(i) a simple management program (through grazing manipulation) could be devised
to control the prairie dogs, without the use of harmful poisons which could affect
other species; and (ii) in many areas prairie dogs are becoming very scarce and their
colonies need to be protected. In addition, the black-footed ferret (Mustela nigripes),
one of the rarest mammals in the world, depends entirely on prairie dogs and it is
thought that their very low population has resulted from the decline in prairie dog
populations. The conservation of both species would benefit from the manipulation
of grazing practices.
In another example where facilitation improved management for wildlife, Anderson
and Scherzinger (1975) showed that ungrazed grassland resulted in tall, low-quality
food in winter for elk. Cattle grazing in spring maintained the grass in a growing
state for longer. If cattle were removed before the end of the growing season, the
grasses could regrow sufficiently to produce a shorter, high-quality stand for elk; their
population increased from 320 to 1190 after grazing management was introduced.
In Australia, rabbits prefer very short grass. Rangelands that have been overgrazed
by sheep benefit rabbits, through facilitation, and rabbit numbers increase. When sheep
are removed and long grass returns, rabbit numbers decline. In Inner Mongolia, China,
the substantial increase in livestock numbers since 1950 has produced a grass height
and growth rate that favors Brandt’s vole (Microtus brandti) populations so that there
has been an increase in the frequency of population outbreaks of this species (Zhang
et al. 2003).
The saltmarsh pastures of Hudson Bay in northern Canada are grazed by lesser
snow geese (Chen caerulescens) during their summer breeding (Bazely and Jefferies
1989; Hik and Jefferies 1990; Wilson and Jefferies 1996). The marshes are dominated
by the stoloniferous grass Puccinellia phryganodesand the rhizomatous sedge Carex
subspathacea. At La Perouse Bay some 7000 adults and 15,000 juvenile geese graze
the marsh, taking 95% of Puccinellialeaves. These are nutritious, with high amounts
of soluble amino acids. From exclosure plots it was found that natural grazing by
geese increased productivity by a factor of 1.3–2.0. Experimental plots with differ-
ent levels of grazing by captive goslings showed that above-ground productivity of
Puccinellia was 30–100% greater than that of ungrazed marsh. In addition, the
biomass (standing crop) of the grass was higher if allowed to regrow for more than
35 days following clipping. Immediately after the experimental grazing the biomass
was less than the ungrazed plots, so that at some point between then and the even-
tual measurements the biomass on the treated and untreated plots was the same. Even
so, the production rate of shoots was higher on the grazed sites. Other experimental
plots where grazing was allowed but from which goose feces were removed showed
that biomass returned to the level of control plots, but no further. Thus, it appears
that goose droppings, which are nitrogen rich and easily decomposed by bacteria,
stimulate growth of Puccinellia. Geese, therefore, benefit each other from their graz-
ing by fertilizing the grass, a form of intraspecific facilitation.
In summary, facilitation occurs when one species alters a habitat, or creates a new
habitat, which allows the same or other species to benefit. We have discussed grazing
systems, in particular, but the concept applies in many other cases. For example, many
hole-nesting birds and mammals in North America such as wood ducks (Aix sponsa)
and flying squirrels (Glaucomys sabrinus) depend on woodpeckers to excavate the
holes, a form of facilitation. Knowledge of such interactions is important for the proper
management and conservation of ecosystems.
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