Lecture #1 Dynamics of Population growth & Feeding

Relationships

Unit 8 Community Interactions

Limiting Factors (resources) – factors

that limit the growth and/or reproduction

of an organism or population.  

Can you think of an example?

There are two types of limiting factors 1. Density Independent Limiting Factors-environmental factors that affect population sizeregardless of the population’s density

• Many density independent limiting factors areabiotic factors.

Examples: long periods of hot or cold weather,natural catastrophes ( forest fires, floods,volcanic eruptions)

Density Dependent Factors

2. Density Dependant Limiting Factors –

Limiting factors that affect large, densepopulations more strongly than small, less crowdedones.

Example: Predators, available food

Summarizing what you’ve learned

Would a drought be a density independentor dependent limiting factor? Explain.

Would an infectious disease be a densityindependent or dependent limiting

factor?Explain.

How Organisms Interact in Communities

In an ecosystem, all members of the webinteract with one another.

• The interactions that we see are the result of a long evolutionary history in which many of the participants of the ecosystem adjust to one another over time.

Example – Floweringplants evolved tohave an efficient

wayto disperse theirpollen(reproduction)

How?

They let animals do it

for them!  

Animals that feed on pollen get a meal out of the relationship that they have with the flowering plants

Coevolution – Theback- and – forthevolutionaryadjustments

betweeninteracting

membersof an ecosystem

Examples - • Wolf & Elk • Bees & Flowers • Cow & bacteria in

their rumen

Feeding Relationships

There are 3 main types of feedingrelationships in nature. 1. Predator- Prey

2. Parasite- Host 3. Plant- Herbivore

1. Predator- Prey

Predation - the act ofone organism feeding on another.

Examples – Lion feeding on aZebra; Spider feedingon a centipede

Predator – anyAnimal that livesby preying onother animals(usually) from alower trophic level.

• Predators have evolved to be stealthy, swift, strong, sharp teeth/ claws, etc

Prey - any animalspecies that is fedupon by another • Prey species have

evolved ways to escape, avoid, or fight off predators.

Predator- prey balance . Predators and theirprey have evolved to keep each other’spopulation in check

time

population

• As prey increases, predators increase. • As predators increase, prey decreases• As prey decreases, predators decrease• As predators decrease, prey increases

Carrying Capacity- the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment

2. Parasite-HostParasitism (a type of predation) – oneorganism feeds on and lives in or onanother organism, bringing them harm.

• Parasites are much more abundant and ecologically important than predators in population control

 • All species of plants and animals (even

microbes) can be infected with parasites

Parasite – an organismthat lives in associationwith another organism(host), from which itobtains organic

nutrition. • This causes harm to

the host.  Host – The organism on/

in which a parasite lives. It is the “prey” of a parasite

Vector - the organismthat carries a parasitefrom one host toanother. Example - Mosquitoes (disease carryingInsects) carry malaria

and West Nile virus

Parasites don’t usually kill their hosts, but

they can in some cases.

Why do you suppose they don’t usually kill

their hosts?

No place like home……..

Answer: Because they

depend on the host for

a place to live and a food source!

2 Types of Parasites

External parasites (Ticks, mosquitoes,fleas, lice, chiggers)

• They feed on the host’s outer surface.

2 Types of Parasites Internal parasites (tapeworm, ascaris,hookworm, heartworm

incanines)

• The live within the body of their host.

• For example, the tapeworm attaches in the small intestine, where it takes nutrients from its host. Tapeworm scolex and suckers

Parasite- host balance

As the population density of a parasitehost increases, parasites and theirvectors have an easy time finding newhosts.

• This means that infection rates increase, and die-off results. This helps to control population growth.

As the population density of the hostsdecrease, transfer of infection is lessbecause there are fewer individuals.

• This means a decrease in infection and the population is usually able to recover

3. Plant- HerbivoreHow the relationship works: Herbivores eatplants. • If the amount of available vegetation

decreases, the population of the herbivore decreases.

• If the population of herbivore increases, the relative amount of vegetation decreases.

Plant-Herbivore balance may only exist if apredator or a parasite is limiting the population ofThe herbivore.

If not, what happens to the ecosystem?

Overgrazing - the practice of grazing too manylivestock for too long a period on land that is

thenunable to recover its vegetation.

Can plants defend themselves against from herbivores?

Plants protect themselves from herbivores in a

Variety of ways.

Examples• Thorns, spines

• “Chemical warfare”. (Toxic chemical compounds that discourage herbivores fromfeeding on them.)

How do herbivores overcome the defenses of plants?

Some herbivores have

evolved the ability to feed on plants thatare “protected” byparticular defensivechemicals.

  

Example

Monarch butterflycaterpillars and milkweed

• The milkweed's "milk“ or white latex is both acidic and somewhat poisonous to many animals.

• A female monarch lays its eggs on a sprouted milkweed plant

• Once the eggs hatch, the monarch caterpillar feeds solely on milkweed, and stores the toxin in its body for life.

• Because of this, predators avoid monarch’s because of their awful taste!

Take 2 minutes to summarize how the interactions of community members may

help to keep populations in check.