Ecosystems Curriculum

• Domino Causality. In a domino pattern, a triggering event initiates a domino of effects that in turn act as causes to precipitate further effects.² Domino causality is important for understanding the process of energy flow in food webs and the important link from the sun to green plants as the source of that energy. It offers a means to think about the far reach of certain events within an ecosystem, such as when the producers die out and have no replacements.

• Cyclic Causality. In a cyclic pattern, a cause precipitates an effect that in turn has an effect on the initial cause. For instance, in ecosystems, plants grow and then die. Decomposers decay them and release nutrients within the plant back into the soil affecting the growth of other plants.

• Two-Way Causality. In a two-way pattern, one event or relationship has mutual, and often simultaneous, effects. Each component has an effect on the other, so each acts as both an effect and a cause. For instance, two-way causality can be seen in symbiotic relationships where an event or action (such as a bee pollinating a flower) results in effects on both organisms (the bee and the flower). It differs from a cyclic model in that the focus is on how one event (a bee pollinating a flower) or relationship (a tick sucking blood from a dog) has mutual, and typically simultaneous, effects (bee gets nectar and flower gets pollinated or tick gets nourishment and dog is weakened). Some two-way relationships might also involve cyclic causality if viewed over the long-term relationship.

Students' tendency towards simple linear causal reasoning makes it unlikely that students will notice these more complex causal patterns on their own. They tend to miss the connectedness within systems, noticing direct, but not extended domino patterns.³ Similarly, they tend to break cyclic patterns apart into linear patterns, thus missing essential parts of the relationship. They often miss two-way causality, by focusing only on one side of the equation and not noticing the other side, or (as explained later) focusing on individual organisms rather than at a population level.

Understanding Related Causal Concepts

Recognizing the causal patterns outlined above is complicated by a set of related concepts that are usually hard for students to grasp. These are outlined below and include indirect effects, time delays, non-obvious causes, reasoning about populations, reasoning about balance and flux, and applying particular causal patterns.

• Indirect effects. Ecosystems are characterized by connectedness, and in ecosystems, causes often have many indirect effects. Students tend to focus on direct effects to the exclusion of indirect effects. This reinforces a simple linear causal model and makes it unlikely that students will notice extended food chain or food web relationships.

• Time delays. There are often significant time delays between causes and effects in ecosystems.⁴ This can make it difficult to trace an effect back to its cause, or even to see the pattern of causality involved. For example, time delays make it harder to recognize indirect, extended effects in domino patterns. Students might realize that the first set of effects is related to a cause. However, the further along an effect is on the domino, (i.e., the further the effect is removed in time and space from its cause), the harder it is for students to notice the effect and to realize that it is connected to the precipitating event. It is common for natural systems to have checks and balances that dampen effects, slowing the appearance of obvious effects. So for instance, the environment may be able to absorb a certain amount of pollution before any obvious effects appear, but eventually there will be enough accumulation that the effects become dramatic.

• Non-obvious causes. Ecosystems also involve non-obvious causes—ones that are hard to detect with the naked eye. Non-obvious causes can make it difficult to recognize that certain causal patterns exist. For instance, recognizing the cyclic model in decay involves recognizing tiny microbes as the primary decomposers. (It also involves dealing with the time delay in nutrient recycling). Students typically do not understand the nature of decay⁵ and the role of microbes as decomposers and recyclers of carbon, nitrogen, and minerals.⁶

• Reasoning about populations. Understanding ecosystem dynamics involves reasoning about what will happen to populations of organisms. A population refers to all the members of a species that live in a particular location. Reasoning about populations is hard for students. When analyzing food chains, they typically think of the predator eating the particular animal as prey, not the population of predators preying on the population of prey.⁷ Reasoning about individuals often leads to feeling sorry for the prey. This makes it hard to see the food web relationships in terms of checks and balances.

• It can also make it hard to see particular causal patterns. For instance, recognizing some two-way patterns involves switching one's focus between individuals and populations, as in the following example. Mice provide energy for owls, while owls might help to maintain population size levels in the mice that can be supported by the available resources. So when an individual mouse is eaten, it does not benefit the individual, but the mouse population, as viewed from an outside perspective, might be better off. On the other hand, the individual owl and the population of owls benefit.

• Reasoning about balance and flux. Ecosystems involve both balance and flux. Typically, studies of ecosystems stress balance. Indeed there is a great deal of redundancy and ability to adapt in ecosystems that provides balance. However, ecosystems typically include some fluctuations as well. Flux is not necessarily negative: it can create patterns in an ecosystem that are ultimately healthy. For instance, it can allow for new species to become established. Students typically reason that flux or change is bad. This can make it difficult to detect the role of flux in positive outcomes.

• Applying particular causal patterns. Ecosystems involve many complex causal patterns. Students may be confused about where to apply particular patterns. For instance, it is common for students to confuse the process of energy transfer with the process of matter recycling. Energy transfer is domino-like. Green plants convert energy from the sun into a form that is usable by organisms in the food web. Energy is transferred along the food web. Energy is lost all along the way (it turns into other kinds of energy, i.e. heat energy and other forms of energy that can’t be eaten) and any remaining energy dissipates as heat energy in the process of decomposition. Matter recycling is cyclic and involves reducing matter to its most basic parts, which can then re-enter the living world. Matter is conserved rather than lost.

Helping students to understand the causal patterns in ecosystems and to master the related causal concepts makes it possible for them to achieve deep understanding of ecosystem concepts. The activities in this module aim to support students' developing understanding.

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*Most ecologists use the individual as the level of analysis in reasoning about natural selection. Each mouse wants to maximize its relative reproduction within the population. You could say that all populations want to maximize their relative ecological importance; however, the intentions are attributed at an individual level and the actions are carried out at an individual level. Therefore, reasoning about cumulative systems effects involves a puzzling juxtaposition of thinking about individuals and populations as "agents," although they differ substantially in how one would attribute goals and intentions. Individuals can be said to have intentions, whereas populations do not. Agency at the population level is an emergent property.