AP Biology 8.7: Disruptions in Ecosystems

Habitat destruction

The process by which a natural habitat becomes unable to support its native species. Populations may relocate or die, causing declines in biodiversity and species abundance.

Distinguish habitat loss, habitat degradation, and habitat fragmentation.

Habitat loss removes suitable habitat, degradation reduces habitat quality, and fragmentation divides a large continuous habitat into smaller isolated patches. All three can reduce population size and biodiversity.

What happens to a population's carrying capacity when its habitat is destroyed?

The carrying capacity, $K$, usually decreases because fewer resources, nesting sites, and shelters are available. Population size may consequently decline or reach extinction.

Biodiversity hotspot

A region with unusually high concentrations of endemic species that is also heavily threatened by habitat loss. Tropical regions contain many such hotspots.

Why are endemic species especially vulnerable to habitat destruction?

Endemic species have restricted geographic ranges and may depend on highly specific environmental conditions. If their local habitat is destroyed, they may have no alternative habitat in which to survive.

Natural causes of habitat destruction

Volcanism, wildfire, climate shifts, and other geological or atmospheric processes can alter or eliminate habitats. Natural destruction is documented in the fossil record and differs from the rapid, large-scale changes often caused by humans.

What human activities are major direct causes of habitat destruction?

Land conversion for agriculture, urban expansion, infrastructure construction, mining, logging, industrial production, and trawling are major direct causes.

What broad categories of underlying drivers contribute to habitat destruction?

Demographic, economic, institutional or policy, technological, cultural, and sociopolitical factors can all contribute. These drivers often interact rather than acting independently.

Why is forest conversion to agriculture a particularly important cause of tropical deforestation?

Agricultural expansion requires clearing land for crops and livestock, and it accounted for the largest share of proximate causes in the cited tropical deforestation studies. Roads and markets often intensify this conversion.

How can road construction create a positive feedback loop that accelerates habitat destruction?

Roads provide access for settlers, logging, agriculture, and commerce. These activities create additional demand for roads and cleared land, causing still more habitat loss.

Why can monoculture agriculture reduce biodiversity even when land remains vegetated?

A monoculture replaces a complex community with one dominant crop species and removes habitat for many plants, animals, and microorganisms. It therefore provides fewer ecological niches and genetic resources.

How can agriculture damage habitat beyond the area directly cleared?

Agricultural runoff can transport nutrients, salts, pesticides, and sediment into surrounding ecosystems. This can cause pollution, eutrophication, salinization, and degradation of aquatic habitats.

What environmental problems can result from desertification?

Desertification converts productive drylands into increasingly degraded, arid environments. It reduces vegetation and biodiversity and can result from unsustainable agriculture, erosion, nutrient depletion, and water mismanagement.

How do erosion, salinization, compaction, and nutrient depletion reduce agricultural productivity?

Erosion removes nutrient-rich topsoil, salinization makes water uptake more difficult for plants, compaction reduces pore space and infiltration, and nutrient depletion limits plant growth. Habitat destruction around farmland can contribute to all of these processes.

How does climate change destroy or alter habitats?

Rising temperatures, melting sea ice, sea-level rise, coastal flooding, erosion, and changing precipitation patterns can make existing habitats unsuitable. Species may be unable to migrate or adapt quickly enough.

Why does melting sea ice threaten marine food webs?

Sea ice provides physical habitat for organisms such as algae and polar bears. Loss of ice removes habitat at the base and higher levels of the food web and changes energy flow through the ecosystem.

Ocean acidification

The decrease in ocean pH caused primarily by absorption of atmospheric $CO_2$. Greater hydrogen-ion concentration can reduce carbonate-ion availability and make calcification more difficult for corals and other marine organisms.

What chemical process links increased atmospheric carbon dioxide to ocean acidification?

Dissolved carbon dioxide reacts with water to form carbonic acid: $CO_2(aq)+H_2O(l)\rightleftharpoons H_2CO_3(aq)$. Carbonic acid dissociates to produce hydrogen ions: $H_2CO_3\rightleftharpoons H^++HCO_3^-$. Additional dissolved $CO_2$ therefore lowers ocean pH.

What is the relationship between pH and hydrogen-ion concentration?

The relationship is $pH=-\log[H^+]$. An increase in $[H^+]$ corresponds to a decrease in pH.

How does ocean acidification threaten coral reefs?

Lower pH reduces the availability of carbonate ions needed to form calcium carbonate structures. Because reefs provide habitat for thousands of species and protect coasts, reef degradation has broad ecological and human consequences.

How does habitat destruction often change community composition?

Specialist species tend to decline first because they require narrow environmental conditions. Generalist species, including many invasive species, are more likely to persist and become a larger fraction of the remaining community.

How can invasive species contribute to ecosystem disruption?

Invasive species can spread through disturbed habitats and compete with, prey on, or otherwise displace native species. Because many are ecological generalists, they may persist across a wide range of conditions and contribute to declining native biodiversity.

How can habitat destruction reduce genetic diversity within a species?

Small, isolated populations experience reduced gene flow and are more likely to mate with relatives. This can increase inbreeding, reduce genetic variation, and lower reproductive success.

Extinction debt

The delayed loss of species after habitat destruction has already occurred. Populations may persist temporarily but remain too small or isolated to survive long term.

Ecosystem service

A benefit humans obtain from ecosystems, such as climate regulation, pollination, water purification, flood control, nutrient recycling, oxygen production, and carbon sequestration.

Why are wetlands important for human populations?

Wetlands store and slow water, reducing flooding, and can filter pollutants from water. Draining or developing them can increase flood damage and degrade water quality.

How can habitat destruction increase the severity of floods and droughts?

Removing vegetation and wetlands reduces water storage, infiltration, and evapotranspiration. More runoff can worsen floods, while less water retention can intensify drought conditions.

How can habitat destruction affect disease transmission?

Changes such as wetland drainage or ecosystem disturbance can alter the abundance and habitat of disease vectors and hosts. Ecosystem management can sometimes reduce opportunities for infectious disease transmission.

How can habitat destruction alter nutrient cycles and water quality?

Removing vegetation and disturbing soils changes the cycling of nitrogen, phosphorus, sulfur, and carbon. Excess nutrients can cause algal blooms and oxygen depletion, while erosion and pollutants can contaminate rivers and oceans.

Eutrophication

The enrichment of an aquatic system with nutrients, especially nitrogen and phosphorus, often from agricultural or sewage runoff. Excessive algal growth followed by decomposition can reduce dissolved oxygen and cause fish kills.

How can algal blooms lead to fish kills?

Nutrient enrichment promotes algal growth. When algae die, microbial decomposition consumes dissolved oxygen, potentially producing hypoxic or anoxic conditions that kill fish and other aerobic organisms.

How does acid rain relate to habitat destruction?

Atmospheric sulfur- and nitrogen-containing pollutants can form acids in water droplets and precipitation. Acidification of soils and aquatic systems can damage vegetation, alter nutrient availability, and harm aquatic organisms.

How does photosynthesis allow forests to regulate atmospheric carbon dioxide?

Photosynthetic organisms remove carbon dioxide from the atmosphere and store carbon in organic molecules: $6CO_2+6H_2O\rightarrow C_6H_{12}O_6+6O_2$. Forest destruction reduces this carbon sink and can release stored carbon.

Carbon sequestration

The capture and long-term storage of carbon, often by photosynthetic organisms or soils. Forests, especially tropical forests, sequester carbon and therefore help limit the accumulation of atmospheric greenhouse gases.

How can deforestation contribute to climate change?

It removes photosynthetic carbon sinks and often releases carbon stored in vegetation and soil through burning or decomposition. The resulting increase in atmospheric $CO_2$ enhances the greenhouse effect.

Why can the destruction of natural habitat harm agriculture itself?

Agriculture depends on ecosystem services such as pollination, soil formation, nutrient recycling, water regulation, and natural pest control. Removing surrounding habitat can weaken these services and reduce crop productivity.

What is the opportunity cost of converting a forest to farmland?

The land may produce more food, but society loses services and resources such as carbon storage, clean water, flood regulation, timber, biodiversity, and ecotourism. The best choice depends on comparing the total benefits and costs of each use.

How can biodiversity increase ecosystem resilience?

Species perform different ecological roles and may respond differently to environmental stress. Greater biodiversity increases the likelihood that some organisms can maintain ecosystem functions after disturbances such as floods, droughts, or fires.

Why can the loss of biodiversity have consequences for medicine and agriculture?

Species may contain genes or chemicals useful for medicines, pest resistance, disease resistance, or improved crop varieties. Extinction permanently removes these potential biological resources.

Why do poorer rural populations often experience habitat destruction more directly?

They frequently depend on nearby forests, fisheries, soils, and clean water for food and income. Wealthier populations may be better able to purchase substitutes or obtain resources from elsewhere.

What is a major long-term challenge connecting population growth, food production, and habitat destruction?

A growing population increases demand for food, but most land suitable for agriculture is already being used or degraded. Increasing production by clearing additional habitat is therefore increasingly difficult to sustain.