Population Dynamics

Producer

Organisms that produce their own source of food (Autotroph)

Consumer

An organism that must eat or consume another organism (Heterotroph)

Food Chain

A linear chain that shows the flow of energy and nutrients through a system

Food Web

All of the food chains of an ecosystem – showing the many different paths

Biotic Selection Pressures

* Competition
* Predation
* Symbiosis
* Commensalism
* Parasitism
* Disease

Competition

When organisms use the same resource in the same way

Competition example

* Food sources, habitat, light
* Introduced foxes and cats compete with Australian native carnivores like quolls for food (possums, rats etc). As a result quolls have declined in distribution and abundance

Competition as a selection pressure

When two different species compete for the same limited resources, they cannot co-exist in the same place. One species will inevitably harvest resources and reproduce more efficiently than the other, so that the weaker competitor will either become extinct or, over time, evolve enough to use a different set of resources

Predation

One organism eats another organism

Predation example

* Wallaby being eaten by a dingo
* Ladybirds consuming an aphid
* Bird eating a worm

Predation as a selection pressure

Predator and prey numbers will oscillate in a similar pattern. Predator numbers will increase they will eat more prey, causing the number of prey to decrease. There is less food available creating competition for food, decreasing the number of predators. With less predators the number of prey increases

Symbiosis

This is a type of interaction between two species that live together in close association

* Mutualism
* Commensalism
* Parasitism

Mutalism

A relationship in which BOTH organisms benefit and neither could survive without the other

Mutalism example

The clownfish is protected by the sea anemone from predators. The anemone benefits as the prey will go near the anemone thinking it is safe, so the anemone gets its prey

Commensalism

A relationship in which one organism benefits and the other is not harmed

Commensalism example

* Barnacles on a whale. The barnacle is exposed to more food sources and the whales are not harmed
* Bird nesting in a tree – the bird benefits by gaining shelter and a location to rear its young, however the tree is not harmed

Parasitism

Where an one organism feeds on a host who is harmed

Parasitism example

* Head lice and humans
* Ticks and dogs
* Aphids and plants

Disease

A disorder of structure or function in a human, animal or plant

Disease example

* A human with chickenpox
* Black spot is a fungal infection that affects plants

Disease vs Parasitism

* Infectious diseases are caused by pathogen which results in an infection to the host
* A parasite is an organism that lives on or in another organism – it usually takes its nutrients at the expense of the host

Predation vs Parasitism

Predators usually have multiple prey throughout a life stage, whilst a parasite usually only has one

Abundance

The number of organisms in a particular area

Distribution

Where an organism is found

Why is abundance and distribution important?

Ecologists use them to provide a measurement of biodivereisty to gain information about species (extinctions, impact of climate change on species)

Transects

A straight line across a habitat or section of habitat that maps indivduals at different distances across the line (used to measure distribution)

Positives of Transects

* Good for vegetation

Negatives of Transects

* Only record organisms directly on it
* May not correctly represent the sit

Random Quadrats

A _______ is randomly placed in an area and the number and type of each species is recorded. The estimated population is ‘scaled-up’ to the total area (used to measure abundance of slow moving animals and plants)

Positives of Random Quadrats

* Beneficial for larger populations (too difficult to count)
* Inexpensive and easy

Negatives of Random Quadrats

* Try for animals that move
* May need lots of samples to get a representative population

Random Quadrats Equation

**Estimated Abundance** = (total number of individuals counted / area of each quadrat x number of quadrants) x total area

Population Density Equation

Sum of indivduals / area of quadrants

Capture-Recapture

A sample of animals is captured, marked in some way (tagged, coloured dot) and released. A sample is later captured and the ratio of marker (used to measure abundance of moving animals)

Positives of Capture-Recapture

* Simple method
* Good for moving organisms (neither quadrats or transects work)

Negatives of Capture-Recapture

* Destructive/damaging
* Time consuming or labour intensive

Capture-Recapture Equation

**Total Population** = (number marked in the first sample x total number of animals recaptured) / number of recaptured animals that are marked

Extinction

The last existing member of a species dies

Australian Megafauna

* In the Pleistocene (2 million to 10,000 years ago) many species of mammals became extinct throughout the world
* There is some debate amongst biologists whether this occured due to climate change at the end of the last Ice Age or to human expansion around the globe
* In Australia, at least 40 species of mammals disappeared during the Pleistocene, and the species that died out were usually the biggest within a group of related species (collectively known as megafauna)

Examples of Australian megafauna

* **Procoptodon**: 3m tall, blunt-faced kangaroo (largest kangaroo ever known), related to the red and grey kangaroos
* **Diprotodon**: largest marsupial ever lived, 3m long, 2m tall at shoulder, possibly related to the wombat
* **Thylacoleo**: “marsupial lion”, size of a leopard, some parts of the skeleton were similar to modern possums (hindlimb was possum-like, with an opposable first toe)
* **Dromornis**: “giant emu”, 3m tall, weighed over 500kg, rrelated to the modern emu.

What caused the extinction of the megafauna?

* Changes in climate – low rainfall, with drought-like conditions – largely affected availability of plants
* Indigenous populations
* Hunting megafauna directly
* Use of fire to change the landscape (seed propagation by First nations people) changed structure of vegetation