Ecology — Practice Problems with Detailed Solutions

1.Question 1. A population has carrying capacity K equal to one thousand and intrinsic growth rate r equal to zero point five. When population size N equals two hundred, what is the population growth rate dN dt? The logistic growth formula is dN dt equals r N times K minus N over K.

2.Answer: eighty individuals per unit time. Substitute: dN dt equals zero point five times two hundred times eight hundred over one thousand, equals one hundred times zero point eight, equals eighty.

3.Follow up: when is growth rate maximized? At N equals K over two, that is five hundred. At that point dN dt equals zero point five times five hundred times zero point five, equals one hundred twenty-five.

4.Question 2. Which of the following statements about biomagnification is INCORRECT? Options: A, lipophilic toxins are more likely to undergo biomagnification than water-soluble ones; B, top predators have the highest toxin concentration; C, DDT causing eggshell thinning in bald eagles is a biomagnification example; D, biomagnification causes the highest toxin concentration in primary producers.

5.Answer: D is incorrect. The defining feature of biomagnification is that higher trophic levels have higher concentrations — primary producers have the LOWEST. A B C are all correct: lipophilic substances accumulate in fat, top predators accumulate the most, and DDT in bald eagles is the classic case.

6.Question 3. A large tree canopy blocks sunlight, killing the seedlings below. The tree itself is unaffected. What type of interaction is this? Options: A, competition; B, commensalism; C, parasitism; D, amensalism.

7.Answer: D, amensalism. The tree is unaffected zero, the seedlings are harmed minus, so the relationship is zero minus — exactly amensalism. Competition would be minus minus on both sides, and the problem says the tree is unaffected, ruling that out.

8.Question 4. On a new volcanic island formed after eruption, which organisms are most likely to colonize first? Options: A, herbaceous plants; B, lichens and mosses; C, shrubs; D, coniferous trees.

9.Answer: B, lichens and mosses. A volcanic island is brand new substrate with no soil — this is primary succession. Pioneer species must grow on bare rock without soil. Lichens secrete acids that break rock down, gradually forming thin soil; mosses then colonize and accumulate organic matter. Herbaceous plants need some soil depth and represent the second or third stage.

10.Question 5. Regarding thermocline and El Nino, which statement is correct? Options: A, thermocline is a deep layer where temperature changes slowly; B, thermocline promotes mixing between surface and deep water; C, during El Nino years, eastern Pacific upwelling strengthens and fish catches increase; D, during El Nino years, equatorial trade winds weaken and eastern Pacific sea temperature rises.

11.Answer: D. A is wrong because thermocline is the middle layer where temperature changes RAPIDLY. B is wrong because thermocline acts as a barrier that PREVENTS mixing. C is wrong because during El Nino, upwelling WEAKENS — nutrients drop and fish catches DECLINE. D correctly describes: weakened trade winds let warm water flow back east, raising eastern Pacific temperature and weakening upwelling.