Commensalism is the type of relationship between two organisms in which one benefits while the other remains neutral. One such example of this is the relationship between whales and barnacles. The barnacles benefit, as they are able to gain mobility and feed off the current generated by movement of the whale. The whale, however, remains neutral; it gains no advantage or disadvantage from the presence of the barnacles.

Cohabitation relationships imply that both species remain neutral, while symbiotic and mutualistic relationships imply that both species gain benefit. In symbiosis, the two species depend on each other for survival. Parasitism implies benefit of one species, at the harm of the other.

Saprotrophs are decomposers that are capable of breaking down dead or dying organisms. Because of this, saprotrophs can obtain energy directly from any other organisms in an ecosystem.

Producers are autotrophs, and do not require organic input to create energy. Carnivores, herbivores, and omnivores are loose classifications of organisms based on diet. Carnivores typically feed on heterotrophs, while herbivores generally feed on autotrophs. Omnivores typically feed on both autotrophs and heterotrophs.

In a mutualistic relationship, both organisms benefit from the relationship. In this example, the human host benefits because (s)he gains access to nutrients and is at a lower risk of hosting dangerous E. coli, while the bacteria species benefits because it gains access to a habitat.

A competitive relationship exists when two species share a predator, are competing for the same resources, or otherwise interfere with one another.

Predation refers to an organism eating another organism; animals can prey on animals, or on plants.

In a parasitic relationship, one species benefits while the other is negatively impacted.

In a commensal relationship, one species benefits while the other is not impacted. 

Though all of these statements are true, the fact that species can occupy different trophic levels depending on what they're eating is not a consequence of the fact that only 10% of all stored energy can ascend from one trophic level to the next. These facts are related, but one does not cause the other.

Food chains only have four or five trophic levels at maximum because the food chain is rapidly depleted of stored energy after each trophic level increase. Logically, producers always have the most biomass of any trophic level because they must produce all of the energy that will sustain the trophic levels above them. Finally, it makes sense that fluctuating population sizes threaten the stability of longer food chains because if even one trophic level suffers a population decrease, then all of the trophic levels above it are potentially jeopardized. 

For a species to be a producer, it must be an autotroph and must be able to convert light or chemicals into storable chemical energy, usually as a form of sugar. Green plants are some of the most familiar producers, but some species of plankton (phytoplankton) are also able to engage in photosynthesis and supply energy to fuel food chains/webs.

As decomposers, all species of fungi are heterotrophs, rather than producers (autotrophs). Ants are also not producers, as they also cannot convert light or chemicals into sugars.  

Biological magnification is most commonly a problem with compounds that are toxic, slow to degrade, and can accumulate in organisms' bodies such as DDT and methylmercury. For example, consider methylmercury. Methylmercury breaks down into less toxic forms very slowly over time and it is excreted from organisms' bodies relatively slowly. Though aquatic plankton may only absorb relatively small or trace amounts of methylmercury over their life spans, they are likely unable to excrete the chemical at the rate that they absorb it into their bodies. Small fish that feed on the plankton must eat large amounts of plankton to survive—in doing so, they are exposed to all of the methylmercury that has accumulated in the bodies of the plankton. They, too, are unable to excrete this methylmercury at the rate they absorb it. When larger fish feed on the small fish, these larger fish are exposed to and absorb the methylmercury contained in the small fish. They are likely unable to excrete the majority of this methylmercury at the rate that they continue consuming mercury found in the small fish. This process continues upwards through the trophic levels, until organisms at high trophic levels, such as birds of prey (or humans!) are exposed to high levels of methylmercury that have been concentrated/magnified up through the trophic levels.

Dominant species are more numerous than their competitors in an ecological community. They can be said to define their communities, and they exert influence over the other species within their communities. For example: mangroves are generally the dominant species in tropical tidal swamps.

Critically, dominant species are different from keystone species. Despite their often relatively low total biomass, keystone species have a large impact on the distribution and abundance of species around them. Sea otters are a good example of a keystone species: even though otters have relatively low total biomass, they are crucial to many marine ecosystems because they prey on sea urchins. If otters are removed from a habitat, sea urchins will eat or destroy large portions of the habitat's kelp, which can threaten species at all trophic levels. 

Because energy transfer between trophic levels is only about 10% efficient, it takes several organisms at a lower trophic level to support one organism at a higher trophic level. If each fish (lower level organism) has 10 toxin molecules in its body, and each eagle (higher level organism) eats 10 fish, the eagle now has 100 toxin molecules in its body. Molecules stored in an organism accumulate at higher trophic levels; this accumulation is known as biological magnification.

Heterotrophs obtain their organic materials by feeding on other organic organisms. Scavengers are an examples of heterotrophs. Autotrophs are self-feeders in that they do not consume organic material but rather create their own organic material from inorganic substances.  

The cascade model, or top-down model, states that there is a unidirectional influence from higher to lower trophic levels. This model follows the idea that predation controls community organization. Predators limit the number of herbivores, and herbivores limit plants, and plants limit the amount of nutrients available in the soil.