Geoscience Reference
In-Depth Information
EXCLOSURES
A slight variation on difference comparisons is the comparison of used sites
and sites where access has been restricted by an exclosure. Exclosures are usu-
ally wire fences or cages that limit access by herbivores but do not hinder plant
recruitment or growth. Exclosures can reveal information on general food
habits of herbivores when based on short-term differences between paired
(fenced and open) plots (Bobek et al. 1975). This technique should not be
applied to estimates of use during the growing season because protected plants
grow at different rates than grazed plants. Probably the more common use of
exclosures is to reveal the effects of herbivory on plant community composi-
tion. Here, investigators usually erect a smaller number of large exclosures that
are monitored for 1-2 years (Huntly 1987) or longer (Alverson et al. 1988;
Brander et al. 1990; Brown and Heske 1990; McInnes et al. 1992). Periodic
inventories are conducted to compare species composition and growth rates
between exclosures and open sites. Exclosure studies have revealed that herbi-
vores can affect the abundance, biomass, and diversity of plants (review by
Huntly 1991). For example, moose (
Alces alces
) in boreal forests affect the
recruitment of forage and nonforage species (Brander et al. 1990; McInnes et
al. 1992). Long-term exclosure studies also have revealed that this herbivore
not only alters the species composition of forest stands, but also can directly
affect the structure of herb, shrub, and canopy layers (McInnes et al. 1992).
Although exclosures can help identify major forage plants and responses in
plant communities, they are not an appropriate method to gain information
on infrequently used forage (table 5.1).
POSTINGESTION SAMPLES
The most common technique for analyzing food habits of terrestrial verte-
brates involves sampling either during or after the digestive process. Samples
may be collected from various stages of digestion for use in identifying food
habits of herbivores and carnivores. All postmastication sampling requires
identification of materials that may not be easily recognized.
Feces or pellets
Examining the content of feces or regurgitated pellets has become widely used
because this approach is nondestructive and large samples can be collected.
This technique has been applied with equal success to waterfowl (Owen
