Populations:

Groups Working As One
A population is a group of organisms that are all the same species. That's it. You're done. A group of ducks would not be a population if there were mallard and wood ducks combined. Scientists would say that there are two populations existing in the same area. 

Population biologists study groups in a specific area. For example, among political regions there are a U.S. population and a Mexico population. Life for these groups is totally different in many ways. We also said a population is all the same species. When you look at the U.S. and Mexico, you might be looking at human populations or dog populations. They all interact, but they are different populations. 

Health Of A Population
You need to remember that populations don't just sit there. Things are always happening. Organisms are born and the population size increases. Other factors that will increase the size of a population are good weather, medicine, geographic isolation and no predators. Organisms will also die of old age and the population will decrease. Populations can also decrease when there are more predators around, after a natural disaster, when competition is too great, or when some of the organisms just leave the area (migration). 

Limiting Factors
No matter how well a population succeeds in its area, it is still limited even if no outside forces are applied. Three main factors limit population expansion. There are others, but htese are some biggies. 

There are physical limits. Sometimes other environments are just bad places to live. They may be too hot, too cold, or too dangerous. 

Next are the competition limits. You might have a great place to live but there may be other organisms competing for your food. If there is a smarter bird in the next area, you might not be able to get all of the food you need. A worse situation would be to meet a bird that wants to eat you. 

Last are the geographic limits. If you are a mouse living in a valley and there are mountains in every direction, are you going to leave your area? Probably not. You live a good life and might die if you tried to get over the mountains. An ocean might be another geographic limitation.
A single species is a population, here are multiple populations because there are multiple species of birds.
Births and immigration add to the population size while death and emigration reduce the population size. These are just some of the interactions that control population sizes

Biotic & Abiotic

The living things in an ecosystem are called biotic factors. Living things include plants, animals, bacteria, fungi and more. The non living parts of an ecosystem are called abiotic factors. In an ecosystem some abiotic factors are sunlight, temperature atmospheric gases water and soil.

Communities & Ecosystems

All of the populations living in an area are considered a community. This includes all of the biotic things in the area; the plant life, animal life and all of the microscopic living things like bacteria. These populations all interact with one another, some interactions include competition for food, resources and space which can limit population sizes. There is also predation/consumption where one organism eats another. Not all interactions limit population sizes, there are some mutualistic relationships where two organisms live/work together and actually increase each others numbers. 

The main factor that determines which populations live in an area is the climate. This includes the average temperature, amount of rainfall and availability of light among other things. When these factors are included with the community of organisms we call it the ecosystem. So an ecosystem includes all of the biotic and abiotic factors in an area. 

Food Chains

Every organism needs to obtain energy in order to live. For example, plants get energy from the sun, some animals eat plants, and some animals eat other animals.
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A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. A food chain starts with the primary energy source, usually the sun or boiling-hot deep sea vents. The next link in the chain is an organism that make its own food from the primary energy source -- an example is photosynthetic plants that make their own food from sunlight (using a process called photosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs or primary producers.

Next come organisms that eat the autotrophs; these organisms are called herbivores or primary consumers -- an example is a rabbit that eats grass. 

The next link in the chain is animals that eat herbivores - these are called secondary consumers -- an example is a snake that eat rabbits.

In turn, these animals are eaten by larger predators -- an example is an owl that eats snakes. The tertiary consumers are are eaten by quaternary consumers -- an example is a hawk that eats owls. Each food chain end with a top predator, and animal with no natural enemies (like an alligator, hawk, or polar bear). 

The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step. A network of many food chains is called a food web.

​ When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.

Some organisms' position in the food chain can vary as their diet differs. For example, when a bear eats berries, the bear is functioning as a primary consumer. When a bear eats a plant-eating rodent, the bear is functioning as a secondary consumer. When the bear eats salmon, the bear is functioning as a tertiary consumer (this is because salmon is a secondary consumer, since salmon eat herring that eat zooplankton that eat phytoplankton, that make their own energy from sunlight). Think about how people's place in the food chain varies - often within a single meal.
Trophic Levels:
The trophic level of an organism is the position it holds in a food chain. 
  1. Primary producers (organisms that make their own food from sunlight and/or chemical energy from deep sea vents) are the base of every food chain - these organisms are called autotrophs. 
  2. Primary consumers are animals that eat primary producers; they are also called herbivores (plant-eaters).
  3. Secondary consumers eat primary consumers. They are carnivores (meat-eaters) and omnivores (animals that eat both animals and plants).
  4. Tertiary consumers eat secondary consumers.
  5. Quaternary consumers eat tertiary consumers.
  6. Food chains "end" with top predators, animals that have little or no natural enemies.
Numbers of Organisms:
In any food web, energy is lost each time one organism eats another. Because of this, there have to be many more plants than there are plant-eaters. There are more autotrophs than heterotrophs, and more plant-eaters than meat-eaters. Although there is intense competition between animals, there is also an interdependence. When one species goes extinct, it can affect an entire chain of other species and have unpredictable consequences.

​Equilibrium
As the number of carnivores in a community increases, they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other's population. A similar equilibrium exists between plants and plant-eaters.