A member of a species that feeds directly on all or part of a living organism

Everything in the natural world is connected. An ecosystem is a community of living and non-living things that work together. Ecosystems have no particular size.

An ecosystem can be as large as a desert or a lake or as small as a tree or a puddle. If you have a terrarium, that is an artificial ecosystem. The water, water temperature, plants, animals, air, light and soil all work together. If there isn't enough light or water or if the soil doesn't have the right nutrients, the plants will die. If the plants die, animals that depend on them will die. If the animals that depend on the plants die, any animals that depend on those animals will die. Ecosystems in nature work the same way. All the parts work together to make a balanced system!

   The More the Merrier

A healthy ecosystem has lots of species diversity and is less likely to be seriously damaged by human interaction, natural disasters and climate changes. Every species has a niche in its ecosystem that helps keep the system healthy. We are learning about new species every day, and we are just figuring out the roles they play in the natural world. By studying and maintaining biodiversity, we help keep our planet healthy.

   Life in a Lake

In a lake ecosystem, the sun hits the water and helps the algae grow. Algae produces oxygen for animals like fish, and provides food for microscopic animals. Small fish eat the microscopic animals, absorb oxygen with their gills and expel carbon dioxide, which plants then use to grow. If the algae disappeared, everything else would be impacted. Microscopic animals wouldn't have enough food, fish wouldn't have enough oxygen and plants would lose some of the carbon dioxide they need to grow.

    Getting Along

Ecosystems have lots of different living organisms that interact with each other. The living organisms in an ecosystem can be divided into three categories: producers, consumers and decomposers. They are all important parts of an ecosystem.

Producers are the green plants. They make their own food. Consumers are animals and they get their energy from the producers or from organisms that eat producers.

There are three types of consumers:  herbivores are animals that  eat plants,  carnivores are animals that eat herbivores and sometimes other carnivores and omnivores are animals that eat plants and other animals.

The third type of living organism in an ecosystem is the decomposers. Decomposers are plants and animals that break down dead plants and animals into organic materials that go back into the soil, which is where we started!

    Parts and Pieces

What are the major parts of an ecosystem? An ecosystem includes soil, atmosphere, heat and light from the sun, water and living organisms.

    Getting Down and Dirty

Soil is a critical part of an ecosystem. It provides important nutrients for the plants in an ecosystem. It helps anchor the plants to keep them in place. Soil absorbs and holds water for plants and animals to use and provides a home for lots of living organisms.

    Give Me a Little Air

The atmosphere provides oxygen and carbon dioxide for the plants and animals in an ecosystem. The atmosphere is also part of the water cycle. Without the complex interactions and elements in the atmosphere, there would be no life at all!

    Getting Some Sun

The heat and light from the sun are critical parts of an ecosystem. The sun's heat helps water evaporate and return to the atmosphere where it is cycled back into water. The heat also keeps plants and animals warm. Without light from the sun there would be no photosynthesis and plants wouldn't have the energy they need to make food.

    Water Everywhere

Without water there would be no life. Water is a large percentage of the cells that make up all living organisms. In fact, you may have heard that humans can go longer without food than they can without water. It's true!  Without water all life would die. In addition to being an important part of cells, water is also used by plants to carry and distribute the nutrients they need to survive.

Food webs are graphical depictions of the interconnections among species based on energy flow . Energy enters this biological web of life at the bottom of the diagram, through the photosynthetic fixation of carbon by green plants.  Many food webs also gain energy inputs through the decomposition of organic matter, such as decomposing leaves on the forest floor, aided by microbes.  River food webs in forested headwater streams are good examples of this. 

Energy moves from lower to higher trophic (feeding) levels by consumption:  herbivores consumes plants, predators consume herbivores, and may in turn be eaten by top predators.  Some species feed at more than one tropic level, hence are termed omnivores.  Figure 1 provides a simplified model of such a food web.
 

We can look at this food web in two ways. It can be a diagram of the flow of energy (carbon) from plants to herbivores to carnivores, and so on. We will take this approach when we examine energy flow in ecosystems. In addition, members of a food web may interact with one another via any of the four interaction types named above. An interaction between two species in one part of the web can affect species some distance away, depending on the strength and sign of the inter-connections. Often, adding a species (as when an exotic species invades a new area) or removing a species (as in a local extinction) has surprisingly far-reaching effects on many other species. This is due to the complex inter-connections of species in ecological webs.

Ecologists use the following terms to describe various categories of the effects of a change (in abundance, or presence vs absence) of one species on another. 

1. Direct effects refer to the impact of the presence (or change in abundance) of species A on species B in a two-species interaction.
2. Indirect effects refer to the impact of the presence (or change in abundance) of species A on species C via an intermediary species (A --> B --> C).
3. Cascading effects are those which extend across three or more trophic levels, and can be top-down (predator --> herbivore --> plant) or bottom-up (plant --> herbivore --> predator).
4. Keystone species are those which produce strong indirect effects.

In the rocky inter-tidal zone of Washington state, and in other, similar areas, starfish have been shown to be keystone species 

Instances are known where a predator so strongly suppresses its prey (herbivores), that the trophic level below (plants) benefits because it is released from the pressures of herbivory.  Such “top-down” trophic cascades, where the community looks more or less ‘green’ depending on the abundance of predators, are well-known in lakes.   We also know of examples where fertilizing a system, which increases plant growth, results in more predators, through the increase in abundance of herbivores.  This is a “bottom-up” trophic cascade.

Our understanding of these complex species interactions gives substance to the popular phrase, the “balance of nature”.  One can also appreciate how a human-induced removal of one species (an extinction event) or the addition on one species (invasion of a community by a non-native species) could result in harm to many additional species, a topic we will consider in the second semester.

We will gain a fuller appreciation of the complex, multi-way interactions among species as we proceed through this series of lectures. However, we can fully appreciate the complexity of these multi-way interactions, it is helpful to first understand the nuances of the various two-way interactions. We will develop our understanding of species interactions in ecological communities based on these building blocks. 

Mutualistic Interactions

Facultative mutualisms are beneficial but not essential to survival and reproduction of either party. Obligate mutualisms are those that are essential to the life of one or both associates. We will examine an example of each. 

A fascinating facultative mutualism involves the Boran people of Africa, and a bird known as the honey guide. According to rock paintings, humans have collected honey in Africa for 20,000 years. H

Many other examples of mutualisms may be familiar to you. 

• Gut symbionts in herbivores: mammals can't digest cellulose 
• endosymbiosis and the origin of eukaryotic cells: mitochondria, flagella, chloroplasts are thought to be derived from free-living bacteria 
• pollination systems 
• the coral polyp and its endosymbiont "alga" (actually a dinoflagellate) 

Commensalism

The clown fish and anemone also illustrates this point. The clown fish hides from enemies within the stinging tentacles of a sea anemone, to which the clown fish is immune. Some report this interaction as a mutualism, arguing that the clownfish drops scraps of food into the mouth of the anemone. Careful studies have failed to find much support for any benefit to the anemone, so this appears to be a commensalism. 

Summary

Species interactions within ecological webs include four main types of two-way interactions: mutualism, commensalism, competition, and predation (which includes herbivory and parasitism). Because of the many linkages among species within a food web, changes to one species can have far-reaching effects. We will next examine competition and predation, and then return to a consideration of more complicated indirect and cascading effects. 

Is a member of a species that feeds directly on all or part of a living organism?

What are the 5 types of interspecific relationships?

What is species interactions?

What is a species interaction that has a strong effect on population sizes and other factors in many ecosystems?