological components of their habitat. For example, the dodo (Raphus cucullatus) was a flightless, turkey-sized bird that used to live on the island of Mauritius in the Indian Ocean. The dodo became extinct by overhunting and introduced predators soon after its island was discovered by Europeans. However, this bird lived in an intimate relationship with a species of tree, the tambalacoque (Calvaria major), that, like the dodo, only occurs on Mauritius. The large, tough fruits of this tree were apparently an important food source for dodos. The dodos found these fruits on the forest floor, ingested them whole, ground up the outer, fleshy coat in their muscular gizzard, and digested and absorbed the nutrients in their alimentary tract. However, the hard, inner seeds were not digested and were defecated by the dodos, prepared for germination in a process that botanists call scarification. Dodos were the only animals on Mauritius that could perform this function. Since the dodo became extinct in the early 1600s, no seeds of tambalacoque were able to germinate since then, although some mature trees managed to persist. In the 1980s, the need of tambalacoque for this sort of scarification was discovered, and seeds can now be germinated after they have been eaten and scarified by passage through a domestic turkey. Seedlings of this species are now being planted in order to preserve this unique species of tree.

Because so many species and natural ecosystems are now endangered by human influences, it is very important that critical habitat needs be identified and understood. This knowledge will be essential to the successful preservation of those many endangered species that now must depend on the goodwill of humans for their survival. An important aspect of the strategy to preserve those species will be the active management and preservation of their critical habitat.