- On an island, a single species of finch feeds on a variety of seed sizes. A second species of finch, which specializes in large seeds, is introduced. Over time, the original finch species evolves a smaller beak size and feeds almost exclusively on small seeds. This is an example of:
Answer: Character displacement, resulting from interspecific competition.
Character displacement is an evolutionary divergence that occurs when two similar species inhabit the same environment. Interspecific competition for resources (in this case, seeds) causes the traits of the competing species to evolve in opposite directions, reducing competition. The original finch species evolving a smaller beak to specialize on a different food resource is a classic example of this phenomenon.
- The removal of sea otters, a predator of sea urchins, from a coastal ecosystem led to a dramatic increase in the sea urchin population. This in turn caused the destruction of kelp forests, as urchins graze heavily on kelp. The loss of kelp drastically reduced the biodiversity of the ecosystem. What term best describes the ecological role of the sea otter in this community?
Answer: Keystone species
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The sea otter's role in controlling the urchin population is critical to the health of the entire kelp forest ecosystem. Its removal causes a trophic cascade that leads to a fundamental change in the community structure, demonstrating its keystone role.
- An ecologist wishes to estimate the population of a rare, territorial orchid species that grows in clumps in a large national park. They propose using random sampling with 1m x 1m quadrats. Why might this sampling strategy lead to a highly inaccurate estimate?
Answer: The clumped distribution violates the assumption of random distribution needed for the method's accuracy.
Random quadrat sampling is most accurate when individuals in the population are distributed randomly or uniformly throughout the study area. When a species has a clumped or aggregated distribution, random placement of quadrats is likely to miss the clumps entirely or fall disproportionately within them, leading to a significant underestimation or overestimation of the true population density. For clumped distributions, systematic sampling (e.g., belt transects) is often more appropriate.
- A biologist determined the carrying capacity (K) of a habitat for a certain species to be 1,000 individuals. The population is currently at 950 individuals and growing logistically. Which statement accurately describes the state of the population?
Answer: The population is experiencing strong environmental resistance and its growth rate is low.
In the logistic growth model, as the population size (N) approaches the carrying capacity (K), limiting factors (environmental resistance) become more pronounced. This causes the population growth rate (dN/dt) to slow down significantly. At N=950, the term (1 - N/K) becomes (1 - 950/1000) = 0.05, which is very small. This indicates strong environmental resistance and a very low population growth rate. The maximum growth rate occurs at N=K/2.
- A population of insects is monitored over several years. Its size fluctuates, but a major population crash is observed following an exceptionally harsh and long winter. How should this environmental factor be categorized in terms of population regulation?
Answer: A density-independent factor because the cold affected all insects regardless of population density.
Density-independent factors are typically abiotic, such as weather and natural disasters, and their effect on an individual's probability of survival is independent of the population density. A harsh winter will affect the survival of insects regardless of whether the population is large or small. In contrast, density-dependent factors like disease or food shortage have a greater effect at higher population densities.
- The black walnut tree (Juglans nigra) produces juglone, a chemical that is leached into the soil and inhibits the growth of many other plant species nearby. What type of interspecific interaction does this represent between the walnut tree and a susceptible neighbouring plant?
Answer: Interspecific competition, as the walnut tree is limiting other plants' access to resources.
This interaction, known as allelopathy, is a form of interference competition. The walnut tree releases a chemical that harms its competitors, thereby increasing its own access to resources like water, light, and nutrients. While sometimes classified as amensalism (-/0), within the main IB categories, it is a clear example of interspecific competition, where one species negatively affects another in the struggle for shared resources.
- Creosote bushes in the desert are often observed to be evenly spaced. This is primarily because their extensive root systems compete intensely for water, and they also release chemicals that inhibit the germination of other creosote seeds nearby. This observation is an example of what type of population distribution pattern?
Answer: Uniform distribution, resulting from strong intraspecific competition.
A uniform (or regular) distribution pattern is characterized by individuals being spaced more evenly than would be expected by chance. This pattern often results from strong negative interactions between individuals, such as intense competition for limited resources (like water in a desert) or direct antagonism, like territoriality or allelopathy. The competition among creosote bushes for water and space leads to this evenly spaced pattern.
- In a stable forest ecosystem, a population of deer is primarily limited by food availability and predation by wolves. If a non-native, contagious disease is introduced that is particularly lethal to deer in crowded conditions, how would this new factor be classified and what would be its effect?
Answer: A density-dependent factor that would have a greater impact when the population is large.
A contagious disease is a classic example of a density-dependent limiting factor because its transmission rate, and thus its impact on mortality and fecundity, increases as the population density increases. It does not change the fundamental carrying capacity (K) of the environment (the amount of food and space), but it is a biotic factor whose effect intensifies at higher population densities, helping to regulate the population.
- An ecologist is studying a community where a species of flowering plant is pollinated by a specific bee species. This plant is the primary food source for a beetle larva, and a predatory wasp exclusively hunts these beetle larvae. If a disease dramatically reduces the population of the predatory wasps, what is the most probable indirect effect on the bee population?
Answer: The bee population will decrease due to a reduction in the number of healthy flowering plants.
This question requires tracing a trophic cascade. 1) A decrease in predatory wasps leads to an increase in their prey, the beetle larvae. 2) An increase in beetle larvae, which are herbivores on the plant, leads to more damage and a subsequent decrease in the flowering plant population. 3) A decrease in the flowering plant population reduces the primary food source (nectar/pollen) for the bees. 4) Consequently, the bee population is likely to decrease due to food scarcity.
- Two species of ciliate protozoans, Paramecium aurelia and Paramecium caudatum, were grown in separate cultures and both thrived. When they were grown together in the same culture with a fixed amount of food, the population of P. aurelia thrived while the population of P. caudatum declined and was eventually eliminated. This experiment is a classic demonstration of:
Answer: The competitive exclusion principle
The competitive exclusion principle, also known as Gause's Law, states that two species competing for the same limiting resource cannot coexist at constant population values, if other ecological factors remain constant. In this experiment, P. aurelia is the superior competitor and drives P. caudatum to local extinction when they are forced to compete for the same food source. Resource partitioning would have allowed them to coexist.
- In the classic predator-prey cycles of lynx and snowshoe hares, the hare population cycle typically peaks and declines before the lynx population does. What is the best explanation for this time lag in the lynx population cycle?
Answer: The lynx population can only increase after the hare population provides an abundant food source.
The predator's population dynamics lag behind the prey's. An increase in the prey (hare) population provides more food for the predator (lynx), leading to higher survival and reproduction rates for the lynx. This causes the lynx population to increase, but only after the prey population has already become abundant. Similarly, the subsequent decline in the lynx population occurs after the hare population has crashed, due to the resulting food scarcity.
- A population is exhibiting logistic growth in an environment with a carrying capacity (K) of 800 individuals. The maximum intrinsic rate of natural increase (r) is 0.4. At which of the following population sizes (N) would the rate of population growth (dN/dt) be highest?
Answer: N = 400
For a population following the logistic growth model, the rate of population growth (dN/dt) is maximal at the inflection point of the S-shaped curve. This point occurs when the population size (N) is exactly half of the carrying capacity (K). In this case, K = 800, so the maximum growth rate occurs at N = K/2 = 400. At N=100 and N=750 the growth rate is positive but lower, and at N=800 (at carrying capacity), the growth rate is zero.
- What is a primary difference between interspecific and intraspecific competition?
Answer: Interspecific competition occurs between species, while intraspecific competition occurs within a population.
The core definition distinguishes the two: 'inter-' means 'between' and 'intra-' means 'within'. Interspecific competition is the competition for resources between individuals of two or more different species. Intraspecific competition is the competition for resources among individuals of the same species (within a population). While intraspecific competition is often more intense because the resource needs are identical, this is not a universal rule (distractor B). Both can lead to various outcomes (distractor D).
- Which set of characteristics describes a population of a typical K-selected species near its environment's carrying capacity?
Answer: Stable population size, intense intraspecific competition, and low reproductive output per individual.
K-selected species are adapted to stable environments and tend to live at densities near the carrying capacity (K). At this point, resources are limited, leading to intense intraspecific competition. Population size is relatively stable. Life history traits include fewer offspring (low reproductive output) but higher investment in each, and longer lifespans. The other options describe traits of r-selected species or populations far below K.
- Two species of barnacles, Chthamalus and Balanus, live on a rocky shore. Chthamalus can survive in both the upper and lower intertidal zones when grown alone. However, in nature, it is only found in the upper zone because Balanus outcompetes it in the lower zone. Which statement correctly describes the niches of Chthamalus?
Answer: The entire intertidal zone represents its fundamental niche, and the upper intertidal zone is its realized niche.
The fundamental niche is the full range of environmental conditions and resources an organism can possibly occupy and use, in the absence of competition. For Chthamalus, this is the entire intertidal zone. The realized niche is the part of the fundamental niche that an organism actually occupies as a result of limiting factors present in its habitat, such as competition. Because competition with Balanus restricts Chthamalus to the upper zone, this represents its realized niche.
- In a capture-mark-recapture study to estimate the population of a fish species, researchers notice that the marking process makes the fish 'trap-shy', meaning a marked fish is less likely to enter a trap again compared to an unmarked fish. Assuming all other conditions for the method are met, how would this behavioural change affect the final population estimate?
Answer: The estimate will be an overestimation of the true population size.
The Lincoln index formula is N = (n1 * n2) / m, where n1 is the number initially marked, n2 is the total size of the second sample, and m is the number of marked individuals in the second sample. If marked fish are trap-shy, 'm' will be artificially low because fewer marked fish are recaptured than expected by chance. A smaller denominator ('m') in the equation results in a larger calculated value for 'N', thus leading to an overestimation of the true population size.
- The relationship between mycorrhizal fungi and plant roots is considered a mutualism. The fungus increases the plant's absorption of mineral ions, while the plant provides the fungus with organic compounds. What would be the expected outcome for a plant if the soil was treated with a fungicide that killed its mycorrhizal partner?
Answer: The plant would show signs of nutrient deficiency, especially for phosphorus.
Since the relationship is mutualistic, both partners benefit. The fungus is crucial for enhancing the plant's ability to absorb minerals, particularly immobile nutrients like phosphorus. Removing the fungus would deprive the plant of this benefit, likely leading to stunted growth and symptoms of nutrient deficiency, even if the nutrients are present in the soil. The other options misinterpret the nature of the mutualistic relationship.
- A yeast culture is grown in a nutrient broth. The population size (N) is 400 cells, the carrying capacity (K) is 2000 cells, and the maximum intrinsic rate of increase (r) is 0.5 per hour. Calculate the current population growth rate (dN/dt).
Answer: 160 cells per hour
The logistic growth equation is dN/dt = rN(1 - N/K). Plugging in the given values: dN/dt = 0.5 400 (1 - 400/2000). First, calculate the term in the parenthesis: 1 - (400/2000) = 1 - 0.2 = 0.8. Then, multiply: dN/dt = 0.5 400 0.8 = 200 * 0.8 = 160. Therefore, the current population growth rate is 160 cells per hour. Choice C represents the initial exponential growth rate (rN) without the limiting factor.
- A non-native grass is introduced to a savanna ecosystem. It grows faster than the native grasses and is also toxic to the native large herbivores, which avoid eating it. What are the most likely impacts on the populations of native grasses and native herbivores?
Answer: Both native grass and herbivore populations will decrease.
The non-native grass will outcompete the native grasses for resources like water, light, and nutrients, causing the native grass populations to decrease. This reduction in their primary food source will negatively impact the native herbivore populations. Furthermore, since the new grass is toxic and inedible for them, it effectively reduces the total available food in the ecosystem, putting further pressure on the herbivores. Therefore, both populations are expected to decline.
- Two species of birds forage for insects on the same species of tree. Species A forages on the upper branches and trunk, while Species B forages on the lower branches and near the ground. This pattern allows them to coexist with reduced competition. Which ecological concept is best demonstrated by this example?
Answer: Resource partitioning, where the species divide a niche to avoid direct competition.
Resource partitioning is the division of limited resources by species to help avoid competition in an ecological niche. In this case, the birds are partitioning the spatial resource of the tree, allowing them to coexist. Competitive exclusion is what would happen if they did not partition the resource. Character displacement is an evolutionary outcome of resource partitioning, but the observation itself is of the partitioning.
