Seasons, Climate, and Biomes

Earth’s Tilt, Orbit, and the Causes of Seasons

• Earth’s tilt (23.5°) and its orbit around the Sun cause different regions to receive varying amounts of sunlight throughout the year, resulting in seasons.

• When a hemisphere is tilted toward the Sun, it experiences summer (more direct sunlight, longer days); when tilted away, it experiences winter (less direct sunlight, shorter days).

• Equinoxes occur when both hemispheres receive equal sunlight (spring and fall).

• Example: In June, the Northern Hemisphere is tilted toward the Sun (summer), while the Southern Hemisphere is tilted away (winter).

Global Circulation Patterns and the Coriolis Effect

• The Sun heats Earth unevenly (more at the equator, less at the poles), causing warm air to rise at the equator and move toward the poles.

• This creates global circulation cells (Hadley, Ferrel, Polar cells) that drive major wind patterns.

• The Coriolis effect is caused by Earth’s rotation, making winds and ocean currents curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• Prevailing winds: Trade winds (east to west near equator), westerlies (west to east in mid-latitudes), polar easterlies (east to west near poles).

• Ocean currents are also shaped by wind, Earth’s rotation, and continental positions.

Geography, Climate, and Biomes

• Latitude and elevation influence temperature and precipitation.

• Rain shadows: Mountains block moist air, causing dry conditions on the leeward side.

• Biomes are major ecological communities defined by climate and dominant vegetation.

• Major terrestrial biomes: Tropical rainforest, desert, savanna, temperate forest, grassland, boreal forest (taiga), tundra, chaparral.

• Example: Deserts occur at 30° latitude due to descending dry air from Hadley cells.

Species Distribution and Adaptations

• Climate (temperature, precipitation), latitude, elevation, and geography determine where species and biomes are found.

• Organisms have adaptations (e.g., water conservation in desert plants, antifreeze proteins in tundra animals) suited to their biome.

• Limiting factors: Temperature, moisture, food, competition, predation, disease, and dispersal ability restrict species’ geographic ranges.

• Within a range, populations may be clumped (resources patchy), evenly spaced (territoriality), or randomly distributed (rare in nature).

Life History and Fitness

Life History Traits and Their Importance

• Life history traits include age at first reproduction, number and size of offspring, lifespan, reproductive frequency, and parental care.

• These traits affect an individual’s fitness—the ability to survive and pass genes to the next generation.

• Trade-offs: Limited resources mean investing in one trait (e.g., many offspring) often reduces investment in another (e.g., parental care).

Life History Strategies: r-Selected vs. K-Selected Species

• r-selected species: Rapid reproduction, many small offspring, little parental care, short lifespan. Favored in unpredictable or disturbed environments.

• K-selected species: Slow reproduction, few large offspring, high parental care, long lifespan. Favored in stable, crowded environments.

• Example: Mice (r-selected) vs. elephants (K-selected).

Population Growth and Life History

• Birth rates, death rates, age at reproduction, and survivorship shape population size and age structure.

• Life history traits influence how quickly populations can grow or recover from declines.

Population Ecology

Population Statistics and Life Tables

• Population size (N): Number of individuals in a population.

• Birth rate (b), death rate (d), survivorship (lx), fecundity (mx): Key demographic parameters.

• Life tables summarize survival and reproduction by age class.

• Survivorship curves:

  • Type I: High survivorship until old age (e.g., humans).

  • Type II: Constant survivorship (e.g., birds).

  • Type III: Low early survivorship, high later (e.g., oysters).

Key Population Parameters and Equations

• Net reproductive rate (R0): Average number of offspring per individual per generation.

  • If , population increases; if , stable; if , decreases.

  • Equation:

• Generation time (T): Average age at which individuals reproduce.

  • Equation:

• Per capita growth rate (r): Rate of population increase per individual.

  • Equation:

Population Growth Models

• Exponential growth (density-independent): Population grows without limits.

  • Discrete:

  • Continuous:

• Logistic growth (density-dependent): Growth slows as population nears carrying capacity (K).

  • Equation:

• Carrying capacity (K): Maximum population size the environment can support.

• Density-independent factors: Affect growth regardless of population size (e.g., weather).

• Density-dependent factors: Effects increase with population size (e.g., competition, disease).

Population Growth Curves

• Exponential curve: J-shaped, rapid increase.

• Logistic curve: S-shaped, levels off at K.

Behavioral Ecology

Proximate and Ultimate Causes of Behavior

• Proximate explanations: How a behavior occurs (mechanism, development).

• Ultimate explanations: Why a behavior evolved (adaptive value, evolutionary history).

• Example: Bird song—proximate: hormonal triggers; ultimate: attracts mates, increases fitness.

Types of Animal Behavior

• Foraging, mating, parental care, communication, territoriality, cooperation, aggression are major behavior types studied in ecology.

Cost-Benefit Analysis in Behavior

• Behaviors evolve when benefits (e.g., increased survival or reproduction) outweigh costs (e.g., energy, risk).

• Costs and benefits can depend on individual condition or environment.

Studying Animal Behavior

• Experiments can test both proximate (mechanism) and ultimate (evolutionary) explanations.

• Behavioral ecologists ask how animals find food, choose mates, avoid predators, communicate, cooperate, and care for offspring.

Kin Selection and Altruism

• Direct fitness: Individual’s own reproduction.

• Indirect fitness: Additional reproduction by relatives due to individual’s help.

• Inclusive fitness: Direct + indirect fitness.

• Kin selection: Natural selection favoring behaviors that help relatives reproduce.

• Hamilton’s Rule: Altruism evolves if (r = relatedness, B = benefit to recipient, C = cost to actor).

• Coefficient of relatedness (r): Probability two individuals share an allele due to common ancestry.

• Relatedness influences likelihood of helping, cooperating, or sacrificing for others.

Community Ecology

Ecological Communities and Species Interactions

• Community: Multiple species interacting in the same area.

• Major interactions: Competition, predation, herbivory, parasitism, mutualism, commensalism.

• Interactions affect survival, reproduction, population size, evolution, and diversity.

Competition and Niches

• Competitive exclusion principle: Two species using the same limiting resource cannot coexist indefinitely.

• Fundamental niche: Full range of resources a species could use.

• Realized niche: Actual resources used, limited by competition.

• Niche differentiation/resource partitioning: Species use resources differently to reduce competition.

• Character displacement: Evolutionary change in traits that reduces competition.

Defense Strategies

• Venomous: Delivers toxin via bite or sting.

• Poisonous: Harmful when eaten, touched, or absorbed.

• Constitutive defenses: Always present (e.g., thorns, toxins).

• Inducible defenses: Produced in response to threat (e.g., chemical release after herbivory).

• Different defenses evolve based on cost-benefit trade-offs.

Keystone Species and Community Structure

• Keystone species: Have a large effect on community structure despite low abundance (e.g., wolves).

• Trophic cascades: Changes at one trophic level affect others (e.g., removing predators increases herbivores, decreases plants).

Species Richness and Latitudinal Gradients

• Species richness: Number of species in a community.

• Generally higher in tropical regions, lower toward poles.

• Hypotheses for latitudinal diversity: More stable climate, higher productivity, longer evolutionary time in tropics.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard ecology textbooks.