Chapert 54
Ecology of Individuals and Populations
Chapter Overview
Learning Objectives (LO):
LO 54.1: Environmental Challenges
LO 54.2: Populations: Individuals of a Single Species in One Place
LO 54.3: Population Demography and Dynamics
LO 54.4: Life History and the Cost of Reproduction
LO 54.5: Environmental Limits to Population Growth
LO 54.6: Factors That Regulate Populations
LO 54.7: Human Population Growth
LO 54.8: Pandemics and Human Health
Main Topics:
The Environmental Challenges
Population dynamics
Life history strategies
Human impact and health
54.1 The Environmental Challenges
Learning Outcomes
List challenges that organisms face in their environments.
Describe ways individuals respond to environmental changes.
Explain species adaptation to environmental conditions.
Key Environmental Factors
Temperature: Most organisms thrive within a narrow temperature range, affecting plant growth and survival.
Water: Essential for all organisms, with availability being a major life influence, especially on land.
Sunlight: Crucial for photosynthesis; limited availability impacts ecosystem productivity, particularly in marine environments.
Soil: Characteristics such as pH and mineral composition limit terrestrial plant growth.
Homeostasis and Environmental Response
Homeostasis: Ability of an organism to maintain a steady internal environment despite external changes. For example:
Behavioral mechanisms: The fog-basking beetle (Onymacris unguicularis) collects moisture from fog in the Namib Desert by holding its abdomen up.
Physiological adjustments: Sweating in response to heat to prevent overheating and acclimatization at high altitudes.
Morphological responses: Thicker fur in mammals during colder seasons for insulation. The trade-off relates to the cost of energy maintaining such adaptations.
Short-term vs Long-term Responses
Short-term: Adjustments to daily or seasonal changes.
Long-term: Natural selection leads to populations becoming better adapted over generations.
Physiological Responses to Environmental Changes
Humans experience acclimatization when moving to high altitudes:
Increased breathing rates, erythrocyte (red blood cell) production, and increased hemoglobin levels.
Table 54.1 outlines physiological changes at high elevation:
Increased breathing.
More erythrocytes and hemoglobin levels.
Higher oxygen delivery effectiveness.
Morphological Capabilities
Organisms alter their physical traits in response to environments:
Thicker fur for insulation in mammals, larger roots in plants during drought.
Norm of reaction: Ability of one genotype to produce different phenotypes under varying environmental conditions. Flexibility has a cost, hence why not all traits are highly plastic.
Behavioral Responses
Organisms may move to different habitats to cope with environmental variation:
Example: Tropical lizards maintain optimal body temperature through basking and seeking shade in different environments.
Spadefoot Toads: Burrowing behavior to avoid harsh conditions, emerging only when favorable next.
Evolution and Natural Selection
Evidence shows human populations have adaptations for high altitudes.
Differences among closely related species exhibit adaptations to specific environmental changes, like Allen's Rule for mammals in cold climates.
54.2 Populations: Individuals of a Single Species in One Place
Learning Outcomes
Differentiate between population and metapopulation.
Understand the changing species geographic range over time.
Defining Populations
A population consists of individuals of a single species living together at a given time and place.
Three key characteristics:
Range
Spacing patterns (random, uniform, clumped)
Changes over time
Population Range and Geographic Distribution
Populations do not occupy entirety of the Earth's habitats. Example species include:
Devil's Hole Pupfish: Lives in a single spring.
Common Dolphin: Found in all oceans.
Geographic Range Changes
Populations undergo range expansions and contractions based on environmental shifts.
Example: North American species' northward expansion after glaciers retreated.
Dispersal Mechanisms
Effective dispersal can occur through various methods, ensuring colonization of suitable areas.
Introduced species' expansion (like the Cattle Egret) showcases human impact on species’ ranges.
Plant seeds exhibit diverse dispersal strategies.
Population Spacing Patterns
Populations demonstrate:
Random spacing: Minimal interactions among individuals, uncommon in nature.
Uniform spacing: Often due to competition, can be seen in plant populations.
Clumped spacing: Associated with resource distribution, common in social animals.
Metapopulation Concept
A metapopulation occurs when local populations reside in different patches connected by dispersal.
Source-sink dynamics: Populations in better habitats (sources) contribute individuals to poorer habitats (sinks).
54.3 Population Demography and Dynamics
Learning Outcomes
Define demography.
Identify factors influencing species' demography.
Explain survivorship curve significance.
Demography and Population Dynamics
Demography is the quantitative study of populations concerning mortality and birth rates.
Population growth = birth rate - death rate + immigration - emigration.
Influences on Population Growth Rates
Influences include:
Sex ratio: More females typically lead to higher birth rates.
Generation time: Shorter generation times result in quicker population growth.
Age Structure Implications
Age structure impacts future growth. A population with many young individuals tends to grow rapidly.
Life Tables: key for understanding cohort survival and fecundity.
Survivorship Curves: Types include:
Type I: High survival until old age (e.g., humans).
Type II: Constant survival rate (e.g., hydra).
Type III: Low survival early (e.g., oysters).
54.4 Life History and the Cost of Reproduction
Learning Outcomes
Describe reproductive trade-offs in an organism's life history.
Compare the costs/benefits of allocating resources to reproduction.
Life History Strategies
Organisms face trade-offs between reproduction and survival, allocating resources strategically.
Semelparity: Reproducing once with a substantial investment.
Iteroparity: Multiple reproductive events over life.
Understanding Reproductive Costs
Increased reproductive effort may lead to reduced future success (cognitive models of energy investment).
54.5 Environmental Limits to Population Growth
Learning Outcomes
Explain exponential growth.
Discuss why populations cannot grow exponentially indefinitely.
Define carrying capacity and changes.
Exponential Growth Model
Under ideal conditions, populations can grow exponentially as expressed by:
where N = population size and r = intrinsic rate of increase.
Limitations to Growth
Growth ceases when populations reach carrying capacity (K) due to limited resources:
54.6 Factors That Regulate Populations
Learning Outcomes
Compare density-dependent and density-independent factors.
Assess cyclic size changes in populations.
Population Regulation
Density-dependent factors: Effects on reproduction and survival based on population size (e.g., competition, predation).
Density-independent factors: Environmental effects unrelated to population size (e.g., natural disasters).
54.7 Human Population Growth
Learning Outcomes
Discuss changes in human growth rates.
Analyze age distribution effects and growth.
Evolution and Human Population Dynamics
Human populations developed slowly until technological advancements initiated exponential growth.
Peak expectations predict a rise to approximately 10 billion by mid-century.
54.8 Pandemics and Human Health
Learning Outcomes
Describe the occurrence of pandemics.
Explain the role of reproduction number in outbreaks.
Disease Dynamics and Population Impact
Understanding pandemics requires assessing the reproduction number of pathogens for controlling spread and infection rates.