Population and Community Ecology

Population Ecology 2

Terrestrial Biomes

• A list of terrestrial biomes is mentioned, including:

  • Ice sheet and polar desert

  • Tundra

  • Taiga

  • Temperate broadleaf forest

  • Temperate steppe

  • Subtropical rainforest

  • Mediterranean vegetation

  • Monsoon forest and desert

  • Xeric shrubland

  • Dry steppe

  • Semiarid desert

  • Grass savanna

  • Tree savanna

  • Subtropical dry forest

  • Tropical rainforest

  • Alpine tundra

  • Montane forests

Life Table Example (Lacerta vivipara)

A life table for female Lacerta vivipara (common lizard) in the Netherlands is presented, including:

• Age Class (x): The age group of the lizards.

• Number of Survivors (Nx): The number of lizards surviving to age x, starting with an initial population of 1000.

• Survivorship (lx): The proportion of lizards surviving to age x, calculated as lx = Nx / N_0.

• Age-Specific Fecundity (mx): The average number of births per year per original female.

• lxmx: The product of survivorship and age-specific fecundity for each age class.

• Net Reproductive Rate (R0): Sum of the lx mx values, representing the average number of offspring produced by an individual during its lifetime. In this case, R_0 = 1.00.

Population Growth Parameters

• R_0: Net reproductive rate.

• r_{max}: Intrinsic per capita rate of increase (or population growth).

  • Calculated as births (b) minus deaths (d), r \approx b - d.

• K: Carrying capacity.

Life-History Continuum

Population Size and Growth

• Population size is denoted as N.

• Examples of r values (intrinsic rate of increase) for different organisms:

  • E. coli: 59 (High r)

  • P. caudatum (Ciliate): 1.6

  • Flour beetle: 0.10 (Moderate r)

  • Domestic cow: 0.001 (Low r)

  • Beech tree: 0.000075 (Very low r)

• Population growth equation:

  • \frac{dN}{dt} = r_{max}N

Density Dependence

• Density dependence: Growth rate slows at high density.

• Logistic growth equation (accounts for carrying capacity K):

  • \frac{dN}{dt} = r_{max}N(\frac{K - N}{K})

  • Alternative form: \frac{dN}{dt} = r_{max}N(1 - \frac{N}{K})

• Illustrative example:

  • Paramecium aurelia alone

  • Paramecium caudatum alone

  • Early growth is rapid

  • Growth begins to slow

  • Later growth falls to zero

Density Dependence Examples

• Survival of gobies declines at high population density.

• Fecundity of sparrows declines at high population density.

• Reindeer introduced to St. Paul Island (Alaska) experienced a population boom followed by a crash due to exceeding carrying capacity.

Density-Dependent Factors Limiting Population Size

• Competition for Resources:

  • Food

  • Territory

  • Water

  • Light

  • Nesting sites

  • Nutrients

  • Oxygen

  • Example: Dense tree saplings compete for limited light, water, and nutrients

• Disease and Parasitism:

  • Stress-related degradation of health

  • Infectious disease

  • Parasitism

  • Example: Pigs in dense confinement are prone to illness

• Predation:

  • Increased predation as prey density increases

  • Example: Hare density influences lynx predation rates

• Toxic Wastes:

  • Ammonia

  • Uric acid

  • Alcohol

  • Carbon dioxide

  • Example: Densities of fruit fly larvae are limited by ammonia build-up from their feces

• Social Behavior:

  • Stress-mediated behavior

  • Dominance behavior

  • Mating behavior

  • Parental-care behavior

  • Predator-avoidance behavior

  • Example: Blue crab cannibalism increases with density

Hare-Lynx Population Cycle

• The hare-lynx populations cycle every 10 years, on average; changes in lynx density lag behind changes in hare density

• Experimental Setup:

  • Document hare population in seven study plots (similar boreal forest habitats, each 1 km^2) from 1987 to 1994 (most of a population cycle).

  • 3 plots: Unmanipulated controls

  • 1 plot: Erect an electrified fence that excludes lynx but allows free access by hares.

  • 2 plots: Supply extra food for hares.

  • 1 plot: Erect an electrified fence that excludes lynx; supply extra food for hares.

• Predictions

  • Experimental Hypothesis:

    • Hare populations are limited by either predation or food availability. When predation and food limitation occur together, they have a greater effect than either factor does independently (predation, food availability or a combination of those two factors).

  • Null Hypothesis

    • Hare populations are not limited by predation nor food availability (Hare populations in all of the plots will be the same).

• Conclusion: Hare populations are limited by both predation and food availability. When predation and food limitation occur together, they have a greater effect than either factor does independently.

Human Population Growth

• Historical Overview:

  • Shows the trend of world population growth through history

  • Includes Old Stone age, New Stone Age, Bronze Age, Iron Age and Middle Ages

  • References the plague

• Significant Milestones:

  • 1804: 1 billion

  • 1927: 2 billion

  • 1960: 3 billion

  • 1974: 4 billion

  • 1987: 5 billion

  • 1999: 6 billion

  • 2011: 7 billion

Demographic Transition Model (DTM)

• Stages:

  • Stage 1: High birth and death rates, low population growth.

  • Stage 2: High birth rates, declining death rates, increasing population growth.

  • Stage 3: Declining birth rates, low death rates, slowing population growth.

  • Stage 4: Low birth and death rates, zero or negative population growth.

Age Structure

• Developed countries (e.g., Sweden) have relatively even age distributions.

• Developing countries (e.g., Honduras) have a pyramid-shaped age structure with a large base (young population).

• Even if birth rates dropped to simple replacement of parents the world’s population would still grow due to Age structure!

Human Population Projections

• The UN has projected human population growth to the year 2050 based on current fertility rates: 2.5 (high), 2.1 (medium), or 1.7 (low) children per woman

• Zero Population Growth:

  • Occurs when fertility at the replacement rate is sustained for a generation.

• The future of the human population hinges on fertility rates - on how many children each woman living today decides, or is allowed to decide, to have.

Human Carrying Capacity

• The question of human carrying capacity is raised.

• Food Web

  • Illustrative example of a food web including decomposers, producers, and consumers (primary, secondary, and high-level).

Community Ecology 1

• Introduction to community ecology.

Species Interactions

• |
  0 | Commensalism |
  |
  0 |

• | Commensalism |
  |

• |

• | Competition |
  |

• |

• | Mutualism |
  |

• |
  0 | Ammensalism |
  |
  0 |

• | Ammensalism |
  |

• |

• | Exploitation |
  |

• |

• | Exploitation |

• Interaction forms are not necessarily settled forever; populations evolve!

• Commensalism: One species benefits while the other is unaffected.

Competition

• Competition is a -/- interaction that lowers the fitness of the individuals involved.

• Intraspecific Competition: Between members of the same species -> density-dependent population growth.

• Interspecific Competition: Members of different species use the same limiting resources.

• Density dependence: Growth rate slows at high density.

• Logistic Growth:

  • \frac{dN}{dt} = r_{max}N(1 - \frac{N}{K})

Competitive Exclusion Principle

• Competing species cannot co-exist in laboratory experiments.

  • Mathematical models predict that one species drives the other to extinction (in three out of four scenarios)

• Two species using the same resources cannot coexist.

• Assumptions:

  • Competition is complete (competitors have exactly the same resource requirements)

  • Environmental conditions constant

• Realistic?

• How similar can species be to coexist?

• What about environmental factors, other than resources?

Fundamental vs. Realized Niche

• Fundamental niche: The full range of environmental conditions and resources that a species can potentially occupy and use, especially when limiting or competitive factors are absent.

• Realized niche: The actual space that an organism inhabits and the resources it can access as a result of limiting pressures from other species (e.g. superior competitors).

Question on Realized vs Fundamental Niche

• Semibalanus can survive only in the lower intertidal zone.

• Semibalanus is inferior to Chthamalus in competing for space on rocks.

• If Chthamalus were removed, Semibalanus' realized niche would become larger.

• Removal of Semibalanus shows that for Chthamalus, realized niche < fundamental niche.

Competition along environmental gradients

• Illustrative Example:

  • Alpine chipmunk

  • Lodgepole pine chipmunk

  • Yellow pine chipmunk

  • Least chipmunk

Niche Differentiation

• Natural selection selects against individuals that compete.

How do species coexist?

• Resource partitioning

  • Over ecological time:

    • Realized niches

    • Combined effects of resource use and environmental requirements/tolerances + the actual environmental conditions (variable!) determine competitiveness

  • Over evolutionary time

    • If natural selection favors resource use at different ends of the range -> Character displacement (Evolutionary change in traits that reduces the amount of niche overlap and the amount of competition)