Population Ecology: Principles, Models, and Human Impact

Population Ecology

Definition and Scope

Population ecology is a branch of ecology that studies populations of organisms, especially in relation to their environment. It focuses on how environmental factors influence population density, distribution, age structure, and overall population size.

• Population ecology examines the interactions between populations and their abiotic and biotic surroundings.

• Population: A group of individuals of the same species living in a defined geographic area.

• Key factors studied include birth rates, death rates, immigration, emigration, and resource availability.

• Example: A population of sea turtles hatching and migrating on a beach.

Survivorship Curves

Patterns of Survivorship

Survivorship curves graphically represent the proportion of individuals in a cohort that survive at each age. These curves help ecologists understand mortality patterns within populations.

• Survivorship curve: A plot showing the number or proportion of individuals surviving at each age for a given population.

• Used to compare survival strategies among species.

• Example: Belding's ground squirrels exhibit a relatively constant rate of death, resulting in a straight-line survivorship curve.

Types of Survivorship Curves

Survivorship curves are classified into three general types, each reflecting different life history strategies.

• Type I: Low death rates during early and middle life, with increased mortality in older age groups. Common in species that produce few offspring but provide substantial parental care (e.g., humans, large mammals).

• Type II: Constant death rate throughout the organism's life span. Typical of some birds, rodents, and reptiles.

• Type III: High death rates for young individuals, with lower death rates for survivors. Found in species that produce many offspring but provide little or no parental care (e.g., oysters, many fish, plants).

• Trade-offs: Type I species invest heavily in offspring survival, while Type III species rely on producing many offspring to ensure some survive.

Comparison Table: Types of Survivorship Curves

Type	Death Rate Pattern	Parental Care	Example Organisms
I	Low early/mid-life, high late-life	High	Humans, elephants
II	Constant throughout life	Moderate	Ground squirrels, some birds
III	High early-life, low for survivors	Low	Oysters, many fish

Population Growth Models

Exponential Growth Model

The exponential growth model describes population increase under ideal, unlimited environmental conditions. It is useful for understanding the potential for rapid population expansion.

• Assumes unlimited resources and no environmental constraints.

• Population growth rate is proportional to the current population size.

• Equation: $\frac{dN}{dt} = rN$ where $N$ is population size, $r$ is the per capita rate of increase, and $\frac{dN}{dt}$ is the rate of change in population size.

• Results in a J-shaped curve when population size is plotted over time.

• Example: Elephant populations in Kruger National Park grew exponentially after hunting was banned.

Logistic Growth Model

The logistic growth model incorporates environmental limits by introducing the concept of carrying capacity (K), the maximum population size an environment can sustain.

• Growth rate decreases as population size approaches carrying capacity.

• Equation: $\frac{dN}{dt} = rN \left( \frac{K-N}{K} \right)$ where $K$ is carrying capacity.

• Produces an S-shaped (sigmoidal) curve.

• When $N$ is small, growth is nearly exponential; as $N$ approaches $K$, growth slows and eventually stops.

• Example: Laboratory populations of Paramecium fit the logistic model, stabilizing near carrying capacity.

Table: Logistic Growth of a Hypothetical Population (K=1,500)

Population Size (N)	Intrinsic Rate of Increase (r)	(K-N)/K	Per Capita Growth Rate	Population Growth Rate
25	10	0.983	0.983	+25
100	10	0.933	0.933	+93
250	10	0.833	0.833	+208
500	10	0.667	0.667	+333
750	10	0.500	0.500	+375
1,000	10	0.333	0.333	+333
1,500	10	0.000	0.000	0

Life History Traits

Evolutionary Strategies

Life history traits are characteristics that affect an organism's schedule of reproduction and survival. These traits are shaped by natural selection and include:

• Age at first reproduction (maturity)

• Frequency of reproduction

• Number of offspring per reproductive episode

• Trade-offs exist between number and size of offspring; species with low offspring survival produce many small offspring, while those with higher survival invest more in fewer, larger offspring.

• Example: Dandelions produce many small seeds; Brazil nut trees produce fewer, larger seeds packed with nutrients.

Human Population Growth

Global Trends and Demographic Transition

The human population has grown rapidly over the past centuries, but the rate of growth is slowing due to demographic transitions.

• Doubling time has decreased dramatically, but growth rate peaked in the 1960s and has since declined.

• Demographic transition: Shift from high birth and death rates to low birth and death rates, often associated with improved healthcare and education.

• Most current growth is concentrated in less industrialized countries.

Age Structure and Population Stability

Age structure diagrams (population pyramids) show the distribution of individuals among age groups and help predict future growth trends.

• Rapid growth: Broad base, many young individuals (e.g., Zambia).

• Slow growth: More even distribution (e.g., United States).

• No growth: Narrow base, fewer young individuals (e.g., Italy).

Table: Age Structure Patterns

Country	Growth Pattern	Age Structure
Zambia	Rapid	Broad base, many young
United States	Slow	Even distribution
Italy	No growth	Narrow base, fewer young

Carrying Capacity and Ecological Footprint

Limits to Human Population Growth

Carrying capacity is the maximum number of individuals an environment can support. For humans, it depends on resource consumption and technological advances.

• Ecological footprint: The total land and water area required to support an individual, city, or nation.

• Measured in global hectares (gha); the sustainable average is 1.7 gha per person, but actual footprints vary widely.

• Resource use above carrying capacity is unsustainable and leads to environmental degradation.

• Example: The average ecological footprint in the United States is 8 gha, compared to 2.7 gha globally.

Additional info: The ecological footprint concept is used to estimate sustainability and guide policy decisions regarding resource use and population management.