Population Ecology - Chapter 36

Introduction to Population Ecology

• Population ecologists focus on the study of natural population structure and dynamics.

• A population is defined as a group of organisms of a single species that fulfills three criteria:

  • They occupy the same general area.

  • They rely on the same resources.

  • They are influenced by the same environmental factors.

  • Individuals in a population are likely to interact and breed with one another to form viable offspring.

• Population ecology is the specific study of how and why populations change.

• Population dynamics involves the study of interactions between biotic factors (living components) and abiotic factors (non-living components) that cause variations in population sizes over time.

• The field is fundamentally concerned with:

  • Changes in population size.

  • Factors that regulate these populations over time.

• Basic population size changes occur through four primary mechanics:

  • Increase: Birth and immigration into an area.

  • Decrease: Death and emigration out of an area.

Population Density and Dispersion Patterns

• Population variables such as density and dispersion are critical for ecological study.

• Population Density is defined as the number of individuals of a species per unit area or volume.

  • Example: The number of oak trees per square kilometer ($km^2$) in a forest.

  • Example: The number of earthworms per cubic meter ($m^3$) in forest soil.

• Ecologists employ various sampling techniques to estimate population densities rather than counting every individual.

• Dispersion patterns describe the way individuals are spaced within their area. There are three primary types:

  • Clumped Dispersion Pattern: Individuals are grouped in patches around shared needed resources such as food, water, or breeding space. This is the most common form of distribution because resources are often unequally distributed.

  • Uniform Dispersion Pattern: This involves individuals being equally spaced in the environment. This pattern most likely arises from interactions between individuals, such as territorial behavior or competition.

  • Random Dispersion Pattern: Individuals are spaced in an unpredictable way without a specific pattern of interaction or resource influence.

Life Tables and Survivorship Curves

• Life tables are used to track survivorship, defined as the chance of an individual in a given population surviving to various age intervals.

• A Survivorship Curve plots the proportion of individuals from an initial population that are alive at each age.

• Table $36.3$: Life Table for the US Population in $2004$ (Selected cohorts based on $100,000$ individuals starting at age interval $0$):

  • Age $0-10$: $100,000$ Living at start; $871$ Dying; $0.991$ chance of surviving.

  • Age $10-20$: $99,129$ Living at start; $419$ Dying; $0.996$ chance of surviving.

  • Age $20-30$: $98,709$ Living at start; $933$ Dying; $0.991$ chance of surviving.

  • Age $30-40$: $97,776$ Living at start; $1,259$ Dying; $0.987$ chance of surviving.

  • Age $40-50$: $96,517$ Living at start; $2,781$ Dying; $0.971$ chance of surviving.

  • Age $50-60$: $93,735$ Living at start; $5,697$ Dying; $0.939$ chance of surviving.

  • Age $60-70$: $88,038$ Living at start; $11,847$ Dying; $0.865$ chance of surviving.

  • Age $70-80$: $76,191$ Living at start; $22,267$ Dying; $0.708$ chance of surviving.

  • Age $80-90$: $53,925$ Living at start; $31,706$ Dying; $0.412$ chance of surviving.

  • Age $90+$: $22,219$ Living at start; $22,219$ Dying; $0.000$ chance of surviving.

• Three main types of survivorship curves are recognized in nature:

  • Type I: Characteristic of species with long life spans and significant parental involvement in rearing offspring (e.g., humans, large mammals).

  • Type II: Characteristic of species where death is relatively equal across all age intervals, showing less parental involvement (e.g., some birds and lizards).

  • Type III: Characteristic of species where death is almost a certainty for the young, involving no parental involvement (e.g., clams, some fish, many insects).

Idealized Models of Population Growth

• Ecologists use two main models to predict patterns of growth:

• Exponential Growth Model: Describes the rate of population increase under ideal conditions. It is calculated using the equation:

  • $G = rN$

  • $G$: Population growth rate.

  • $N$: Population size.

  • $r$: Per capita rate of increase (average contribution of each individual to the growth).

• Logistic Growth Model: Represents idealized population growth that is slowed by limiting factors as the population size ($N$) increases. The formula is:

  • $G = rN \frac{K - N}{K}$

  • $K$: Carrying Capacity, defined as the maximum population size a particular environment can sustain.

• Table $36.4B$: Effect of $K$ on Growth Rate (where $K = 1,000$ and $r = 0.1$):

  • When $N = 10$; $rN = 1$; $\frac{K - N}{K} = 0.99$; $G = 0.99$.

  • When $N = 100$; $rN = 10$; $\frac{K - N}{K} = 0.9$; $G = 9.00$.

  • When $N = 400$; $rN = 40$; $\frac{K - N}{K} = 0.6$; $G = 24.00$.

  • When $N = 500$; $rN = 50$; $\frac{K - N}{K} = 0.5$; $G = 25.00$ (Max growth rate).

  • When $N = 700$; $rN = 70$; $\frac{K - N}{K} = 0.3$; $G = 21.00$.

  • When $N = 1,000$; $rN = 100$; $\frac{K - N}{K} = 0.00$; $G = 0.00$.

• Limiting factors eventually restrict growth, preventing populations from expanding indefinitely.

Factors Limiting Population Growth

• The logistic model implies that growth slows as density increases.

• Density-dependent rates result in declining birth rates and increasing death rates at higher densities.

• Intraspecific Competition: Competition between individuals of the same species for limited resources. It is a critical density-dependent factor limiting growth.

• Limiting factors in the environment include:

  • Food and nutrients.

  • Retreats for safety.

  • Nesting sites.

• Density-independent factors affect population size regardless of population density. Most often these are abiotic, such as:

  • Weather events (e.g., aphids experience sudden declines due to seasonal changes).

  • Fires.

  • Storms.

  • Habitat destruction caused by human activity.

Boom-and-Bust Cycles

• Some populations exhibit regular fluctuations in density known as boom-and-bust cycles.

• A classic example is the Snowshoe Hare and the Lynx.

  • Hare populations peak (boom) and then collapse (bust) in regular intervals.

  • Lynx population cycles follow the hare cycles because they are primary predators.

• These cycles are generally attributed to two main causes:

  • Food shortages.

  • Predator-prey interactions.

Evolution of Life Histories

• Life history refers to the traits affecting an organism’s schedule of reproduction and death.

• Key life history traits include:

  • Age of first reproduction.

  • Frequency of reproduction.

  • Number of offspring.

  • Amount of parental care.

• The Two Ends of the Life History Spectrum:

  • r-selected life history traits: Species produce more offspring and grow rapidly. They are typically found in unpredictable environments.

  • K-selected life history traits: Species raise fewer offspring but provide more care, maintaining relatively stable populations near carrying capacity ($K$).

  • Most species fall on a continuum between these two extremes.

Practical Applications of Population Ecology

• Sustainable Resource Management: This involves harvesting crops and resources while eliminating damage to the long-term viability of that resource.

• Resource managers use population ecology principles to determine sustainable yields.

• Case Study - The Newfoundland Cod Fishery:

  • The fishery was overfished due to poor management.

  • The population collapsed in $1992$.

  • Despite management efforts, the population still has not recovered as of the report date.

Human Population Trends

• The human population grew rapidly during the $20\text{th}$ century.

• As of the report, the total population stands at approximately $7$ billion.

• The Demographic Transition:

  • The shift from high birth and death rates to low birth and death rates.

  • This transition has lowered growth rates in developed nations.

  • In developing nations, death rates have dropped significantly, but birth rates remain high, leading to rapid growth.

• Estimated Population Changes in $2009$:

  • World: Birth Rate $19.5$ (per $1,000$); Death Rate $8.3$; Rate of Increase $11.2$.

  • More Developed Nations: Birth Rate $10.9$; Death Rate $10.5$; Rate of Increase $0.4$.

  • Less Developed Nations: Birth Rate $21.4$; Death Rate $7.8$; Rate of Increase $13.6$.

• Population Momentum: This is the continued growth occurring despite reduced fertility because of the large proportion of young girls ($0-14$ age group) reaching childbearing years.

Age Structure and Social Trends

• Age structure diagrams (population pyramids) show the proportion of individuals in different age groups.

• These diagrams reveal growth trends and social conditions such as the future labor force or retirement needs.

• Trends in Mexico (Age Structure):

  • $1985$: Population size $76,767,225$.

  • $2010$ (Estimated): Population size $112,468,855$.

  • $2035$ (Projected): Population size $139,457,070$.

• Trends in the United States (Age Structure):

  • $1985$: Population size $238,466,283$.

  • $2010$ (Estimated): Population size $310,232,863$.

  • $2035$ (Projected): Population size $389,531,156$.

Ecological Footprint and Resources

• The U.S. Census Bureau projects a global population of $8$ billion within the next $20$ years and $9.5$ billion by the middle of the $21\text{st}$ century.

• Resource Sustainability: To accommodate the population expected by $2025$, the world must double its food production.

• Ecological Footprint: An estimate of the land area required to provide the raw materials an individual or nation consumes. This includes:

  • Food.

  • Fuel.

  • Water.

  • Housing.

  • Waste disposal.

• The United States has a very large ecological footprint that exceeds its own land area, resulting in a large ecological deficit.

• Standard of Living Comparisons: Researchers estimate that if every person on Earth maintained the same standard of living as residents of the United States, we would require the resources of approximately $4.5$ planet Earths.