The terms habitat loss and habitat reduction are also used in a wider sense, including loss of habitat from other factors, such as water and noise pollution.
Impacts on organisms
In
the simplest term, when a habitat is destroyed, the plants, animals,
and other organisms that occupied the habitat have a reduced carrying capacity so that populations decline and extinction becomes more likely. Perhaps the greatest threat to organisms and biodiversity is the process of habitat loss. Temple (1986) found that 82% of endangered bird species were significantly threatened by habitat loss. Most amphibian species are also threatened by habitat loss, and some species are now only breeding in modified habitat. Endemic
organisms with limited ranges are most affected by habitat destruction,
mainly because these organisms are not found anywhere else within the
world, and thus have less chance of recovering. Many endemic organisms
have very specific requirements for their survival that can only be
found within a certain ecosystem, resulting in their extinction.
Extinction may also take place very long after the destruction of
habitat, a phenomenon known as extinction debt.
Habitat destruction can also decrease the range of certain organism
populations. This can result in the reduction of genetic diversity and
perhaps the production of infertile
youths, as these organisms would have a higher possibility of mating
with related organisms within their population, or different species.
One of the most famous examples is the impact upon China's giant panda,
once found across the nation. Now it is only found in fragmented and
isolated regions in the southwest of the country, as a result of
widespread deforestation in the 20th century.
Geography
Satellite photograph of deforestation in Bolivia. Originally dry tropical forest, the land is being cleared for soybean cultivation.
Biodiversity hotspots are chiefly tropical regions that feature high concentrations of endemic species and, when all hotspots are combined, may contain over half of the world’s terrestrial species.
These hotspots are suffering from habitat loss and destruction.
Most of the natural habitat on islands and in areas of high human
population density has already been destroyed (WRI, 2003). Islands
suffering extreme habitat destruction include New Zealand, Madagascar, the Philippines, and Japan. South and East Asia — especially China, India, Malaysia, Indonesia, and Japan — and many areas in West Africa have extremely dense human populations that allow little room for natural habitat. Marine areas close to highly populated coastal cities also face degradation of their coral reefs or other marine habitat. These areas include the eastern coasts of Asia and Africa, northern coasts of South America, and the Caribbean Sea and its associated islands.
Regions of unsustainable agriculture or unstable governments, which may go hand-in-hand, typically experience high rates of habitat destruction. Central America, Sub-Saharan Africa, and the Amazonian tropical rainforest areas of South America are the main regions with unsustainable agricultural practices and/or government mismanagement.
Areas of high agricultural output tend to have the highest extent
of habitat destruction. In the U.S., less than 25% of native
vegetation remains in many parts of the East and Midwest. Only 15% of land area remains unmodified by human activities in all of Europe.
Ecosystems
Jungle burned for agriculture in southern Mexico
Tropical rainforests
have received most of the attention concerning the destruction of
habitat. From the approximately 16 million square kilometers of tropical
rainforest habitat that originally existed worldwide, less than 9
million square kilometers remain today. The current rate of deforestation is 160,000 square kilometers per year, which equates to a loss of approximately 1% of original forest habitat each year.
Other forest ecosystems have suffered as much or more destruction as tropical rainforests. Farming and logging have severely disturbed at least 94% of temperate broadleaf forests; many old growth forest stands have lost more than 98% of their previous area because of human activities. Tropical deciduous dry forests are easier to clear and burn and are more suitable for agriculture and cattle ranching than tropical rainforests; consequently, less than 0.1% of dry forests in Central America's Pacific Coast and less than 8% in Madagascar remain from their original extents.
Farmers near newly cleared land within Taman Nasional Kerinci Seblat (Kerinci Seblat National Park), Sumatra.
Plains and desert areas have been degraded to a lesser extent. Only 10-20% of the world's drylands, which include temperate grasslands, savannas, and shrublands, scrub, and deciduous forests, have been somewhat degraded.
But included in that 10-20% of land is the approximately 9 million
square kilometers of seasonally dry-lands that humans have converted to
deserts through the process of desertification. The tallgrass prairies of North America, on the other hand, have less than 3% of natural habitat remaining that has not been converted to farmland.
Wetlands
and marine areas have endured high levels of habitat destruction. More
than 50% of wetlands in the U.S. have been destroyed in just the last
200 years. Between 60% and 70% of European wetlands have been completely destroyed.
In the United Kingdom, there has been an increase in demand for coastal
housing and tourism which has caused a decline in marine habitats over
the last 60 years. The rising sea levels and temperatures have caused soil erosion, coastal flooding, and loss of quality in the UK marine ecosystem. About one-fifth (20%) of marine coastal areas have been highly modified by humans. One-fifth of coral reefs have also been destroyed, and another fifth has been severely degraded by overfishing, pollution, and invasive species; 90% of the Philippines’ coral reefs alone have been destroyed. Finally, over 35% of the mangrove ecosystems worldwide have been destroyed.
Natural causes
Habitat destruction through natural processes such as volcanism, fire, and climate change is well documented in the fossil record.
One study shows that habitat fragmentation of tropical rainforests in
Euramerica 300 million years ago led to a great loss of amphibian
diversity, but simultaneously the drier climate spurred on a burst of
diversity among reptiles.
Human causes
Deforestation and roads in Amazonia, the Amazon Rainforest.
Habitat destruction caused by humans includes land conversion from forests, etc. to arable land, urban sprawl, infrastructure development, and other anthropogenic changes to the characteristics of land. Habitat degradation, fragmentation, and pollution
are aspects of habitat destruction caused by humans that do not
necessarily involve over destruction of habitat, yet result in habitat
collapse. Desertification, deforestation, and coral reef degradation are specific types of habitat destruction for those areas (deserts, forests, coral reefs).
Geist and Lambin (2002) assessed 152 case studies of net losses
of tropical forest cover to determine any patterns in the proximate and
underlying causes of tropical deforestation. Their results, yielded as
percentages of the case studies in which each parameter was a
significant factor, provide a quantitative prioritization of which
proximate and underlying causes were the most significant. The proximate
causes were clustered into broad categories of agricultural expansion (96%), infrastructure expansion (72%), and wood extraction (67%). Therefore, according to this study, forest conversion
to agriculture is the main land use change responsible for tropical
deforestation. The specific categories reveal further insight into the
specific causes of tropical deforestation: transport extension (64%),
commercial wood extraction (52%), permanent cultivation (48%), cattle ranching (46%), shifting (slash and burn) cultivation (41%), subsistence agriculture (40%), and fuel wood extraction for domestic use (28%). One result is that shifting cultivation is not the primary cause of deforestation in all world regions, while transport extension (including the construction of new roads) is the largest single proximate factor responsible for deforestation.
Global warming
Rising global temperatures, caused by the greenhouse effect, contribute to habitat destruction, endangering various species, such as the polar bear. Melting ice caps promote rising sea levels and floods which threaten natural habitats and species globally.
Drivers
Nanjing Road in Shanghai
While the above-mentioned activities are the proximal or direct
causes of habitat destruction in that they actually destroy habitat,
this still does not identify why humans destroy habitat. The forces
that cause humans to destroy habitat are known as drivers of habitat destruction. Demographic, economic, sociopolitical, scientific and technological, and cultural drivers all contribute to habitat destruction.
Demographic drivers include the expanding human population; rate of population increase over time; spatial distribution of people in a given area (urban
versus rural), ecosystem type, and country; and the combined effects of
poverty, age, family planning, gender, and education status of people
in certain areas. Most of the exponential human population growth worldwide is occurring in or close to biodiversity hotspots. This may explain why human population density accounts for 87.9% of the variation in numbers of threatened species across 114 countries, providing indisputable evidence that people play the largest role in decreasing biodiversity. The boom in human population and migration of people into such species-rich regions are making conservation efforts not only more urgent but also more likely to conflict with local human interests.
The high local population density in such areas is directly correlated
to the poverty status of the local people, most of whom lacking an
education and family planning.
From the Geist and Lambin (2002) study described in the previous
section, the underlying driving forces were prioritized as follows (with
the percent of the 152 cases the factor played a significant role in):
economic factors (81%), institutional or policy factors (78%),
technological factors (70%), cultural or socio-political factors (66%),
and demographic factors (61%). The main economic factors included
commercialization and growth of timber markets (68%), which are driven
by national and international demands; urban industrial growth (38%);
low domestic costs for land, labor, fuel, and timber (32%); and
increases in product prices mainly for cash crops (25%). Institutional
and policy factors included formal pro-deforestation policies on land
development (40%), economic growth including colonization and
infrastructure improvement (34%), and subsidies for land-based
activities (26%); property rights and land-tenure insecurity (44%); and
policy failures such as corruption, lawlessness, or mismanagement (42%).
The main technological factor was the poor application of technology
in the wood industry (45%), which leads to wasteful logging practices.
Within the broad category of cultural and sociopolitical factors are
public attitudes and values (63%), individual/household behavior (53%),
public unconcern toward forest environments (43%), missing basic values
(36%), and unconcern by individuals (32%). Demographic factors were the
in-migration of colonizing settlers into sparsely populated forest
areas (38%) and growing population density—a result of the first
factor—in those areas (25%).
There are also feedbacks and interactions among the proximate and
underlying causes of deforestation that can amplify the process. Road
construction has the largest feedback effect, because it interacts
with—and leads to—the establishment of new settlements and more people,
which causes a growth in wood (logging) and food markets.
Growth in these markets, in turn, progresses the commercialization of
agriculture and logging industries. When these industries become
commercialized, they must become more efficient by utilizing larger or
more modern machinery that often has a worse effect on the habitat than
traditional farming and logging methods. Either way, more land is
cleared more rapidly for commercial markets. This common feedback
example manifests just how closely related the proximate and underlying
causes are to each other.
Impact on human population
The
draining and development of coastal wetlands that previously protected
the Gulf Coast contributed to severe flooding in New Orleans, Louisiana
in the aftermath of Hurricane Katrina.
Habitat destruction vastly increases an area's vulnerability to natural disasters like flood and drought, crop failure, spread of disease, and water contamination.
On the other hand, a healthy ecosystem with good management practices
will reduce the chance of these events happening, or will at least
mitigate adverse impacts.
Agricultural land can actually suffer from the destruction of the
surrounding landscape. Over the past 50 years, the destruction of
habitat surrounding agricultural land has degraded approximately 40% of
agricultural land worldwide via erosion, salinization, compaction, nutrient depletion, pollution, and urbanization. Humans also lose direct uses of natural habitat when habitat is destroyed. Aesthetic uses such as birdwatching, recreational uses like hunting and fishing, and ecotourism
usually rely upon virtually undisturbed habitat. Many people value the
complexity of the natural world and are disturbed by the loss of natural
habitats and animal or plant species worldwide.
Probably the most profound impact that habitat destruction has on people is the loss of many valuable ecosystem services. Habitat destruction has altered nitrogen, phosphorus, sulfur, and carbon cycles, which has increased the frequency and severity of acid rain, algal blooms, and fish kills in rivers and oceans and contributed tremendously to global climate change. One ecosystem service whose significance is becoming better understood is climate regulation. On a local scale, trees provide windbreaks and shade; on a regional scale, plant transpiration
recycles rainwater and maintains constant annual rainfall; on a global
scale, plants (especially trees from tropical rainforests) from around
the world counter the accumulation of greenhouse gases in the atmosphere by sequestering carbon dioxide through photosynthesis. Other ecosystem services that are diminished or lost altogether as a result of habitat destruction include watershed management, nitrogen fixation, oxygen production, pollination, waste treatment (i.e., the breaking down and immobilization of toxic pollutants), and nutrient recycling of sewage or agricultural runoff.
The loss of trees from the tropical rainforests alone represents a
substantial diminishing of the earth’s ability to produce oxygen and
use up carbon dioxide. These services are becoming even more important
as increasing carbon dioxide levels is one of the main contributors to global climate change.
The loss of biodiversity
may not directly affect humans, but the indirect effects of losing many
species as well as the diversity of ecosystems in general are enormous.
When biodiversity is lost, the environment loses many species that
perform valuable and unique roles in the ecosystem. The environment and
all its inhabitants rely on biodiversity to recover from extreme
environmental conditions. When too much biodiversity is lost, a
catastrophic event such as an earthquake, flood, or volcanic eruption
could cause an ecosystem to crash, and humans would obviously suffer
from that. Loss of biodiversity also means that humans are losing
animals that could have served as biological control agents and plants
that could potentially provide higher-yielding crop varieties,
pharmaceutical drugs to cure existing or future diseases or cancer, and
new resistant crop varieties for agricultural species susceptible to
pesticide-resistant insects or virulent strains of fungi, viruses, and bacteria.
The negative effects of habitat destruction usually impact rural populations more directly than urban populations.
Across the globe, poor people suffer the most when natural habitat is
destroyed, because less natural habitat means fewer natural resources
per capita, yet wealthier people and countries simply have to pay more
to continue to receive more than their per capita share of natural
resources.
Another way to view the negative effects of habitat destruction
is to look at the opportunity cost of destroying a given habitat. In
other words, what are people losing out on by taking away a given
habitat? A country may increase its food supply by converting forest
land to row-crop agriculture, but the value of the same land may be much
larger when it can supply natural resources or services such as clean
water, timber, ecotourism, or flood regulation and drought control.
Outlook
The rapid expansion of the global human population
is increasing the world’s food requirement substantially. Simple logic
dictates that more people will require more food. In fact, as the
world’s population increases dramatically, agricultural output will need
to increase by at least 50%, over the next 30 years.
In the past, continually moving to new land and soils provided a boost
in food production to meet the global food demand. That easy fix will no
longer be available, however, as more than 98% of all land suitable for
agriculture is already in use or degraded beyond repair.
The impending global food crisis
will be a major source of habitat destruction. Commercial farmers are
going to become desperate to produce more food from the same amount of
land, so they will use more fertilizers
and show less concern for the environment to meet the market demand.
Others will seek out new land or will convert other land-uses to
agriculture. Agricultural intensification will become widespread at the
cost of the environment and its inhabitants. Species will be pushed out
of their habitat either directly by habitat destruction or indirectly by
fragmentation, degradation, or pollution.
Any efforts to protect the world’s remaining natural habitat and
biodiversity will compete directly with humans’ growing demand for
natural resources, especially new agricultural lands.
Solutions
Chelonia mydas
on a Hawaiian coral reef. Although the endangered species is protected,
habitat loss from human development is a major reason for the loss of green turtle nesting beaches.
In most cases of tropical deforestation, three to four underlying causes are driving two to three proximate causes.
This means that a universal policy for controlling tropical
deforestation would not be able to address the unique combination of
proximate and underlying causes of deforestation in each country.
Before any local, national, or international deforestation policies are
written and enforced, governmental leaders must acquire a detailed
understanding of the complex combination of proximate causes and
underlying driving forces of deforestation in a given area or country.
This concept, along with many other results of tropical deforestation
from the Geist and Lambin study, can easily be applied to habitat
destruction in general. Governmental leaders need to take action by
addressing the underlying driving forces, rather than merely regulating
the proximate causes. In a broader sense, governmental bodies at a
local, national, and international scale need to emphasize the
following:
- Considering the many irreplaceable ecosystem services provided by natural habitats.
- Protecting remaining intact sections of natural habitat.
- Educating the public about the importance of natural habitat and biodiversity.
- Developing family planning programs in areas of rapid population growth.
- Finding ecological ways to increase agricultural output without increasing the total land in production.
- Preserving habitat corridors to minimize prior damage from fragmented habitats.
- Reducing human population and expansion. Apart from improving access to contraception globally, furthering gender equality also has a great benefit. When women have the same education (decision-making power), this generally leads to smaller families.
