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What's in a name?
Herbivores are divided into generalists, animals that eat a wide variety of plants
more or less equally, and specialists, animals that rely on one food source, often to the
exclusion of all other available foods. An example of a specialist would be the
Koala bear (Phascolarctos vulpecula) of Australia. They eat Eucalyptus (Eucalyptus
spp.) plants only, though there are several species of Eucalyptus available.
Koalas are an example of an extreme specialist, though there are shades of gray involved
here as with anything else in nature, meaning that there are specialists that eat other
plants at different times of the year, or times of their lives. An example of one of
these specialists would be the pygmy rabbit (Brachylagus idahoensis) found in the
Great Basin region of the western U.S. These critters eat a variety of forbs and
grasses in the spring and summer, but during winter exist almost solely on sagebrush (Artemesia
spp.) plants.
There are relatively few mammals that specialize on a single plant species as their
primary food source, and popular thinking holds that this is because of the high
concentrations of plant toxins that would naturally be present in such a diet. It is
thought that exclusive intake of one plant species would exceed the animals' ability to
remove or bypass the compounds plants that are toxic to many animals. It would seem
that those that have adapted to be specialists would be very sensitive to changes in their
habitat, particularly changes that affect their food.Interestingly,
most specialists eat foods that would be poisonous to other animals. Many plants
contain what are known as anti-quality factors, or allelochemicals, that discourage
herbivores from foraging on them. These anti-quality factors include tannins,
terpenoids, alkaloids, and oxalates, and many others. Tannins and terpenoids are
known as quantitative defenses because they act in a dosage dependent fashion, in other
words the amount the animal eats determines how sick they will feel. Specialist
herbivores have co-adapted with their preferred food source, they have acquired special
traits or organs that allow them to detoxify these allelochemicals. These can
include enlarged livers, enlarged parotid salivary glands, and many others, all of which
allow specialists to use foods others cannot.
Allelochemicals
Tannins bind with proteins in the gastrointestinal tract of
herbivores, making them indigestible. They also tend to taste astringent, which
allows animals to recognize and avoid them. Terpenoids are volatile compounds found
in some plants that can cause the microbes in ruminant's stomach to stop growing or
die. These compounds are readily recognizable by their scent, for example if you
crush a pine needle in your fingers, the distinctive smell is due to the terpenes.
Some specialists have been shown to use this smell as a method to identify their preferred
food source.
The problems with foragers
Herbivores can affect vegetation structure by modifying
plant community structure, composition, density, and patchiness. Toxins found in
plants can affect what the herbivores choose to eat. Both generalist and specialists
can affect what plants exist in local stands, but recent studies suggest that they can
also affect what plants exist in smaller patches, and even how the plants themselves
grow. While generalist can affect the structure of stands of vegetation through
selective foraging, specialists can also exert pressure on specific plants and alter their
defensive mechanisms.
Problems between foragers
It has been shown that foraging pressure from novel herbivores can cause plants to shift
their chemical composition, increasing or changing the allelochemicals to try and deter
the new threat. Cases in several areas have shown that even when native animals are
encouraged to increase population sizes, this increase in pressure can cause the
vegetation structure to change, which in turn causes other species to decline.
Yellowstone National Park has recently seen declines in mule deer (Odocoileus hemionus)
numbers because elk (Cervus elaphus) are increasing and eating more sagebrush
than before. The sage plants are not able to keep up with the increase in foraging
pressure, which has caused a shift in vegetative composition in the region.
Meanwhile mule deer rely on sagebrush for their winter forage, and since the plant is on
the decline, so to are the deer.
And then there is us...
In the modern world there has been a lot of movement of plants and animals to areas that
are not their traditional homes. Humans have moved plants to the American and
Australian continents from Europe and Asia as food sources. These introductions of
non-native species often cause problems, since we don't also move their control agents,
such as predators. These invasive species are often able to out compete the local
species because they have nothing around them that limits their growth, while the native
species have all the usual diseases and predators. Since these introduced plants are
often food sources for humans, they are often encouraged to grow as well, though they are
often able to migrate away from the farmer's fields and into the wild.
Conclusion
Any person interested in wildlife management needs to take into consideration the habitat
requirements, but with specialist herbivores it is critical to think about
everything. These animals are adapted to fill a very narrow niche in the habitat,
and when humans or other wildlife cause their one food to change, then the specialist
becomes threatened. As never before we see that ecosystems are interconnected
circles, with every aspect having an impact on all others. We need to consider that
this plant that may be inedible and toxic to most animals, may be the key to life for
other animals, and anything we do to cause or increase the changes in the vegetation of a
region can have serious impacts.
Suggested readings
Dearing, M. D., A.M. Mangione, and W.H. Karasov. 2000. Diet breadth of
mammalian herbivores: nutrient versus detoxification constraints. Oecologia.
123:397-405.
Lawler, I. R., W.J. Foley, and B.M. Eschler. 2000. Foliar concentrations of a
single toxin creates habitat patchiness for a marsupial folivore. Ecology.
81:1327-1338.
Vourc'h, G., J.L. Martin, P. Duncan, J. Escarre, and T.P. Clausen. 2001.
Defensive adaptations to Thuja plicata to ungulate browsing: a comparative study between
mainland and island populations. Oecologia. 126:84-93
Wambolt, C. L. 1998. Sagebrush and ungulate relationships on Yellowstone's
northern range. Wildlife Society Bulletin. 26:429-437.
Zhang, X., and J.S. States. 1991. Selective herbivory of ponderosa pine by
Abert squirrels: a re-examination of the role of terpenes. Biochemical systematics
and ecology. 19:111-115. |
