February 11, 2004
The stability of grazing systems - overview
1. Is grazing a problem to plants?

No, plants are eaten all the time, they have evolved numerous mechanisms to persist
in the face of continuous predator attack (indeterminant growth, low palatability,
toxins, tree-form, predation-safe storage, spines …)

At the same time, predators have co-evolved to get around some of the obstacles
(toxin tolerance, new digestive abilities, skills or morphologies to get to hidden plant
parts, …).

Naturally evolved grazing systems tend to be quite diverse.
2. What sort of ecological mechanisms maintain species diversity?

In the most general sense, diversity is maintained if there are mechanisms that limit
species growth when they are abundant (have high density) and foster species growth
when they are rare (have low density). These feedback mechanisms have to work on
all species in the community.

One of the most important mechanisms is the differentiation of species by the
ecological niches they occupy, i.e. by their differences in resource use. Because
species tend to be restricted to their niches, and because different species have
different niches, species tend to suppress themselves the most when they are
abundant, rather than drive other species to extinction. On the other hand, when a
species is rare, its niche is relatively empty and it can achieve very high growth rates
when conditions are good.

Grazers and other predators also help to maintain diversity: Behaviorally: predators
tend to select species that are more abundant and ignore rare species. Through
population growth responses: High abundance in one plant species tends to increase
the birth rates of its predators. Eventually, the higher number of predators reduces
plant density. Conversely, when a species is rare, its predators also tend to even rarer.

There are many other conceivable feedback mechanisms that help to put bounds on
population growth and help species at low density to recover: Fire could set back
dominant grasses and give rare grasses a chance to recover, disease spreads faster in
populations at high density than at low density, free-ranging grazers migrate from
regions of low forage abundance to regions of higher abundance, top predators check
the population density of grazers, which could limit overgrazing by hungry animals.
3. Do some of these feedback mechanisms break down in man-managed grazing
systems?

There may be a lack of feedback control on alien plants that have not co-evolved with
indigenous species, both plant and animals (e.g. fewer natural enemies, broader
niches).
February 11, 2004

Equally, there could be a lack of feedback control on alien grazers: grazer densities
could be held artificially high by supplementary feeding and installation of water
sources.

Plants may be intolerant to the way alien animals graze.

Top predator elimation could reduce control over indigenous plant-eaters.

Fences limit migratory movements, predominantly of the managed grazers.

Reseeding with competitively superior monocultures may swamp out natural
population control in the vegetation (through intra- and interspecific competition).

Fire suppression could weaken suppression of dominant species.
4. What is the actual evidence that the introduction of cattle and sheep grazing
changes plant communities?

Evidence is hard to come by, because diversity may have been lost before anybody
paid attention. Areas never grazed may be different for other reasons. Changes in
vegetation released from grazing may be different than changes associated with firsttime cattle/sheep grazing.

In an Australian study long-term exposure to grazing was assumed to vary with
distance from the well. The study found that a) there are many rare species, b) more
of the rare species are found furthest away from the well than close to it, c) most
alien species increased in abundance with proximity to the well.

The analysis of packrat middens from Utah indicates a major shift in vegetation in
the past 200 years: severe reduction in vegetation that grazers like to eat, severe
reduction too in other species that are not so good to eat. Other species were rare 200
years ago, but are abundant now, some of them exotics, some not.

Overall, it is possible, but ultimately unproven, that some species that were rare
before cattle/sheep grazing went locally extinct on rangelands. It is certain that major
shifts in species dominance occurred, and that overall vegetation cover declined.