Population Ecology & Distribution of Organisms – Key Vocabulary

Scope of Ecological Study

• Levels of ecological organization, studying interactions from individual organisms to the entire planet:

  • Organismal Ecology: Focuses on how an organism's structure, physiology, and behavior meet environmental challenges. Sub-fields include:

  • Physiological Ecology: Investigates how an organism's physical and chemical functions interact with its environment.

  • Evolutionary Ecology: Examines how natural selection shapes an organism's adaptations to its environment.

  • Behavioral Ecology: Studies how animal behavior is influenced by and influences ecological processes.

  • Population Ecology: Analyzes factors affecting population size, density, distribution, and age structure.

  • Community Ecology: Studies how interactions between different species (e.g., predation, competition) affect community structure and organization.

  • Ecosystem Ecology: Examines energy flow and chemical cycling among the various biotic and abiotic components of an ecosystem.

  • Landscape Ecology: Focuses on the exchanges of energy, materials, and organisms across multiple ecosystems, considering spatial arrangement.

  • Global Ecology (Biosphere): Investigates regional and global exchanges of energy and materials, influencing the functioning and distribution of organisms across the Earth.

Climate & Biomes

• Climate (long-term prevailing weather conditions, including factors like temperature (T), precipitation, sunlight intensity, and wind patterns) is the primary determinant of biomes' global distribution.

• Global Climate Patterns:

  • Solar Energy and Latitudinal Variation: Intense solar radiation at the equator heats air and causes it to rise, creating a low-pressure zone. This warm, moist air cools and releases precipitation, leading to the formation of Hadley cells (convection cells).

  • Descending Dry Air: As the now-dry air descends around \pm 30^\circ latitude (both north and south), it absorbs moisture, leading to the formation of Earth's major deserts.

  • Mid-latitude Westerlies: At mid-latitudes (e.g., between 30^\circ and 60^\circ), prevailing westerlies blow from west to east.

• Regional and Local Modifiers of Climate:

  • Seasonality: Earth's 23.5^\circ axial tilt causes seasonal variations in light and temperature, most pronounced at mid- to high latitudes.

  • Large Water Bodies: Oceans and large lakes moderate the temperature of nearby terrestrial environments due to water's high specific heat, absorbing and releasing heat slowly.

  • Mountains: Influence air flow and precipitation patterns:

  • Rain Shadow Effect: Moist air from oceans rises on the windward side of mountains, cools, and releases precipitation. As the now-dry air descends on the leeward side, it results in a 'rain shadow' region with very little precipitation.

  • Lapse Rate: Temperature generally decreases with increasing elevation at an average rate of 6^\circ\text{C} per 1000\,m (or 3.6^\circ\text{F} per 1000\,ft).

• Terrestrial Biomes:

  • Defined by dominant vegetation (which reflects climate adaptations), characteristic climate bands, and prevalent disturbance regimes (e.g., fire, hurricanes, logging).

  • Major types include:

    • Tropical Forest: Occur in equatorial and subequatorial regions with high temperatures and high annual precipitation (often 200-400 cm annually), showing little seasonal variation in temperature. Characterized by vertically layered broadleaf evergreen trees, incredibly high biodiversity, and rapid decomposition.

    • Savanna: Found in equatorial and subequatorial regions, with scattered trees and large grass fields. Precipitation (30-50 cm annually) is seasonal, with a prolonged dry season and a wet season. Temperatures average 24-29^\circ\text{C}. Large herbivore populations (e.g., zebras, wildebeests) and carnivores are prominent, with regular fires playing a crucial role in maintaining the biome structure.

    • Desert: Occur near 30^\circ north and south latitude or in the interior of continents, and also in the rain shadows of mountains. Precipitation is very low (less than 30 cm annually) and highly variable, while temperatures vary widely daily and seasonally. Plants are adapted for water conservation (e.g., succulents, deep roots). Organisms show physiological and behavioral adaptations to heat and desiccation.

    • Chaparral: Found in mid-latitude coastal regions with highly seasonal precipitation (rainy winters, long, hot, dry summers). Summers average 30^\circ\text{C} or more; fall, winter, and spring are cool. Dominated by evergreen shrubs and small trees that are drought-adapted and fire-adapted, often regenerating quickly after fires.

    • Temperate Grassland: Occur in areas with seasonal precipitation in mid-latitude regions (cold, dry winters (-10^\circ\text{C})with hot, wet summers (30^\circ\text{C}). Dominated by grasses and shrubs, with large grazing mammals (e.g., bison, horses). Periodic droughts and fires are common, preventing tree establishment.

    • Temperate Broadleaf Forest: Found in mid-latitudes, characterized by significant amounts of precipitation (70-200 cm annually) spread evenly throughout the year. Temperatures range from 0^\circ\text{C} in winter to 30^\circ\text{C} in hot, humid summers. Dominated by deciduous trees that drop their leaves in autumn, leading to distinct vertical layers, including a relatively open canopy and a rich understory.

    • Northern Coniferous Forest (Taiga): The largest terrestrial biome, stretching across northern North America and Eurasia, just south of the Arctic Tundra. Winters are long and cold, while summers may be hot. Precipitation varies (30-70 cm annually), often in the form of snow. Dominated by cone-bearing trees (conifers) like pine, spruce, and fir, which are well-adapted to cold and short growing seasons.

    • Tundra: Covers expansive areas of the Arctic or high mountaintops (alpine tundra). Characterized by extremely cold winters (below -30^\circ\text{C} typical) and cool summers (less than 10^\circ\text{C}). Precipitation is low (20-60 cm annually). A permanently frozen layer of soil, called permafrost, restricts root growth. Vegetation includes mosses, lichens, grasses, and dwarf shrubs. Large grazing musk oxen and reindeer are typical.

• Aquatic Zonation:

  • Light Penetration: Photic zone (sufficient light for photosynthesis) vs. Aphotic zone (little to no light).

  • Benthic vs. Pelagic: Benthic zone (bottom substrate) vs. Pelagic zone (open water column).

  • Lake Specific Zones: Littoral zone (shallow, well-lit waters close to shore with rooted plants) vs. Limnetic zone (open, offshore, well-lit surface waters).

  • Thermocline: A narrow layer of abrupt temperature change that separates warm upper water from cold deeper water in many temperate lakes and oceans, leading to thermal stratification.

Factors Limiting Species Distribution

• Dispersal: The movement of individuals away from their area of origin or centers of high population density. Absence of a species from an area may simply reflect historic barriers (e.g., oceans, mountain ranges, urban development) to dispersal, rather than unsuitable biotic or abiotic conditions. Transplant experiments (moving a species to a new area) are used to determine if its potential range is greater than its actual range.

• Biotic Factors: Living components that can restrict distribution:

  • Predation: Consumers (predators, herbivores) directly limiting prey/plant distribution.

  • Herbivory: Animals consuming plants, impacting plant distribution and abundance.

  • Competition: Interspecific competition for limited resources (e.g., food, space, light) can exclude species.

  • Mutualism: The absence of necessary symbiotic partners (e.g., mycorrhizal fungi for plants, pollinators for flowering plants) can limit distribution.

  • Parasitism/Pathogens: Diseases or parasites can reduce survival or reproduction, limiting a species' range.

• Abiotic Factors: Non-living physical and chemical components of the environment:

  • Temperature: Influences metabolic rates, enzyme activity, and protein stability. Extreme temperatures (too hot or too cold) can be lethal or reduce fitness.

  • Water and Oxygen: Essential for all life. Water availability (too little or too much) and oxygen levels (especially in aquatic or soil environments) are critical.

  • Salinity: The concentration of salt in water or soil affects water balance (osmoregulation) within organisms. Too high or too low can be detrimental.

  • Sunlight: Crucial for photosynthesis, thus limiting primary production in aquatic environments (depth) and affecting terrestrial plant distribution.

  • Soil/Rocks: Influence nutrient availability, pH, and physical structure, impacting plant growth and indirectly animal inhabitants.

Population Characteristics

• A population is defined as a group of individuals of the same species living in the same geographic area.

• Populations are characterized by their boundaries (natural or arbitrary) and size (number of individuals).

• Density: The number of individuals per unit area or volume. Density changes are driven by:

  • Births (increase density)

  • Deaths (decrease density)

  • Immigration (in-migration, increases density)

  • Emigration (out-migration, decreases density)

• Dispersion Patterns: The spatial arrangement of individuals within a population's boundaries:

  • Clumped: Individuals aggregated in patches, often due to resource availability (e.g., water sources), social behavior (e.g., schooling fish), or successful reproduction in localized areas.

  • Uniform (Even): Individuals are evenly spaced, usually resulting from antagonistic social interactions like territoriality (e.g., nesting seabirds, creosote bushes secreting toxins).

  • Random: The position of each individual is independent of other individuals, occurring in the absence of strong attractions or repulsions among individuals or where key physical factors are relatively constant (e.g., dandelions dispersed by wind).

• Demography: The study of vital statistics of populations and how they change over time. Key tools include:

  • Life Tables: Age-specific summaries of the survival patterns of a population, often constructed by following a cohort (a group of individuals of the same age).

  • Survivorship Curves: Graphical representations of the number of individuals in a cohort still alive at each age.

  • Type I Curve: Low mortality rates during early and middle life, followed by a sharp increase in mortality among older age groups (e.g., humans, large mammals with parental care).

  • Type II Curve: Constant mortality rate throughout the lifespan (e.g., some squirrels, hydra, many perennial plants).

  • Type III Curve: High mortality rates for the young, with a much lower mortality rate for those few individuals that survive to old age (e.g., oysters, many fish species, insects, plants that produce vast numbers of seeds).

Population Growth Models

• Per-capita change (r): The per capita rate of increase, representing the difference between per capita birth rate and per capita death rate.

• Exponential Growth Model:

  • Represented by the equation \frac{dN}{dt}=rN , where \frac{dN}{dt} is the rate of change in population size, r is the intrinsic rate of increase, and N is the population size.

  • Produces a J-shaped growth curve, characteristic of populations in ideal, unlimited environments with abundant resources.

  • Assumes continuous reproduction and non-limiting resources.

  • Common in new populations or those rebounding from a catastrophe.

• Carrying Capacity (K): The maximum population size that a particular environment can sustain indefinitely, given the available resources and environmental conditions.

• Logistic Growth Model:

  • Represented by the equation \frac{dN}{dt}=rN\left(\frac{K-N}{K}\right) . This model incorporates the concept of carrying capacity.

  • When N is small, (K-N)/K is close to 1, and growth is nearly exponential. As N approaches K, (K-N)/K approaches 0, and the growth rate slows.

  • Produces an S-shaped (sigmoid) growth curve, which is more realistic for most natural populations, showing an initial rapid increase, followed by a slowing growth rate as N approaches K.

  • The population growth rate is fastest when the population size (N) is approximately half of the carrying capacity (N = K/2).

  • As N approaches K, the growth rate (\frac{dN}{dt}) approaches zero.

  • Real populations may temporarily overshoot K due to time lags between resource depletion and a population's response, leading to fluctuations around carrying capacity.

Life History Strategies

• Life History refers to the traits that affect an organism's schedule of reproduction and survival.

• Key Life History Traits include:

  • Age at first reproduction (maturity).

  • Frequency of reproduction (number of reproductive events).

  • Number of offspring produced per reproductive episode.

  • Size of offspring.

• Trade-offs: Organisms face physiological and energetic limitations, leading to trade-offs between different life history traits.

  • Many small offspring vs. few large offspring: Producing many small offspring (e.g., dandelions, marine invertebrates) often means less parental investment per offspring. Producing few large offspring (e.g., primates, coconuts) typically involves significant parental investment, increasing the survival probability of each.

  • Reproduction vs. Survival: Investing more energy in reproduction often comes at the cost of survival. For example, a kestrel study showed a negative relationship between the number of eggs laid and the survival rate of parent birds in the following year.

• r-selection (Density-Independent Selection):

  • Favors traits that maximize the intrinsic rate of increase (r) and reproductive success.

  • Common in uncrowded environments, new or disturbed habitats, or those with highly variable conditions.

  • Typically characterized by early maturity, short generation times, many small offspring, a single reproductive event (semelparity), and little to no parental care.

  • Examples: Most insects, bacteria, weeds.

• K-selection (Density-Dependent Selection):

  • Favors traits that are advantageous when the population is living at or near its carrying capacity (K).

  • Common in stable, predictable environments where competition for resources is high.

  • Typically characterized by delayed maturity, longer generation times, fewer but larger offspring, repeated reproductive events (iteroparity), and significant parental care.

  • Examples: Large mammals (humans, elephants), long-lived plants (oak trees).

Density Dependence & Regulation

• Density-Independent Factors: Factors that affect birth and death rates regardless of population density.

  • Examples: Natural disasters (floods, fires, earthquakes), extreme weather (droughts, severe cold).

• Density-Dependent Mechanisms: Mechanisms where the birth rate falls or the death rate rises as population density increases, forming a negative feedback loop that regulates population growth (growth rate decreases when N is high).

  • Competition for Resources: As population density increases, competition for limited resources (e.g., food, water, nesting sites, light) intensifies, leading to reduced birth rates and increased death rates.

  • Territoriality: When space is a limited resource, aggressive interactions or the establishment of territories can limit population density by ensuring only certain individuals reproduce or survive well.

  • Disease & Parasites: Transmission rates of diseases and parasites often increase with higher population density, as individuals are in closer contact.

  • Predation: Predator populations may increase in response to abundant prey, or predators may spend more time hunting in high-density prey areas, leading to an increased death rate for the prey population.

  • Toxic Wastes: Accumulation of metabolic wastes can become toxic at high population densities (e.g., alcohol for yeast, ammonia for aquatic animals).

  • Intrinsic Physiological Factors: High population densities can induce stress, leading to hormonal changes that reduce fertility, suppress the immune system, or increase aggressive behavior, ultimately leading to lower birth rates or higher death rates.

Population Dynamics & Metapopulations

• Population Dynamics: The study of how populations change in size and composition over time. Fluctuations in population size arise from the complex interplay of interacting biotic and abiotic factors.

  • Examples: The classic predator-prey cycles, such as the moose-wolf dynamics on Isle Royale, where wolf population changes follow those of moose, and climate factors also influence both populations.

• Metapopulation: A network of spatially separated local populations, or patches, interconnected by occasional dispersal of individuals between them.

  • The persistence of a metapopulation depends on a dynamic balance between local extinctions (when a population in a patch dies out) and recolonizations (when a patch is re-occupied by individuals from other patches).

  • Example: The Glanville fritillary butterfly, which exists in a metapopulation structure where local populations frequently go extinct, but new populations are established in vacant habitat patches through dispersal from existing populations.