Section 6: Background Information

The Fates of Populations are Linked

There are many interdependencies within ecosystems. Events that affect one population typically have ripple effects—affecting other populations. When the size of one population becomes too large or too small for its niche (or role) in the ecosystem, it is out of balance and may throw others out of balance too. The simulations in Section 5 revealed some of the patterns of boom (when a population size is too big ¹) and bust (when a population size is too small) that can occur.

The lesson in this section introduces a story describing how the World Health Organization inadvertently interrupted the balance of an ecosystem when trying to solve a health issue. The story illustrates the complex connections that existed and what happened when those were disrupted. The consequences came as a surprise to the officials, who had not anticipated the many non-obvious connections.

Ecosystems Involve Both Balance and Flux

Ecosystems are not only about balance. Ecosystems involve both balance and flux. Typically, studies of ecosystems stress balance. Indeed there is a great deal of redundancy and adaptability in ecosystems that provides balance. Redundancy means that organisms have multiple acceptable food sources or habitats. The ability to adapt means it is possible for organisms to switch food sources or habitats. However, ecosystems typically include some fluctuations. Flux is not necessarily harmful to an ecosystem: it can create patterns in an ecosystem that are ultimately healthy. For instance, flux can allow for new species to become established. For some populations, flux is a necessary part of their existence. Further, flux doesn't always mean that a population will die out. It can simply mean that the population size changes over time. Sometimes flux occurs in a cyclical pattern, with seasonal or yearly cycles. Sometimes flux occurs when there have been changes to the environment. This type of change can be a one-time event, such as a fire or a very harsh winter from which an ecosystem eventually recovers to a previous state. This type of change can also be permanent, forcing organisms to adapt in new ways or die.

Patterns of Balance and Flux Can Tell Us How Things Are Connected in Ecosystems

School curricula often focus on the connections in ecosystems. This makes sense because ecology is about connections. However, it is important to pay attention to how things are connected, rather then just assuming that all things are connected in one way, and that an event involving that particular thing will have a strong and direct effect on other things.² There are events that don't necessarily affect other ecosystem components because there is a fair amount of "insurance" built into ecosystems. A predator does not necessarily die off because its prey dies off. It may change its diet. One food source may dwindle while another flourishes. For example, perhaps a storm washes an abundance of mussels on shore, resulting in a feeding frenzy among shorebirds. The "surprise" mussel supply becomes depleted, and the birds must shift to their regular diet, which might have included only small amounts of mussels. This example stresses the insurance that provides balance, but it is also about flux.

Patterns of balance and flux are interesting because they offer information about the nature of the connections within the ecosystem, both their limits and their flexibility. While balance or equilibrium might be viewed as a system's "goal," flux is probably the more common state of the system. The dynamics of flux can play out in many ways. There may be shifts in the proportion of prey to predators. For instance, more mice than their predators can keep in check may result in an unbalanced population size until an event such as a dry spell causes large numbers of mice to die off. This re-establishes a balance of prey population size and predator population size, without the predators being the cause.

It is important to help students learn to reason about both balance and flux. The activities in this section are designed to address that need. The understanding goals build upon those taught in the previous lessons, so this particular lesson set should be taught after those understandings are evident.