Population Growth and Dispersion

Population Growth

• Population Growth: The change in the number of individuals of a species in a defined area over a certain time period.
• Factors limiting growth: Limited resources, competition, predation, and disease.
• Spekboom: Serves as habitat for numerous organisms, forming a dense thicket biome.
• Elephant's impact: Elephants clear spekboom, allowing for movement between biomes.

Population Dispersion

• Distribution of individuals within a geographical area.
• Random: Individuals spread randomly, as in a uniform habitat.
• Clumped: Individuals group together due to patchy resources, social behavior, or reproductive patterns.
• Uniform: Individuals are evenly spaced, often due to competition for limited resources.

Demography

• The statistical study of population changes concerning size, age structure, and sex ratios.
• Used to predict population trends and manage resources.
• Exponential growth: Occurs when a population has high birth rates and low death rates, resulting in a rapid increase in numbers.
• J Curve: A graphical representation of exponential growth.
• Formula: dN/dt=(b−d)NdN/dt = (b-d)NdN/dt=(b−d)N
• r > 0: Population is growing.
• r < 0: The population is decreasing.
• r = 0: Zero population growth.
• South African Rhinos: Used as an example of population growth, highlighting birth, death, and growth rates.
• Poaching impact: Example of how human activity negatively influences a population.

Do Populations Ever Grow Exponentially?

• Exponential growth: Occurs only under ideal conditions, such as unlimited resources.
• Logistic growth: Occurs in environments with limited resources.
• Carrying capacity (K): The maximum population size that a specific environment can support.

Population Growth Under Ideal Conditions

• High birth rates and low death rates.
• r: Represents the maximal per capita growth rate.
• Formula: dN/dt=rmaxNdN/dt = r_{max}NdN/dt=rmax​N

Carrying Capacity

• Population sizes are limited by factors like resource scarcity.
• The maximum number of individuals that can be sustained in a given environment.

Logistic Model

• Per capita growth rate decreases as population size approaches carrying capacity.
• S-shaped curve: Represented graphically.
• Formula: dN/dt=rN(K−N)/KdN/dt = rN (K - N)/KdN/dt=rN(K−N)/K

Intraspecific Competition

• Competition for limited resources between individuals of the same species.
• Animals: Food, water, territories, and nesting sites.
• Plants: Sunlight, water, nutrients, and growth space.
• Results in uniform distribution and increased predation/disease rates.

Exponential vs Logistic Growth

• Density-dependent mortality (logistic): Mortality rates increase with population density due to limited resources, competition, and disease.
• Density-independent effects (exponential): Mortality rates are not affected by population density, often due to catastrophic events.

Population Cycles

• Fluctuations in population size over time influenced by density-dependent and density-independent factors.
• Stable: Relatively constant population size.
• Cyclic: Fluctuations in population size driven by interactions between predator and prey populations.
• Chaotic: Unpredictable fluctuations due to dominant density-independent effects.

Metapopulation Dynamics

• Interconnected populations of the same species that exchange individuals.
• Source population: A population with a surplus of individuals that migrates to other habitats.
• Sub-optimal population: A population with a deficit of individuals that relies on immigration from suitable habitats.
• Unsuitable: Habitats where populations cannot survive for extended periods.

Number of Breeding Seasons

• Iteroparous: Organisms reproduce multiple times throughout their lifespan.
• Semelparous: Organisms reproduce only once in their lifespan, often producing a large number of offspring.
• Agave: A semelparous species that releases seeds once, maximizing survival chances.

r-K Scheme of Life History Strategies

• Different species exhibit distinct life history strategies.
• r-selection: Rapid growth, high reproductive rate, and low parental care.
• K-selection: Slow growth, low reproductive rate, and high parental care.

Population Growth and Age Structure

• Negative growth: Occurs when death rates exceed birth rates, resulting in a shrinking population.
• Rapid growth: Characterized by a high proportion of young individuals, leading to a rapid population increase.

Demographic Transition Model

• Describes the stages of population growth as societies develop.
• Stage 1: Preindustrial, characterized by high birth and death rates.
• Stage 2: Transitional, characterized by declining death rates and stable birth rates.
• Stage 3: Industrial, characterized by declining birth rates and continued low death rates.
• Stage 4: Postindustrial, characterized by low birth and death rates.