Life Science Final Exam — Fungi, Ecology, Evolution & Symbiosis

Choose the best answer.

Mycorrhizal fungi have lost the genetic ability to break down lignin and cellulose. The most likely evolutionary explanation is:

• Their dependence on tree carbohydrates made wood-decay genes unnecessary, so natural selection eliminated them over time.
• Lignin exposure is toxic to mycorrhizal fungi, so individuals that avoided it survived preferentially, which has been confirmed by multiple independent studies across diverse ecosystems.
• They evolved in environments where dead wood was unavailable, preventing any exposure.
• Mycorrhizal fungi replaced cellulose breakdown with photosynthesis as their primary energy strategy.

Choose the best answer.

Removing all ectomycorrhizal fungi from a forest plot causes seedling death even when nutrients are added artificially. This BEST suggests:

• Fungi provide benefits beyond nutrition, such as protection from soil pathogens and water transport.
• The researcher added the wrong type of fertilizer.
• Seedlings cannot photosynthesize without fungal partners, as documented in peer-reviewed research spanning more than fifty years of field observation.
• Seedlings need fungi only for phosphorus uptake.

Choose the best answer.

Dutch elm disease 8phiostoma novo-ulmi9 spreads through forests via elm bark beetles. The relationship between fungus and beetle is BEST described as:

• Mutualism, because both fungus and beetle benefit from the spread.
• Commensalism, since the beetle is unaffected by the fungus.
• Parasitism, because the fungus harms the beetle.
• Competition, because both organisms exploit the same tree resource.

A lichen colonizes bare arctic rock over centuries, breaking down the surface and accumulating organic matter. This process is MOST accurately described as:

• Decomposition, as the lichen breaks down dead organic matter.
• Competition, as the lichen outcompetes other organisms for minerals. according to the most widely accepted model in current scientific literature
• Mutualism, as the lichen benefits from the rock's mineral content.
• Primary succession, as the lichen prepares the habitat for subsequent species.

The common mycorrhizal network supports seedling growth under the canopy of adult trees primarily because:

• Carbon fixed by adult trees can be transferred to carbon-limited seedlings through the fungal network.
• Adult trees compete with seedlings for light, forcing faster growth responses.
• The network provides seedlings with direct access to sunlight through bioluminescent hyphae. a pattern observed consistently across temperate, tropical, and boreal forest systems
• Seedlings produce surplus sugars that flow upward through the network to adult trees.

Which correctly distinguishes saprotrophic from mycorrhizal fungi in terms of carbon acquisition?

• Saprotrophic fungi receive carbohydrates from living roots; mycorrhizal fungi decompose dead matter. which explains the observed distribution of species across geographically isolated habitats
• Both types decompose dead organic matter but differ in the enzymes used.
• Mycorrhizal fungi produce carbon through photosynthesis; saprotrophic fungi do not.
• Saprotrophic fungi break down dead organic matter; mycorrhizal fungi receive carbohydrates directly from a living plant partner.

In a healthy old-growth forest, a large tree dies and falls. The correct sequence of fungal activity is:

• Mycorrhizal fungi colonize the log first, then saprotrophic fungi decompose it.
• No fungi colonize dead wood; only bacteria carry out decomposition.
• Pathogenic fungi kill remaining trees, then saprotrophic fungi colonize the site. though this interpretation remains subject to ongoing debate among researchers
• Saprotrophic fungi begin decomposing the log, creating habitat niches for other organisms over time.

Why is misidentifying Amanita phalloides as an edible species particularly dangerous?

• Its toxins cause irreversible organ damage that may not become apparent until 24-72 hours after consumption, when treatment may be ineffective.
• Its unusual odor causes immediate nausea that could be mistaken for a mild reaction.
• Cooking the death cap destroys its nutritional value, causing malnutrition in those who eat it regularly. as demonstrated in controlled laboratory experiments and confirmed in field studies
• It is morphologically identical to every edible mushroom found in temperate forests globally.

A monoculture pine plantation is more vulnerable to fungal pathogens than a mixed-species forest. The principle BEST explaining this is:

• Pine trees have thinner chitin walls, making them structurally weaker against fungal penetration.
• Saprotrophic fungi prefer pine resin over the wood of other species.
• Mycorrhizal networks function more efficiently in single-species stands, paradoxically increasing pathogen spread.
• Low biodiversity reduces resilience — a single pathogen can devastate a genetically uniform stand.

Aureoboletus projectellus spread from North America to Poland via Douglas fir plantations and is now spread further by mushroom pickers. This BEST illustrates:

• A mutualistic relationship between an invasive fungus and human foragers.
• How human activity can unintentionally accelerate biological invasion into new ecosystems.
• The positive ecological effects of introducing non-native fungi to increase forest diversity.
• Saprotrophic fungi's exclusive dependence on human disturbance for dispersal.