- Two species of desert ants compete for seeds. Where they occur separately (allopatry), both species forage during the cool hours of the early morning. Where they occur together (sympatry), one species continues to forage in the early morning, while the other has shifted to foraging during the hotter, midday period. This shift in foraging time in the sympatric population is an example of:
Answer: niche partitioning allowing for species coexistence.
The correct answer is C. The two species are avoiding direct competition by using the same resource (seeds) at different times. This is a classic example of temporal niche partitioning, an outcome that allows competitors to coexist rather than one excluding the other. Competitive exclusion (A) is what is being avoided. The principle is not violated (B); rather, this is a common result of the selective pressures it describes. The fundamental niches are not expanding (D); instead, the realized niche of one species has shifted within its fundamental niche.
- The concept of an ecological niche is often described as an 'n-dimensional hypervolume', where 'n' is the number of environmental factors important to a species' survival. Which of the following could not be considered a dimension of a species' fundamental niche?
Answer: The intensity of competition from a sympatric species.
The correct answer is B. The fundamental niche is defined by a species' physiological tolerances and resource requirements, independent of biotic interactions. Therefore, abiotic factors like temperature (A), soil moisture (C), and pH (D) are all valid dimensions. In contrast, the intensity of competition is a biotic factor that determines the size and shape of the realized niche by restricting access to parts of the fundamental niche.
- The competitive exclusion principle is most likely to apply and result in the elimination of one species when:
Answer: a key resource for both species is stable and limited in supply.
The correct answer is C. Competitive exclusion operates most strongly when there is intense, constant competition for a resource that is in short supply (i.e., it is limiting). A stable and limited resource prevents the weaker competitor from finding refuge in times or places of temporary abundance. Fluctuating resources (B) can allow for coexistence. High niche differentiation (D) is the opposite of the condition for exclusion; it facilitates coexistence. The relatedness of species (A) is less important than the degree of niche overlap.
- A scientist observes that when two species of fish are in the same pond, they both exhibit stunted growth compared to when they are in separate ponds with identical resources. Which statement is the most valid conclusion?
Answer: The two species are engaged in interspecific competition that reduces their fitness.
The correct answer is B. Stunted growth is a measure of reduced fitness. Since this occurs when they are together but not when they are apart, it strongly indicates that interspecific competition is occurring. The fundamental niche is a physiological property and is not reduced by a competitor (A). If their realized niches were completely separate, they would not compete and their growth would not be affected (C). While competition is occurring, we cannot conclude that extinction is inevitable; they might coexist at lower population densities (D).
- A species is described as a 'specialist'. What does this imply about its ecological niche and its potential response to environmental change?
Answer: It has a narrow fundamental niche and is vulnerable to changes in its key resource.
The correct answer is A. A specialist species has a narrow fundamental niche, meaning it has very specific requirements for food, habitat, or other abiotic factors. This specialization makes it highly efficient in a stable environment but very vulnerable if that specific environment or resource changes. A broad niche is characteristic of a 'generalist' (B). Specialization often evolves to reduce, not increase, interspecific competition (C). The realized niche can never be larger than the fundamental niche (D).
- In a rocky intertidal zone, two species of limpets, Species A and Species B, graze on algae. Species A can tolerate greater exposure to air and desiccation, while Species B is a more efficient grazer and competitor in areas that remain moist. In areas where they coexist, Species A is found only in the upper, drier zone. What does this distribution suggest?
Answer: The realized niche of Species A is restricted by interspecific competition.
The correct answer is B. Species A can physiologically tolerate lower zones (implied by its competition with B there), but is outcompeted by Species B. Therefore, its actual distribution (realized niche) is restricted to the upper zone by this biotic interaction. The fundamental niche is always greater than or equal to the realized niche (making A incorrect). We cannot conclude anything definitive about the relative widths of their fundamental niches from the information given (C). There is no information to suggest hybridization (D).
- An invasive plant species is introduced into a grassland ecosystem. It is a highly effective competitor for soil nitrogen, a key limiting resource for native plant species. What is the most probable immediate effect on the native plant species that also rely on soil nitrogen?
Answer: Their realized niches will contract.
The correct answer is B. The introduction of a superior competitor increases interspecific competition. This pressure will restrict the ability of the native species to access resources and occupy space, causing their realized niches (the conditions they actually live in) to contract. The fundamental niche (A, C), which is the potential range of conditions based on physiology, does not change in the short term. Expansion of the realized niche (D) would occur if a competitor were removed, not added.
- A disease eliminates a species of grass that was the dominant competitor in a meadow. Following its removal, a smaller, less competitive species of wildflower, which was previously rare, becomes abundant and widespread throughout the meadow. This change for the wildflower represents:
Answer: an expansion of its realized niche due to competitive release.
The correct answer is A. The removal of a superior competitor is termed 'competitive release'. This allows the previously suppressed species to access resources and occupy habitats from which it was formerly excluded. This is an expansion of its realized niche, which grows to more fully occupy its pre-existing fundamental niche. The fundamental niche itself (B, D) does not change. The niche expands, not contracts (C).
- A species of barnacle can physiologically tolerate a wide range of water depths and temperatures when raised in a laboratory aquarium free from other species. However, in its natural marine environment, it is only found within a narrow tidal zone, primarily due to the presence of a more aggressive, competing barnacle species in deeper water. The narrow tidal zone where the barnacle is found in nature represents its:
Answer: realized niche, because its distribution is restricted by biotic interactions.
The correct answer is B. The realized niche is the actual range of conditions and resources an organism uses, which is often restricted by biotic interactions like competition. The fundamental niche is the potential range without such restrictions, described by its laboratory tolerance. 'Ecological equivalent' refers to species with similar niches in different geographical locations. 'Resource partition' is a mechanism to avoid competition, not a description of the niche itself.
- Two species of insect larvae live on the same host plant. Species A feeds on the leaves, while Species B feeds on the roots. Which concept does this best illustrate?
Answer: Resource partitioning reducing interspecific competition.
The correct answer is B. The two species are utilizing different parts of the same resource (the host plant). This is a form of resource partitioning that allows them to coexist by minimizing direct competition for food. It is an example of competition being avoided, not competitive exclusion in action (A). Their fundamental niches are clearly not identical, as they are specialized on different parts of the plant (C). There is no information suggesting a predator-prey relationship (D).
- Which statement most accurately distinguishes between a species' habitat and its niche?
Answer: A habitat is the physical place where an organism lives, while a niche is the role it plays in its ecosystem.
The correct answer is A. This is the classic distinction. A habitat is the 'address' of an organism (e.g., a coral reef, a temperate forest floor), whereas a niche is its 'profession' or functional role, which includes its trophic position, resource use, and interactions with other species within that habitat. B reverses the definitions. C is incorrect as a niche encompasses both biotic and abiotic factors. D is incorrect as the terms have distinct meanings.
- A sudden blight eliminates a species of tree that provided the main food source for a specialist insect species. In the absence of this food, the insect begins to feed on a less nutritious, secondary plant species that it previously avoided. How can this change in feeding behaviour be described in terms of ecological niches?
Answer: A shift in the realized niche towards the edge of the fundamental niche.
The correct answer is D. The fundamental niche includes all potential food sources, even less preferred ones. The realized niche was previously focused on the primary food source. The loss of this resource forces the species to utilize other parts of its fundamental niche that were previously unoccupied. This is best described as a shift of the realized niche towards the physiological limits, or edge, of its fundamental niche. It is not an expansion (A), as no new capability is gained. The fundamental niche itself does not contract (B). The shift is driven by resource loss, not a change in competition (C).
- Which of the following scenarios is the clearest example of temporal niche partitioning?
Answer: A hawk species that hunts during the day and an owl species that hunts in the same area at night.
The correct answer is B. Temporal partitioning involves using the same resource at different times. The hawk and owl hunt in the same area (sharing a spatial niche) but at different times (day vs. night). Feeding in different parts of a tree (A) is spatial partitioning. Eating different foods (C) is resource partitioning. Living at different depths (D) is also a form of spatial partitioning.
- What is the key difference between the conditions that lead to competitive exclusion and the conditions that lead to resource partitioning?
Answer: Competitive exclusion occurs when niche overlap is high, while partitioning is a response that reduces niche overlap.
The correct answer is B. Competitive exclusion is the outcome predicted when niche overlap between two species is nearly complete, and competition is intense. Resource partitioning is an evolutionary or behavioural response that allows species to coexist by differentiating their niches and thus reducing the degree of overlap and competition. Both processes involve biotic and abiotic factors (A). Partitioning maintains or increases biodiversity, while exclusion reduces it (C). Both processes are driven by resource scarcity, not abundance (D).
- A field experiment is designed to determine the fundamental niche of a specific plant species, Plantago maritima. Which experimental procedure would be most effective for this purpose?
Answer: Growing Plantago maritima in a greenhouse across a range of abiotic conditions.
The correct answer is B. The fundamental niche is defined by the physiological tolerance of a species to abiotic factors, in the absence of biotic interactions like competition and predation. Growing the plant in a controlled greenhouse environment without competitors allows researchers to determine the full range of physical conditions (temperature, moisture, pH, etc.) under which it can survive and reproduce. Mapping its current distribution (A) determines the realized niche. Removing competitors (D) or herbivores (C) would measure the effect of those specific interactions on the realized niche, but not the full range of abiotic tolerance.
- Two species of finch live on the same island. Species X has a slightly larger beak and eats slightly larger seeds than Species Y. An environmental change causes the extinction of plants producing medium-sized seeds, leaving only very small and very large seeds. Which ecological concept describes the most likely evolutionary response in these finch species over many generations?
Answer: Character displacement, as beak sizes will diverge further to specialize on available seeds.
The correct answer is B. Character displacement is an evolutionary divergence in characteristics (like beak size) that occurs when two competing species co-occur. The pressure to reduce competition for the remaining food sources would likely select for even larger beaks in Species X (to specialize on large seeds) and smaller beaks in Species Y (to specialize on small seeds), causing their traits to diverge. Competitive exclusion (A) is a possible outcome, but character displacement is the specific evolutionary response described. Niche expansion (C) is unlikely as specialization would be favored.
- Two similar species of coral are found on a reef. Species P thrives in high-light, shallow water, while Species Q excels in low-light, deeper water. In the zone where their distributions overlap, the growth rate of both species is significantly reduced compared to their growth rates in their preferred zones. What can be deduced from this observation?
Answer: There is a zone of niche overlap where interspecific competition occurs.
The correct answer is C. The reduced growth rate for both species in the overlap zone is a classic sign of interspecific competition. This indicates that their niches overlap in this intermediate zone and they are negatively impacting each other's fitness. While competition occurs, it may not lead to complete exclusion (A) if they each have a refuge in their preferred zones. Their niches are not completely different, otherwise they would not interact negatively (B). A mutualistic relationship would result in increased, not decreased, growth rates (D).
- If a keystone predator is removed from a community, a prey species that was previously controlled by predation may become a dominant competitor. How would this event affect the realized niches of other species that compete with the newly dominant species?
Answer: Their realized niches would likely contract due to increased interspecific competition.
The correct answer is B. The removal of the keystone predator releases one species from predatory control, allowing its population to grow and become a superior competitor. This increase in competitive pressure on other species will restrict their access to resources and habitats, causing their realized niches to contract. The predator's absence does not directly benefit them (A). Fundamental niches are defined by physiology and do not change in response to short-term ecological shifts (C, D).
- In many ecosystems, different species of predators (e.g., lions and hyenas) often prey on the same species (e.g., wildebeest). This sharing of a food resource indicates that:
Answer: there is an overlap in one dimension of their niches.
The correct answer is B. A niche is multi-dimensional. Sharing a food source means their niches overlap in the 'food resource' dimension. However, they may differ in other dimensions, such as hunting time (temporal), specific hunting locations (spatial), or preference for other prey, which allows them to coexist. Their fundamental niches are unlikely to be identical in all dimensions (A). They are competitors for that shared resource (C). Competitive exclusion does apply to predators, but coexistence is possible if niche overlap is not complete (D).
- According to the competitive exclusion principle, what is the most likely long-term outcome when two different species are forced to occupy the exact same niche within a stable, unchanging ecosystem?
Answer: One species will be eliminated from the ecosystem due to resource competition.
The correct answer is A. The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist indefinitely in the same place; the species with even a slight competitive advantage will eventually drive the other to local extinction. Coexistence (C) is not predicted under these strict conditions. Mutualism (B) and partitioning (D) are ways to avoid or evolve out of direct niche overlap, but the principle itself predicts exclusion if the niche overlap is complete.
