Ecosystems Curriculum

Section 4: Background Information

Time Delays Make it Difficult to Detect Causal Patterns

Many events in ecosystems do not have immediate effects or immediately noticeable effects. For instance, the effects of microbes take a long time to become observable because they are cumulative or aggregate. (This is similar to other processes like erosion by natural events or the effects of acid rain). Therefore it is easy to assume that nothing is happening, when in fact, effects are accumulating which will eventually become quite dramatic. The time delay makes it difficult to recognize the effects of the decay process.

Exponential growth patterns are another type of causal pattern that can be difficult to notice at first. In exponential growth, the size of the population typically keeps doubling (in natural systems). The effects are hard to see at first. Eventually, there are startling effects. Imagine there is one duckweed plant on a pond, and next there are two, then four, then eight and so on. Suddenly, the pond will dramatically go from being half covered to fully covered. The time delay between the start of the process and noticeable effects makes it hard to foresee the outcome or to understand the process that led to it. Initially small effects (new plants) cause new effects (more plants) and ultimately result in a startling change. An invader species such as Purple Loose-strife, a plant with bright purple flowers seen in late summer in many New England wetlands, when introduced to a swamp would at first grow alongside native species such as Eupatorium. However, the Loose-strife growth would be exponential, as each new plant begets numerous new plants. All of a sudden, the continued doubling of the Loose-strife would be enough to make it impossible for the Eupatorium to live.

Time-Lapses Illuminate the Cyclic Pattern of Decay

The activity in this lesson engages students in thinking about the nature of time delays and how such delays can make it difficult to see the underlying causal interaction pattern. Time-lapse videos are introduced as a means to help students clearly see the cyclic nature of decay despite the time delay.¹ Linking the concepts in this lesson to those of the lesson on non-obvious decomposers helps students to see that slow accumulation of effects is the result of many microbes doing their work, and will eventually result in a discernible change.