Problem 12 - Campbell Biology

Question

Synthesis/Evaluation SCIENTIFIC INQUIRY An ecologist studying desert plants performed the following experiment. She staked out two identical plots, containing sagebrush plants and small annual wildflowers. She found the same five wildflower species in roughly equal numbers on both plots. She then enclosed one plot with a fence to keep out kangaroo rats, the most common grain-eaters of the area. After two years, four of the wildflower species were no longer present in the fenced plot, but one species had become much more abundant. The control plot had not changed in species diversity. Using the principles of community ecology, propose a hypothesis to explain her results. What additional evidence would support your hypothesis?

Step-by-Step Solution

Answer

Hypothesis: Kangaroo rats maintain wildflower diversity by grazing on dominant species. Additional evidence: feeding patterns and growth rates with and without rats.

Step 1: Understand the Experiment

Review the details of the experiment. The ecologist has two identical plots with sagebrush plants and wildflowers. One plot was fenced to keep out kangaroo rats, while the other was left open.

Step 2: Observe the Experimental Results

After two years, the fenced plot had its wildflower species reduced from five to one, while the remaining species flourished. In contrast, the control plot retained all five species.

Step 3: Consider Community Ecology Principles

Reflect on the principles of community ecology, such as competition, predation, and niche differentiation. The absence of kangaroo rats likely interfered with the natural dynamics of the ecosystem.

Step 4: Formulate a Hypothesis

Propose a hypothesis: Kangaroo rats help maintain wildflower species diversity by grazing on the dominant wildflower species, preventing it from outcompeting the others.

Step 5: Identify Supporting Evidence

Consider what additional evidence could support the hypothesis. Possible evidence includes observing kangaroo rat feeding patterns, studying the growth rates of each wildflower species in the presence and absence of the rats, and examining the competitive interactions among the wildflower species.

Key Concepts

competition

In community ecology, competition refers to the struggle between organisms for the same resources within an environment. This can be food, water, shelter, or any other necessity. When resources are limited, species have to compete, leading to survival of the fittest.

The experiment with desert plants serves as an example of competition among wildflower species. Without the fence, kangaroo rats kept the dominant wildflower species in check by feeding on it. This allowed other wildflower species to thrive. Without the rats, the dominant species had free reign over resources, leading to its unchecked growth.

Understanding competition helps explain why certain species thrive and others diminish when an external factor (like the kangaroo rats) is removed. The rat-free environment led to intense competition among the wildflowers, resulting in a single species outcompeting the rest for survival.

predation

Predation is another key aspect of community ecology. It refers to the interaction where one organism, the predator, hunts and feeds on another organism, the prey.

In the given experiment, kangaroo rats play the role of predators, preying on the seeds and plants of the wildflowers. This predation helps control the population of the wildflower species, maintaining a balance in the ecosystem.

The fenced plot, free from kangaroo rats, shows what happens when a predator is removed from the environment. Without these predators, the dominant wildflower faced no threats, allowing it to proliferate and outcompete the other species. This reveals how predation keeps potential dominant species in check and maintains biodiversity. Removing a predator can cause dramatic shifts in the community structure.

niche differentiation

Niche differentiation, or resource partitioning, is the process by which competing species use the environment differently to coexist. By occupying slightly different niches, species minimize direct competition for resources, allowing them to live together in the same habitat.

In our experiment, with the presence of kangaroo rats, the wildflower species coexisted by occupying slightly different niches, possibly evading the rats in various ways.

However, in the absence of rats, the dominant wildflower no longer needed to differentiate its niche. It could monopolize resources without restraint. The concept of niche differentiation explains why diverse environments often maintain high species diversity. When niche differentiation breaks down, such as when a predator is removed, competition intensifies, leading to a reduction in species diversity as seen in the fenced plot.

Understanding niche differentiation helps illustrate how species adapt their behaviors and characteristics to coexist and maintain balanced ecosystems.