Population Ecology and Biome Distribution

Scope and Hierarchical Organization of Ecology

• Ecology is the scientific study of the interactions between organisms and their environment. These interactions determine both the distribution of organisms and their abundance. Ecology relies on a combination of observation and experimentation to answer fundamental biological questions.

• Levels of Ecological Study:

  • Global Ecology: Examines the biosphere, which is the sum of all the planet's ecosystems and landscapes. It focuses on how energy and materials are exchanged across the global ecosystem.

  • Landscape Ecology: Focuses on the exchanges of energy, materials, and organisms across multiple ecosystems. A landscape (or seascape) consists of a mosaic of connected ecosystems.

  • Ecosystem Ecology: Emphasizes energy flow and chemical cycling between living (biotic) and non-living (abiotic) components. An ecosystem is the community of organisms in an area along with the physical factors with which they interact.

  • Community Ecology: Studies the whole array of interacting species in a given area. A community is defined as a group of populations of different species living close enough to interact.

  • Population Ecology: Analyzes the factors that affect population size and how and why it changes over time. A population is a group of individuals of the same species living in a specified geographic area.

  • Organismal Ecology: Includes physiological, evolutionary, and behavioral ecology. It investigates how an organism's structure, physiology, and behavior meet the challenges posed by its environment.

• Case Study - Harlequin Toad (Atelopus) in Costa Rica:

  • The rediscovery of the nearly extinct harlequin toad raises critical ecological questions regarding species survival:

  • What specific environmental factors limit their geographic distribution?

  • What biological factors (such as food availability or emerging pathogens) dictate population size and survival?

Earth's Climate and Global Biome Distribution

Global Climate Determinants

• Climate is defined as the long-term prevailing weather conditions in a given area. It is governed by four major abiotic factors: temperature, precipitation, sunlight, and wind.

• Abiotic vs. Biotic Factors:

  • Abiotic factors: The nonliving chemical and physical attributes of the environment.

  • Biotic factors: All living organisms that inhabit an environment and interact with one another.

• Macroclimate: Refers to climate patterns on global, regional, and landscape scales.

Latitudinal Solar Variation and Atmospheric Circulation

• Global climate patterns are primarily driven by solar energy input and Earth's movement in space.

• Latitudinal Variation in Sunlight: Earth's curved surface leads to uneven solar radiation distribution:

  • Sunlight strikes the tropics—regions located between $23.5^\text{o}\,\text{N}$ (Tropic of Cancer) and $23.5^\text{o}\,\text{S}$ (Tropic of Capricorn)—most directly, delivering intense solar energy per unit surface area.

  • At higher latitudes (near the North and South Poles), sunlight strikes Earth at an oblique angle, spreading the light energy over a larger surface area and resulting in cooler temperatures.

• Air Circulation and Precipitation Patterns:

  • Intense solar radiation near the equator heats surface air, causing warm, moist air to rise. As this air rises, it cools and releases moisture, creating abundant precipitation in tropical regions.

  • The now-dry high-altitude air masses move north and south toward $30^\text{o}\,\text{N}$ and $30^\text{o}\,\text{S}$ latitude, where they descend. As the dry air descends, it absorbs moisture from the land, creating arid conditions that give rise to Earth's major desert belts.

  • This pattern of rising moist air and descending dry air repeats at $60^\text{o}\,\text{N}$ and $60^\text{o}\,\text{S}$ latitudes and at the polar zones.

• Global Wind Patterns:

  • As Earth rotates on its axis, land surfaces near the equator move faster than those at higher latitudes. This differential surface velocity deflects vertical air currents and creates persistent surface winds:

  • Trade winds: Blow from east to west in tropical latitudes.

  • Westerlies: Blow from west to east in temperate zones.

Regional Climate Influences

• Seasonality:

  • Seasonal variations in light and temperature increase toward the poles due to Earth's permanent axis tilt of $23.5^\text{o}$ relative to its orbital plane around the sun.

  • During the June solstice, the Northern Hemisphere tilts toward the sun, resulting in longer days and summer; during the December solstice, it tilts away, bringing winter.

  • Belts of wet and dry air straddling the equator shift north and south throughout the year with the changing solar angle, generating distinct wet and dry seasons in tropical environments.

• Bodies of Water:

  • Ocean currents modify regional coastal climates by carrying cold or warm water between equatorial and polar regions:

  • The California Current transports cold water southward along western North America, cooling coastal ecosystems.

  • The Gulf Stream carries warm water from the equator to the North Atlantic, moderating northwestern Europe's climate.

  • Large bodies of water moderate land temperatures due to water's high specific heat:

  • Daytime: Warm air rises over heated land, drawing a cooler sea breeze from the ocean across the land.

  • Nighttime: As land cools rapidly, warm air rises over the ocean, drawing cooler land breezes back out toward the water.

• Mountains:

  • Rain Shadow Effect: When warm, moist air approaches a mountain range, it rises, cools, and drops precipitation on the windward side. On the leeward side, dry air descends, absorbing moisture and producing a dry environment known as a rain shadow, where many deserts are situated.

• Solar Exposure: In the Northern Hemisphere, south-facing mountain slopes receive significantly more direct sunlight than north-facing slopes, resulting in warmer, drier microclimates.

• Elevation Changes: Temperature drops by approximately $6^\text{o}\,\text{C}$ for every $1,000\,\text{m}$ increase in elevation.

Terrestrial Biomes

• Biomes are major life zones categorized by vegetation type (terrestrial) or physical environment (aquatic).

• Distribution of Terrestrial Biomes:

• Climographs: A climograph plots annual mean temperature against annual mean precipitation for a region to display climatic bounds for biomes.

• General Features of Terrestrial Biomes:

  • Terrestrial biomes are defined by dominant physical factors, climate parameters, and characteristic vegetation.

  • Terrestrial biomes transition into one another without sharp borders; the intergradation zone is called an ecotone.

  • Vertical Layering: Forests usually feature distinct structural layers, including:

  1. Upper canopy

  2. Low-tree layer

  3. Shrub understory

  4. Ground layer of herbaceous plants

  5. Forest floor

  6. Root layer

  • Layering creates microhabitats supporting specialized animal adaptations.

  • Disturbance: An event such as fire, storm, or human activity that removes organisms and alters resource availability. For instance, periodic wild fires prevent woody plants from invading temperate grasslands or savannas.

Profiles of Terrestrial Biomes

Tropical Forest

• Distribution: Equatorial and subequatorial regions.

• Precipitation: In tropical rain forests, rainfall is relatively constant ($200\text{--}400\,\text{cm/year}$); in tropical dry forests, precipitation is highly seasonal ($150\text{--}200\,\text{cm/year}$ with a 6–7 month dry season).

• Temperature: High year-round, averaging $25\text{--}29^\text{o}\,\text{C}$.

• Vegetation & Animals: Vertically layered with intense light competition. Home to millions of animal species, including an estimated 5–30 million undescribed arthropod species. Rapid human deforestation poses a severe threat.

Savanna

• Distribution: Equatorial and subequatorial zones.

• Precipitation: Seasonal rainfall averaging $30\text{--}50\,\text{cm/year}$ with dry seasons lasting up to 8–9 months.

• Temperature: Averages $24\text{--}29^\text{o}\,\text{C}$ with seasonal variation.

• Vegetation & Animals: Dominated by drought-tolerant, fire-adapted grasses and scattered thorny trees. Inhabited by herbivorous mammals (zebras, wildebeests), predators (lions, hyenas), and abundant termites. Fire plays a major role in maintaining this biome.

Desert

• Distribution: Bands near $30^\text{o}\,\text{N}$ and $30^\text{o}\,\text{S}$ latitude and in continental interiors.

• Precipitation: Low and variable, generally less than $30\ authorize\,\text{cm/year}$.

• Temperature: Varies widely; hot deserts exceed $50^\text{o}\,\text{C}$ in summer, while cold deserts drop below $-30^\text{o}\,\text{C}$ in winter.

• Vegetation & Animals: Succulents (cacti), deep-rooted shrubs, and reduced leaf surfaces for water conservation. Animals include scorpions, snakes, lizards, seed-eating rodents, and birds; many exhibit nocturnal behavior.

Chaparral

• Distribution: Midlatitude coastal regions.

• Precipitation: Highly seasonal; rainy winters and dry summers ($30\text{--}50\,\text{cm/year}$).

• Temperature: Cool winters ($10\text{--}12^\text{o}\,\text{C}$) and hot summers ($30^\text{o}\,\text{C}$).

• Vegetation & Animals: Dense thickets of evergreen shrubs and small trees adapted to fire and drought. Animals include browsing herbivores (deer), small mammals, birds, amphibians, and insects.

Temperate Grassland

• Distribution: Midlatitude continental interiors.

• Precipitation: Highly seasonal; dry winters and wet summers ($30\text{--}100\,\text{cm/year}$).

• Temperature: Cold winters (averaging $-10^\text{o}\,\text{C}$) and hot summers (near $30^\text{o}\,\text{C}$).

• Vegetation & Animals: Grasses and forbs adapted to periodic fires and seasonal drought. Large grazing mammals (bison, wild horses) and burrowing mammals (prairie dogs) predominate. Most grasslands have been converted to agricultural fields.

Northern Coniferous Forest (Taiga)

• Distribution: Spans northern North America and Eurasia; it is the largest terrestrial biome on Earth.

• Precipitation: Ranges from $30\,\text{cm}$ to $70\,\text{cm/year}$ (coastal temperate rain forests can receive over $300\,\text{cm}$).

• Temperature: Very cold winters; summers can be warm. Siberia ranges from $-50^\text{o}\,\text{C}$ in winter to $20^\text{o}\,\text{C}$ in summer.

• Vegetation & Anatomy: Dominated by needle-bearing conifers such as pine, spruce, fir, and hemlock.

  • Conical Tree Architecture: The conical shape prevents heavy, wet snow from accumulating and breaking branches.

  • Needle Adaptations: Waxy, narrow needles arranged in clusters (e.g., sets of 2 or 5) reduce exposed surface area, suppressing transpiration and reducing water loss in cold weather.

• Animals & Threats: Migratory birds, moose, brown bears, and Siberian tigers. Fungal pathogens and periodic insect outbreaks can defoliate vast forest stands. Industrial logging remains a primary ecological disturbance.

Temperate Broadleaf Forest

• Distribution: Midlatitudes in the Northern Hemisphere, with smaller areas in Chile, South Africa, Australia, and New Zealand.

• Precipitation: Significant amounts during all seasons as rain or snow ($70\text{--}200\,\text{cm/year}$).

• Temperature: Winter averages $0^\text{o}\,\text{C}$; summers are hot and humid (near $35^\text{o}\,\text{C}$).

• Vegetation & Animals: Deciduous trees dominate in the Northern Hemisphere (dropping leaves before winter), while evergreen Eucalyptus dominates in Australia. Animals undergo winter hibernation or seasonal migration.

Tundra

• Distribution: Covers extensive areas of the Arctic; alpine tundra sits atop high mountains at all latitudes.

• Precipitation: Low in arctic tundra ($20\text{--}60\,\text{cm/year}$) and higher in alpine tundra ($>100\,\text{cm/year}$).

• Temperature: Cold winters ($<-30^\text{o}\,\text{C}$) and cool summers ($<10^\text{o}\,\text{C}$).

• Vegetation & Soil: Herbaceous vegetation including mosses, grasses, forbs, dwarf shrubs, lichens, and stunted trees.

  • Permafrost: A permanently frozen layer of subsoil that restricts plant root penetration and limits primary productivity.

• Animals: Resident musk oxen, migratory caribou and reindeer, polar bears, wolves, and nesting migratory birds.

Aquatic Biomes and Zonation

Physical and Chemical Properties

• Salinity Differentiation:

  • Marine Biomes: Average salt concentrations of approximately $3\%$. Oceans cover about $75\%$ of Earth's surface.

  • Freshwater Biomes: Salt concentrations of less than $0.1\%$. Linked directly to surrounding terrestrial soils and biomes.

Aquatic Zonation

• Stratification Layers:

  • Photic Zone: Upper region with sufficient light penetration for photosynthesis.

  • Aphotic Zone: Deeper region receiving little to no sunlight.

  • Pelagic Zone: Comprises the photic and aphotic zones together.

  • Abyssal Zone: Deep ocean floor between $2,000\,\text{m}$ and $6,000\,\text{m}$ depth.

  • Benthic Zone: The organic and inorganic sediment layer at the bottom of all aquatic environments, inhabited by organisms collectively termed the benthos.

  • Thermocline: A narrow vertical layer characterized by a rapid temperature drop that separates warm upper waters from cold deeper waters.

  • Lake-Specific Zones:

  • Littoral Zone: Shallow, well-lit waters close to shore supporting rooted aquatic plants.

  • Limnetic Zone: Deep, offshore waters where light supports floating phytoplankton rather than rooted plants.

Profiles of Aquatic Biomes

Wetlands and Estuaries

• Wetlands: Environments submerged by water at least occasionally, supporting plants adapted to water-saturated soil. Types include basin wetlands, riverine wetlands, and coastal fringe wetlands.

• Estuaries: Transition zones between rivers and the sea; salinity fluctuates with tidal cycles.

• Chemical Attributes: High organic production combined with rapid microbial decomposition leads to low dissolved oxygen levels.

• Organisms: Wetlands host cattails and sedges; estuaries host saltmarsh grasses. Both support diverse invertebrates, waterfowl, fish, and aquatic mammals.

Lakes

• Oligotrophic Lakes: Nutrient-poor and oxygen-rich; clear water with low organic sediment levels.

• Eutrophic Lakes: Nutrient-rich and oxygen-depleted in deeper zones or under ice cover in winter; high organic content and higher surface-area-to-depth ratios.

• Threats: Anthropogenic runoff causes nutrient enrichment (eutrophication), leading to algal blooms, oxygen depletion, and fish kills.

Streams and Rivers

• Headwaters: Cold, clear, swift, turbulent, oxygen-rich, narrow, and rocky.

• Downstream: Warmer, more turbid, wider, meandering, with silty bottoms; salt and nutrient contents increase while dissolved oxygen decreases.

• Threats: Dam construction disrupts natural flow regimes and impairs migratory fish pathways.

Intertidal Zones

• Environment: Periodically submerged and exposed by daily tides. High oxygen and nutrient levels.

• Substrate & Organisms: Rocky substrate supports attached marine algae, sponges, anemones, barnacles, and starfish with structural attachment mechanisms. Sandy zones allow clams, worms, and crustaceans to bury themselves.

Coral Reefs

• Formation: Built from calcium carbonate skeletons secreted by corals (cnidarians).

• Environment: Requires high oxygen levels and solid substrates. Shallow reef-building corals reside in photic waters ($20\text{--}30^\text{o}\,\text{C}$); deep-sea corals exist at depths of $200\text{--}1,500\,\text{m}$.

• Mutualism: Corals form obligate mutualisms with unicellular endosymbiotic algae (zooxanthellae) that provide them organic nutrients.

• Succession: Coral reefs progress over time from fringing reefs to barrier reefs to coral atolls.

Oceanic Pelagic Zone

• Environment: Vast open water mixed by ocean currents, covering roughly $70\%$ of Earth's surface with high oxygen levels.

• Organisms: Dominant primary producers are photosynthetic phytoplankton; consumers include zooplankton (protists, krill, jellies, larvae), squids, fishes, sea turtles, and marine mammals.

Marine Benthic Zone

• Environment: Seafloor ecosystems. Abyssal zone organisms are adapted to continuous cold and high hydrostatic pressures.

• Deep-Sea Hydrothermal Vents: Dark, high-temperature benthic environments along mid-ocean ridges. Primary producers are chemoautotrophic sulfur-oxidizing prokaryotes. Heterotrophs include giant tube worms, specialized crustaceans, and echinoderms.

Factors Governing Species Distribution

Diagnostic Schema for Species Absence

To evaluate why a given species $X$ is absent from a specific geographic region, ecologists follow a systematic diagnostic process:

1. Dispersal Limits: Is the area geographically inaccessible or has there been insufficient time for colonization?

2. Biotic Limits: Are biological interactions—such as predation, parasitism, competition, herbivory, or disease—preventing establishment?

3. Abiotic Limits: Are physical or chemical parameters outside the species' physiological tolerance limits?

   • Chemical Factors: Water availability, oxygen concentration, salinity, soil pH, or mineral nutrient levels.

   • Physical Factors: Temperature, sunlight intensity, soil structure, moisture levels, or fire frequency.

Dispersal and Range Expansion

• Dispersal is the movement of individuals away from their area of origin or centers of high population density.

• Species Transplants: To determine if dispersal limits a species' distribution, ecologists perform transplant experiments, intentionally or accidentally moving a species to an unoccupied site:

  • If the transplant thrives and reproduces, the species' potential range is larger than its actual range.

  • Transplants can disrupt target ecosystems if the introduced species acts as an invasive pest.

• Channel Island Fox Example: Channel Island foxes reside on islands off Southern California. They remain absent from mainland California due to geographic barriers to dispersal across ocean channels, despite the availability of suitable habitat and food resources on the mainland.

Biotic Drivers of Distribution

• Herbivory Exclusion Experiment (Sea Urchins and Limpets):

  • Field manipulations examined whether herbivores restrict the growth and distribution of seaweeds (algae).

  • Experimental Treatments & Results:

  • Control (Both sea urchins and limpets present): Seaweed cover remained near $0\%$.

  • Limpets Removed: Seaweed cover showed minor increases ($<10\%$).

  • Sea Urchins Removed: Seaweed cover increased substantially over time.

  • Both Urchins and Limpets Removed: Seaweed cover increased rapidly, reaching near $100\%$ coverage.

  • Conclusion: Sea urchins act as the primary biotic factor limiting seaweed cover and distribution in rocky intertidal zones.

• Sea Otter, Sea Urchin, and Kelp Trophic Cascades:

  • Sea otters regulate sea urchin density via top-down predation, preventing urchins from overgrazing kelp forests.

  • Data across four coastal study sites demonstrate this relationship:

Abiotic Drivers of Distribution

• Temperature: Dictates metabolic rates. Freezing water below $0^\text{o}\,\text{C}$ lyses cell walls and membranes; temperatures above $45^\text{o}\,\text{C}$ denature essential structural proteins and enzymes.

• Water and Oxygen: Desiccation risk restricts non-adapted plants and animals. Oxygen diffuses slowly in aquatic environments; deep lake waters can become hypoxic or anoxic.

• Salinity: Regulates intracellular water balance through osmotic stress. Osmoconformers and osmoregulators are typically confined to either freshwater or marine systems.

• Sunlight: Limits photosynthetic energy capture. In forest ecosystems, canopy shading drives competition for light. In aquatic systems, light extinction limits photosynthesis to the photic zone.

• Rocks and Soil: Soil pH, mineral composition, and physical particle structure limit plant growth and dictate herbivore distribution.

Population Ecology and Demographics

Population Density and Dispersion

• Density is the number of individuals per unit area or volume.

• Dispersion is the pattern of spacing among individuals within population boundaries.

• Density Dynamics: Population size changes via four core processes:

  • Additions: Births ($B$) and Immigration ($I$).

  • Removals: Deaths ($D$) and Emigration ($E$).

• Patterns of Dispersion:

1. Clumped: Individuals aggregate in patches. Driven by localized resource availability, mating behavior, or predator-defense swarms.

2. Uniform: Individuals maintain even spacing across the landscape. Often results from direct behavioral interactions such as territoriality (the defense of a bounded space) or allelopathy in plants.

3. Random: Position of each individual is independent of others. Occurs in homogenously uniform environments in the absence of strong attractions or repulsions.

Demographics and Life Tables

• Demography is the study of vital statistics of a population and how they change over time.

• Life Tables: Age-specific summaries of survival patterns within a population. Constructed by tracking a cohort—a group of individuals of the same age—from birth until death.

• Survivorship Curves: A graphic representation plotting the proportion or log number of a cohort alive at each age step.

• Type I: Low mortality early and middle in life; high mortality in older age groups (e.g., humans, large mammals providing intensive parental care).

• Type II: Constant death rate over the organism's lifespan (e.g., rodents, Belding's ground squirrels, songbirds).

• Type III: Extremely high mortality rates for young, followed by high survival for the few individuals reaching adulthood (e.g., oysters, marine invertebrates, many plants).

  • Reproductive Tables (Fertility Schedules): Age-specific summaries of reproductive rates within female cohorts.

  • Reproductive Table Data for Belding's Ground Squirrels at Tioga Pass:

Mathematical Models of Population Growth

Per Capita Rate of Increase

• Omitting immigration and emigration, the net population change over a given time interval $Δt$ is: $\frac{\Delta N}{\Delta t} = B - D$

• Expressed using per capita birth rate ($b$) and per capita death rate ($m$) for population size $N$: $B = bN$ $D = mN$ $\frac{\Delta N}{\Delta t} = (b - m)N$

• Defining the per capita rate of increase as $r = b - m$: $\frac{\Delta N}{\Delta t} = rN$

• Zero Population Growth (ZPG) occurs when $r = 0$ ($b = m$).

• Differential calculus models instantaneous population growth as: $\frac{dN}{dt} = r_{\text{inst}}N$

Exponential Growth Model

• Exponential Population Growth describes population expansion under ideal, unlimited resource conditions. The per capita rate of increase operates at its theoretical maximum ($r_{\text{max}}$).

• Differential Equation: $\frac{dN}{dt} = r_{\text{max}}N$

• Produces a characteristic J-shaped curve.

• Rebounding African Elephant Example: Following a strict hunting ban in Kruger National Park, South Africa, the protected elephant population grew exponentially for decades.

Logistic Growth Model

• Unrestricted growth cannot continue indefinitely. Real environments impose a carrying capacity ($K$), defined as the maximum population size an environment can sustain given available resources.

• Logistic Population Growth Model: Incorporates resource limitation by reducing the per capita growth rate as population size ($N$) approaches carrying capacity ($K$).

• Differential Equation: $\frac{dN}{dt} = r_{\text{max}}N \left(\frac{K - N}{K}\right)$

• Mathematical Behavior:

  • When $N \ll K$, the term $\frac{K - N}{K} \approx 1$, and growth approaches exponential rates.

  • When $N \approx K$, the term $\frac{K - N}{K} \approx 0$, reducing the overall growth rate to zero.

  • Maximum overall population growth rate occurs at $N = \frac{K}{2}$.

• Logistic Growth Table for a Hypothetical Population ($K = 1,500$, $r_{\text{max}} = 1.0$):

Real Population Fit

• Laboratory Paramecium: Small microorganisms grown under steady conditions without predators closely follow the smooth S-shaped logistic curve.

• Laboratory Daphnia (Water Flea): Daphnia store energy reserves, causing a time lag before high density inhibits reproduction. Consequently, populations temporarily overshoot carrying capacity ($K$) before experiencing elevated mortality and settling back to baseline.

Life History Strategies and Population Regulation

Life History Trade-offs

• An organism's life history comprises the evolutionary traits that determine its schedules of reproduction and survival:

  1. The age at which reproduction begins.

  2. How frequently the organism reproduces.

  3. How many offspring are produced per reproductive cycle.

• Resource Allocation Trade-offs: Organisms possess finite energy budgets, driving trade-offs between offspring quantity and investment per offspring.

  • Dandelion Strategy: Produces vast numbers of tiny, wind-dispersed fruits. Low per-seed energy investment ensures wide dispersal, though individual survival probability is low.

  • Brazil Nut Tree Strategy: Produces a moderate number of large seeds packaged in heavy protective pods. High energy investment equips seedlings to establish under competitive forest canopy conditions.

Selection Frameworks

• $K$--selection (Density-Dependent Selection): Selects for life history traits sensitive to population density. Favors competitive ability and resource efficiency in crowded environments near carrying capacity.

• $r$--selection (Density-Independent Selection): Selects for traits that maximize reproductive output ($r$) in uncrowded, unpredictable, or disturbed environments far below carrying capacity.

Mechanisms of Density-Dependent Regulation

• Density-Independent Parameters: Birth or death rates do not change with population density.

• Density-Dependent Parameters: Birth rates fall and death rates rise as population density increases, serving as negative feedback to stabilize population size around equilibrium ($Q$).

• Regulatory Mechanisms:

  1. Competition for Resources: High density reduces per capita nutrient supply, decreasing birth rates.

  2. Territoriality: Space limits breeding sites; non-territorial individuals fail to reproduce.

  3. Disease: Pathogen transmission rates rise in dense populations.

  4. Predation: Predators may target species that become locally abundant.

  5. Toxic Wastes: Accumulation of metabolic byproducts (e.g., ethanol accumulation in yeast cultures) limits population survival.

  6. Intrinsic Factors: Physiological stress responses induced by crowding can depress reproductive output via hormonal changes.

Long-Term Dynamics and Metapopulations

• Isle Royale Wolf and Moose Dynamics: Long-term tracking reveals that populations of large mammals fluctuate due to combinations of severe winter weather, forage shortages, and predator-prey oscillations.

• Metapopulations: Networks of local populations linked by immigration and emigration across patches of suitable habitat separated by unsuitable terrain.

  • Glanville Fritillary Butterfly (Melitaea cinxia): Inhabits hundreds of suitable meadow patches in Finland's Åland Islands. Local extinctions in small patches are balanced over time by colonization of unoccupied patches via dispersal.

Scope and Organization of Ecology

• Ecology: Scientific study of interactions between organisms and their environment, determining organism distribution and abundance.

• Ecological Hierarchy:

  • Global Ecology: Examines the biosphere and global energy/material exchanges.

  • Landscape Ecology: Focuses on energy, material, and organism exchange across connected ecosystems.

  • Ecosystem Ecology: Emphasizes energy flow and chemical cycling between biotic (living) and abiotic (nonliving) components.

  • Community Ecology: Studies interactions among species populations in a given area.

  • Population Ecology: Analyzes factors affecting population size and dynamics over time.

  • Organismal Ecology: Investigates physiological, evolutionary, and behavioral adaptations to environmental challenges.

Climate and Biome Distribution

• Global Climate Determinants: Climate is governed by four major abiotic factors: temperature, precipitation, sunlight, and wind.

  • Latitudinal Solar Variation: Direct solar radiation hits the tropics ($23.5^\text{o}\,\text{N}$ to $23.5^\text{o}\,\text{S}$), whereas higher latitudes receive diffuse light at oblique angles.

  • Global Air Circulation: Warm moist air rising at the equator creates heavy tropical precipitation. Descending dry air at $30^\text{o}\,\text{N}$ and $30^\text{o}\,\text{S}$ forms global desert belts.

  • Global Winds: Trade winds blow east-to-west in tropics; Westerlies blow west-to-east in temperate zones.

• Regional Influences:

  • Seasonality: Driven by Earth's permanent $23.5^\text{o}$ axial tilt during orbital revolution.

  • Bodies of Water: Moderate terrestrial temperatures via water's high specific heat. Ocean currents (e.g., California Current, Gulf Stream) redistribute heat globally.

  • Mountains: Cause a rain shadow effect (moist windward slopes vs. arid leeward slopes). Temperature drops approximately $6^\text{o}\,\text{C}$ per $1,000\,\text{m}$ elevation gain.

Terrestrial Biomes

• General Features: Transition gradually across ecotones (zones of intergradation) and feature vertical layering (canopy, understory, floor, root layer).

• Biome Profiles:

  • Tropical Forest: High temperature ($25\text{--}29^\text{o}\,\text{C}$); constant rainfall ($200\text{--}400\,\text{cm/year}$) in rain forests vs. seasonal rainfall in dry forests. Highest species diversity.

  • Savanna: Seasonal rainfall ($30\text{--}50\,\text{cm/year}$); dominated by fire- and drought-adapted grasses and scattered thorny trees.

  • Desert: Low rainfall ($<30\,\text{cm/year}$); severe temperature extremes ($>50^\text{o}\,\text{C}$ to $<-30^\text{o}\,\text{C}$); succulents and nocturnal fauna.

  • Chaparral: Coastal midlatitudes; rainy winters and hot, dry summers ($30\text{--}50\,\text{cm/year}$); fire-adapted evergreen shrubs.

  • Temperate Grassland: Seasonal precipitation ($30\text{--}100\,\text{cm/year}$); cold winters, hot summers; grazing mammals and deep soils.

  • Northern Coniferous Forest (Taiga): Earth's largest terrestrial biome; needle-bearing conifers with conical architecture to shed heavy snow and waxy needles to limit transpiration.

  • Temperate Broadleaf Forest: Significant year-round rain/snow ($70\text{--}200\,\text{cm/year}$); dominated by deciduous trees in the Northern Hemisphere.

  • Tundra: Arctic/alpine; low rainfall; features permafrost (permanently frozen subsoil) restricting root growth.

Aquatic Biomes and Zonation

• Salinity Differentiation: Marine biomes ($\approx 3\%$ salt, covering $75\%$ of Earth) vs. Freshwater biomes ($<0.1\%$ salt).

• Aquatic Zonation:

  • Photic Zone: Upper layer with sufficient sunlight for photosynthesis.

  • Aphotic Zone: Deep layer with minimal light penetration.

  • Pelagic Zone: Combined photic and aphotic zones.

  • Benthic Zone: Organic/inorganic bottom sediment layer inhabited by benthos.

  • Thermocline: Temperature boundary separating warm upper waters from cold deep waters.

• Aquatic Biome Highlights:

  • Wetlands & Estuaries: High primary productivity, low oxygen levels due to organic decomposition.

  • Lakes: Oligotrophic (nutrient-poor, oxygen-rich) vs. Eutrophic (nutrient-rich, oxygen-depleted).

  • Coral Reefs: Calcium carbonate structures formed by cnidarians with mutualistic photosynthetic zooxanthellae.

  • Deep-Sea Vents: Dark benthic habitats supported by chemoautotrophic sulfur-oxidizing prokaryotes.

Factors Governing Species Distribution

• Diagnostic Framework for Species Absence:

  1. Dispersal Limits: Geographic barriers or insufficient time prevent colonization (e.g., Channel Island fox absent from mainland California).

  2. Biotic Factors: Predation, herbivory (e.g., sea urchins limiting seaweed coverage), competition, disease.

  3. Abiotic Factors: Chemical (salinity, oxygen, pH, soil nutrients) and physical (temperature, sunlight, moisture, fire) constraints.

Population Ecology and Dynamics

• Density Dynamics: Population size changes via additions (Births $B$ + Immigration $I$) and removals (Deaths $D$ + Emigration $E$).

• Dispersion Patterns:

  • Clumped: Aggregated patches driven by resources or social behavior.

  • Uniform: Even spacing maintained by direct interactions like territoriality.

  • Random: Independent positioning in homogeneous environments.

• Demographics & Survivorship Curves:

  • Type I: Low early/middle mortality, steep decline in old age (e.g., humans, large mammals).

  • Type II: Constant mortality rate throughout lifespan (e.g., rodents, songbirds).

  • Type III: Massive early mortality followed by high survival for adults (e.g., oysters, plants).

Mathematical Models of Population Growth

• Exponential Growth Model: Idealized growth under unlimited resources; produces a J-shaped curve:   $\frac{dN}{dt} = r_{\text{max}}N$

• Logistic Growth Model: Incorporates environmental carrying capacity ($K$) to produce an S-shaped (sigmoid) curve:   $\frac{dN}{dt} = r_{\text{max}}N \left(\frac{K - N}{K}\right)$

Life History Strategies and Regulation

• Life History Trade-offs: Finite energy forces trade-offs between offspring count and parental investment per offspring (e.g., dandelion's many small seeds vs. Brazil nut's few large seeds).

• Selection Frameworks:

  • $K$--selection: Favors competitive adaptations sensitive to population density near carrying capacity.

  • $r$--selection: Favors high reproductive output in uncrowded or disturbed environments.

• Density-Dependent Regulation: Negative feedback mechanisms regulating population equilibrium ($Q$), including resource competition, territoriality, disease, predation, toxic waste accumulation, and intrinsic physiological stress.

• Metapopulations: Spatial networks of local populations linked by immigration and emigration across habitat patches.