Chpater 53 & 54

Chapter 53: Population Ecology

• Population Size Changes: Population size is affected by births, deaths, immigration, and emigration. Over time, a population may reach a stable size, fluctuate irregularly, or rise and fall in regular cycles.

• Density: Number of individuals per unit area or volume. Factors include births, deaths, immigration, and emigration.

• Dispersion: Pattern of spacing among individuals within the population boundaries.

• Demographics: Attributes (biotic and abiotic) influencing density/dispersion, summarized by life tables.

• Estimating Density:

  • Counting individuals in random plots, calculating density, and extending to the entire area.
  • Using indicators like nests, burrows, tracks, or fecal droppings.
  • Employing the mark-recapture method.

• Life Table: Summarizes the survival pattern of a cohort (group of individuals of the same age) in a population.

• Survivorship Curves: Graphically represent the number of individuals alive at each age.

• Exponential Growth Model: Describes population growth in an idealized, unlimited environment.

  • The population growth rate is given by: \frac{\Delta N}{\Delta t} = B - D where \Delta N is the change in population size, \Delta t is the time interval, B is the number of births, and D is the number of deaths during the time interval.

• Logistic Growth Model: Describes how population growth slows as it nears carrying capacity.

  • Carrying Capacity (K): The maximum population size an environment can sustain, varying with space and time.
    • Limiting factors include energy, shelter, refuge from predators, nutrient availability, water, and nesting sites.

• The logistic model produces a sigmoid (S-shaped) curve when population size (N) is plotted over time.

• New individuals are added most rapidly at intermediate population size, when the breeding population is substantial and resources are abundant.

• The population growth rate \frac{dN}{dt} decreases as N approaches K.

• Life History: Traits affecting reproduction and survival (development, physiology, behavior).

  • Key components: age at first reproduction (maturity), frequency of reproduction, number of offspring per reproductive episode.
  • Trade-offs: Resources are limited; resources for one function reduce those available for others.

• K-selection: Advantageous when density is high (near K), resources are low, and competition is strong.

• r-selection: Maximizes reproductive success when density is low and there is little competition.

• Density-Dependent Factors:

  • Population growth is regulated by environmental factors.
  • Growth occurs when birth rate > death rate; decline when death rate > birth rate.
  • Density-independent factors: Birth/death rates do not change with population density.
  • Density-dependent factors: Death rate increases or birth rate decreases with increasing density.

• Mechanisms of Density-Dependent Regulation:

  • Competition for resources
  • Disease
  • Intrinsic factors
  • Territoriality
  • Toxic wastes
  • Predation

• Human Population Growth:

  • The human population is still increasing rapidly, though not exponentially any more.
  • Doubling time decreased from 200 years in 1650 to 45 years in 1975.

• Regional Patterns:

  • Zero population growth = High birth rate - High death rate, or Low birth rate - Low death rate.
  • Demographic transition: Move from the first state to the second state.
  • Age structure: Relative number of individuals of each age in a population.
  • Global carrying capacity and ecological footprint are important considerations.

Chapter 54: Community Ecology

• Community structure is affected by the number, composition, and relative abundance of different species within a community.

• Interspecific Interactions: Interactions between individuals of different species, including competition, predation, herbivory, parasitism, mutualism, and commensalism.

  • Interactions can be positive (+), negative (–), or neutral (0) for the individuals involved.

• Categories of Ecological Interactions:

  • Competition (–/–)
  • Exploitation (+/–)
  • Positive interactions (+/+ or +/0)

• Competition:

  • Occurs when individuals of different species use a resource that limits survival and reproduction of both.
  • Species do not compete for resources that are not in short supply.

• Exploitation:

  • +/– interaction where one species benefits by feeding on another.
  • Includes predation, herbivory, and parasitism.

• Predation:

  • An individual of one species (predator) kills and eats another (prey).
  • Predators have acute senses and adaptations like claws, fangs, or poison.
  • Prey species have behavioral and morphological defenses.

• Herbivory:

  • An herbivore eats parts of a plant or alga.
  • Herbivores harm but usually do not kill the plants/algae.

• Parasitism:

  • One organism (parasite) derives nourishment from another (host), which is harmed.
  • Endoparasites live within the host; ectoparasites live on the external surface.

• Commensalism:

  • + /0 interaction where one species benefits and the other is neither harmed nor helped.

• Mutualism:

  • + /+ interaction that benefits both species.
  • Species may depend on each other for survival and reproduction, or both can survive alone.

• Ecological Niche: An organism’s specific set of biotic and abiotic environmental resources it uses.

  • The niche of a tropical tree lizard includes the temperature range it tolerates, the size of branches it perches on, the time it is active, and the size and kind of insects it eats.
  • Competitive exclusion principle: Two species cannot coexist permanently if their niches are identical.
  • Ecologically similar species can coexist if there are significant differences in their niches.

• Resource Partitioning: Differentiation of niches that enables similar species to coexist.

• Fundamental Niche: The niche potentially occupied by a species.

• Realized Niche: The portion of the fundamental niche actually occupied by a species.

• Competition may cause a species’ fundamental niche to differ from its realized niche.

• Trophic Structure:

  • The feeding relationships between organisms in a community.
  • Energy is transferred from autotrophs (primary producers) to herbivores (primary consumers) to carnivores (secondary and higher consumers).
  • Decomposers are the final link in this chain (food chain).
  • Trophic level: The position an organism occupies in a food chain.

• Food Webs:

  • A group of food chains linked together forming complex trophic interactions.
  • Arrows link species according to who eats whom.
  • Species may play a role at more than one trophic level.
  • Food webs can be simplified by grouping species with similar trophic relationships or isolating a portion of the community.

• Species Diversity:

  • The variety of organisms in a community.
  • Species richness: The number of different species.
  • Relative abundance: The proportion each species represents of all individuals.
  • Communities can have the same species richness but different relative abundance.

• Species with a Large Impact:

  • Certain species have a large impact due to their abundance or pivotal role.
  • Foundation species: Have strong effects due to their size or abundance, providing habitat or food; may be competitively dominant.
  • Keystone species: Exert strong control by their pivotal ecological roles but are not usually abundant.
    *Example a sea star affects its community by feeding on and limiting the abundance of a competitively dominant species, a mussel

• Disturbance: Influences species diversity and composition.

  • Disturbance keeps many communities from reaching equilibrium.
  • A disturbance is an event that changes a community by removing organisms or altering resource availability.
  • The nonequilibrium model describes communities as constantly changing after disturbance.