Population Ecology

Fundamentals of Population Ecology

• Population ecology is a primary field within ecology that investigates how and why the number of individuals in a population changes over time and how populations interact with their environment.

• It serves as a connection between biology, environmental science, and human impact studies by exploring organisms' survival, reproduction, and the maintenance of ecological balance.

• The field specifically focuses on fluctuations in population size and the specific factors that regulate these changes.

• Population ecology allows for:

  • Predicting how populations respond to environmental changes.

  • Managing wildlife and conservation efforts.

  • Understanding the effects of human activity on species survival.

  • Maintaining the balance of natural ecosystems through sustainable resource use.

Levels of Ecological Organization and Key Definitions

• Understanding population ecology requires distinguishing between five levels of organization:

  • Organism: A single individual capable of independent existence.

  • Population: A group of organisms of the same species living in the same area and capable of interbreeding freely.

  • Community: All the populations of different species that live and interact in a specific area.

  • Ecosystem: A community of living organisms interacting with each other and their non-living (abiotic) environment.

  • Biosphere: The part of Earth where all life exists, representing all ecosystems combined.

• Core Terminology:

  • Species: A group of organisms with similar characteristics which can interbreed to produce fertile offspring.

  • Habitat: The specific type of environment or area in which a plant or animal species normally occurs.

  • Ecology: The study of mutual interaction among living organisms and the environment in which they live.

  • Ecological Niche: All the conditions necessary for an organism to survive and reproduce.

  • Size of a population: The total number of individuals in a population.

  • Population Density: The number of individuals of the population per unit of area.

• Density Formula:

  • $\text{Population Density} = \frac{\text{Number of People}}{\text{Land Area}}$

Population Parameters and Change

• Population parameters are variables that determine population size and its changes over time, helping ecologists understand if a population is growing, shrinking, or stable.

• Natality (Birth Rate):

  • Increases population size.

  • Expressed as the number of live births per thousand individuals per year.

  • In animals, it is measured by birth rate; in plants, it is represented by the number of seeds or new plants produced.

• Mortality (Death Rate):

  • Decreases population size.

  • Expressed as the number of deaths per thousand individuals per year.

  • Mortality rate by age graph: Mortality rates increase exponentially as age increases. There is also a notable spike in infant deaths due to vulnerability to diseases and underdeveloped immune systems.

  • Mortality Calculation Formula: $\frac{\text{Deaths}}{\text{population size}} \times 1000$

  • Example: A city with a population of $20\,000$ people had $560$ deaths in a year.

  • $\text{Crude Mortality Rate} = \frac{560}{20\,000} \times 1000 = 28\,\text{deaths per 1000 people}$

• Immigration:

  • One-way movement of organisms into an area, increasing population size.

• Emigration:

  • One-way movement of organisms out of an area, decreasing population size.

• Migration:

  • The movement of an organism from one place to another and then returning to the original place.

  • It is a temporary factor and not a permanent change to population size.

  • Population size decreases when organisms migrate away and increases when they return.

Population Trends and formulas

• Population change occurs when the balance between parameters shifts:

  • Increase: \text{Birth} + \text{Immigration} > \text{Death} + \text{Emigration}

  • Decrease: \text{Birth} + \text{Immigration} < \text{Death} + \text{Emigration}

  • Stable: $\text{Birth} + \text{Immigration} = \text{Death} + \text{Emigration}$

• General Change Formula:

  • $\text{Pop Size Change} = (\text{Births} + \text{Immigration}) - (\text{Deaths} + \text{Emigration})$

• Population Growth Rate (R):

  • $R = \frac{(B + I) - (D + E)}{N}$

  • Where $B$ = births, $I$ = immigration, $D$ = deaths, $E$ = emigration, and $N$ = population size.

• Open vs. Closed Populations:

  • Open Population: Natality, mortality, immigration, and emigration all occur freely (e.g., birds migrating between regions).

  • Closed Population: Only natality and mortality affect the size because no immigration or emigration occurs (e.g., fish in a small pond).

Methods for Estimating Population Size

• Direct Methods: Accounting for every individual to determine exact size.

  • Census: Counting all individuals (common for human populations).

  • Aerial Photography: Used for counting large animals in nature reserves.

  • Helicopters: Used for counting larger animals.

• Indirect Methods: Using samples to estimate the total size.

• Quadrant Method (Simple Sampling):

  • A square area is used to count individuals. Multiple quadrants are sampled, and the mean is calculated.

  • Formula: $N = \frac{\text{Average number in sample size } \times \text{ size of habitat}}{\text{Size of quadrant}}$

• Mark-Recapture Method:

  • Animals are captured, marked, and released. Later, a second group is captured, and the marked individuals are counted.

  • Formula: $N = \frac{C \times M}{R}$

  • $C = \text{total individuals captured second time}$

  • $M = \text{individuals originally captured and marked}$

  • $R = \text{marked individuals found in the second capture}$

• Validity of Mark-Recapture:

  1. Marking must not injure the organism.

  2. Marks must remain clearly visible until the second capture.

  3. Marking must not affect the animal's movement.

  4. Marked organisms need enough time to mix back into the original population.

  5. The population must be closed (no immigration/emigration).

  6. The period between captures must be brief enough that no births or deaths occur.

Population Growth Forms and Regulation

• Exponential Growth (J-Shaped Curve):

  • Occurs when a population has unlimited resources, ideal conditions, no disease, and no predation.

  • Each generation grows faster than the last, increasing the growth rate with population size.

  • This is unsustainable and usually found in small populations entering new habitats for a short period.

• Environmental Resistance:

  • The sum of all factors that stop a population from reproducing at its maximum rate.

  • Factors include limited food/space, predation, disease, climate changes, and competition.

  • This causes birth and immigration rates to decrease, and death and emigration rates to increase.

• Carrying Capacity (K):

  • The population density that the environment can support.

  • Once reached, growth slows or stops, and the population fluctuates around $K$.

  • If population exceeds $K$, negative feedback mechanisms (increased death/emigration, decreased birth/immigration) reduce growth until equilibrium is restored.

• Logistic Growth (S-shaped Curve):

  • As environmental resistance builds, growth slows and stabilizes around the Carrying Capacity ($K$).

• Biotic Potential: The maximum rate at which a population can grow under ideal conditions.

Limiting Factors of Population Growth

• Limiting factors regulate populations exceeding carrying capacity and are categorized by density dependence.

• Density-Independent Factors:

  • Result from natural factors rather than the number of organisms.

  • Affect populations regardless of size or density (e.g., climate, environmental disasters, pollutants).

• Density-Dependent Factors:

  • Result from high population density.

  • As density rises, these factors cause birth rates to fall or death rates to rise (e.g., lack of food/water, space, shelter, disease, parasitism, predation, competition).

• Stable Population: Fluctuates around carrying capacity; numbers decrease when above $K$ and increase when below $K$.

• Unstable Population: Develops if a population exceeds $K$ without limiting factors; results in population crash or extinction once resources are exhausted.

Population Interactions

• Importance: Interactions maintain balance, prevent uncontrolled growth, promote biodiversity through niche differentiation, and drive evolution.

• Predation:

  • One species (predator) kills and eats another (prey).

  • Low prey levels decrease predator populations; high prey levels increase them.

  • Predators maintain diversity by regulating prey distribution, preventing dominance of one species, and keeping the prey population genetically fit by catching the weakest individuals.

  • Predator vs. Prey Graph: Prey numbers always spike or decrease before predator numbers do.

• Adaptations:

  • Predators: Speed, camouflage, claws, teeth, venom, keen senses.

  • Prey: Camouflage, warning coloration, mimicry, herd behavior, speed, physical defenses (spines, shells, toxins).

• Feeding Levels:

  • Primary producers: Autotrophic, make their own food.

  • Primary Consumers: Prey of secondary consumers.

  • Secondary Consumers: Prey of tertiary.

  • Tertiary Consumers: Prey of quaternary.

  • Quaternary Consumers: Top predators with no natural hunters. Energy decreases moving up these levels.

Competition and Symbiosis

• Intraspecific Competition: Between individuals of the same species. Regulates size and ensures only the fittest reproduce (e.g., male impalas fighting for a mate).

• Interspecific Competition: Between different species with similar niches (e.g., lions and hyenas competing for prey).

• Competitive Exclusion Principle: Two species requiring identical resources cannot coexist indefinitely. One will outcompete the other, forcing the loser to change its niche or move.

• Resource Partitioning: An evolutionary process where species coexist by using shared resources slightly differently (e.g., different root depths in plants, different feeding heights for giraffes vs. kudu).

• Symbiosis:

  • Parasitism: One benefits (parasite), one is harmed (host). Endoparasites (tapeworms) live inside; ectoparasites (ticks) live on the surface.

  • Mutualism: Both benefit. Facultative (can survive alone) vs. Obligate (cannot survive without each other).

  • Commensalism: One benefits, the other is unaffected (e.g., barnacles on whales, remora fish on sharks).

Community Ecology and Succession

• Community Structure: Interaction between living and non-living organisms, defined by species richness (number of species) and evenness (relative abundance).

• Stability: Depends on biodiversity and Keystone species (species like bees that play a critical role in structure).

• Ecological Succession: The gradual replacement of one community by another until a stable Climax Community is reached.

• Primary Succession:

  • Occurs in areas with no soil or previous life (bare rock, volcanic lava).

  • Pioneer species: Lichens, mosses, or algae colonize first to form soil.

  • Takes hundreds to thousands of years.

• Secondary Succession:

  • Occurs after a disturbance (fire, flood, farming) that leaves soil intact.

  • Happens much faster (decades) due to existing soil and nutrient banks.

• Climax Community: The final stable stage with high diversity, complex food webs, and efficient nutrient cycling.

Human Impacts and Behavior

• Human Overpopulation: Exponential growth due to medicine and technology leads to resource overexploitation, deforestation, and pollution.

• Habitat Destruction: Species isolation and fragmentation reduce breeding success and genetic diversity.

• Invasive Alien Species: Non-native species (e.g., Water hyacinth, European starlings) spread rapidly without natural predators, disrupting food webs.

• Social Organization:

  • Pack Behavior: Coordinated hunting/defense with a dominant breeding pair (e.g., wolves).

  • Herds/Flocks: Safety in numbers and increased foraging efficiency (e.g., sheep, buffalo).

  • Colonies: Division of labor with a reproductive hierarchy (e.g., ants, bees).

  • Solitary: Maintain large territories to reduce competition (e.g., leopards).

• Territoriality: Defense of resources to prevent overcrowding.

• Dominance Hierarchies: Restrict reproduction to the strongest individuals, maintaining genetic quality.

• Parental Care:

  • Minimal: Many offspring, low survival (fish, insects).

  • Intensive: Few offspring, high survival (mammals).

• Communication: Essential for warning of predators, sharing food locations, and mating (visual, auditory, chemical/pheromones, tactile).