Chapter 59: The Ecological Impact of Humans

Human Population Growth

Learning Outcomes

1. Understanding Differences in Age Structure and Fertility: Ability to describe how variations in age structures and human fertility across countries impact population growth.

2. Ecological Footprint Concept: Explanation of the ecological footprint concept and its implications.

3. Solutions to Lower Population Growth Rates: Discussion of potential solutions available for reducing growth rates of human populations.

Overview of Global Population Growth

The global human population continues to increase, albeit at a slower rate than historical averages recorded since 1950. A significant two-thirds of the anticipated population growth will happen even if the birth rate immediately decreases to two births per female. This phenomenon is attributed to the substantial number of children and youths in high-fertility regions. Notably, over half of the anticipated growth until 2050 is expected to occur in nine countries:

• Democratic Republic of the Congo

• Egypt

• Ethiopia

• India

• Indonesia

• Nigeria

• Pakistan

• United Republic of Tanzania

• United States

This section will delve deeper into the trends in human population growth, elucidate how the age structure and fertility levels can forecast future growth, and explore the carrying capacity of Earth along with the concept of ecological footprint, which quantifies human resource utilization to evaluate this carrying capacity.

Historical Context of Human Population Growth

Before the dawn of agriculture and the domestication of animals, around 10,000 B.C.E., the average human population growth rate was minimal. The advent of agriculture catalyzed a gradual increase in the world's population, which reached approximately 800 million by 1750. The Industrial Revolution and the emergence of modern medicine radically accelerated population growth rates. From 1750 to 2022, the world witnessed a surge in human population from 800 million to 8 billion. The continued interest among scientists is to ascertain when and at what population size human growth will stabilize.

Age Structure and Future Population Predictions

Defining Age Structure

Age structure in populations refers to the relative distribution of individuals within defined age categories. This information is often visualized through a population pyramid, which provides insights into growth potential.

Example Comparison: Sub-Saharan Africa vs. Western Europe

In Sub-Saharan Africa, individuals below the age of 15 constitute over 40% of the total population, leading to a population pyramid characterized by a wide base and a narrow apex (Figure 59.2a). Even with declining fertility rates, the population is likely to see substantial increases as this sizeable cohort matures into childbearing age. Conversely, Western Europe features a more uniform age structure (Figure 59.2b). Even if fertility rates among young females in this region exceed those of their predecessors, the overall number of births will remain low due to the limited number of females in childbearing age.

Conceptual Application

• If the population pyramid in Figure 59.2a were inverted, one could deduce that the population has a higher proportion of older individuals, indicative of lower fertility rates and potentially stagnant or declining growth.

Fertility Rates and Global Population Growth

Total Fertility Rate (TFR)

The total fertility rate (TFR) quantifies the average number of live births a female is expected to have In her lifetime (Figure 59.3). Fertility rates across different geographic areas exhibit considerable variability.

Fertility Rates Across Regions:

• Africa (2015-2020): TFR of 4.4 children per female, a marked decline from 6.7 in the 1970s.

• Latin America and Southeast Asia: Current rates are around 2.1, reflecting significant declines since the 1970s.

• Canada & Most European Countries: TFR is below 2.0.

• United States (2015-2020): TFR of 1.78.

• Russia: TFR of 1.82.

• China: TFR of 1.69, yet population growth is projected to continue until at least 2025 due to a substantial number of women of reproductive age.

Historical Trend and Future Projections

Currently, the global TFR has decreased from 4.47 in the 1970s to 2.47 in 2015-2020, still above the 2.3 benchmark necessary for zero population growth. The replacement rate, which slightly exceeds 2.0 due to natural mortality rates, varies geographically from 2.1 to 3.3.

Population Growth Scenarios by the UN

A UN report from 2010 projected global population outcomes to 2100 under three distinct growth scenarios based on varying fertility rates:

• Low scenario (TFR of 1.5): Population peaks around 8 billion by 2050.

• Medium scenario (TFR of 2.0): Population grows likely to 10 billion.

• High scenario (TFR of 2.5): Potential population of nearly 16 billion.

The UN’s most recent figures predict a population peak of 10.4 billion in the 2080s, representing a 30% increase over the current population figure of roughly 8 billion (Figure 59.4).

Influences on Population Behavior

Family Size Preferences

Surveys indicate that actual family size aligns closely with desired family size among older females, though this may stem from rationalization of existing family structures. In contrast, younger females (aged 15-19) globally show a preference for having fewer than three children. In the United States, desired family sizes have declined to about 2.5, down from 3.5 in the 1930s.

Strategies for Mitigating Population Growth

Recommendations and Strategies

To dampen population growth rates, various strategies can be implemented:

1. Secondary Education for Young Females: For instance, uneducated African females typically have an average of 5.4 children, while those with secondary schooling have 2.7, and college-educated females average 2.2 children.

2. Eradication of Gender Bias: Addressing legal and economic inequalities promotes smaller family sizes; women with property rights and equal access to credit and divorce tend to have fewer children.

3. Family Planning Programs: As of 2019, 76% of women globally had access to modern contraceptive methods; this figure drops substantially in high-fertility nations. Noteworthy examples include Iran, which experienced a decline in TFR from 5.6 to 2.6 after introducing a national family planning program, and Thailand, which reduced its TFR by 75% in two generations through similar methods.

4. Child Mortality Reduction: By decreasing child mortality rates, the necessity for high birth rates to ensure offspring survival diminishes.

Understanding Earth's Carrying Capacity

Definition of Carrying Capacity

Carrying capacity is the maximum sustainable population size that an environment can support. It is crucial to understand Earth's carrying capacity for humans and when it may be reached. There are vast variances in estimates, primarily depending on lifestyle choices.

Lifestyle Examples

For instance, if global consumption trends mirror a diet high in meat and car dependency, the carrying capacity would be lower compared to a population that predominantly consumes a vegetarian diet and utilizes bicycles for transportation.

Ecological Footprint Concept

In the 1990s, Mathis Wackernagel and associates devised a method to calculate the land necessary to sustain each person on the planet, termed the ecological footprint. This encompasses the consumption of land resources, including crops, forest products, fossil fuels, and minerals. The average ecological footprint was approximately 2.75 hectares per person in 2021 (with 1 ha = 10,000 m²). A global ecological footprint exceeding 1.73 hectares indicates unsustainability—resources are being consumed faster than they can be regenerated. The ecological footprint for the average American was cited at 8.1 hectares.

Challenges and Implications of Overconsumption

If the population lived with an ecological footprint analogous to the average American, it would necessitate the resources of five Earths to sustain them. The current global trajectory of unsustainable living—characterized by overconsumption—indicates we have exceeded Earth's carrying capacity. This trend contributes to grave environmental shifts, including habitat destruction, overfishing, pollution, and, significantly, climate change.

Global Warming and Climate Change

Learning Outcomes

1. Greenhouse Effect and Global Warming: Understanding the relationship between the greenhouse effect and global warming.

2. Impacts of Global Warming: Effects of global warming on sea levels, precipitation patterns, and extreme weather events.

3. Climate Change and Ecology: Predicting climate change effects on species distribution, phenology, and prevalence of pests and diseases.

4. Mitigation Strategies: Discussing strategies to lower atmospheric carbon dioxide levels.

Introduction to Global Warming

Global warming refers to an increase in Earth's average surface temperature. Ecologists are investigating the role of human activities in accelerating global warming, which is a significant catalyst for climate change—a long-term shift in Earth's climate.

Greenhouse Effect

The greenhouse effect is crucial for understanding global warming. This phenomenon operates similarly to the glass panes of a greenhouse:

• Mechanism: Solar radiation enters Earth's atmosphere as short-wave energy, heating the surface. Earth then radiates this energy back into space as long-wave infrared radiation. Atmospheric gases absorb a considerable portion of this radiation and subsequently re-radiate it back towards Earth's surface, further increasing temperature.

• Importance: Without the greenhouse effect, Earth's average temperature would be approximately -17°C. In contrast, the current average temperature is around +15°C, showcasing the greenhouse effect's crucial role in sustaining life.

Major Greenhouse Gases

The greenhouse effect primarily involves particular gases in Earth's atmosphere, consisting of less than 1% of its total volume. The following table summarizes critical greenhouse gases:

Established Human Impact on Greenhouse Gases

Human activities significantly increase greenhouse gas concentrations in the atmosphere, notably through the burning of fossil fuels. Fossil fuels (oil, coal, natural gas) contain high carbon content, contributing greatly to atmospheric CO₂ levels when combusted. From the late 18th century to present, atmospheric concentrations of CO₂ rose from approximately 280 ppm to 420 ppm, with notable increases in the last 60 years.

• Mauna Loa Observations: Since 1957, atmospheric CO₂ levels recorded at Mauna Loa have shown a 34% increase from 313 ppm to 420 ppm, illustrating this trend, and seasonal variations in CO₂ levels reflect plant absorption patterns between hemispheres.

Rising Sea Levels Due to Global Warming

Projections indicate that, by the year 2100, atmospheric CO₂ could double to around 700 ppm, leading to an anticipated global temperature increase of 2.1-3.5°C, comparable to the end of the last Ice Age. Consequences include significant sea-level rise driven by thermal expansion of water and glacial melting—estimated to be about 45-75 cm due to this temperature increase. Currently, sea levels have risen approximately 15-25 cm over the past century, further accentuating coastal issues, such as tree kill-off in coastal forests.

• Time Lag Effect: It's crucial to note a lag between warming and ice melt rates, where even ceasing greenhouse gas emissions won't stop glacier melting immediately; for example, Greenland's ice sheets will continue melting for centuries despite reaching target temperatures in the near future.

Alterations in Precipitation Patterns

Global warming is expected to modify precipitation patterns globally: warmer temperatures raise evaporation rates, with air holding 7% more moisture for every 1°C increase. This could yield heavier rainfall but might also exacerbate droughts in already dry regions. Historical data shows a global average increase in precipitation of 2 mm per year since 1901.

• Analogy to a Sponge: The atmosphere acts like a sponge that absorbs more moisture in humid conditions, while dry areas struggle to receive adequate rainfall.

• Shifting Patterns: Some regions, such as northern and southern temperate zones, are experiencing increased precipitation, while arid regions like the Sahel are becoming drier, indicating a trend where wet regions become wetter and dry areas exhibit further aridity.

Increase in Extreme Weather Events

Global warming will likely escalate the frequency and severity of extreme weather events, including heavier rainfall, flooding, drought, and stronger hurricanes. As ocean temperatures rise, so do hurricane strengths, resulting in elevated storm surges and compounded flooding in coastal areas.

• Seasonal Changes in Precipitation: Changes in winter precipitation types (from snow to rain) contribute to less snow and ice cover, affecting global glacier dynamics. Over approximately 40 years, Arctic sea ice has diminished by 40%.

Climate Change and Species Distribution

As Earth's climate becomes warmer, the distribution patterns of many plant and animal species are expected to shift. However, many plants, especially trees, take an extended time for seed dispersal needed to thrive in new climatic regions.

• Margaret Davis's Research: Paleobotanist Margaret Davis highlighted that in response to CO₂ doubling, tree species like sugar maples could suffer range contractions, losing southern ranges while potentially gaining new habitats in Canada, but the speed of climate change might outpace their ability to migrate or adapt. Davis’s studies indicate that both temperature and altered precipitation significantly affect distribution predictions for such species.

Effects on Phenology and Life Cycle Events

Global warming has affected phenology, the timing of life cycle events in many species:

• Phenological Changes: Observed shifts include earlier flowering in plants, earlier arrival of butterflies and migrating birds, and changes in egg-laying dates for birds. These alterations could disrupt important ecological interactions, such as mismatches in the timing of herbivore emergence and plant blooming.

Pest and Disease Dynamics

Global warming may accelerate the spread and intensity of pests and diseases:

• Emergence of New Disease Vectors: Warmer temperatures could facilitate the expansion of disease vectors such as mosquitoes, which carry diseases like malaria, yellow fever, and dengue fever, leading to both increased incidence and broader geographic ranges for these diseases.

• Outbreaks of Pathogens: Fungal pathogens have been noted to appear more frequently due to climate impacts, and pests like mountain pine beetles have devastated vast pine forests due to their increased survival rates in warmer temperatures.

Strategies for Lowering Atmospheric CO₂ Levels

Several strategies exist to mitigate global warming effects by reducing atmospheric CO₂:

1. Renewable Energy: Transitioning from fossil fuel reliance to renewable energy sources (solar, wind, hydroelectric) can substantially mitigate CO₂ emissions. While nuclear power can also help, it presents long-term waste disposal challenges.

2. Energy Conservation: Implementing energy-saving practices (e.g., using efficient transportation) aids in CO₂ reduction. Nevertheless, the longevity of greenhouse gas effects necessitates broader systemic changes.

3. CO₂ Removal: The Intergovernmental Panel on Climate Change (IPCC) suggests a crucial need to remove 730 billion tonnes of CO₂ to keep warming below 1.5°C. Suggested methods include reforestation and restoring degraded lands to natural forests to increase carbon capture.

4. Carbon Tax: Enacting a carbon tax is proposed as an effective economic approach to curtail fossil fuel usage while incentivizing sustainable practices, although public acceptance remains an issue as demonstrated during the 2018 yellow vest protests in France.

Learning Outcomes

1. Outline the steps of the carbon cycle and describe the environmental effects of elevated atmospheric concentrations of CO₂.

2. Describe the processes of the water cycle and explain how they are affected by humans.

3. Describe the role of phosphorus in lake eutrophication.

4. List the five main steps of the nitrogen cycle and identify the human influences on it.

5. Describe some methods used to combat pollution.

Overview of Energy Flow and Chemical Cycles in Ecosystems

A unit of energy moves through an ecosystem only once, passing through the trophic levels of a food web from producer to consumer and dissipating as heat. In contrast, chemical elements such as carbon or nitrogen follow a cycle, moving from the physical environment to organisms and back to the environment, where the cycle begins again. The movements of chemicals through ecosystems involve biological, geological, and chemical transport mechanisms, thus termed biogeochemical cycles.

Mechanisms Involved in Biogeochemical Cycles

• Biological mechanisms are involved in the absorption of chemicals by living organisms and their subsequent release back into the environment.

• Geological mechanisms include the weathering and erosion of rocks and transportation of elements by both surface and subsurface drainage.

• Chemical transport mechanisms consist of dissolved matter in rain and snow, atmospheric gases, and dust blown by the wind.

Essential Elements for Life

In addition to the basic building blocks of carbon, hydrogen, and oxygen, the elements required in the greatest amounts by living organisms are phosphorus and nitrogen. There are two broad types of biogeochemical cycles:

1. Local cycles, such as the phosphorus cycle, which involve elements without an atmospheric mechanism for long-distance transfer.

2. Global cycles, which involve interchange between the atmosphere and the ecosystem.

Human activities, including burning fossil fuels and fertilizer use, impact these cycles significantly.

The Carbon Cycle

The movement of carbon from the atmosphere into organisms and back again is known as the carbon cycle. The rates of the processes in this cycle vary greatly.

Carbon Reservoirs

• Some carbon becomes locked up in reservoirs with low turnover rates, such as:

  • Fossil fuels (coal, natural gas, and oil).

  • Deposits formed from marine organisms incorporating carbon into their shells, resulting in limestone deposits on the ocean floor or terrestrial rocks.

Rapid Turnover in Phototrophs

• Carbon in phototrophs (plants, algae, and cyanobacteria) turns over rapidly as these organisms acquire CO₂ via photosynthesis, removing approximately one-seventh of the atmospheric CO₂ each year.

• Respiration and decomposition of phototrophs recycle a similar amount back into the atmosphere as CO₂.

Human Impact

• Human activities, primarily burning fossil fuels, increase atmospheric CO₂. Deforestation also raises CO₂ levels due to decreased vegetation for CO₂ absorption.

• Over recent decades, direct measurements show a steady rise in atmospheric CO₂ levels.

Environmental Consequences of Elevated CO₂

• Elevated CO₂ levels are linked to global warming, increased ocean acidity due to the formation of carbonic acid, which affects shell growth in marine species, and potential reproductive disorders in fish.

• While higher CO₂ might enhance plant growth, it can also lead to reduced herbivory.

The Water Cycle

The water cycle, or hydrological cycle, is characterized by minimal chemical change in the water itself. It is mainly driven by the Sun's energy, involving both evaporation and precipitation.

Key Components

• Evapotranspiration: 90% of the moisture reaching the atmosphere from land is through plants.

• Only about 2% of Earth's water is found in organisms or frozen states. The remainder cycles from bodies of water to the atmosphere and back.

Human Influences on the Water Cycle

• Climate change alters global precipitation patterns and accelerates glacier melting.

• Human developments, such as dams, significantly alter water flow and disrupt fish migration.

• Over-extraction of groundwater for drinking water can lead to dried lakes and aquifers, exacerbating water shortages.

The Phosphorus Cycle

The phosphorus cycle differs from other biogeochemical cycles as phosphorus has no gaseous phase and is viewed as a local cycle. The Earth’s crust is the primary storehouse for phosphorus.

Phosphorus Sources and Transport

• Weathering and erosion of rocks release phosphorus into the soil.

• Human-made fertilizers increase phosphorus levels beyond natural processes.

• Phosphate (HPO₄²⁻ or H₂PO₄⁻) is the main form absorbed by plants.

Eutrophication

Phosphorus is often a limiting nutrient in aquatic ecosystems. Increased phosphorus from human activities can result in:

• Algal blooms, leading to high primary production.

• Oxygen depletion as decomposing algae consume dissolved oxygen, potentially resulting in fish kills, a process known as eutrophication.

Case Studies

• Lake Washington suffered severe eutrophication due to sewage and phosphorus-rich detergents, leading to habitat degradation. Post-1960s interventions improved water quality.

• Lake Erie experienced similar issues due to runoff and pollutants, leading to the depletion of fish populations. U.S. and Canadian efforts in the 1980s saw some recovery, but issues of eutrophication have resurfaced since the mid-1990s.

The Nitrogen Cycle

Nitrogen is crucial for proteins, nucleic acids, and chlorophyll, yet nitrogen is often a limiting nutrient in ecosystems.

Five Key Steps in the Nitrogen Cycle

1. Nitrogen Fixation: Certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium (NH₄⁺), which can be utilized by plants.

2. Nitrification: Soil bacteria convert ammonia into nitrite (NO₂⁻) and then into nitrate (NO₃⁻), a form accessible to plants.

3. Assimilation: Organisms assimilate nitrogen by absorbing NH₃, NH₄⁺, or NO₃⁻ for incorporation into organic molecules such as amino acids.

4. Ammonification: Decomposition of organic matter releases ammonia back into the soil, primarily through bacteria and fungi.

5. Denitrification: Anaerobic bacteria convert nitrate back into N₂, completing the cycle and returning nitrogen to the atmosphere.

Human Impact on the Nitrogen Cycle

• Human activities have doubled nitrogen input into ecosystems, primarily through the industrial fixation of nitrogen in fertilizers.

• Nitrogen runoff contributes to eutrophication, and excess nitrate in drinking water poses health risks, especially to infants, potentially causing methemoglobinemia.

• Combustion of fossil fuels increases atmospheric nitrogen in the form of nitrous oxide (N₂O), contributing to air pollution and acid rain.

Solutions for Reducing Human Influences on Nutrient Cycles

Strategies for mitigating human impacts on biogeochemical cycles include:

1. Reforestation: Reduces CO₂ levels and stabilizes the water cycle.

2. Reducing Eutrophication: Necessary actions include decreasing phosphorus and nitrogen use in agriculture, eliminating phosphorus from detergents, and improving wastewater treatment processes.

59.4 Pollution and Biomagnification

Learning Outcomes

1. Define Biomagnification and Explain Its Relationship to Pollution
Biomagnification refers to the process where certain chemicals accumulate in the bodies of organisms and increase in concentration at successive levels of the food chain, particularly at higher trophic levels. It signifies how pollution, especially from human activities, affects biogeochemical cycles and impacts ecological health by increasing the concentration of harmful substances in organisms.

Case Study: DDT (Dichlorodiphenyltrichloroethane)

Discovery and Historical Use

• DDT, a synthetic insecticide first created by chemists in 1874, gained prominence in the 20th century for its effectiveness against malaria and agricultural pests.

• Its insecticidal properties were discovered in 1939 by Paul Müller, leading to his award of the 1948 Nobel Prize in Physiology or Medicine.

• Post World War II, DDT was used in public health programs, particularly in the fight against mosquito-borne diseases such as malaria. Its application in agriculture also commenced around this time.

• The production of DDT peaked in 1970, at 175 million kilograms globally, highlighting its widespread usage.

Chemical and Environmental Properties of DDT

• DDT is persistent in the environment; it does not break down quickly under standard degradation processes.

• The persistence in soil: approximately 10 years, which is two to three times longer compared to many other insecticides.

• It exhibits low solubility in water and high solubility in lipids, which means it tends to accumulate in the biological tissues of living organisms.

Mechanism of Biomagnification

• The biomagnification process results in a higher concentration of DDT in organisms at higher trophic levels.

• Example from Lake Michigan Food Chain:

  • Tertiary consumers (gulls) which feed on fish (secondary consumers) exhibit the highest concentrations of DDT in their lipids.

  • DDT's cumulative presence from lower food chain levels results in alarming concentrations at apex predator levels.

Impact on Wildlife and Human Health

• A significant negative impact of DDT was its interruption of the eggshell formation in birds, leading to thin-shelled eggs that frequently broke under the weight of incubating birds.

• This specific impact contributed to a dramatic decline in the populations of birds, particularly those relying on DDT-contaminated prey, due to reproductive failure.

• Additionally, high concentrations of DDT raised concerns about game fish unfitness for human consumption.

Graphical Representation

• The relationship of DDT concentrations at various trophic levels can be visualized in diagrams (Roughly illustrated in Figure 59.16). This aids in understanding the principles of biomagnification through visual data regarding concentrations measured in gulls, fish, and other organisms.

Consequences of DDT Usage

• Bans and Recovery: Due to increased awareness regarding DDT's harmful ecological effects, most industrialized nations (including the U.S.) implemented bans on its usage by the early 1970s.

• After the ban, there has been a notable recovery in the populations of the most affected bird species, suggesting that earlier intervention may have prevented some population declines.

Other Substances Related to Biomagnification

• Biomagnification is not limited to DDT alone. Other persistent pollutants like mercury, primarily emitted from coal-fired power plants, and persistent organic pollutants (POPs) found in certain herbicides and pesticides also exhibit this phenomenon.

• Contaminants released from underwater mining processes, specifically sulfide and selenium, can also biomagnify through food chains, underscoring a broader environmental concern.

Solutions for Preventing Biomagnification

1. Reduction of Pesticide Use

• Limiting the application of pesticides, herbicides, and heavy metal-containing compounds can significantly decrease biomagnification risks.

2. Biological Control

• Utilizing natural predators or parasites as biological control agents to manage pest populations can reduce reliance on chemical pesticides. For instance:

  • Bacillus thuringiensis (Bt), which produces a non-toxic insecticidal toxin deadly to certain species but safe for non-insects, has been integrated into the genetic makeup of some crops like corn and cotton.

  • Conservation efforts are underway to attract natural predators, such as the American kestrel. This small falcon, known for preying on pests like grasshoppers and starlings, exemplifies how integrated pest management can benefit agricultural productivity while minimizing chemical pesticide use. Farmers in Michigan cherry orchards have used nesting boxes to encourage kestrels, yielding fruit crops at lower costs compared to traditional pesticide applications.

Habitat Destruction

Learning Outcomes

1. Identify the main causes of habitat destruction by humans.

2. Explain why tropical deforestation is a particularly destructive form of habitat loss.

3. Describe how the impact of agriculture causes ecological changes.

Definition of Habitat Destruction

Habitat destruction is predominantly a human-driven process that alters natural habitats to a state that they can no longer support the indigenous species that previously thrived there. This alteration leads to the displacement or demise of organisms originally inhabiting the area, which in turn reduces biodiversity. Habitat destruction is the leading cause of species extinction, and a significant proportion of existing species are currently threatened by this ongoing process.

Threats to Species Survival

Habitat destruction is one of several human-driven practices that jeopardize species survival. Other critical threats include:

• Pollution

• Overexploitation

• Invasive species

These categories overlap, meaning a species may face multiple threats concurrently, which complicates the overall picture of species survival and threat percentages as illustrated in Figure 59.18, which shows the percentage of plant and animal species threatened by various causes in the U.S. Notably, species can be impacted by multiple threats, so the percentages across categories do not sum to 100%.

Core Skill: Science and Society

The impact of human activity significantly raises the proportion of species facing extinction, further underscoring the urgency of addressing habitat loss.

Types of Habitat Destruction

Habitat destruction manifests in various forms, including but not limited to:

• Deforestation

• Conversion of land to agricultural use

• Urbanization

• Mining operations (strip mining, quarrying)

• Bottom trawling

Among these, urbanization is noted as the most human-dominated and rapidly expanding form of land use globally, inflicting severe damage on natural landscapes. The alteration of freshwater habitats through dam construction and river channelization further exacerbates habitat loss, while wetlands have faced significant degradation through drainage for agriculture and filling for urban and industrial purposes.

In the United States, approximately 90% of freshwater marshes have been lost in states such as Iowa and California, with a national average loss of around 53%. The two most prevalent forms of habitat destruction that significantly influence species extinctions are deforestation and land conversion for agriculture.

Deforestation by Humans Threatens Species

Forests are crucial, covering 80% of the planet's terrestrial biodiversity. Deforestation involves converting forested areas into non-forested land, contributing directly to the extinction of numerous species. Currently, forests encompass one-third of the Earth’s land surface, with substantial portions under threat.

Rate of Deforestation

Forests are disappearing at an alarming rate, translating to the loss of an area equivalent to 27 soccer fields every minute. Over the last 50 years, the Amazon rainforest has experienced a loss of 17% of its forest cover. In North America, oak trees play an essential role, with at least 100 species of birds and mammals relying on acorns as a vital food source.

• Example: The ivory-billed woodpecker (Campephilus principalis), North America's largest woodpecker, is presumed extinct, a loss attributed to habitat destruction resulting from heavy logging activities.

Importance of Tropical Forests

Tropical forests, primarily rainforests located in Africa, Asia, and Latin America, are among the most biologically diverse terrestrial habitats. Latin America hosts more tropical forest than Africa and Asia combined, with Brazil holding the largest expanse. The Amazon rainforest is often referred to as "the lungs of the planet," as it generates over 20% of the global oxygen supply. Originally, tropical forests covered 14% of Earth's land but now occupy only about 7%.

Trends in Tropical Deforestation

Despite the alarming trends, there are signs of improvement; from 1990 to 2000, the rate of tropical deforestation was 7.8 million hectares per year, which decreased to 4.7 million hectares per year from 2010 to 2020.

Causes of Tropical Deforestation

The principal driver of tropical forest loss is land clearing for agricultural purposes, while logging—often exceeding the regrowth rate—contributes significantly, especially in Asian forests. Other factors include:

• Collection of wood for fuel (more detrimental in lightly wooded regions)

• Construction of mines, dams, and oil installations, which, while minor direct deforestation causes, lead to harmful indirect effects via chemical discharge and sedimentation.

Conservation Benefits

Conserving tropical forests not only prevents the extinction of numerous rare species but also yields significant benefits for humanity:

1. Agricultural Genetic Resources: Many globally significant crops, such as oranges, bananas, and cacao, originated in tropical rainforests, which serve as reservoirs of genetic variation vital for future crop breeding.

2. Medicinal Properties: Tropical forests harbor numerous plants with unique chemical compounds that are beneficial for medicinal applications. Yet less than 1% of tropical plant species have been evaluated for their medicinal potential.

3. Economic Value: Tropical forests have been estimated to have a lifetime value of approximately $6330 per hectare when sustainably harvested for renewable resources. In contrast, the value drops to $1000 per hectare for timber alone and $2960 per hectare for conversion to agricultural grazing land.

4. Disease Spread: Increased forest clearing leads to closer contact between wildlife and humans, contributing to outbreaks of zoonotic diseases, such as SARS and avian influenza. Following deforestation, larger species often disappear, leaving smaller, disease-carrying species, such as bats and rodents, to proliferate.

For these reasons, strategies aimed at slowing tropical deforestation make significant ecological and economic sense.

Ecological Impacts of Agriculture

Agriculture requires vast land use more than any other human activity. Since 1945, more land has been converted for agricultural purposes than in the total land use change during the 18th and 19th centuries combined. Currently, about 11.5% of the global land area is under crop cultivation, with an additional 25.8% utilized for animal grazing, varying significantly by region (Table 59.2).

Table 59.2: Percentage of Land Area Used for Agricultural Purposes

Ecological Effects of Agriculture

The expansion of agricultural fields leads to numerous ecological impacts, including:

• Soil erosion

• Decreased soil fertility

• Flooding

• Silting of rivers

• Desertification

• Loss of wildlife habitat

Runoff from agricultural land is often contaminated with high levels of nitrogen, phosphorus from fertilizers, and residual pesticides, threatening aquatic ecosystems, with approximately 4 billion tons of topsoil washing into U.S. waterways annually.

Case Study: The Rocky Mountain Grasshopper

The Rocky Mountain grasshopper (Melanoplus spretus), once a predominant species on the Great Plains, became extinct due to agricultural expansion. An estimated 1874 swarm was reported to be twice the size of Colorado. Agricultural practices devastated the grasshoppers' breeding grounds, impeding egg hatching and limiting successful development.

Complex Effects of Agriculture on Wildlife

The expansion of agricultural lands also creates secondary effects on wildlife. For instance, to meet dairy demands in regions lacking suitable land for production, lands in northern Spain were converted for alfalfa cultivation. This change attracted pest rodents (voles), prompting the use of rodenticides that inadvertently harmed nontarget bird species such as endangered raptors. Population declines of species like red kites (Milvus milvus) and hen harriers (Circus cyaneus) have been correlated with the usage of these rodenticides.

Solutions for Reducing Habitat Destruction

Habitat destruction can be mitigated through various strategies:

1. Recycling Initiatives: Implementing recycling systems can help reduce the demand for raw materials, subsequently lowering the need for deforestation. For example, recycling paper products lessens the dependence on natural forests while also addressing mining impacts on endangered species such as gorillas and chimpanzees.

2. Land Preservation Efforts: Measures to constrain urban expansion, minimize deforestation, and curb agricultural development can significantly reduce habitat loss.

Overexploitation

Learning Outcomes

1. Outline the main causes of overexploitation of plant and animal populations.

2. Describe examples of the overexploitation of animals and plants.

Definition of Overexploitation

In ecology, overexploitation refers to the practice where humans harvest a particular species at a rate that is unsustainable, given the natural rate of mortality and capacity for reproduction of that species. It is particularly impactful in the context of hunting animals, as this practice has historically led to many extinctions. This section will explore various examples of overexploitation, focusing on land mammals, whales, birds, fishes, and plants.

Effects of Overexploitation on Species

General Impact of Hunting and Fishing

Hunting and fishing are the primary methods facilitating overexploitation among animals. For plants, human activities involve identifying valuable species and subsequently removing them from their native habitats.

In a 2015 study conducted by conservation biologist Chris Darimont and colleagues, they examined the impact of humans as hunters of terrestrial mammals and fishers of marine fishes. Their research assessed 2,125 estimates of exploited animal populations and revealed that humans kill adult prey at rates up to 14 times higher than those of other predators. The researchers labeled humans as "super predators" and cautioned that such levels of hunting and fishing are unlikely to be sustainable in the long term.

Human Predation vs. Nonhuman Predation

Humans significantly impact populations across various trophic levels:

• Herbivores

• Carnivores

• Top predators

This impact is distal when compared to nonhuman predators, with humans exploiting species typically classified as the top predators in their ecosystems. For instance, through hunting practices, human activities have drastically reduced wolf populations, animals that usually thrive in diverse habitats.

Examples of Overexploitation

Land Mammals

A lengthy list of mammals has faced threats of extinction or has become extinct due to human hunting. A notable example includes the North American buffalo (Bison bison), whose populations dwindled from around 70 million in the 18th century to just 1,150 by 1899. Currently, their numbers have bounced back to about 500,000. Conversely, the Eurasian wild horse (Equus ferus), known as the tarpan, was hunted to extinction by the 1860s.

Many experts assert that the last of the woolly mammoth populations succumbed due to overhunting towards the end of the last glacial period, approximately 12,000 years ago. In contemporary scenarios, poaching remains a significant threat to various land mammals.

• Rhinos: The black market value for rhino horn in Southeast Asia is approximately $65,000 per kilogram, making it more valuable than gold, diamonds, or cocaine. Roughly 10,000 African elephants are poached each year, though the annual poaching mortality rate has decreased from over 10% in 2011 to less than 4% in 2017, partly due to a ban on ivory imports to China from 2017 onwards.

Whales

Research conducted in 2014 by Roberta Rocha, Director of Science at the New Bedford Whaling Museum, revealed that nearly 3 million whales were killed during the last century (1900-1999), marking the most significant cull by biomass in human history.

Steve Palumbi's laboratory performed estimations of past population sizes of North Atlantic whales by analyzing mitochondrial DNA sequences. They found that pre-whaling populations of fin and humpback whales were likely 6-20 times larger than current estimates, with full recovery anticipated to take 70-100 years.

Historically, whaling has shifted focus from more valuable or easily caught species to others as original populations dwindled. For instance, blue whales (Balaenoptera musculus) predominated whaling catches up until the 1930s, but by the mid-1950s, few were taken, despite legal protection beginning only in 1965. As blue whale numbers fell, whalers turned to fin whales (B. physalus), whose populations subsequently declined during the 1960s. Finally, as larger species were exhausted, minke whales (B. acutorostrata) — previously neglected by whalers — emerged as the most frequently caught whales in more recent history.

In 1982, the International Whaling Commission (IWC) implemented a moratorium on all commercial whaling, which began taking effect in 1988. This initiative has seen some positive results, with populations of some whale species, such as blue whales, reportedly quadrupling off the California coast during the 1980s, now nearing the historic population levels estimated at around 2,200 individuals.

Birds

A poignant illustration of overexploitation is that of the extinct dodo (Raphus cucullatus), a flightless bird unique to Mauritius with no native predators. Its extinction occurred within 200 years of human arrival, due to a combination of human hunting and the introduction of species that fed on its eggs. Sailors hunted the dodo for its meat, while rats and pigs devastated the nests.

Two common species of North American birds, the passenger pigeon (Ectopistes migratorius) and the Carolina parakeet (Conuropsis carolinensis), were both hunted to extinction in the early 20th century. The passenger pigeon, once possibly accounting for over 40% of North America's native bird population, was subject to extensive hunting practices. In 1876, for instance, over 1.6 million of these birds were killed in Michigan alone for market distribution. The Carolina parakeet, the sole parrot species indigenous to the eastern U.S., suffered a similar fate.

Fishes

Various fish species are harvested unsustainably where removal rates outpace reproduction. A historical case is the Canadian cod fishery that faced collapse in the early 1990s after centuries of fishing. A pivotal study in 2010 revealed that populations near Newfoundland and Labrador were at just 10% of their original sizes.

Similarly, documented historical photographs from the Florida Keys demonstrate a severe reduction in large trophy fish sizes from 1956 to 2007, with the average size declining from 19.9 kg to 2.3 kg. This shift also included a move from landing large groupers and sharks to smaller snapper species.

Plants

Several species of valuable plants have suffered significant overexploitation for human usage. For example, the West Indian mahogany (Swietenia mahogani) in Florida and the Chile sandalwood (Santalum fernandezianum), which became extinct in the 1800s. Additionally, rare plants such as cacti and orchids are often threatened due to collectors. A notable example is Rothschild's orchid (Paphiopedilum rothschildianum), which can sell for as much as $5,000 per stem, driving collectors to deplete its wild populations.

Plants are also overharvested for their supposed medicinal properties. The American ginseng (Panax quinquefolius), native to eastern North America, is sought for its perceived health benefits, leading to threats of extinction in certain states due to overcollection.

The Botanic Gardens Conservation International has claimed that over 400 medical plant species are at risk of extinction due to overcollection and deforestation, which includes various species of yew trees (genus Taxus), whose bark is integral for cancer drugs.

Solutions for Reducing Overexploitation

Overexploitation is an enduring issue that is challenging to mitigate. For instance, the Maori in New Zealand drove all ten species of flightless moa birds to extinction in just 200 years. Strategies to curb overexploitation include:

1. Establishment of protected areas: National parks and national marine parks protect native species by making hunting, fishing, and logging illegal within their confines.

2. Legislation: Although some laws exist to protect endangered species, enforcing stringent laws against illegal trade of plants and animals and poaching could significantly bolster conservation efforts.

Invasive Species

Learning Outcomes

1. Define introduced species and invasive species.

   • Introduced species are those moved by humans from their native habitat to a different location. They are often deliberately introduced for purposes such as agriculture, landscaping, timber sourcing, meat, or wool. Additionally, many species are brought in as part of the pet trade. Furthermore, plants, insects, or aquatic species may be unintentionally transported through cargo movements by ships or planes.

2. Compare and contrast invasive species as competitors, as predators, and as pathogens.

   • Invasive species can impact native populations in various ways, including competition for resources, predation, and acting as pathogens.

Introduction to Invasive Species

Introduced species can become invasive species once they spread through their new habitat and affect native species adversely. The characteristics that often contribute to the invasive success of these species include:

• Rapid Reproduction: Invasive species may reproduce at a much faster rate than native species.

• Absence of Natural Enemies: In their new environments, invasive species frequently encounter limited or no natural predators or diseases that could control their populations.

• Lack of Native Defenses: Certain native species may lack the evolutionary adaptations to defend against invasive species, making them vulnerable.

• Resource Competition: Invasive species can outcompete native species for essential resources like nutrients, water, and sunlight.

• Habitat Tolerance: Many invasive species can thrive in a variety of habitat conditions, allowing them to spread effectively.

Statistics on Invasive Species

In the United States alone, over 4,500 invasive species have been documented, with approximately 15% causing severe ecological or economic harm. Notable examples include:

• 142 species of introduced vertebrates that have established self-sustaining populations in the wild, such as the ring-necked pheasant (Phasianus colchicus) and the Burmese python (Python molurus).

• Of the 300 most invasive weeds, more than half were introduced for horticultural purposes, including:

  • Purple loosestrife (Lythrum salicaria) and Japanese honeysuckle (Lonicera japonica) (Northeast U.S.)

  • Kudzu (Pueraria lobata) (Southeast U.S.)

  • Chinese tallow (Sapium sebiferum) (South U.S.)

  • Leafy spurge (Euphorbia esula) (Great Plains U.S.)

Economic and Ecological Impacts

Invasive species can exert a significant toll on the economy and the environment. For example:

• Tamarisk (Tamarix chinensis) outcompetes native plants along rivers in the western U.S., altering riverbank habitats and causing localized flooding.

• The** brown tree snake (Boiga irregularis)** has been responsible for the extinction of numerous vertebrates on Guam.

• Invasive species can also pose health risks; for example, an invasive snail species can carry schistosomiasis, while the Asian tiger mosquito (Aedes albopictus) is a vector for diseases such as West Nile virus, encephalitis, and dengue fever.

• The Asian giant hornet (Vespa mandarinia), known as the