Page 308 - Environment: The Science Behind the Stories

in decline, over 1900 are threatened, and roughly 170 spe-
cies studied just years or decades ago are thought to be extinct
(Figure 11.15a). As these creatures disappear before our eyes,
scientists are racing to discover the reasons. Recent studies
implicate a wide array of factors, including habitat destruc-
tion, chemical pollution, disease, invasive species, and climate
change (Figure 11.15b). Biologists suspect that multiple factors
may be interacting and multiplying one another’s effects.
Many amphibian populations are vanishing in remote
and pristine environments when no direct damage is appar-
ent. In many of these cases, the culprit seems to be a fun-
gal disease called chytridiomycosis, caused by the pathogen
Batrachochytrium dendrobatidis. Researchers do not yet
Figure 11.14 The polar bear became the first species listed
as threatened under the Endangered Species Act as a result know whether its rapid spread is due to human influence.
of climate change. As Arctic warming melts the sea ice from which As scientists learn more, they are designing responses
they hunt seals, polar bears must swim farther and farther for food. to amphibian declines. A conservation action plan published
by the IUCN recommended protecting and restoring habitat,
cracking down on illegal harvesting, enhancing disease moni-
parasitism, and competition, an introduced species may prolif- toring, and establishing captive breeding programs.
erate and become invasive, displacing native species. Invasive All of the main causes of biodiversity loss are intensi-
species cause billions of dollars in economic damage each year. fied by human population growth and rising per capita
The Serengeti has so far had little problem with inva-
sive species because of its large size, but ecologists fear that
a highway built through it would change this by introduc-
ing weed seeds from passing vehicles. Park managers there
already are concerned about several American plant species,
such as datura, parthenium, and prickly poppy, that are toxic
to native herbivores and that have spread rapidly in grassland
areas of Africa where they have appeared.

Climate change The preceding four types of human
impacts affect biodiversity in discrete places and times. In con-
trast, our manipulation of Earth’s climate system (Chapter 18)
is having global impacts on biodiversity. As our emissions of
greenhouse gases from fossil fuel combustion cause tempera- (a) Male golden toad from Monteverde, Costa Rica
tures to warm worldwide, we modify weather patterns and
increase the frequency of extreme weather events.
Extreme weather events such as droughts and storms All habitat loss
increase stress on populations. In the Arctic, where tem- Pollution
peratures have warmed the most, melting sea ice and other Fires
impacts (p. 529) are threatening polar bears and people alike Invasive species
(Figure 11.14). Across the world, warming temperatures are Disease
forcing organisms to shift their geographic ranges toward the Other causes
poles and higher in altitude. Some species will not be able to Unknown
adapt. Mountaintop organisms cannot move further upslope None Non-threatened species
to escape warming temperatures, so they may perish. Trees Threatened species CHAPTER 11 • Bi odiv ER si T y A nd Cons ER vAT i on Bi ology
may not be able to move toward the poles fast enough. As 0 1000 2000 3000 4000
ranges shift, animals and plants find themselves among new Number of species
communities of prey, predators, and parasites to which they (b) Causes of amphibian declines
are not adapted. All in all, scientists predict that a 1.5–2.5°C
(2.7–4.5°F) global temperature rise could put 20–30% of the Figure 11.15 The world’s amphibians are declining. The
world’s plants and animals at increased risk of extinction. golden toad (a) is one of about 170 species of amphibians that
have suddenly gone extinct in recent years. This brilliant orange
Amphibians are vanishing toad of Costa Rican cloud forests disappeared due to drought,
climate change, and/or disease. Habitat loss (b) is the main reason
Reasons for the decline of a population or species can be for amphibian declines, but many declines remain unexplained. Data
complex and difficult to determine. The worldwide collapse from IUCN, 2008. Global amphibian assessment.
of amphibians provides an example. Today entire populations What is the second greatest known cause of amphibian
of frogs, toads, and salamanders are vanishing without a trace. declines, after habitat loss? What is the greatest cause for
More than 2600 of the 6400 known species of amphibians are threatened species? 307

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