Still and Silent Ecosystems: Declining Grassland Biodiversity
Healthy grassland ecosystems teem with plant and animal life. Some
of the best-known, most dramatic, and unique large herbivores roam
grasslands: antelopes and zebras in Africa; gazelles, goats, camels,
bison, and wild horses in Eurasia; and deer in North America. Grasslands
emanate the songs of birds—often birds found nowhere else.
In fact, grasslands contain 11 percent of the world’s endemic
bird areas (areas encompassing the range of two or more bird species
that have relatively small ranges)(White et al. 2000:40). The flora
of grasslands is just as rich and important as the fauna. Grasslands
were the seedbeds for the ancestors of major cereal crops, including
wheat, rice, rye, barley, sorghum, and millet. These ecosystems
continue to provide genetic material necessary to breed cultivated
varieties of cereals that are resistant to crop diseases.
But there are growing concerns about the ability of grasslands to
sustain such a rich assemblage of species. Although there are no
globally comprehensive measures of the condition of grassland biodiversity,
restricted regional studies suggest that increasing conversion of
grasslands to agricultural and urban areas, fragmentation of grasslands
with roads, and invasive species are responsible for declines in
biodiversity (White et al. 2000:45–47). Although relatively
small areas of grasslands have been designated as "protected,"
to date this strategy appears insufficient to prevent grasslands
globally from becoming more still, much quieter, and far simpler
ecosystems—ecosystems less able to supply the game species,
medicinal plants, tourism, and genetic material we routinely enjoy.
Pressures on Grassland Biodiversity
Globally, grasslands have been heavily modified by human activities;
few large expanses of unaltered grasslands remain. Even small areas
are frequently fragmented. Although forest fragmentation has been
the source of recent and often heated discussions regarding the
merits and drawbacks of road building, grassland fragmentation has
received relatively little attention, despite the fact that the
fragmentation can be pervasive. In the United States, roads have
so fragmented the Great Plains that 70% of those grasslands are
in blocks less than 1,000 km² in area, and none of the blocks are
greater than 10,000 km² in area (White et al. 2000:47). (See Figure
1.)
Grassland fragmentation can lead to:
Another pressure on grassland biodiversity is the introduction
of non-native plants and animals. Though not all non-native species
are detrimental, some do affect the capacity of grasslands to sustain
biodiversity when they become invasive, spread rapidly, change the
composition of grasslands, and prevent growth of native species.
The extent of nonnative species in grasslands is tremendous. Looking
just at non-native plant species in North America, researchers have
found that at least 10% of the species in the Great Plains and more
than 20% of species in the California Central Valley Grasslands
are nonnative (Ricketts et al. 1997: 81–84). (See Figure 2.)
Trends in Grassland Biodiversity
As noted above, there is a dearth of information about global trends
in grassland biodiversity. However, some regional studies of bird
populations suggest declines in grassland biodiversity, and evaluations
of remaining large grassland mammals are not encouraging.
The North American Breeding Bird Survey (BBS) provides population
trends for a wide range of bird species in the United States and
Canada. Survey data from 1966 to 1995 for 28 bird species that breed
in grasslands show declines throughout most of the region (Sauer
et al. 1997). In fact, grassland birds showed the most consistent
declines of any group of birds monitored by this survey. Only a
few small areas showed increases in breeding grassland birds. Habitat
loss and increased mowing of grasslands for hay production on the
breeding grounds, as well as problems along migratory routes or
on the wintering grounds, may be responsible for many of the declines
(Sauer et al. 1997; White et al. 2000:43).
In some parts of the world, grasslands have developed largely because
browsing by wild herbivores has prevented the establishment and
growth of trees. Main areas where grassland formation has been influenced
by large herbivores are the savannas of Africa, steppes of Eurasia,
and prairies of North America (WCMC 1992: 280).
Human activity, however, has seriously impaired populations of some
of these large grassland vertebrates. The spectacular migrations
of large vertebrates in the temperate grasslands and steppes of
North America and Eurasia now occur only in isolated pockets--in
the Daurian Steppe and Tibetan Plateau (Olson and Dinerstein 1997:
16). The large-scale migration of herbivores, such as wildebeest
and zebra, across the savannas of Africa now occur over a much less
extensive area in East Africa and the central Zambezian region (Olson
and Dinerstein 1997: 16).
Is Grassland Biodiversity Sustainable?
It already may be too late for some grasslands to provide goods
or services related to biodiversity in areas where conversions to
agriculture and urbanization, as well as fragmentation and invasive
species, have considerably altered grassland biodiversity.
Many research programs have identified grassland areas that still
contain outstanding biodiversity, but the continued existence of
these areas is not guaranteed. A challenge is to conserve the flora
and fauna in protected areas such as the Centers of Plant Diversity,
Endemic Bird Areas, Global 200 Ecoregions, and biologically distinctive
areas. Protected areas (designated by IUCN--The World Conservation
Union) with sizeable amounts of grassland make up only 3 percent
of the global land area, or 7.6 percent of the world’s total
grassland area (White et al. 2000:43). Some of these areas may be
more vulnerable than others and require extra attention. For example,
Centers of Plant Diversity in Madagascar are subject to clearing
for agriculture, grazing, mining, erosion, and invasive exotic species
(Davis et al 1994: 271). Supporting grassland biodiversity in these
protected areas may require tailored, targeted protection, monitoring,
and maintenance activities.
Another priority is the collection of better global information
and data on grassland biodiversity. Some indicators available to
evaluate grassland condition rely on subjective data rather than
on quantitative measures. Just identifying and mapping large, intact
grasslands is difficult because there is no consistent, up-to-date
road data for the world.
Grassland biodiversity protection would benefit from the universal
adoption of quantitative indicators of grassland condition, as well
as regularly collected, reliable data. Such a global effort might
be modeled after some regional data-gathering efforts like the Breeding
Bird Survey for North America, which provides high-quality information
on species abundance and population trends. These survey data permit
evaluation of long-term trends across several habitats. Other datasets
on grassland wildlife populations are of good quality but have limited
coverage.
Datasets on invasive species must be expanded to cover the entire
globe and must distinguish between introduced species and harmful
species. Several workshops have addressed invasive species as an
international problem, and databases related to the study and documentation
of such species have been reviewed (Ridgway et al. 1999; Ricciardi
et al. 2000). The
National Biological Information Infrastructure (NBII) website
provides information on these databases. Ricciardi et al. (2000:
240) call for a global information system for invasive species and
provide a list of invasive species databases available on the Internet.