- Which of these populations is most likely to exhibit exponential growth?
Answer: A colony of bacteria newly inoculated onto a petri dish with abundant nutrients.
Exponential growth (J-shaped curve) occurs when there are no limiting factors on a population's growth. This is typical for populations in new or newly available environments with unlimited resources and space. A new bacterial colony on a nutrient-rich agar plate is the classic example. The other scenarios all involve significant limiting factors (stable prey, competition for light, limited territory) that would lead to logistic growth.
- In an ecosystem, a primary producer has a biomass of 10,000 kg. A primary consumer feeds on it, and a secondary consumer feeds on the primary consumer. Assuming a 10% trophic efficiency, what is the biomass of the secondary consumer that can be supported?
Answer: 100 kg
Energy and biomass decrease at each successive trophic level. With a 10% efficiency, the biomass of the primary consumers would be 10% of the producers' biomass: 10,000 kg 0.10 = 1,000 kg. The biomass of the secondary consumers would be 10% of the primary consumers' biomass: 1,000 kg 0.10 = 100 kg. Thus, 100 kg of secondary consumer biomass can be supported.
- The individuals of a plant species that is a poor competitor for soil nutrients but requires high light levels are most likely to exhibit which spatial distribution pattern?
Answer: A clumped distribution in forest gaps where sunlight can reach the ground.
The plant's survival depends on a critical resource (sunlight) that is not uniformly available in a forest. It will only be able to grow in light-filled gaps. This leads to individuals being aggregated, or clumped, in these suitable patches. This is a classic example of a resource-driven clumped distribution.
- A population introduced to a new environment with limited resources is expected to follow a logistic growth curve. What happens as the population size (N) approaches the carrying capacity (K)?
Answer: The birth rate and death rate become approximately equal.
As N approaches K, resources become scarcer, competition increases, and predation or disease may become more prevalent. These density-dependent factors cause the birth rate (natality) to decrease and the death rate (mortality) to increase. At K, the population growth rate is zero, which means natality equals mortality, and the population is at a stable equilibrium.
- In a predator-prey cycle, the population of the predator often lags behind the population of the prey. What is the primary biological reason for this time lag?
Answer: It takes time for the increased energy intake from abundant prey to be converted into predator offspring.
The lag exists because population growth is not instantaneous. When the prey population increases, it provides more food for the predators. This increased energy source leads to higher predator survival and reproduction. However, there is a delay associated with gestation periods and raising young to maturity. This biological delay means the predator population's growth response occurs after the prey population has already increased.
- A population of yeast cultured in a flask with a fixed amount of nutrient broth exhibits logistic growth. At which point is the population growth rate the highest?
Answer: When the population size (N) is approximately half the carrying capacity (K/2).
In the logistic growth model, the rate of population increase is greatest at the inflection point of the S-shaped curve. This occurs when the population size is half the carrying capacity (N = K/2). At this point, there is an optimal balance between the number of reproducing individuals and the availability of resources, before density-dependent limiting factors become severe.
- The mortality rate of a certain fish population increases significantly as the population density in a lake rises. Which of the following is the most likely cause of this density-dependent mortality?
Answer: A depletion of the available food supply for the fish.
Density-dependent factors are those that have a greater effect as population density increases. Food supply is a classic example. At low densities, food is plentiful. As density increases, competition for the limited food intensifies, leading to starvation and a higher mortality rate. The other options describe density-independent factors (temperature, pollution) or factors not directly related to density (a non-contagious disorder).
- In many legume plants, Rhizobium bacteria inhabit root nodules. The bacteria fix atmospheric nitrogen into a form the plant can use, and the plant provides the bacteria with carbohydrates from photosynthesis. This relationship is best described as:
Answer: Mutualism, because both the plant and the bacteria derive a significant benefit from the interaction.
This is a classic example of mutualism, a type of symbiotic relationship where both species benefit. The plant receives usable nitrogen, a crucial limiting nutrient, and the bacteria receive carbohydrates (energy) and a protected environment. Since both organisms gain a significant advantage, the interaction is mutualistic.
- A biologist is using quadrat sampling to estimate the population of a rare orchid in a forest. To minimize bias and ensure the estimate is as accurate as possible, what is the most important consideration in the experimental design?
Answer: Establishing a grid over the area and using a random number generator to select quadrat coordinates.
The cornerstone of representative sampling is the avoidance of bias. Random sampling ensures that every part of the study area has an equal chance of being selected. Using a grid and random number generator is the standard method for achieving this. Placing quadrats deliberately (A) introduces severe bias. While quadrat size (B) is a factor, random placement is more critical for accuracy. Transects (D) are useful for studying gradients but may not be representative of the entire area.
- Which set of characteristics is most representative of a K-selected species?
Answer: Long lifespan, extensive parental care, and a population size near carrying capacity.
K-selected species are adapted to stable environments where population size is near the carrying capacity (K). Their life history traits reflect this, typically including long lifespan, slow development, fewer but larger offspring, and extensive parental care to ensure high offspring survival. Elephants and humans are classic examples.
- In a capture-mark-recapture study, 80 deer were marked and released. A month later, in a second sample of 100 deer, 20 were found to be marked. What is the estimated deer population size?
Answer: 400
The population size (N) is estimated using the Lincoln index: N = (number marked in first sample × total number in second sample) / number of marked recaptures in second sample. N = (80 × 100) / 20. N = 8000 / 20. N = 400. Therefore, the estimated population size is 400 deer.
- An ecologist performs a chi-squared test to determine if two plant species are associated. The null hypothesis is that there is no association. The calculated chi-squared value is 9.49, and the critical value from the table at p=0.05 is 3.84. What is the correct interpretation of this result?
Answer: The null hypothesis is rejected; there is a significant association between the species.
In a chi-squared test, if the calculated value is greater than the critical value, the null hypothesis is rejected. Here, 9.49 > 3.84. Therefore, the null hypothesis of no association is rejected. This means there is a statistically significant association between the two species (they are either found together more often or less often than expected by chance).
- A large herbivore grazes on grass. In the process, it stirs up insects from the grass, which are then eaten by cattle egrets (a type of bird) that follow the herbivore. The herbivore is unaffected by this activity. What are the interactions between the herbivore and the egret, and the egret and the insects?
Answer: Commensalism and Predation
The relationship between the herbivore and the egret is commensalism: the egret benefits (gets food) while the herbivore is unaffected. The relationship between the egret and the insects is predation: the egret (predator) consumes the insects (prey). Therefore, the correct pair of interactions is commensalism and predation.
- Two species of finches live on the same island and feed on seeds of similar sizes. Over time, one species evolves a slightly larger beak to specialize on larger seeds, while the other evolves a slightly smaller beak to specialize on smaller seeds. This allows them to coexist. What ecological concept does this scenario illustrate?
Answer: Character displacement, a form of resource partitioning that reduces interspecific competition.
This scenario describes character displacement, an evolutionary divergence of characteristics in two competing species where their ranges overlap. This divergence reduces competition by facilitating resource partitioning (using different resources), allowing for coexistence. It is a specific outcome of interspecific competition, avoiding competitive exclusion.
- A remote island has a stable population of ground-nesting birds. A volcanic eruption covers a large part of the island in a thick layer of ash, destroying habitats. Which statement best analyzes the limiting factors affecting the bird population immediately after the eruption?
Answer: The eruption is a density-independent factor, but the resulting scarcity of nesting sites will become a density-dependent factor.
The volcanic eruption is a catastrophic event whose initial impact is independent of the population density. However, the consequence of this event is a reduction in a critical resource (nesting sites). The increased competition among the surviving birds for these few remaining sites is a density-dependent limiting factor, as its effect will be more severe at higher population densities.
- In a forest ecosystem, a species of fungus grows on the roots of an oak tree. The fungus benefits by absorbing sugars from the tree. The oak tree benefits because the fungal hyphae greatly increase the surface area for water and mineral absorption. Which term best describes this interaction?
Answer: Mycorrhiza, a form of mutualism.
The relationship described is a mycorrhizal association, which is a specific type of mutualistic symbiosis between a fungus and a plant root. Both organisms benefit significantly: the fungus gets carbohydrates, and the plant gets improved nutrient and water uptake. This makes it a clear case of mutualism.
- A species of barnacle can physiologically tolerate living in both deep and shallow water (its fundamental niche). However, due to predation by a starfish that lives only in deep water, the barnacle is only found in shallow water (its realized niche). What would happen if the starfish predator were removed from the ecosystem?
Answer: The barnacle's realized niche would expand to become the same as its fundamental niche.
The fundamental niche represents the full range of environmental conditions under which a species can survive and reproduce. The realized niche is the portion of the fundamental niche that is actually occupied, limited by interactions like competition and predation. Removing the predator eliminates the biotic constraint, allowing the barnacle population to expand into the deep water, making its realized niche equal to its larger fundamental niche.
- A community consists of grass, rabbits that eat grass, and foxes that eat rabbits. A prolonged drought severely reduces the grass population. What is the most likely immediate effect on the rabbit and fox populations?
Answer: The rabbit population will decrease, followed by a decrease in the fox population.
This describes a 'bottom-up' control scenario. The drought reduces the producer (grass) population. This leads to a decrease in the food source for the primary consumer (rabbits), causing the rabbit population to decline due to starvation and lower birth rates. Subsequently, the secondary consumer (foxes) will face a shortage of their food (rabbits), leading to a decrease in the fox population as well.
- A sea otter has a disproportionately large effect on its marine community by controlling sea urchin populations, which would otherwise overgraze kelp forests and destroy the habitat for many other species. The sea otter's biomass is not particularly high. This is an example of the role of a:
Answer: keystone species.
A keystone species is a species that has an effect on its community that is disproportionately large relative to its biomass or abundance. The sea otter's role in maintaining the entire kelp forest ecosystem by preying on urchins is a classic example. A dominant species is the most abundant, while a foundation species creates or defines an ecosystem (like kelp itself).
- Researchers use capture-mark-recapture to estimate a fish population in a lake. In the first sample, 200 fish are marked and released. In the second sample, 250 fish are captured, of which 50 are marked. It is later discovered that the marks made the fish more visible to predators, increasing their mortality rate compared to unmarked fish. How does this violation of assumptions affect the population estimate?
Answer: The estimate is an overestimate of the true population size.
The Lincoln index formula is N = (n1 n2) / m. The initial estimate is (200 250) / 50 = 1000. Increased mortality of marked fish means that the proportion of marked fish in the overall population decreases between samples. Consequently, fewer marked fish are caught in the second sample (m is artificially low) than would be expected if mortality were equal. A smaller denominator (m) in the formula leads to a larger calculated N, thus the population size is overestimated.
