Ecosystem Learning Objectives
- Understand that the biological communities present in an ecosystem interact with the ecosystem’s climate, soil, atmosphere, and/or water
- Define primary producer, primary consumer, secondary consumer, and omnivory and be able to accurately identify these in a food web
- Read and interpret a food web diagram with multiple trophic levels and how these interact using top-down and bottom-up terminology.
- Be able to interpret food chains and food webs, and be able to locate a food chain within a food web
Biological communities interact with the climate, soil, atmosphere, and water to create an ecosystem. We consider ecosystems at the end of the ecology module because ecosystems bring together every element of ecology we’ve learned so far: physical environment, individual behaviour, population ecology, and community ecology.
Each population in an ecosystem can be labeled by how it fits into the trophic interactions, or feeding interactions, between species. Species that capture sunlight energy (or energy from inorganic molecules) and build it into chemical bonds through photosynthesis (or chemosynthesis) are called primary producers. Primary producers form organic matter from inorganic matter using the energy gained from outside sources. All other organisms are consumers that gain energy from the organic matter they consume. Primary consumers eat primary producers, while secondary consumers eat primary consumers, and so on up the scale of trophic levels. Detritivores are species that consume dead organic matter. Some species, such as humans, are omnivores because they can feed on producers and consumers at more than one trophic level. The top level in a chain or web of feeding interactions is often called the top carnivore.
Trophic feeding interactions can be mapped out into a food web that groups species by trophic level and connects “enemy” (herbivore or predator) to “victim” (producer or prey) with arrows that point in the direction of energy flow.Within the food web, you can trace carbon and energy flow up each food chain by starting with any primary producer and following an arrows from it up to the next trophic level, and the next. For simplicity, producers are depicted at the bottom of the web, while the “top” consumers, usually carnivores, are shown at the top of the web diagram.
- Select a food chain and trace it through the food web above. What’s the maximum length food chain you can make?
- Imagine if the population size of the tree in the food web above decreased dramatically because of logging practices or an invasive pest that kills the trees. How would altering the bottom level of the food web affect population sizes up at the higher trophic levels of the food web?
- How would removal of the coyote, the predator at the top of this food web, change population sizes and composition of species lower in the food web?
Ecosystem Learning Objectives
- Explain why energy flows but matter cycles
- Explain that transfer of energy is not efficient and the effect of this on the length of food chains; be able to give a rule of thumb for how much energy is transferred to the next trophic level.
- Explain the distinction between NPP and GPP
- Describe the global pathways for cycling of nitrogen and carbon between living organisms, atmosphere, oceans and continental crust
Energy flows but matter cycles
As enemies consume their victims in a community, they digest the matter of their victim and use some of it for energy for their own growth and reproduction. For instance, when the squirrel eats the conifer seed in the food web above, the transfer of energy is not efficient. For the average trophic interaction, roughly 90% of energy is lost at each trophic level transfer, and this loss of energy to the consumer limits the length of food chains within a food web. Also, if only 10% of the energy makes it to the next level, the population size of the top predator(s) remains small, while the population size and biomass of producers needs to be huge! In ecology, biomass is the combined mass of all the organisms of that species or group in the ecosystem. Note that in the biofuel industry, the term biomass is used a little differently than in by an ecologist. Ecologists refer to the entire organism, including roots and seeds, but biofuel biomass almost always refers to the mass of animal waste and harvested plant material used to make energy.
All the matter in living organisms, made up mostly of carbon, hydrogen, oxygen, and nitrogen in organic molecules, is either incorporated into the enemy that consumes it or left behind in the environment. Each atom ends up somewhere, as described below in the nutrient cycles section. The energy obtained by each organism is:
- used for maintenance of the organisms
- used for growth and reproduction
- lost as heat or excreted waste from the organism
While energy is transferred very inefficiently up a food chain, chemical toxins in the eaten organisms are incorporated into the consumer. Consumers eat many prey and retain all the toxins in those prey, accumulating higher toxin concentrations with each trophic position, a phenomenon called biomagnification.
Globally, we quantify all the primary production, which is organic matter formed from inorganic matter by primary producers, to find the total, or gross primary production (GPP). Those producers metabolize (or use) some of the energy they acquire for their own growth and maintenance. The remainder is the net primary production (NPP), the amount of energy resources left for the consumers in the ecosystem to acquire through herbivory. Most of global NPP occurs in the world’s oceans, but the community with the greatest NPP per unit area is the tropical rain forest.
The entire planet can be thought of as one huge ecosystem, called the biosphere, where energy flows into and out of the system openly, but matter cycles within the system.
What happens to a protein molecule in a plant seed that the squirrel consumes? Energy flows but matter cycles, meaning that matter is not lost the way that energy can leave the system as heat. Instead, matter is retained in some form in the ecosystem. Matter is stored in “compartments” such as carbon stored in rock, plants, the ocean, and the atmosphere, while the movement of matter between compartments is called “flux.” Carbon fluxes because of respiration, photosynthesis, decomposition, and burning.Carbon, shown here, moves around when organisms—which contain carbon—eat or die. Carbon cycles quickly through organisms but very slowly in the environment, often staying in rocks for millennia. Until recently, scientists didn’t know where some of the carbon on earth was stored, but current evidence shows that more carbon is tied up in tropical forests than we had previously realized. So, tropical forests are the missing “carbon sink” or compartment. Tropical forests are being removed at a high rate to grow more lucrative if short term crops, which could add an additional disruption to the carbon cycle.
Summing the fluxes of carbon moving into and out of the atmosphere, we can see that the fluxes are just over 200 Gt/y, while the atmosphere contains 750 Gt of carbon. Therefore, the residence time of carbon in the atmosphere is 750 Gt/200 Gt•y^-1, or 3-4 years. More interestingly, the actual flux into the atmosphere is 217 by these data, a little higher than the 214 leaving the atmosphere.
- How could changes to carbon usage reverse the trend of more carbon being added to the atmosphere than is being removed?
The annual global pattern of C in the atmosphere (mostly in the form of CO2) is recorded at the Mauna Loa observatory in Hawaii.
The trend is for increase in CO2 over time—dramatic increase. However, within each year the global atmospheric carbon cycles with a steady pattern (see yellow inset) that represents the global terrestrial photosynthesis, which occurs largely during the northern hemisphere during summer and fall, when photosynthesis reduces atmospheric CO2.
Here’s Hank Green’s Crash Course video on ecosystems for review, and while you are watching, cast a critical eye on the food web diagram in the video and note what’s not right about it.