Community Ecology
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AP Biology › Community Ecology
In a temperate pond, larval dragonflies consume mosquito larvae. When dragonflies are experimentally removed from several enclosures, mosquito larvae density increases and grazing on algae by mosquito larvae increases, causing algae biomass to decrease compared with control enclosures. No other species are added or removed, and abiotic conditions are similar across enclosures. Which interaction best explains the initial decrease in mosquito larvae in control enclosures?
Predation by dragonflies reduces mosquito larvae abundance
Commensalism allows mosquito larvae to benefit without affecting dragonflies
Competition between dragonflies and mosquito larvae for algae lowers both
Mutualism between dragonflies and algae increases algae biomass
Parasitism of dragonflies by mosquito larvae reduces mosquito survival
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Predation by dragonflies on mosquito larvae directly reduces mosquito abundance in control enclosures, as dragonflies consume the larvae, limiting their population. When dragonflies are removed, mosquito larvae increase, leading to higher grazing on algae and decreased algae biomass, confirming the predatory relationship. This interaction explains the initial decrease in mosquito larvae, as no other species or abiotic changes account for the difference. A tempting distractor is choice C, which suggests competition for algae, but this is wrong due to the misconception that consumption of one species by another equates to resource competition rather than predation. To identify interactions in community ecology, always examine experimental manipulations and their direct effects on population densities.
In a kelp forest, sea urchins feed on kelp holdfasts, reducing kelp density. In areas where sea otters are common, urchin density is low and kelp density is high. In nearby areas without otters, urchin density is high and kelp density is low. Abiotic conditions are similar across areas. Which interaction best explains the difference in kelp density between areas?
Mutualism between otters and kelp directly increases kelp reproduction
Predation by otters on urchins indirectly increases kelp density
Commensalism allows urchins to benefit without affecting kelp density
Competition between otters and kelp reduces urchin grazing pressure
Parasitism by urchins on otters reduces otter abundance and raises kelp
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Predation by otters on urchins indirectly increases kelp density by reducing urchin populations that graze on kelp, leading to higher kelp in otter-present areas. Without otters, high urchin density lowers kelp, with similar abiotic conditions confirming the trophic cascade. This explains the density differences through top-down control in the food chain. A tempting distractor is choice B, implying competition between otters and kelp, but this is wrong due to the misconception that indirect effects via herbivores are confused with direct competition for resources. To understand trophic interactions, compare predator presence with prey and resource densities across similar environments.
In a kelp forest, sea urchins graze on kelp holdfasts. Sea otters prey on sea urchins. In areas where otters are common, kelp cover is high and urchin density is low. In nearby areas with few otters, urchin density is high and kelp cover is reduced. Wave exposure and water temperature are similar between areas. Which community interaction best explains the difference in kelp cover between the two areas?
Primary succession, with otters initiating kelp establishment on newly exposed rock
Mutualism, with kelp feeding otters and otters feeding kelp through nutrient exchange
Commensalism, with otters unaffected while kelp benefits from urchin presence
Trophic cascade, with otter predation reducing urchins and indirectly increasing kelp
Competition, with otters and kelp competing for space on the seafloor
Explanation
This question assesses the skill of analyzing community ecology by identifying multi-level effects in marine food webs. Sea otters prey on urchins, reducing urchin density and allowing kelp to flourish, exemplifying a trophic cascade where top predator control of herbivores indirectly benefits primary producers. Areas with otters show high kelp cover and low urchins, while otter-scarce areas have the opposite, with similar abiotic conditions confirming the cascade's role. This interaction logic traces the indirect positive effect on kelp through predator-herbivore dynamics. A tempting distractor is C, mutualism, which is wrong because otters and kelp do not directly exchange benefits, stemming from the misconception that all indirect positives indicate direct symbiosis. To uncover cascades, compare ecosystems with varying predator densities to map abundance changes across trophic levels.
In a coral reef, a cleaner fish removes ectoparasites from larger client fish. Reefs with cleaner fish present show lower parasite loads on client fish, and cleaner fish have higher feeding rates when client fish are abundant. When cleaner fish are experimentally excluded from a reef section, client fish parasite loads increase, while client fish density remains similar over the short study period. Which interaction best explains the relationship between cleaner fish and client fish?
Commensalism, because only client fish benefit while cleaner fish feeding is unchanged.
Mutualism, because cleaner fish gain food and client fish experience reduced parasites.
Competition, because both fish species consume the same plankton resources.
Parasitism, because cleaner fish increase parasite loads by transferring parasites to clients.
Predation, because cleaner fish kill clients to obtain parasites as prey.
Explanation
This question tests analysis of community ecology through cleaning symbiosis interactions. The data shows cleaner fish remove parasites from client fish (reducing client parasite loads), while cleaner fish have higher feeding rates when clients are abundant (gaining food resources). This reciprocal benefit where cleaners gain food by removing parasites that harm their clients defines mutualism - both species experience fitness benefits from the interaction. Students often incorrectly choose commensalism (E) thinking only one species benefits, but the data clearly shows cleaners gain food (increased feeding rates) while clients gain parasite removal (reduced parasite loads). To identify mutualism in service-resource exchanges, verify both partners show measurable benefits.
In a freshwater lake, a fungal pathogen infects a dominant zooplankton species. During weeks with high fungal infection, zooplankton grazing pressure decreases and phytoplankton biomass increases, while fish abundance remains stable. In weeks with low infection, zooplankton grazing increases and phytoplankton biomass decreases. Which interaction best explains the reduced zooplankton grazing during high-infection weeks?
Competition between fungus and phytoplankton lowers zooplankton grazing
Commensalism allows fungus to benefit without affecting zooplankton grazing
Parasitism by the fungus reduces zooplankton performance and grazing
Mutualism between fungus and zooplankton increases zooplankton feeding
Predation by zooplankton on fungus reduces zooplankton grazing rates
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Parasitism by the fungus reduces zooplankton performance and grazing, as infection harms the host, leading to decreased grazing pressure and increased phytoplankton biomass during high-infection weeks. Low-infection weeks show opposite trends, with stable fish abundance ruling out predation changes. This explains the grazing reduction, as the fungus benefits at the zooplankton's expense without directly affecting phytoplankton. A tempting distractor is choice C, proposing competition with phytoplankton, but this is wrong due to the misconception that infection effects mimic resource competition, ignoring the host-parasite dynamic. To identify parasitism, correlate infection levels with host performance and cascading ecosystem effects like grazing rates.
In a forest understory, a shrub species grows poorly beneath a canopy tree that releases leaf litter containing inhibitory chemicals. In plots where the tree’s litter is removed weekly but shade remains, shrub growth increases compared with unmanipulated plots. No evidence of herbivory differences is observed between treatments. Which interaction best explains the reduced shrub growth in unmanipulated plots?
Commensalism allows shrubs to benefit without affecting the tree
Predation by shrubs on tree seedlings reduces shrub growth indirectly
Mutualism between tree and shrub increases shrub growth under litter
Competition for light is eliminated when litter is removed
Allelopathic amensalism from the tree suppresses shrub growth
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Allelopathic amensalism from the tree suppresses shrub growth through inhibitory chemicals in leaf litter, harming the shrub without benefiting or harming the tree. Removal of litter increases shrub growth while shade remains, indicating the chemicals, not light competition, cause the suppression. No herbivory differences support that the interaction is chemical-based and one-sided. A tempting distractor is choice E, suggesting competition for light, but this is incorrect due to the misconception that physical factors like shade override chemical inhibition, despite litter removal not altering light. When investigating amensalism, isolate potential mechanisms like chemicals by manipulating specific variables in experiments.
In a tropical rainforest, a species of ant nests in hollow thorns of an acacia tree and patrols the tree’s leaves. When ants are experimentally removed from some trees, leaf-chewing insects increase on those trees and leaf area decreases compared with control trees that retain ants. Soil nutrients and rainfall are similar for all trees. Which interaction best explains the higher leaf area in control trees?
Predation by ants on herbivorous insects reduces leaf damage on acacias
Mutualism between insects and acacias increases leaf loss in controls
Competition between ants and insects reduces herbivory on control trees
Parasitism by ants increases insect feeding and leaf area on controls
Commensalism allows insects to benefit without affecting acacia leaf area
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Predation by ants on herbivorous insects reduces leaf damage on acacia trees, as ants patrol and consume insects, preserving leaf area in controls. Removal of ants increases insects and decreases leaf area, with similar soil and rainfall isolating the protective effect. This interaction benefits the acacia by lowering herbivory, while ants gain nesting sites. A tempting distractor is choice A, suggesting competition between ants and insects, but this is incorrect due to the misconception that consumption is equated to resource competition rather than predation. When evaluating protective interactions, use removal experiments to quantify changes in damage and herbivore abundance for clarity.
In a grassland, flowering plants receive visits from native bees that collect nectar and pollen. In cages that exclude bees but allow wind, plants produce fewer seeds per flower than in uncaged controls, while bee abundance outside cages remains unchanged. Herbivore damage and soil moisture are similar between treatments. Which interaction best explains the higher seed set in the uncaged controls?
Predation by bees on herbivores increases seed set directly
Parasitism by bees decreases plant reproduction in controls
Amensalism by plants reduces bee foraging and raises seed output
Mutualism between bees and flowering plants increases pollination success
Competition between bees and plants for pollen limits seed production
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions based on experimental outcomes. Mutualism between bees and flowering plants enhances pollination, as bees transfer pollen while collecting nectar, leading to higher seed production in uncaged controls. Exclusion of bees reduces seed set, showing that the interaction benefits both the plants (via reproduction) and bees (via food), with no changes in herbivores or moisture. This explains the difference in seed output, as wind alone is insufficient for effective pollination. A tempting distractor is choice B, proposing competition for pollen, but this is wrong due to the misconception that mutual benefits are confused with resource rivalry, ignoring the facilitative nature of pollination. To evaluate mutualistic interactions, assess how exclusion affects reproductive success and resource acquisition in both species.
In a coral reef, cleaner wrasse remove ectoparasites from larger client fish at cleaning stations. Observations over several weeks show that client fish visit stations and then display fewer visible parasites. When cleaner wrasse are experimentally excluded from a section of reef, client fish in that section show increased parasite loads and spend less time feeding. Cleaner wrasse feed primarily on the parasites they remove. Which interaction best describes the relationship between cleaner wrasse and client fish?
Mutualism, with both species benefiting from parasite removal and food acquisition
Predation, with cleaner wrasse killing client fish to obtain parasites
Parasitism, with cleaner wrasse increasing client parasite loads while feeding
Commensalism, with cleaner wrasse benefiting while client fish experience no change
Competition, with both species competing for parasites as a shared resource
Explanation
This question assesses the skill of analyzing community ecology by evaluating symbiotic relationships through observational and exclusion data. Cleaner wrasse remove parasites from client fish, benefiting clients by reducing parasite loads and allowing more feeding time, while wrasse gain food, illustrating mutualism with reciprocal advantages for both species. Exclusion of wrasse increases client parasite loads, confirming the cleaning service's positive impact on client health and the wrasse's reliance on parasites as sustenance. This interaction logic emphasizes mutualism as both parties gaining fitness benefits without harm. A tempting distractor is E, parasitism, which is wrong because wrasse help rather than harm clients, due to the misconception that feeding on another species always constitutes exploitation. To classify symbioses, measure fitness outcomes like health or behavior changes in presence versus absence to confirm mutual benefits.
On a rocky intertidal shore, two barnacle species occupy similar substrate. When species A is removed from small plots, species B expands downward into the lower zone within one month. When species A is present, species B remains mostly in the upper zone even though temperature and wave exposure are similar across zones. Predators were not observed during the study period. Which interaction best explains why species B expands only when species A is removed?
Mutualism, with both barnacles increasing each other’s survival in the upper zone
Interspecific competition, with species A excluding species B from lower-zone substrate
Parasitism, with species B harming species A and spreading after removal
Predation, with species A consuming species B in the lower zone
Facilitation, with species A creating attachment sites required by species B
Explanation
This question assesses the skill of analyzing community ecology by identifying species interactions through removal experiments in zoned habitats. When species A is removed, species B expands into the lower zone, suggesting that species A was competitively excluding species B from preferred substrate, as interspecific competition occurs when one species limits another's access to shared resources like space. The similar environmental conditions across zones indicate that the restriction of species B to the upper zone is due to species A's superior competitive ability in the lower zone, preventing species B from occupying it. This interaction logic demonstrates competition as the mechanism, with species A's presence directly suppressing species B's distribution. A tempting distractor is A, facilitation, which is incorrect because species A harms rather than helps species B, arising from the misconception that one species' dominance always enables another's success. For evaluating distributions, use removal studies to test if one species' absence allows another's expansion, revealing competitive exclusion.