# Ecosystems

## Sampling Populations

In order to make decisions about an organism, like the horseshoe crab, scientists must have an accurate count of the population size. There are several methods used, such as guessing, counting one-by-one, and grouping. However, the best and most accurate method is sectioning. In this method, a certain area is divided into several sections. Using the most average group, scientists count the number of organisms in the section. Scientists take this number and multiply by the number of sections in order to calculate the population size.

The video below shows examples of how scientists use sampling techniques to count deer.

Basic Sampling Techniques - Counting Deer

## Interactions Among Organisms

In an ecosystem, organisms interact with each other in many ways. One type of interaction involves predator and prey relationships. Predators hunt, catch, kill, and eat their prey. Population sizes of organisms can greatly change based off of predator/prey relationships. Populations sizes can change due to other factors as well. In all environments, organisms with similar needs may compete with one another for resources including food, water, air, space and shelter. This competition results in natural population fluctuations.

Organisms can also benefit from each other. Symbiosis is a relationship between two species that exist closely together where at least one of the species benefits from the relationship. There are three different types of symbiosis- commensalism, mutualism, and parasitism. Commensalism is a relationship between species where one is helped and the other is neither helped nor harmed. Mutualism is a relationship between two species where both species benefit. Parasitism is a relationship between two species where one species benefits and the other is harmed.

Symbiosis: Mutualism, Commensalism, and Parasitism

## Movement of Energy in an Ecosystems

In an ecosystem, organisms can be defined several way. One way can be by what the organism feeds on. There are three main feeding types- herbivore, omnivore, carnivore. Herbivores eat plants only, herbivores eat plants and animals, and carnivores eat animals only.

Organisms can also be defined by their energy role, or how they move energy in an ecosystem. The three main energy roles include producers, consumers, and decomposers. The source of all energy in an ecosystem can be traced back to the sun. However, not all organisms can use energy directly from the sun. Producers are organisms that use the sun's energy and transforms it into a useable form that all organisms can use through the process of photosynthesis. Consumers feed directly or indirectly on producers in order to gain energy. Scavengers are a special type of consumer that only eat dead animals. Decomposers are organisms that break down dead organic matter. They return nutrients to the ecosystem to be used again by plants. If there were no decomposers, the plants would take nutrients out of the ecosystem and it would never be returned. Eventually there would be no more nutrients left, the plants would die. If the plants all died, animals would soon follow!

These interactions can be traced through a complex diagram called a food web. Food webs show all possible connections within an ecosystem to follow the flow of energy.

How Ecosystems Work | Biology | Ecology