Chapter 40 Notes: Population Ecology and the Distribution of Organisms
Concept 40.1 Global climate patterns and terrestrial biomes
Climate vs. biomes
Climate = long-term prevailing weather conditions in an area; four key abiotic factors shape climate: temperature, precipitation, sunlight, and wind.
Biotic factors (living organisms) also influence distributions alongside abiotic factors.
Global climate patterns depend on solar input and Earth’s movement in space.
The sun warms the atmosphere, land, and water, establishing latitudinal temperature patterns, air and water movements, and evaporation.
Seasonal variation is driven by Earth's tilted axis (tilt = 23.5°) and orbital position.
Seasonal effects and regional climate features
In middle to high latitudes, strong seasonal cycles in day length, solar radiation, and temperature occur due to axial tilt.
The sun’s changing angle shifts belts of wet and dry air; roughly around 20° N and 20° S, producing wet and dry seasons in tropical deciduous forests.
Seasonal wind changes can alter ocean currents, causing upwelling of nutrient-rich deep water that fuels surface phytoplankton and their consumers.
Upwelling zones are ecologically important, covering a small ocean area but contributing to >25% of global fish catches.
Coastal climates and ocean currents
Ocean currents heat or cool air masses over continental coasts, shaping coastal climates.
Coastal regions tend to be wetter than inland areas at the same latitude.
Large bodies of water moderate nearby climates due to high specific heat, leading to sea breezes and milder temperatures.
Mountains and climate
Mountains influence air flow: warm, moist air rises on windward slopes, releasing moisture as rain/snow; leeward sides form rain shadows with drier conditions.
Slopes differ in sun exposure: south-facing slopes (NH) receive more sunlight than north-facing slopes, leading to warmer and drier conditions on those slopes.
Elevation effect: ~1,000 m rise in elevation drops average temperature by ~6°C, equivalent to ~880 km of latitudinal change; high-elevation communities near the equator can resemble low-elevation communities farther from the equator.
Concept 40.1: Earth’s climate influences the distribution of terrestrial biomes
Primary drivers of biome distribution on land: climate, especially long-term patterns in temperature and precipitation, plus sunlight and wind.
Abiotic vs. biotic factors:
Abiotic: temperature, precipitation, sunlight, wind, geology, soils, etc.
Biotic: interactions with other organisms, food webs, pathogens, pollinators, etc.
Global climate patterns (summary)
Global patterns are determined largely by solar energy input and Earth’s orientation in space, creating latitudinal variation in climate that affects biome distribution.
Figure references (conceptual): latitudinal bands (e.g., 60°N, 30°N, Equator, 30°S, 60°S) and solstices/equinoxes describe day length and solar input variations across the year.
Concept 40.1 (cont.) Regional effects on climate mechanisms
Upwelling and nutrients
Coastal upwelling brings cold, nutrient-rich water to the surface, stimulating phytoplankton growth and supporting rich marine food webs.
Rain shadows and deserts
Mountain ranges create rain shadows on their leeward sides, contributing to desert formation in many ranges (e.g., Mojave, Gobi).
Climographs and climate interpretation
Climographs plot mean annual temperature vs. mean annual precipitation for a region, illustrating how climate variables co-vary to define biomes.
Key interpretation points:
Grasslands are generally drier than forests.
Deserts are drier than grasslands.
Areas with seasonal precipitation can support different biomes than regions with year-round precipitation.
Climographs capture temporal variation, not just annual means;
The pattern of wet vs. dry seasons can strongly influence biome type even if the mean values are similar.
Concept 40.1 (continues) Disturbances and climate
Disturbances alter biome distribution
Storms, fires, and human activity can change community structure and resource availability.
Example: frequent fires can prevent woody plants from establishing in savannas, maintaining grass-dominated landscapes.
Hurricanes and storms create openings that allow new species to establish in tropical and temperate forests.
Human impacts on biomes
Human activity has replaced many natural ecosystems with urban and agricultural land, altering biome distributions and climate feedbacks.
Major terrestrial biomes (overview)
Biome definitions and latitudinal patterns
Terrestrial biomes are major life zones characterized by vegetation type and climate.
Latitudinal patterns arise because climate varies with latitude; deserts tend to form at ~30° N and S where descending dry air occurs.
Biomes typically grade into one another via ecotones (transitional zones).
Vertical layering is common in forests and creates multiple habitats for diverse organisms.
Tropical forest
Climate: typically year-round warm temperatures; high rainfall in rain forests (≈200–400 cm/year); dry forests have a pronounced dry season (~150–200 cm/year).
Organisms: vertically layered; broadleaf evergreen trees predominate in rain forests; many animal species (estimates suggest 5–30 million undescribed arthropod species).
Humans: extensive deforestation due to population growth, agriculture, and development.
Savanna
Climate: equatorial/subequatorial regions with rainfall 30–50 cm/year; pronounced dry season lasting up to 9 months; temperatures 24–29°C.
Organisms: scattered thorny trees; grasses and forbs; large herbivores (e.g., wildebeest, zebras) and predators (lions, hyenas); termites as dominant herbivores; fires are common in the dry season.
Humans: early human habitation; habitat changes due to fires, cattle ranching, and overhunting reducing large mammals.
Desert
Climate: low, highly variable precipitation (<30 cm/year); temperature extremes (can exceed 50°C in hot deserts or drop below -30°C in cold deserts).
Organisms: sparse, drought-tolerant vegetation (succulents, deep-rooted shrubs, drought-tolerant herbs); many plants possess C4 or CAM photosynthesis; animals include scorpions, insects, reptiles, birds; many desert animals nocturnal to avoid heat.
Humans: water extraction and irrigation reduce biodiversity; urbanization and agriculture alter natural desert ecosystems.
Chaparral (mediterranean climate)
Climate: midlatitude coastal regions with cool, wet winters and dry summers.
Organisms: shrub-dominated with fire-adapted species; some shrubs release seeds only after hot fires; resprouting plants tolerate drought.
Distribution: midlatitudes, interior of continents.
Temperate grasslands
Climate: midlatitudes with 30–100 cm/year precipitation; seasonal variation; winters often below -10°C, summers up to 30°C.
Organisms: grasses and forbs; adaptations to drought and fire; large grazers (e.g., bison, wild horses); burrowing mammals (prairie dogs).
Humans: extensive conversion to farmland; grazing can convert to desert in drier regions.
Temperate broadleaf forest
Climate: moderate temperatures and precipitation; deciduous trees common in NH; some regions experience fire and drought disturbances.
Organisms: layered forests; rich insect, bird, mammal communities; broadleaf trees drop leaves seasonally.
Humans: extensive deforestation and land-use change; forests are regrowing in some regions.
Northern coniferous forest (taiga)
Climate: cold winters; 30–70 cm precipitation; pine, spruce, fir, and other conifers dominate; many species rely on fire in some regions.
Organisms: conifers; animals adapted to cold and seasonal light; predators include large mammals.
Disturbances: fire can play a role in regeneration.
Tundra
Climate: Arctic tundra with 20–60 cm/year precipitation (some alpine tundra in mountains); cold winters (< -30°C) and cool summers (< 10°C).
Vegetation: mostly herbaceous plants, mosses, grasses, forbs; permafrost limits root growth; migratory caribou/reindeer and musk oxen are key herbivores; predators include bears, wolves, and snowy owls.
Humans: mineral and oil extraction; sparse human settlement.
High mountains and polar ice
High mountains: climate and biotic patterns change with elevation; communities resemble low-latitude patterns at different elevations.
Polar ice: extreme cold and limited life; unique biota adapted to cold and seasonality.
Figure 40.7 and Figure 40.8 (Climographs)
Figure 40.7 shows latitudinal distribution of major biomes.
Figure 40.8 climograph example highlights how annual mean temperature and precipitation define biome boundaries and help explain why identical mean values can support different biomes depending on seasonal patterns.
Concept 40.2 Aquatic biomes are diverse and dynamic systems
Key distinctions from terrestrial biomes
Aquatic biomes are primarily defined by their physical and chemical environments, notably salinity: marine ≈3% salt; freshwaters <0.1% salt.
Vertical and horizontal zoning is common; light availability decreases with depth.
Zonation in aquatic systems
Photic zone: sufficient light for photosynthesis; aphotic zone: little light.
Pelagic zone: open water; benthic zone: seafloor or bottom substrate.
Littoral zone: near-shore, shallow with rooted plants; limnetic zone: open water away from shore; both part of the littoral/limnetic distinction in lakes.
Thermocline: depth with abrupt temperature change separating warmer surface water from deeper water.
Aquatic biomes and their characteristics
Oceans: largest biome (~75% of Earth's surface); open ocean (pelagic) vs deep-sea benthic zones; surface waters support most photosynthesis by phytoplankton and photosynthetic bacteria; oxygen produced and CO2 consumed significantly affect global climate and carbon cycling.
Freshwater biomes include lakes, streams, rivers, wetlands, estuaries; productivity varies with nutrients and flow.
Wetlands and estuaries: highly productive, nutrient-rich; waterlogged soils; plant adaptations to saturated soils; estuaries are brackish with tidal mixing; degradation from drainage and pollution.
Coral reefs: built by corals and algae; require high oxygen and stable salinity; vulnerability to warming, pollution, and sedimentation; overfishing impacts; macro- and microdiversity.
Open ocean vs pelagic zone: extensive, nutrient-poor surface waters; nutrient mixing renews productivity seasonally (temporal patterns).
Deep-sea environments: aphotic; hydrothermal vent communities rely on chemoautotrophic bacteria; deep-sea corals and benthic communities inhabit rocky substrates; high pressure and low temperature dominate.
Concept 40.2 (examples of zones and features)
Examples include headwater streams (cold, clear, fast), oligotrophic lakes (nutrient-poor, oxygen-rich), eutrophic lakes (nutrient-rich, possible seasonal anoxic conditions in deep layers), rocky intertidal zones, salt marshes, mangroves, coral reefs, open ocean, and deep-sea habitats.
Concept 40.3 Interactions between organisms and the environment limit distribution of species
Core idea: species distributions reflect both ecological factors and evolutionary history.
Dispersal and geographic isolation shape where species occur; transplants can reveal potential ranges; successful transplants imply potential ranges larger than actual ranges.
Biotic factors: predation, parasitism, competition, disease, pollinators, food resources, parasites, pathogens, and competing organisms can limit distribution.
Abiotic factors: temperature, water, oxygen, salinity, pH, soil nutrients; light; fire; moisture; etc. can limit distributions.
Flowchart of limiting factors (Figure 40.12)
Initial question: Is dispersal limiting distribution?
If yes, the species might not reach some areas; if no, move to biotic/abiotic checks.
Biotic factors: predation, parasitism, competition, disease, pollinators; abiotic factors: temperature, light, water, oxygen, salinity, soil, etc.
The questions are not mutually exclusive; multiple factors can limit distribution.
Examples and investigations
Sea urchins as a biotic factor limiting seaweed distribution; removal experiments show seaweed invasion increases when urchins are removed, with a larger effect when both urchins and limpets are removed.
Key interpretation: biotic interactions (e.g., predation pressure) can shape distributions even when abiotic conditions are suitable.
Abiotic biases and water balance
Temperature constraints (enzymatic activity, membrane stability) influence geographic distributions.
Water availability and oxygen content critically affect aquatic organisms and desiccation risk for terrestrial species.
Salinity affects osmoregulation and limits the range of many species to either freshwater or saltwater environments.
Rock/soil properties (pH, minerals) influence plant distributions and, indirectly, consumer distributions.
Concept 40.3 (wrap-up)
Dispersal, biotic interactions, and abiotic conditions together determine species distributions.
Aquatic and terrestrial ecosystems differ in how these factors weigh on distributions due to viscosity, movement capabilities, and habitat connectivity.
Concept 40.4 Biotic and abiotic factors affect population density, dispersion, and demographics
Population basics
Population = a group of individuals of a single species in a defined area.
Boundaries (natural or investigator-defined) and size (N) are critical for population studies.
Density and dispersion
Density = number of individuals per unit area or volume.
Dispersion = pattern of spacing among individuals within the population’s range.
Common dispersion patterns: clumped (patchy), uniform (even spacing), and random.
Causes of dispersion patterns:
Clumped: favorable microhabitats, social/mating behavior, resource aggregation.
Uniform: territoriality, negative interactions, and chemical inhibition (e.g., plant allelopathy).
Random: absence of strong attractions or repulsions; seeds dispersed by wind may be randomly placed.
Population dynamics: births, deaths, immigration, emigration
Change in population size over a time interval: where B = births, D = deaths, I = immigration, E = emigration.
Often we focus on net growth rate R = B - D (ignore immigration/emigration) and express per-capita terms.
Life tables and demography
Life table summarizes age-specific survival and reproductive rates for a cohort.
Often constructed from females only (as they produce offspring) when modeling sexually reproducing species.
Survivorship curves (Types I, II, III):
Type I: low early mortality, high survival to old age (e.g., humans, large mammals).
Type II: constant survival probability regardless of age (some birds, lizards, certain rodents).
Type III: high juvenile mortality, great investment in early reproduction; many offspring with little parental care (many invertebrates, fishes).
Examples and data
Table 40.1 (life table for female Belding’s ground squirrels) shows age-specific survival and female offspring numbers; illustrates how demography is summarized.
Survivorship curves and sex-specific reproduction are used to understand population trends.
Concept 40.5 The exponential and logistic models describe the growth of populations
Exponential growth (ideal conditions)
When resources are unlimited, populations grow exponentially: where r is the intrinsic rate of increase (per capita growth rate).
Per-capita growth is constant, but total growth accelerates as N increases, yielding a J-shaped curve.
Demonstrative example: initial exponential growth in certain populations after release from hunting or disturbance (e.g., elephants in Kruger NP for ~60 years), followed by resource limitations and management actions.
From births/deaths to per-capita growth and rate of increase
Net growth rate: over a time interval, so .
Per-capita change in population size over a time interval: , hence If we define per-unit-time growth without scaling by Δt, we obtain .
Carrying capacity and logistic growth
Real environments have limited resources; carrying capacity K is the maximum population size the environment can sustain indefinitely.
When N approaches K, per-capita growth rate declines due to resource limitation, leading to a slowdown in population growth.
Logistic growth model (incorporating limiting resources):
Start with the exponential model and introduce a factor that reduces growth as N increases:
One common form:
Key behavior:
When N is small (N << K), growth is near exponential (growth rate near r).
When N is around K/2, the population grows fastest (maximum positive growth rate); as N nears K, growth slows to zero.
Example: In a hypothetical population with K = 1500 and r = 1.0, the maximum growth rate occurs near N = 750, giving a growth rate of about +375 individuals per unit time (rN(K - N)/K at N = 750).
Overshoot and real-world deviations
Real populations can overshoot carrying capacity due to delays in the response of reproduction to resource limitation.
Examples and exercises (Paramecium, Daphnia) illustrate how real populations may overshoot and then stabilize at or around K, or oscillate around it due to time lags and other factors.
Concept 40.6 Life history traits and population density
Life history and trade-offs
Life history includes three main variables: when reproduction begins (age at maturity), how often reproduction occurs, and how many offspring are produced per reproductive event.
Trade-offs: energy allocated to reproduction reduces resources for survival; between number and size of offspring; plants with numerous small seeds vs. plants with fewer large seeds illustrate this trade-off.
Examples:
A plant like dandelion produces many tiny seeds to maximize dispersal and colonization chances.
Brazil nut trees produce a moderate number of large seeds with substantial nutrient reserves, improving seedling establishment.
r-selection vs. K-selection
r-selection maximizes intrinsic rate of increase (r); occurs in disturbed or underpopulated environments; many small seeds or high fecundity (weeds, many invertebrates).
K-selection maximizes performance near carrying capacity; traits favored at higher densities; fewer offspring with more parental investment (longer-lived and larger organisms such as many trees and large mammals).
Population change and density dependence
Density-dependent factors: birth rates decrease and/or death rates increase with population density (e.g., competition for resources, disease transmission, territoriality).
Density-independent factors: population regulation by abiotic factors, such as weather, drought, or floods, that affect birth/death rates independently of density.
Interaction example: dune fescue grass showed density-dependent reproduction with density-independent mortality, leading to a stable equilibrium density (Q) where births balance deaths.
Mechanisms of density-dependent regulation (Figure 40.23)
Competition for resources reduces reproduction as density increases.
Predation can increase mortality at higher prey densities or lead to selective predation on common species.
Intrinsic physiological factors: crowding can trigger hormonal changes that reduce reproduction or immune function.
Toxic wastes and territoriality: accumulation of wastes or territorial defense limits population size.
Disease can spread more easily in denser populations, contributing to density-dependent mortality.
Population dynamics and metapopulations
Population fluctuations arise from many interacting factors and have implications for harvests, conservation, and ecosystem function.
Isle Royale moose-wolf dynamics illustrate predator-prey and density-dependent interactions; oscillations reflect complex interactions and external factors (climate, weather).
Metapopulations concept: populations exist in a network of habitat patches; local extinctions and recolonizations maintain species presence across a landscape.
Glanville fritillary (Melitaea cinxia) on the Åland Islands: metapopulation dynamics with patches repeatedly becoming occupied or vacant; illustrates immigration and emigration shaping gene flow and persistence.
Connections and applications
Ecological perspective of population regulation
Negative feedback (density dependence) is essential to prevent unlimited growth and to explain stabilization or fluctuations in natural populations.
Density-dependent regulation interacts with density-independent factors to determine equilibrium densities or cycles.
Practical implications
Understanding carrying capacity and density dependence helps manage wildlife populations and fisheries.
Life-history strategies influence how species respond to environmental change and disturbances.
Metapopulation concepts guide conservation in fragmented landscapes by emphasizing connectivity and recolonization potential.
Key equations and concepts (summary)
Change in population size (births, deaths, immigration, emigration):
Net growth rate (simplified):
Exponential growth (intrinsic rate of increase):
Carrying capacity and logistic growth:
Relation between per-capita growth and total growth (per-interval):
Maximum growth rate under logistic model (at N = K/2): example value with K = 1500, r = 1.0 gives (for illustrative parameters)
Concepts of r-selection vs K-selection
r-selection favors high r (rapid population growth) in disturbed or open environments.
K-selection favors traits that enhance survival near carrying capacity in crowded environments.
Population dynamics terminology
Density-dependent regulation: births/deaths tied to population density.
Density-independent regulation: births/deaths driven by abiotic factors regardless of density.
Ecotone: transitional zone between adjacent biomes.
Cohort: group of individuals born in the same time period tracked in a life table.
Survivorship types (I, II, III): patterns of survival across ages.
Practical connections and study tips
Use climographs to predict biome distribution in your region by comparing mean annual temperature and precipitation and considering seasonality.
When evaluating a species’ distribution, consider three factors in order: dispersal potential, biotic interactions (predation, competition), and abiotic conditions (temperature, water, salinity, soil).
Disturbance regimes (fire, storms) interact with climate to shape biome boundaries and vegetation structure.
Remember the logistic model’s implication: growth slows as N approaches K; populations rarely remain at K due to time lags, environmental changes, and demographic stochasticity.