Energy Flow
One of the fundamental things that determines an organism's role in its ecosystem is the way that it gets its food. Two major categories that living things can be split into are based on whether they make their own food or consume other organisms for food.
Autotrophs
Autotrophs ("self-feeders"), also known as producers, make up the lowest trophic level where they make their own food. Organisms such as plants, algae, and cyanobacteria do so using photosynthesis, where they will capture solar energy to create sugars. Some bacteria use chemosynthesis where they use geothermal energy from hot springs and deep-sea vents to produce their food.
Heterotrophs
Heterotrophs ("other-feeders"), also known as consumers, are organisms that consume other organisms. Primary consumers are those that eat producers. Secondary consumers eat the primary consumers. Tertiary consumers eat the secondary consumers and so on. Ecosystems are rarely that simple and have many consumers that feed at multiple trophic levels.
Productivity
Producers are essential to an ecosystem as they create the fundamental supply of energy that is then passed on to consumers. The types of producers that are present in an ecosystem determine the types of consumers that can survive in an ecosystem.
Gross primary productivity (GPP) is the rate the ecosystem's producers convert solar energy into chemical energy useable by themselves and other organisms (sugars); it is essentially the rate of photosynthesis in an area over a specific amount of time. It is measured in energy/area/time. The plants need to use some of the energy available in these sugars themselves, though, so it is not all available to other organisms in the ecosystem. Net primary productivity (NPP) is the rate of energy storage by plants after subtracting the rate at which the plants use energy for cellular respiration (respiration loss [RL]). It can be calculated using the following equation:
NPP = GPP - RL
If you were to think about it in terms you might be more familiar with, gross primary productivity is like how much money someone makes from their job. The net primary productivity is then how much money they have to save or use as spending money after they've paid for bills, gas, and food.
Primary productivity is highly linked to the amount of biodiversity in an ecosystem. The higher the primary productivity, the more plants will grow, which will lead to a more biodiverse area. There are various factors that affect productivity levels. Higher water availability, higher temperature, and higher nutrient availability will lead to a higher NPP. It is because of this that areas like the tropical rainforest, which have high water availability and temperature, are incredibly biodiverse.
Food Chains/Webs
The energy produced by the producers will be transferred from one organism to the next as organisms eat each other. A sequence of organisms that shows how this energy is passed on is called a food chain.
The consumers present in the food chain can be put into different categories depending on what types of organisms they eat. The organisms will have different anatomical structures and varying physiologies that allow them to eat and break down certain types of foods.
Herbivore
Herbivores are primary consumers. They are organisms that feed on plant tissue.
These organisms can be split into categories based on what it is that they eat specifically, such as algivores (eat algae), frugivores (eat fruit), folivores (eat leaves), nectarivores (eat nectar), granivores (eat seeds), graminivores (eat grass), palynivores (eat pollen), mucivores (eat plant fluids like sap), and xylophages (eat wood).
Many herbivores practice osteophagy - the consuming of bones. Most plants do not have sufficient phosphorus levels, and herbivores will consume the bones of dead animals in order to increase their phosphorus (and calcium) intake.
Carnivore
Carnivores are consumers that eat other animals.
They can be further categorized based on what type of organisms they eat, such as insectivores (eat insects) or piscivores (eat fish). Carnivores do not have to be animals, as there are carnivorous plants and fungi. An obligate carnivore is one that relies on meat exclusively for its nourishment, while organisms such as a hypercarnivore need roughly 70% of their food intake to be meat.
Omnivore
Omnivores are organisms that can survive by eating both plants and animals.
They can also be further categorized into different categories, including the categories found with herbivores and carnivores, depending on what the organism's primary food source is.
Due to eating a wider variety of food, these organisms tend to deal with stressful environments and food scarcities better than other types of organisms.
Decomposer / Detritovore
Decomposers, such as bacteria and fungi, break down wastes or the remains of living things and get their nourishment and energy from that.
Detritivores, such as earthworms and vultures, eat the waste or dead bodies (detritus) of other organisms and get their nourishment and energy from it.
These organisms are essential for the recycling of nutrients from other organisms back into the ecosystem.
Ecosystems are more complicated than food chains let on. There will be multiple types of producers, consumers will have more than one organism they use as food, and there are multiple decomposers involved in the ecosystem. Food webs are a group of interconnected food chains that shows the more complex feeding relationships that exist in natural ecosystems.
Trophic Levels
As organisms eat each other, energy moves from one trophic level to another. These are essentially hierarchal levels in the ecosystem that correlate to where the organisms fall in the food chain.
A trophic pyramid, also known as an energy pyramid, displays the progression of trophic levels. As energy is passed from one trophic level to the next, energy is lost. The percent of energy transferred from one level to the next is known as ecological efficiency. It varies from ecosystem to ecosystem, and trophic level to trophic level, but, on average, only about 10% of energy is passed on to each subsequent level due to most of it being expended as heat or used in metabolism. This is known as the 10% rule. Due to this loss of energy, there is a limit to what the highest level of a trophic pyramid can be. Most ecosystems can typically only support up to tertiary consumers.
"Lost Energy" and Thermodynamics
The energy that is lost isn't really lost, it is merely converted into a different form of energy. Some of it is converted into and expended as heat. Energy is also used up by organisms for things like movement, development, physiological processes, etc.
Energy cannot be created or destroyed - this is the First Law of Thermodynamics and is not violated by food webs. Nor is the Second Law, the idea that entropy increases as energy changes from one form to another, violated. Energy will change form, and transfer between organisms (or between the sun and producers), and some will be lost as heat with each transfer.
The 10% rule applies to both energy and biomass - the total mass of organisms in the trophic level. Energy amount or biomass can be determined at another level by multiplying or dividing by 10. If the producers, the trophic level with the most energy, has 95,000 J of energy available, only 10% will be passed on, so the primary consumers would have 9,500 J available at that level.
When a major change occurs at the top of the trophic pyramid, or at the top of the food chain, there can be massive ripple effects that will affect the lower layers of the chain. This is known as a trophic cascade.
Imagine the food chain of trees -> deer -> wolves. If the wolves were killed off, there would be fewer predators eating the deer, resulting in a larger deer population. More deer results in more feeding on the trees, which will result in lowered tree populations.
Trophic cascades will result in dramatic changes in the ecosystem structure, which can also affect how nutrients are cycled via biogeochemical cycles as producers and consumers are essential parts of the cycles.
