Question

When old-growth forests are cleared of tall trees, more sunlight reaches the forest floor. This results in a sharp increase in the population of leafy shrubs on which the mule deer depend for food. Yet mule deer herds that inhabit cleared forests are less well-nourished than are herds living in old- growth forests. Which one of the following, if true, most helps to resolve the apparent paradox? (A) Mule deer have enzyme-rich saliva and specialized digestive organs that enable the deer to digest tough plants inedible to other deer species. (B) Mule deer herds that inhabit cleared forests tend to have more females with young offspring and fewer adult males than do other mule deer populations. (C) Mule deer populations are spread throughout western North America and inhabit hot, sunny climates as well as cool, wet climates. (D) As plants receive more sunlight, they produce higher amounts of tannins, compounds that inhibit digestion of the plants' proteins. (E) Insect parasites, such as certain species of ticks, that feed primarily on mule deer often dwell in trees, from which they drop onto passing deer.

Short answer

Option (D) resolves the paradox.

Step-by-step solution

Understand the Paradox

The situation presents a paradox where mule deer, despite having more leafy shrubs (their food source) in cleared forests, are less well-nourished compared to those in old-growth forests. This paradox needs to be resolved by identifying a plausible explanation.

Interpret the Effect of Clearing Forests

Clearing old-growth forests allows more sunlight to reach the forest floor, promoting the growth of leafy shrubs. This is expected to improve the food availability for mule deer, yet their nourishment is less than expected.

Evaluate Option (A)

Option (A) suggests that mule deer can digest tough plants thanks to enzyme-rich saliva and specialized organs. This does not address why deer are less nourished despite more food.

Evaluate Option (B)

Option (B) notes demographic changes in the herd populations, such as more females with offspring and fewer adult males, but it doesn't directly relate to the issue of nutrition.

Evaluate Option (C)

Option (C) discusses the wide distribution of mule deer but does not address the specific nutrition issue related to cleared versus old-growth forests.

Evaluate Option (D)

Option (D) explains that plants exposed to increased sunlight produce more tannins, which inhibit protein digestion in the plants. This could mean that deer in cleared forests struggle to digest these leafy shrubs fully, leading to poorer nutrition.

Evaluate Option (E)

Option (E) involves the presence of insect parasites from trees, which could affect the health of deer, but this doesn't directly explain the paradox about food and nutrition.

Conclusion and Final Choice

Option (D) best explains the paradox: the increase in tannins due to more sunlight in cleared areas could make the shrubs less digestible, leading to poorer nutrition for the deer.

Key concepts

Old-Growth Forests

Old-growth forests are unique ecosystems characterized by their age, physical features, and biodiversity. These forests contain large and old trees, provide rich habitats, and support numerous species.
Old-growth forests remain largely untouched and continue to thrive because of their natural balance. They have thick canopies that regulate the amount of sunlight reaching the forest floor, which can affect the plant life and animals depending on them.
These forests play a crucial role in supporting species like the mule deer. The consistent environment helps provide stable food sources, climate conditions, and minimizes extreme environmental changes. When forests are cleared, the balance of this ecosystem suffers, influencing various species and their interactions.

Mule Deer Nutrition

Mule deer depend heavily on leafy shrubs for their diet, sourcing essential nutrients to stay healthy and active. They have adapted to digest tough plant materials using specialized enzymes.
In old-growth forests, the shrubs that deer feed on are not only abundant but also nutrient-rich. The quality of the shrubs can significantly influence the overall health and nutrition of the mule deer population.

• Enzymes help break down complex plant materials.
• Proper nutrition leads to healthier and more robust deer.
• Mule deer rely on a consistent supply of nutritious plants.

When these forests are cleared, the sudden increase in shrubs does not necessarily equate to better nutrition, especially if the plants have higher tannin levels.

Environmental Factors

Environmental factors such as sunlight, temperature, and soil condition play a vital role in the growth of plants that support wildlife.
In old-growth forests, the stable environment supports the balanced growth of shrubs. These shrubs have a consistent nutritional profile, crucial for the mule deer's health.
When forests are cleared, more sunlight reaches the forest floor, potentially altering the chemical composition of plant life. For instance, as discussed in the original exercise, plants may produce more tannins.

• Additional sunlight can change plant structures.
• Increased tannins can limit protein absorption in deer.
• Environmental stability is crucial for maintaining healthy wildlife populations.

Changes in these factors can, therefore, lead to differences in deer nutrition.

Critical Thinking

Critical thinking is essential when resolving paradoxes and understanding complex ecosystems. It involves evaluating information, recognizing assumptions, and distinguishing between facts and hypotheses.
The exercise presents a puzzle where, despite apparent food abundance, mule deer are less nourished in cleared forests. Resolving this requires critical thinking to identify underlying causes, such as higher tannin levels in the plants, preventing adequate digestion.
Critical thinking steps include:

• Analyzing the facts and questioning assumptions.
• Evaluating each option methodically and interpreting its relevance.
• Recognizing indirect relationships and seeking more information if needed.

By using critical thinking, we can understand the broader impacts of environmental changes on animal nutrition.