BIOL 109 - Ch. 52: Population Ecology Notes

Population Ecology

Population Dynamics

• Population dynamics describes how population characteristics change over time and vary geographically.

• A population's characteristics include:

  • Geographical range: Overall spatial boundaries within which a population lives.

  • Habitat: Specific environment characterized by biotic and abiotic features.

  • Size: The number of individuals in a population at a specified time.

  • Density: The number of individuals per unit area or volume of habitat.

  • Distribution: The spatial distribution of individuals within the geographical range.

  • Age structure: Statistical description of the relative numbers of individuals in each age class.

Population Characteristics: Density

• Population density provides information about the relationship to resources.

• Generally, species with large body sizes have lower population densities than species with smaller body sizes.

Estimating Population Size & Density

• Head Count: For large-bodied species, a simple head count gives accurate information.

• Extrapolation: For tiny organisms at high densities (e.g., phytoplankton), population size can be extrapolated from water samples.

• Mark-Release-Recapture: Used in other cases to estimate population size.

Dispersion Patterns

• Dispersion is the spatial distribution of individuals within the geographical range.

• Random Dispersion: Individuals are distributed unpredictably within a uniform habitat.

• Clumped Dispersion: Individuals group together due to patchy habitats, social groups, or reproductive patterns.

• Uniform Dispersion: Individuals repel each other and are evenly spaced because resources are limited.

Age Structure

• Age structure is a statistical description of the relative numbers of individuals in each age class.

• Individuals are categorized as prereproductive, reproductive, or postreproductive.

• A population's age structure reflects its recent growth history and predicts future growth potential.

• Populations with many prereproductive individuals will likely grow as these individuals mature and reproduce.

Generation Time & Sex Ratio

• Generation time is the average time between an organism's birth and the birth of its offspring.

• Species that reach sexual maturity at a small body size usually have short generation times, leading to rapid population growth.

• Sex ratio is the relative proportions of males and females. The number of females has a greater impact on population growth than the number of males.

Demography

• Demography is the statistical study of the processes that change a population's size and density through time.

• Populations grow through births and immigration (movement into the population).

• Populations shrink through deaths and emigration (movement out of the population).

Life Tables

• A life table summarizes the demographic characteristics of a population.

• For short-lived organisms, demographers mark a cohort of individuals of similar age and monitor them until death.

• For longer-lived organisms, researchers sample the population for a shorter period (e.g., one to two years) and extrapolate the results.

Age-Specific Mortality and Survivorship

• Age-specific mortality is the proportion of individuals alive at the start of an age interval that died during that interval.

• Age-specific survivorship is the proportion of individuals alive at the start of an age interval that survived until the start of the next interval.

• The sum of age-specific mortality and age-specific survivorship always equals 1.

Life Table Data

• Life tables summarize the proportion of a cohort that survives to a particular age, indicating the probability that a newborn will still be alive at that age.

• They also include data on age-specific fecundity, the average number of offspring produced by surviving females during each age interval.

Survivorship Curves

• A survivorship curve shows the rate of survival for individuals over a species' average lifespan.

• Type I curves: High survivorship until late in life (typical of large animals producing few young).

• Type II curves: Constant rate of mortality in all age classes (seen in small animals subject to predation).

• Type III curves: High juvenile mortality, followed by a period of low mortality once offspring reach a critical size.

The Evolution of Life Histories

• Life histories describe the lifetime patterns of growth, maturation, and reproduction of an organism.

• Every organism is constrained by an energy budget – the total amount of energy it can accumulate to fuel its activities.

• Excess energy is stored as starch, glycogen, or fat.

• Organisms use energy for three general functions: growth, maintenance, and reproduction.

Variations in Life History Patterns

Examples:

• Coho Salmon: Larvae feed and grow for a year before swimming to the ocean, then return to spawn and die.

• Oak Trees: Grow throughout their lives, producing thousands of seeds annually for many years.

• European Red Deer: Adult females produce one or two offspring annually until about 16 years old, then die.

Analyzing Life Histories

• Ecologists compare fecundity (number of offspring produced) with parental care.

• Passive parental care includes energy invested in offspring before birth, such as yolk in an egg or nutrients via the placenta.

• Active parental care involves care provided after birth, such as nursing.

Geometric & Exponential Models of Population Growth

• Geometric Growth:

  • Bacteria reproduce by binary fission, doubling the population size each generation if no bacteria die.

  • Generation time can be as short as 20 minutes.

• Exponential Growth:

  • Population size increases steadily by a constant ratio.

  • A graph exhibits a J shape, becoming steeper over time.

Logistic Population Growth

• Population sizes are limited by factors such as resource shortages.

• Carrying capacity is the maximum number of individuals that an environment can support indefinitely.

• Carrying capacity is specific to each population and varies by habitat and time.

Logistic Population Growth

• The logistic model predicts an S-shaped graph of population size over time.

• Population grows slowly when small due to few reproducing individuals.

• Population grows slowly when large because the per capita population growth rate is low.

• Population grows quickly at intermediate sizes.

Population Dynamics: Density-Dependent Factors

• Population size, density, age structure, and ranges may change seasonally or annually.

• Many factors affecting population dynamics are density-dependent, with their influence increasing or decreasing with population density.

• The logistic model includes density-dependence, assuming per capita birth and death rates change with density.

Effects of Crowding

• Crowding (high population density) decreases individual growth rate, adult size, and survival.

• Crowding negatively affects reproduction: females produce fewer, smaller offspring with lower survival rates.

• In some species, crowding stimulates developmental and behavioral changes, such as locusts developing into a migratory form and leaving high-density areas.

Effects of Crowding on Fecundity

• Capsella bursa-pastoris (Shepherd's Purse): The number of seeds produced decreases dramatically with increasing density.

• Parus major (Great Tits): The mean number of eggs produced declines as the number of breeding pairs increases.

Other Density-Dependent Factors

• Interspecific Competition: Competition between different species has density-dependent effects.

• Predation: As prey becomes more numerous, predators consume more due to easier finding and catching.

• Parasitism and Disease: Infectious microorganisms spread quickly in crowded populations, causing density-dependent regulation.

Density-Independent Factors

• Density-independent factors (e.g., fires, earthquakes, storms) reduce population size regardless of density.

• Density-independent factors have strong effects on small-bodied species that cannot buffer themselves against environmental change.

• Example: Australian Thrips populations crash predictably during summer due to intolerance of hot, dry conditions.

Global Climate Change

• Rapid warming since the mid-20th century significantly impacts population ecology.

• Example: Increased mortality rates in 200- to 1,000-year-old coniferous trees in western North America, attributed to climate warming and longer summer droughts.

Interacting Factors

• Density-dependent and density-independent factors can interact, limiting population growth.

• Example: Food shortage (density-dependent) leading to malnourishment, increasing vulnerability to extreme weather (density-independent).

Interacting Factors (cont.)

• Populations can also be affected by density-independent factors in a density-dependent manner.

• Example: In large populations, a smaller proportion may find suitable shelter from environmental stress, increasing the percentage experiencing stress.

Life History Patterns: r-Selection

• r-selected species are adapted to rapidly changing environments.

• Generally small, with short generation times, producing numerous, tiny offspring in a single reproductive event.

• Offspring receive little or no parental care.

• High r_{max} – populations grow exponentially when conditions are favorable.

• Most offspring die before sexual maturity (Type III survivorship).

• Survivorship and fecundity are greatly influenced by density-independent factors; population size fluctuates.

Life History Patterns: K-Selection

• K-selected species thrive in more stable environments.

• Generally large, with long generation times, producing offspring repeatedly during their lifetimes.

• Offspring receive substantial parental care (Type I or Type II survivorship).

• Low r_{max} – populations often grow slowly.

• Produce a relatively small number of high-quality offspring that join a well-established population.

• Use behavioral and physiological mechanisms to buffer against environmental change; survivorship and fecundity do not fluctuate greatly.

Characteristics of r-Selected & K-Selected Species

Metapopulation Structure

• A metapopulation is a group of neighboring populations that exchange individuals.

• Populations within a metapopulation differ in size, growth rates, habitat suitability, predator exposure, and other factors.

• Individuals typically move from source populations to sink populations; sink populations persist because they receive immigrants from source populations.

Cycles in Population Size

• Population densities of insects, birds, and mammals in the Northern Hemisphere fluctuate between species-specific lows and highs in multiyear cycles.

• Factors influencing population growth include food availability and quality, predator abundance, prevalence of disease-causing microorganisms, and weather variations.

• A cycling population’s food supply and predators are themselves influenced by the population’s size.

Cycles & Time Lags

• Mathematical models suggest that predator-prey cycles are induced by time lags in each population’s response to changes in the density of the other.

• The 10-year cycles of snowshoe hares and Canada lynxes were a classic example, but hare populations fluctuate even where lynxes are absent.

• The lynx cannot be solely responsible for the hare’s cycle, although cycles in hare populations may trigger cycles in populations of their predators.

Human Population Growth

• For most of human history, population grew slowly, but it has grown exponentially over the past two centuries.

• The worldwide human population surpassed 7 billion in 2011 and is projected to reach 9.7 billion by 2050.

• More than a billion people are malnourished, lack clean drinking water, and lack adequate shelter or health care.

Human Population Growth Rate

• The annual growth rate for the human population averaged roughly 1.2% (r = 0.012) between 2001 and 2011.

• Annual population growth rates vary widely among nations, ranging from less than 0% to 3.6% in 2009.

• Age structure reflects differences in population growth rates:

  • Uniform age structure: zero population growth

  • Narrow-based age structure: negative growth

  • Broad-based age structure: rapid growth

2014 Age Pyramids: US & Africa

• The United States’ age structure falls between those for countries with zero growth and countries with rapid growth.

• The average number of children per family has declined to two, the number necessary to replace their parents.

• The U.S. population will continue to grow slowly due to continued immigration.

The Demographic Transition Model

• The demographic transition model shows the relationship between population growth and economic development:

  • Preindustrial stage: High birth and death rates, slow population growth.

  • Transitional stage: Rising food production, improved health care and sanitation, declining death rate.

  • Industrial stage: Decreasing birth rate, slowing growth.

  • Postindustrial stage: Zero population growth; eventually, birth rate falls below the death rate, r falls below zero, and population size begins to decrease.