Lesson 20

Life History & Population Dynamics

• Life history refers to the schedule of an organism's growth, development, reproduction, and survival. Traits evolve to maximize organism fitness.
• Natural selection favors traits that increase an organism's survival and reproductive success.

Principle of Allocation

• Organisms acquire a finite amount of resources.
• Resources are allocated to life processes, including maintenance, growth, repair, acquiring resources, escaping predators, and reproduction.
• Allocation impacts survival and fecundity. Resources allocated to one will come at the expense of the other.
• Abundant resources allow for growth and reproduction.
• Scarce resources prioritize basic maintenance.

Resource Allocation Trade-offs

• Allocation of resources to different needs results in trade-offs.
• Allocation to acquiring resources is prioritized over other tasks.
• Maintaining needs must be prioritized over the other activities.
• When resources are abundant, more resources can be allocated to reproduction.

Reproductive Strategies - Semelparity

• Organisms reproduce only once before death. (Example: salmon)
• Semelparity is adaptive when environments are unpredictable, and offspring survival is low.

Reproductive Strategies - Iteroparity

• Organisms reproduce multiple times before death. (Example: humans)
• Iteroparous organisms invest in fewer, larger offspring which often increases offspring survival rates.

Life History Trade-offs

• Trade-offs between reproduction and survival are common.
• Energy put into reproduction cannot be used to fuel growth or repair.
• Reproductive decisions can impact an individual's winter survival.

Life History Strategies - K-Selection

• Organisms in stable environments, near carrying capacity.
• Examples: Bison, elephants, humans, whales, coconut palms
• Characterized by: fewer offspring, longer gestation periods, and greater parental care.

Life History Strategies - r-Selection

• Organisms in unstable or unpredictable environments, with low offspring survival.
• Examples: Weeds, grasses, insects, rodents
• Characterized by: many offspring, small body size, short time to maturity, short generation time, and little parental care
• More r-selective organisms may have higher intrinsic growth rate (r).
• Invasive species often have high r and perform well in disturbed environments.

Factors Regulating Population Growth - Density-Independent Factors

• Factors that do not depend on population density, like floods, fires, drought, human development.
• Seasonal cycles (wet/dry, cold/warm) are important density-indepedent factors.
• Natural disasters like floods, drought, fire, and volcanic eruptions affect all sizes of populations.
• Human effects/activities can influence population size, including clear-cutting, damming, and paving.

Factors Regulating Population Growth - Density-Dependent Factors

• Factors whose impact depends on population density, such as competition, predation, herbivory, parasitism, disease, and waste accumulation.
• Intraspecific competition occurs between members of the same species for limited resources.
• Predation involves one species (predator) killing and consuming another species (prey). Predation increases as the density increases.
• Herbivory is the interaction where an organism eats part of a plant.
• Waste accumulation is a risk as population densities increase.

Density-Dependent Factors: Intrinsic Physiological Factors

• At high densities, aggressive interactions increase and some organisms might not reproduce given limited resources and increased competition.

Density-Dependent factors: Competition

• Limited resources may make competition intense.
• Competition may cause decreased survival and reproduction.
• Competition is less fierce if the resource is abundant or not shared.
• Intraspecific competition occurs within a single species.
• Interspecific competition occurs between different species.

Density-Dependent Factors: Waste Accumulation

• High population density can lead to toxic waste accumulation which can endanger the population.
• Waste accumulation can spread disease as concentrations increase

Density-Dependent Factors: Parasitism & Disease

• Disease transmission rates increase in dense populations.
• Parasites often facilitate disease transmission. High parasite density can cause high mortality.

Density-Dependent Factors: Predation & Herbivory

• Predation rates increase as the density of predators or prey increases.
• Herbivory impacts plants

Allee Effect

• Individuals have more difficulty surviving or reproducing when population size is too small.

Living in Groups

• Animals vary in the degree of sociality. Some are solitary, while others live in groups.
• Benefits of group living include: increased foraging efficiency, increased awareness of predators, and defense against predators.
• Costs of group living include exposure to parasites, wastes, greater predator detection risk, and more competition for resources.

Many Eyes Hypothesis & Flocking

• Larger flock size can decrease time spent on vigilance.
• Intermediate flock size yields optimal feeding time.

Many Eyes Hypothesis & Predator Confusion

• Success in capturing prey is dependent on the flock size.

Age Structure of Populations

• Age structure, plus sex, is important to understand population.
• Different classifications of age structure can be used to categorize the overall population trends. This can be used to predict population growth/decline.

Survivorship Curves

• Graphs showing how the number of individuals surviving changes over time.
• Type 1 survivorship curves: Low mortality in early and middle years, high mortality in older individuals.
• Type 2 survivorship curves: Constant mortality rates throughout the lifespan.
• Type 3 survivorship curves: High mortality in early life, low mortality in older individuals.