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Exclosure Observations

By Ranger Steve Mueller


Helping students and the public understand the nature of science was an important goal for me as director of Howard Christensen Nature Center. Education deals with many misconceptions and the scientific process helps people understand that science, based on verifiable evidence, is self-correcting.

To see science in action we constructed an “exclosure.”  It helped people observe natural changes in nature niches. It was a ten by twenty foot fenced area. Fence posts were five feet apart. In the first section we did not disturb the area during construction and left it untouched for the next twenty years. The other five by ten-foot sections were cleared of vegetation on a rotating three-year cycle.

Sheep sorrel was an early colonizing ground cover. Plants, insects and small animals could enter and colonize without our influence. Large animals like deer were excluded. After the plants were cleared and roots filtered from the soil, we observed colonizing plants and animals. Ants made about 20 small mounds about 2 to 3 inches in diameter. Sheep sorrel, pigweed, and five clumps of grass also took hold within three weeks.

Rather than describe details of what was observed, lets focus attention on nature niche establishment. John Curtis first described the process of plant succession. By documenting the species and the order they colonized sand dunes, he established predictable models. Plants changed the soil and made it more hospitable for different species. It could be predicted how long it would take for particular species to establish.

Plant colonizers were replaced by mosses, perennial grass or other plants when soil conditions improved. Later woody shrubs and trees could establish. With each new plant species various insects, birds, and mammals could make a living.

I mentioned science is self-correcting. Over the years, repeated studies showed a predictable sequence of colonizing plants and animals. It was found the sequence was somewhat variable based on surrounding vegetation and animal populations. Generally species arrived in the same sequence to occupy nature niches. Continued long-term studies corrected initial conclusions.

Using HCNC’s exclosure, students learned the process of collecting and analyzing data. As humans, we are prone to draw conclusions based on how things appear or how we want them to be. Often they are correct but frequently we have not collected adequate data to confirm our conclusions. Scientific process slowly builds valid conclusions that get modified and corrected when studies indicate our conclusions are not perfect.

The process allows us to discard misconceptions and support what is shown to be correct. That is where climate change discussions frustrate many. Some people deny it is greatly influenced by human activities. Most scientists acknowledge climate change is greatly human influenced but qualify the statement with “pending further data collection.” Science process is always open to modification pending further data collection. Many people accept unsupported absolute conclusions because they do not like science being open to modification or not being what they desire to think.

The exclosure experiment at HCNC helped people learn how scientific process works and how new data collection modified our understanding of how nature functions. It would have been nice if the experiment were continued after I left. We could have documented changes in growing season as well as plant and animal composition. Many scientific studies take decades or centuries to make valid predications. We tend to be impatient and want absolute answers now.

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net Ody Brook, 13010 Northland Dr, Cedar Springs, MI 49319-8433.  616-696-1753.


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