Population Ecology

• Ecology - the study of interactions between organisms and the living and nonliving components of their environment

  • Oikos (Greek for house, home, or place to live)

  • logos (Greek for the study of)

• Levels of organization

  • Biosphere - The entire spherical location where life is located (i.e. EARTH)

  • Ecosystem - all of the biotic and abiotic factors of a relatively defined geographic area

  • Community - All living organisms that live in a relatively defined geographic area

  • Population - a group of organisms, all of the same species, located in a relatively defined geographic area

• The key to ecology is that everything is interconnected (one of the 7 themes, interdependence)

• Population Ecology - focuses on how populations change (i.e. how they grow or shrink)

• Populations are described by their location, species, size, density, dispersion, and dynamics (how they change)

  • Emperor Penguins (Aptenodytes forsteri)

  • Outer edges of Antarctica

  • Populations can vary from the dozens to the thousands!

• 4 factors lead to population change:

  • B = # of births

  • D = # of deaths

  • I = # of immigrants

  • E = # of emigrants

    • ΔN = B - D + I - E

    • Simplified version (closed system): ΔN = B - D

    • Birth rate (b) and death rate (d) represent the number of births per individual per unit time (b = B/N, d = D/N)

    • r = b - d (where r = growth rate of the population)

• Exponential growth - a population’s growth rate remains constant

  • Exponential growth can operate in short periods of time

  • If b and d are equal to each other, population growth stops.

• This is the equilibrium point or carrying capacity of the population (aka K)

  • The carrying capacity of a population is the maximum number of organisms that can be sustained in a population when considering limiting factors.

  • When a population is at carrying capacity, growth rate = 0, because births = deaths.

• Logistic growth builds on the exponential growth model by including the carrying capacity (K) of the population

• per capita growth rate - r

  • dN (death x population size) / dt (death x time) = rN (growth rate * population size)

  • Slow growth at low N

    • r is near its max, but few individuals

  • Quick growth at moderate N

    • rate of growth is at its max, nothing holding growth back

  • Slow growth as N → K (population size approaches the carrying capacity)

    • Limiting factors restrict growth

• why does r (the growth rate) decrease as the carrying capacity approaches?

  • Density-Dependent Factors

    • Competition for:

      • food

      • space

      • shelter

    • Stress increases in d due to:

      • predation

      • parasitism

      • starvation

      • disease

  • Exponential growth is not stable long term because:

    • limiting factors: density-dependent / biotic factors are living factors that increase in severity the higher the density of the population.

    • limiting factors: density-independent / abiotic factors are non-living factors that affect populations regardless of their size.

• The way a population grows also entirely depends on its life history;

  1. type 1

     1. high survivorship to low survivorship (i.e. humans)

     2. we are much more likely to die in old age

  2. type 2

     1. steady survivorship (i.e. birds)

  3. type 3

  4. low survivorship to high survivorship (i.e. trees)

  5. are most likely to die young

  6. usually organisms that produce a large amount of offspring but only a few survive

• Stages of a population:

1. Stage 1: Total population is low but it is balanced due to high birth rates (36/37 per 1,000) and high death rates (36/37 per 1,000). Countries at this stage will usually be undeveloped.

2. Stage 2: Total population will start to rise because the death rates will start to fall (to around 18/19 per 1,000). Birth rates will remain high. Death rates fall due to:

   1. medical care will be improved (vaccinations, scientific improvements, doctors and new drugs);

   2. sanitation and water supplies will be much better;

   3. the quality and security of food will be improved;

   4. there will be a noticeable decrease in child mortality.

3. Stage 3: Total population is rising rapidly. The gap between birth and death rates will narrow. Natural increase is high. Death rates will now remain low and steady (to 15 per 1,000) but birth rates will fall quickly (down to around 18 per 1,000). Birth rates fall due to:

   1. increased use of family planning methods;

   2. much lower infant mortality rates will mean that more children will survive and there is less need to have as many babies;

   3. increased opportunity for employment in factories means that fewer people (and children) are required to work on the land;

   4. changes to society put more desire on material possessions than large families;

   5. changes to equality mean that women are increasingly in the workforce and not ‘staying at home’ to look after the children.

4. Stage 4: Total population is high and growing slowly. It is balanced by a low birth rate (15 per 1,000) and a low death rate (12 per 1,000). Contraception is widely available and there is a social desire to have smaller families.

Community Ecology

• Communities consist of different populations of different species interacting with each other.

  • you are a walking community

• Quantifying a community:

  • Species richness - is the number of species in a community

  • Species diversity (or biodiversity) relates the number of species in a community to the relative abundance of each species

  • the difference - richness is just the # of species and diversity is the # of species in comparison to the amount of each species there is too

  • Species Interactions (aka Symbioses)

    • predation (- +)

    • parasitism (- +)

    • competition (- -)

    • mutualism (+ +)

    • commensalism (neutral +)

• PREDATION - Predation is when one individual (predator) captures, kills and consumes another individual (prey)

• Because of this evolutionary pressure, camouflage or the ability to blend into the background, becomes VERY important to survival!

• Predator and prey populations change based on the abundance of each other according to Predator-Prey Cycles

• Selective pressures can cause an “arms race” where prey evolve to be better than predators, then predators evolve to catch up!

• Ecological niche: A unique set of ecological requirements and roles for a given species.

  • Aspects of an animal’s niche include the temperature range it can tolerate, the species it eats, and the places it can breed.

  • Competition comes from a niche overlap; the use of a limiting resource by two or more species

    • Interspecific competition comes in two forms: Interference and Exploitative (aka resource)

    • A result of competition can be competitive exclusion: when one species is driven to extinction (or near extinction) due to competition over a limited resource.

      • Competitive exclusion can result in resource (or niche) partitioning

      • resource partitioning - where species can minimize competition

    • Because of resource partitioning, natural selection may lead to a directional selection on a trait, resulting in character displacement

• PARASITISM - is when one individual (parasite) harms another (host) but the organism is not immediately killed

  • Parasites benefit from dense populations (where transmission can increase), by reducing noticeable impact on the host (pain, immune response), keeping the host alive (so it can keep feeding on it), and even manipulating behavior!

  • Brood parasites lay their eggs in another species’ nest, relying on others to raise their offspring

• MUTUALISM - is a cooperative relationship in which both species derive some benefit

• COMMENSALISM - is when one species benefits from interactions while the other is “unaffected”

• How do communities change?

  • Succession is the sequential, gradual growth or regrowth of species (usually plants) in an area

  • The first organisms in primary succession are pioneer species; quick growing, small, and quick reproducing. They also change the environment to allow the start of a succession

  • Climax Community – the end of the succession; a “stable” endpoint... Until disturbance “resets” the process

    • pioneer species - quick growing, small, fast in reproduction

    • intermediate species - i.e. young trees

    • climax community - mature

• Disturbance impacts biodiversity; the intermediate disturbance hypothesis states that diversity is highest with a moderate level of disturbance

• What impacts biodiversity?

  • habitat size

  • distance from a colonizing source

• Indicator Species (aka bioindicator) - a species (or group of species) whose status gives humans insight into the health of the community

• Keystone Species - a species that has a very large effect on the community. The removal of a keystone species may lead to community collapse

• Endemic species - species that are native to a specific location, and only that location

  • basically a species in which the location is their home

• Exotic species are those that are NOT native to a specific location.

  • These species evolved elsewhere, later establishing in a different place.

  • Some exotic species are invasive, which harm members of the new community they move to

    • i.e. spotted lanternfly

Ecosystem Ecology

• Ecosystem ecology focuses on how living things interact with their non-living (abiotic) environment.

• Non-living factors include:

  • Energy

    • Heat

    • Light

    • Radiation (!)

  • Water

  • Nutrients

    • Phosphorus

    • Nitrogen

    • Carbon

    • Oxygen

  • Climate

    • Weather

    • Precipitation

    • Wind

    • Humidity

    • Air pressure

• While energy flows through a system, nutrients (water, nitrogen, phosphorus, and carbon) are cycled in a system

  • The energy captured by photosynthesis and released by cell respiration powers the cycling of nutrients on Earth!

• Energy flows from the sun through trophic feeding levels.

• Nutrient cycle, since there is no new source of nutrients.

• No living things create energy, but organisms called autotrophs can capture the energy and turn it into something living cells can use.

• All organisms can be broken into trophic levels, or the position the species has in a food web.

  • At each trophic level (or feeding level), energy is lost. Most are lost as heat, but some are in inedible parts of the organism (bones, fur, poop).

  • 90% of energy is lost at each trophic level. This limits the number of trophic levels an ecosystem has!

    • third-level consumers, second-level consumers, first-level consumers, primary producers

    • A food web best illustrates the transfer of energy and nutrients between producers and consumers.

    • It is important to note the vital role decomposers play. Since many consumers are not consumed while alive, decomposers recycle the vital nutrients trapped in the dead consumers, allowing primary producers to reuse them and build new organic molecules.

• What nutrients get cycled?

  • Water: needed by all living things for all reactions in the body.

  • Carbon: makes up all organic molecules, which make up all living things.

  • Nitrogen: used in all proteins, DNA, RNA, ATP, and chlorophyll.

  • Phosphorus: used in nucleic acids, in the phospholipid bilayer (i.e. all cell/organelle membranes), and ATP. Vital for bones and teeth.

• WATER:

  • Your body is >70% water, yet only 2.5% of all the water on Earth is available to use

  • Water cycles from a solid/liquid state to a gaseous state due to weather (rain/snow/hail) and evaporation.

    • Remember: condensation, evaporation, precipitation!

• NITROGEN

  • Nitrogen is used in all proteins (and all human tissues are made of protein).

    • Also, DNA/RNA, ATP/ADP, and chlorophyll contain nitrogen!

    • Nitrogen is needed to make proteins and nucleic acids, so it is also cycled as organisms are born/die

    • Nitrogen fixation - Some decomposers (like bacteria and fungi) can “fix” nitrogen gas into sources that organisms can use (such as nitrates and nitrites).

• PHOSPHORUS

  • Phosphorus is vital for bones and teeth, but is also used in nucleic acids, in the phospholipid bilayer that makes up all cell/organelle membranes, and in ATP/ADP.

  • Phosphorus is in a terrestrial cycle, meaning it is never in a gaseous state.

    • It is mostly found in water and in rock.

    • because of its rarity, its a known limiting factor.

Human Impact

• We have identified 9 planetary boundaries

1. Land-system change - size of forest area

2. Freshwater use - amount of water available for human and plants

3. Biogeochemical flow - outflow of nitrogen and phosphorous in synthesized fertilizers

4. Ocean acidification - carbonate ion concentration in the ocean

5. Atmospheric aerosols - amount of air pollution

6. Ozone Depletion - stratospheric ozone concentration

   1. the ozone layer protects us from UV radiation, and if we were in the red for this boundary, skin cancer levels would increase drastically

   2. luckily, this is one of the only boundaries we are going in the right direction with.

7. Novel Entities - pollution created by compounds like plastic

8. Climate change - co2 concentration, energy balance between earth and space

9. Biosphere integrity - percentage of functional diversity, speed of extinction

• going beyond these boundaries risks destabilizing Earth and going to a hothouse Earth that amplifies changes instead of stabilizing them

• we are already very close, if not already destabilizing Earth and destroying it’s precious biodiversity

• “Global Warming” is a gradual increase in the average global temperature that is due to a higher concentration of gases (such as CO2) in the atmosphere

  • More accurately this should be known as climate change, since weather patterns are changing across the planet; some places will warm, while some places may get colder!

  • Climate change is driven by greenhouse gases (CO2, CH4) trapping heat energy that would have normally radiated into space. This is the greenhouse effect.

    • Ozone (O3), a natural gas that absorbs most ultraviolet radiation, is becoming depleted due to chlorofluorocarbons (or CFCs)

    • UV radiation can lead to DNA degradation/mutation, which can lead to increased cancer rates and/or sterility

    • Extinction, or the elimination of an entire species from the planet, is a natural process that has happened throughout Earth’s history

    • Humans have contributed to a higher-than-normal rate of extinction in recent years due to pollution, habitat destruction, invasive species, disasters and hunting.

      • Humans reducing ecosystem evenness (such as through monocultures) reduces biodiversity

      • Humans’ role in habitat destruction is the greatest threat to biodiversity. This can lead to fragmentation, limiting gene flow between populations and diversity in habitats