Ecosystems Curriculum
Section 5: Background Information
Two-Way Relationships in Ecosystems
There are many ways in which relationships in an ecosystem are two-way or mutual. This means that two organisms affect each other in some way. While domino models are helpful for conceptualizing the one-way process of energy flow, a different type of model is needed to address relationships where there are mutual effects.
In a two-way or mutual causal model, each organism acts as both a cause and an effect. For instance, when a bee pollinates a flower, the bee and the flower are affected. The bee gets the nectar it needs for food energy and the flower gets pollen that the bee picks up from other flowers. The pollen enables the flower to reproduce. Each organism affects the other, so each acts as both a cause and an effect. Different types of mutually causal relationships in ecosystems are outlined below.
Mutualism
One of the easiest types of two-way relationships to understand is a symbiotic relationship that involves mutualism. These are instances where two organisms are interdependent in a way that benefits both organisms.1 For instance, ants known as Attini live in tropical regions in North, South and Central America (although some are found as far up the Atlantic coast as Long Island). These ants cultivate fungus gardens by chewing up leaf cuttings into a pulp and dropping bits of fungus onto this pulp, which is then deposited around the base of gardens. The ants eat the fungi and the fungi thrive in the garden that the ants create for it. The fungi that are cultivated by these ants are not found anywhere else and without the ants, would be overtaken by other types of fungi and bacteria.
Another example of mutualism is the relationship between the ox pecker bird and large animals, such as antelopes and zebras. The bird lands on the antelopes and zebras, where it feeds on the ticks and parasites that live on these animals. The antelopes and zebras benefit from the pest control that the ox pecker provides, while the ox pecker gets a meal. The ox pecker may also warn the antelope or zebra of an impending predator by flying upward and sounding a warning call.
Many Different Organisms Have Mutualistic Relationships
Mutual symbiotic relationships are found between plants and animals, animals and animals, and plants and plants. Lots of plants have symbiotic relationships with either bacteria or fungi that are found in soil. The bacteria and/or fungi incorporate themselves into the roots of the plants, helping the plants to thrive while the plants provide the bacteria/fungi with a food source. Lichen is actually made up of two different types of organisms, fungus and algae, and together these two organisms are able to thrive in conditions where neither could survive on its own.
As humans, we have symbiotic relationships with other organisms that are mutually beneficial. Bacteria live inside the intestines of humans and other animals such as cows. These bacteria help in the process of digestion as humans and other animals are not always capable of digesting all that they consume. The bacteria do this by digesting the food that is not broken down by the human's or other animal's digestive system. The animal's intestines provide food for the bacteria that in turn help the animal by finishing the job of digestion. (In humans, the bacteria "consume" the products of digestion and produce vitamins as a by-product. So they help more than by just digesting. Cows, on the other hand, are aided much more in digestion.)
Mutual Relationships can be Essential for Survival
Some mutual symbiotic relationships are so interdependent that one population cannot exist without the other. Not only is the two-way relationship beneficial, in these cases it is essential for the populations to survive. On the other hand, some mutually causal relationships happen infrequently, once a year or every few years.
Parasitism
Another two-way symbiotic relationship is that of parasitism. In this case, one species benefits and the other is harmed in some way. For instance, fleas and ticks consume blood from their host animals and receive, in turn, matter and energy. They benefit, however, the host animal is typically weakened as an effect of the parasites.
Two-Way Causality in Food Web Relationships
One of the more difficult types of two-way relationships to understand involves foodweb relationships where one type of organism (prey) provides food (or energy) for another (predator), and at the same time the predators help to keep the prey population in balance. For instance, rabbits provide energy for foxes and foxes cull the rabbit population of its weakest members so that the strongest rabbits reproduce, improving the population. This may not be best for the individual, but for the population of rabbits as a whole, it is beneficial.
What makes this so difficult to grasp? The two populations mutually affect each other; however, the effects are positive at the population level for the prey, but negative at the individual level. For the predator, the effects are positive at the individual and population level. This pattern is difficult to understand because:
- On the level of individual organisms, the effects are not parallel. As with parasitism, the effects are beneficial to one organism and detrimental to the other. So while there are two-way effects, the effects are not the same in substance or in quality of outcome.
- It juxtaposes reasoning about populations and reasoning about individuals. Many students have a hard time reasoning about populations at all,2 and here is an instance where what is good for the population is bad for the individual. This can be confusing to students.*
Students typically miss the two-way connections in an ecosystem. It is a common tendency to reason in a unidirectional, linear way. Students need to see that the fates of populations as a whole are linked together in many different ways. This is why it is important to introduce two-way causality. Understanding it is a building block for other ecosystem concepts, such as the dynamics of balance and flux as explored in the next section.
*Further, when reasoning about evolution, scientists reason at the level of the individual’s "aims"—that each organism is seeking to maximize its ability to reproduce. These individual aims give rise to population effects. Reasoning from the "aims" of individual animals, there aren’t good reasons in terms of evolution to be eaten. So the actuality—that both populations ultimately benefit—is difficult to arrive at when reasoning from an individual’s point of view. Note that the word "aims" is in quotation marks because scientists don’t conceptualize it as intentional—that the organisms do it on purpose— just that this is how they are genetically programmed to behave.
