PSAT Writing Question of the Day

Ecosystems are made of a complex system of energy and nutrient transfers from one organism to another. Some of these transfers are in the form of predation: one organism eating another – while others are in the form of ecosystem engineering, an organism changing the environment around them. One major form of ecosystem engineering is niche creation, the process of an organism changing its environment to create a competitive advantage. In order for an organism to be considered as creating a niche, three things must be true: the organism must significantly modify its environment, those changes must affect other organisms’ survival, and those changes must lead to an evolutionary response in another organism. 1

One of the world’s most influential “niche constructors” - species known for changing the environment around them - is the earthworm, an organism found almost everywhere on the planet. Earthworms were originally aquatic organisms that were ill-equipped to survive on land. A scientist only concerned with evolution would predict that, in order to live on land, earthworms would have to significantly change. Earthworms didn’t change their physiology a great amount, however, instead, they changed the soil to make it more like the ocean in order to survive. Land with earthworms is less compacted, is more nutrient rich, and is better mixed than land without them, leading to monumental changes in the ecosystem.

To be considered true niche constructors, however, earthworms must change the environment in such a way as to alter the evolution of another species. One of the easiest ways to measure this effect on evolution is in the effect that the number of earthworms has on soil fertility, a measure of how hospitable an environment is to plant growth. Even the least fertile soil has around 62 worms per square meter, and as the number of worms increases so does soil fertility. As worms move through the different layers of soil, they eat, digest, and excrete massive amounts of organic matter. They leave their excretions behind in the form of nutrient-rich droppings known as casings. As these casings decompose, they release nutrients into the soil. This process not only moves nutrients from one layer of the soil to another but also converts the nutrients to forms that plants can absorb and process more easily. Because it is easier for plants to get the proper nutrients, plants don’t have to invest time and energy into making better root systems to gather nutrients. As a result, plants have, over time, lost some of these mechanisms – a form of evolution.

Without earthworms, modern land-based ecosystems would look very different. There is no way to predict the exact changes that would exist without earthworms, but it is easy to recognize earthworms’ impact. As research continues into the effect of earthworms, scientists are confident that it will show an even more complex picture of the earthworm’s contribution to the land-based ecosystem.

The author is considering the addition of the following table:

The table above outlines the unique ways in which several organisms have impacted their ecosystems to the benefit of other species.

Should the author make this addition?