Biological Society of Washington - Department of Botany ...

NUMBER 17 3
and are so unusual that they inspired a fictional
scientific expedition referred to one as ‘‘The Lost
World’’ (Doyle 1912), a term sometimes applied to all
tepuis. Notable waterfalls of the region include Angel
Falls (979 m) on Auyán-tepui in Venezuela and
Kaieteur Falls (226 m), which flows year around, on
the eastern-most edge of the Roraima formation
known as the Potaro Plateau in Guyana.
Granitic dome mountains occur on the Shield in the
southern part of the three Guianas (Guyana, Suriname,
French Guiana), where they are known as
‘‘inselbergs,’’ as well as in the western extreme of the
Shield in the Puerto Ayacucho region in Venezuela
where they are called ‘‘lajas.’’ Deposits of low-nutrient
white sands occur inland of the coastal plain, in belts
across the Shield, and in isolated pockets. Large areas
of savanna are found in the region, particularly the
complex of savannas that includes the Rupununi
Savanna in southwestern Guyana, the Gran Sabana
in eastern Venezuela, and the savannas of northern
Roraima, Brazil. In some of these areas the sands
overlay a clay hardpan that is resistant to penetration
by tree roots and that floods during the heavy rainy
season, resulting in limited forest growth. Tertiary and
Quaternary sediments separate the southern edge of
the Guiana Shield from the Amazon River and the
eastern edge from the Atlantic Ocean.
Climate
As a whole, the Guiana Shield region has a tropical
climate characterized by a relatively high mean annual
temperature exceeding 25uC at sea level, an annual
monthly maximum temperature range of less than 5uC,
and an average daily temperature range of approximately
6uC (Snow 1976). Because of the Guiana
Shield’s location just north of the equator, its climate
varies primarily according to elevation and effects of
the trade winds that combine to affect rainfall patterns.
The trade winds blow consistently from the east and
northeast, off of the Atlantic Ocean onto northeastern
South America, with wind speeds averaging from 3–4
m per second. Due to orographic effects, the easternmost
escarpments of the mountains of the Guiana
Shield are generally localities of increased precipitation
where these moisture-laden winds meet the slopes
(Clarke et al. 2001). Seasonal oscillations of the
Intertropical Convergence Zone (ITCZ) also bring
variations in rainfall as the locations of low pressure
zones near the equator change (Snow 1976). Varying
primarily by latitude, one or two rainy seasons result
from shifts in the ITCZ. The heaviest rains usually
occur between May and August, whereas the rainy
season running from December to January is shorter
and less intense, with rains that do not penetrate as far
inland. Even during most dry seasons, frequent storms
provide adequate moisture to allow evergreen tropical
moist forests to persist in most low elevation parts of
the region.
Biological Diversity
The variety of landscapes of the Guiana Shield
includes sandstone tepuis, granite inselbergs, white
sands, seasonally flooded tropical savannas, lowlands
with numerous rivers, isolated mountain ranges, and
coastal swamps, each supporting a characteristic
vegetation (Huber 1995b, Huber et al. 1995). This
variety accounts for a great deal of the high diversity
and endemicity of the Shield’s biota. The highlands of
the Shield have a flora and fauna with numerous
endemic species. Some tepui endemic species occur as
low as 300 m in elevation, with increasing numbers by
1500 to 1800 m, and fully developed communities
occurring by 2000 m. Few if any plant or animal
specimens have been collected from most medium to
high elevation areas of the Guiana Shield. Many parts
of the Shield are poorly explored, including parts of
Brazil north of the Amazon River, much of eastern
Colombia, and the southern parts of Venezuela,
Guyana, Suriname, and French Guiana.
Conservation
With the exceptions of the populated localities such
as Puerto Ayacucho, Ciudad Guayana, Ciudad Bolívar,
Boa Vista, Georgetown, Paramaribo, Cayenne, the
agricultural coastal areas, and open areas like the
Rupununi Savanna, the environment of the Guiana
Shield has benefited from limited access and low
population densities, although this same isolation has
hindered biodiversity research. Estimates vary, but
much of the vegetation is still relatively undisturbed by
human activities. Recently, however, the pace of
disturbance has greatly increased. Current threats to
the environment include large-scale logging by Asian
and local companies, large- and small-scale gold and
diamond mining, oil prospecting, bauxite mining,
hydroelectric dams, wildlife trade, and populationrelated
pressures such as burning, grazing, agriculture,
and the expansion of towns and villages. Taken
together, these impacts have begun to take their toll,
with vast areas vulnerable to increasing disturbance a
fact easily observed by using Google Earth and
‘‘flying’’ over the area.
The status of conservation efforts varies by country.
Throughout the Guiana Shield, many areas that are
designated as protected or otherwise restricted are
often only ‘‘paper’’ parks because of a lack of
infrastructure, funds, and will to actually protect the
areas. Over the last four decades, Venezuela has
established seven national parks, 29 natural monuments,
and two biosphere reserves covering about
142,280 km 2 , more than 30% of its share of the