Community Ecology

Populations rarely, if ever, live in isolation from populations of other species. In most cases, numerous species share a habitat. The interactions between these populations play a major role in regulating population growth and abundance. All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time. The number of species occupying the same habitat and their relative abundance is known as species diversity. Areas with low diversity, such as the glaciers of Antarctica, still contain a wide variety of living things, whereas the diversity of tropical rainforests is so great that it cannot be counted. Ecology is studied at the community level to understand how species interact with each other and compete for the same resources.

Predation and Herbivory

Perhaps the classical example of species interaction is predation: the consumption of prey by its predator. Nature shows on television highlight the drama of one living organism killing another. Populations of predators and prey in a community are not constant over time: in most cases, they vary in cycles that appear to be related. The most often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), using nearly 200 year-old trapping data from North American forests, shown below. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, they kill so many hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase again due, at least in part, to low predation pressure, starting the cycle anew.

Extended description

The chart’s y-axis is labeled ‘Thousands of animals’ and runs from 0 to just over 140, with gridlines at 50 and 100; the x-axis is labeled ‘Time (years)’ and is marked at 1845, 1865, 1885, 1905, and 1925, extending to roughly 1935. A legend in the upper right marks the red-filled curve ‘Hare (red)’ and the pale blue-filled curve ‘Lynx (blue).’ Both curves rise and fall together across the plotted span: each time the hare curve climbs from a low trough near the bottom of the chart to a peak — the tallest peaks, just after 1865 and just before 1885, reach close to the top of the chart — then drops steeply back down; the lynx curve follows the same up-and-down rhythm a little later than the hare curve and never climbs as high, so the red hare curve is visible above the blue lynx curve through most of each cycle.

Some researchers question the idea that predation models entirely control the population cycling of the two species. More recent studies have pointed to undefined density-dependent factors as being important in the cycling, in addition to predation. One possibility is that the cycling is inherent in the hare population due to density-dependent effects such as lower fecundity (maternal stress) caused by crowding when the hare population gets too dense. The hare cycling would then induce the cycling of the lynx because it is the lynxes’ major food source. The more we study communities, the more complexities we find, allowing ecologists to derive more accurate and sophisticated models of population dynamics.

Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.

Defense Mechanisms against Predation and Herbivory

The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten. The figure below shows some organisms’ defenses against predation and herbivory.

Many species use physical appearance, such as body shape and coloration, to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary against a background of real twigs. In another example, the chameleon can, within limitations, change its color to match its surroundings. Both of these are examples of camouflage, or avoiding detection by blending in with the background. There are many behavioral adaptations to avoid or confuse predators. Playing dead and traveling in large groups, like schools of fish or flocks of birds, are both behaviors that reduce the risk of being eaten.

Some species use coloration as a way of warning predators that they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemicals, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration.

While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation.

In Müllerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. The figure below shows a variety of foul-tasting butterflies with similar coloration. In Emsleyan/Mertensian mimicry, a deadly prey mimics a less dangerous one, such as the venomous coral snake mimicking the nonvenomous milk snake. This type of mimicry is extremely rare and more difficult to understand than the previous two types. For this type of mimicry to work, it is essential that eating the milk snake has unpleasant but not fatal consequences. Then, these predators learn not to eat snakes with this coloration, protecting the coral snake as well. If the snake were fatal to the predator, there would be no opportunity for the predator to learn not to eat it, and the benefit for the less toxic species would disappear.

Competitive Exclusion Principle

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown below, with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

Extended description

Panel (a), titled ‘P. aurelia alone,’ plots number of cells (0 to 300) against time in days (0 to 20): the curve climbs steeply from 0 to about 175 cells by day 4, then more gradually to a peak near 270 cells around day 14, before dipping slightly to settle near 250 by day 16. Panel (b), titled ‘P. caudatum alone,’ uses the same day axis but a number-of-cells axis running only 0 to 80: the curve rises unevenly to about 40 cells by day 4, climbs to a peak near 70 cells by day 10, dips to about 53 cells by day 14, then recovers slightly to around 57–60 by day 16. Panel (c), titled ‘Both species grown together,’ plots both species on one 0–250 cell axis: the P. aurelia curve climbs to about 140 cells by day 4 and continues rising to roughly 225 cells by day 14, staying high through day 16; the P. caudatum curve rises only to about 25 cells by day 3–4, then declines steadily toward zero by day 16.

This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different microniches. These organisms coexist by minimizing direct competition.

Symbiosis

Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the broader definition will be used.

Commensalism

A commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship, shown below. The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators. Another example of a commensal relationship is the pilot fish and the shark. The pilot fish feed on the leftovers of the host’s meals, and the host is not affected in any way.

Mutualism

A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut. The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite. Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria. As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae.

Parasitism

A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits, but the host is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle by spreading to another host.

The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat is consumed, shown below. The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is eating, and may grow to be over 50 ft long by adding segments. The parasite moves from species to species in a cycle, making two hosts necessary to complete its life cycle.

Another common parasite is Plasmodium falciparum, the protozoan cause of malaria, a significant disease in many parts of the world. Living in human liver and red blood cells, the organism reproduces asexually in the gut of blood-feeding mosquitoes to complete its life cycle. Thus malaria is spread from human to human by mosquitoes, one of many arthropod-borne infectious diseases.

Extended description

The cycle has six numbered steps, each set in its own small icon, joined by parallel blue and red arrows (one pathway through pigs, one through humans) that mostly run alongside each other. Step 1, at the bottom in a boxed icon of a round tapeworm egg beside a rectangular tapeworm segment, reads ‘Eggs or tapeworm segments in feces are passed into the environment.’ Two arrows lead up to step 2, an outline of a pig beside a small egg icon, reading ‘Eggs or segments are ingested by pigs or humans.’ Arrows continue upward to step 3, a circular hatching-embryo icon, reading ‘Tapeworm embryos hatch, penetrate the intestinal wall, and circulate to musculature in pigs or humans,’ which leads to an oval muscle-tissue icon showing an embedded larva, labeled ‘Embryos develop into larvae in muscles of pigs or humans.’ From there, one blue arrow leads to a human figure at the upper right whose brain, throat, lungs, and digestive tract are highlighted, labeled ‘Cysts may develop in any organ, and are most common in subcutaneous tissue as well as in the brain and eyes’; a second arrow leads down to a central human figure with the same organs highlighted, marking step 4, ‘Humans acquire the infection by ingesting raw or undercooked meat from an infected animal host.’ A line from that figure’s intestine marks step 5, ‘The tapeworm attaches itself to the intestine via hooks on the scolex,’ with a separate hook-and-sucker icon labeled ‘Scolex’ drawn to the right of the figure. Step 6, below the figure, reads ‘Adults in small intestine’ beside a coiled strip of tapeworm segments and an egg, and a blue arrow leads from it back down to the step 1 box, closing the cycle. Outside the numbered cycle, at lower right, is a black-background photograph of several long, pale, ribbon-like preserved tapeworm specimens laid out beside a measuring scale bar.

Characteristics of Communities

Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively.

Foundation Species

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, or brown algae, is a foundation species, forming the basis of the kelp forests off the coast of California.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef, shown below. Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

Biodiversity, Species Richness, and Relative Species Abundance

Biodiversity describes a community’s biological complexity: it is measured by the number of different species (species richness) in a particular area and their relative abundance (species evenness). The area in question could be a habitat, a biome, or the entire biosphere, the sum of all ecosystems. Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the globe, shown below. One factor in determining species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, which often have warmer temperatures, large amounts of rainfall, and low seasonality. The lowest species richness occurs near the poles, which are much colder, drier, and thus less conducive to life in recent geologic time (time since glaciations). The predictability of climate or productivity is also an important factor. Other factors influence species richness as well. For example, the study of island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galápagos Islands that inspired the young Darwin. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest relative abundance of species.

Extended description

The map’s legend, titled ‘Number of mammal species per sq km,’ lists nine shaded classes from lightest to darkest: 0, 1–23, 24–42, 43–60, 61–93, 94–128, 129–154, 155–178, and 179–228. On the map, North America is shaded in the lighter classes throughout, lightening further toward the Arctic; Central America and the Amazon basin of South America carry the darkest shading (155–228 species per square kilometer), with shading lightening again toward the southern tip of South America. A north-pointing arrow and a 500 km scale bar sit in the upper left.

Keystone Species

A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species, shown below. Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.

Everyday Connection. Invasive Species

Invasive species are nonnative organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. Many such species exist in the United States, as shown below. Whether enjoying a forest hike, taking a summer boat trip, or simply walking down an urban street, you have likely encountered an invasive species.

One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.

Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to their extinction. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed on, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become the most predominant, effectively outcompeting native fishes for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community’s biomass. Although edible, the fish is bony and not a desired food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.

The Great Lakes and their prized salmon and lake trout fisheries are also being threatened by these invasive fish. Asian carp have already colonized rivers and canals that lead into Lake Michigan. One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.

The issues associated with Asian carp show how population and community ecology, fisheries management, and politics intersect on issues of vital importance to the human food supply and economy. Socio-political issues like this make extensive use of the sciences of population ecology (the study of members of a particular species occupying a particular area known as a habitat) and community ecology (the study of the interaction of all species within a habitat).

Community Dynamics

Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.

Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly exposed or newly formed land is colonized by living things; in secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain.

Primary Succession and Pioneer Species

Primary succession occurs when new land is formed or rock is exposed: for example, following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually being formed. On the Big Island, approximately 32 acres of land is added each year. First, weathering and other natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species, shown below. These species help to further break down the mineral rich lava into soil where other, less hardy species will grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.

Secondary succession

A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire, shown below. Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, the area will soon be ready for new life to take hold.

Before the fire, the vegetation was dominated by tall trees with access to the major plant energy resource: sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species. After the fire, though, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed within a few years by quickly growing and spreading grasses and other pioneer species. Due to, at least in part, changes in the environment brought on by the growth of the grasses and other species, over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which will remain stable until the next disturbance.

Extended description

Three framed illustrations run left to right, each linked to the next by a gray right-pointing arrow. The first, captioned ‘Pioneer species — Annual plants grow and are succeeded by grasses and perennials,’ shows a strip of bare brown soil with low grasses and tall yellow-flowered perennial stalks. The second, captioned ‘Intermediate species — Shrubs, then pines, and young oak and hickory begin to grow,’ shows the same strip now covered with low shrubs, several young conifers, and a few young broadleaf trees of increasing height. The third, captioned ‘Climax community — The mature oak and hickory forest remains stable until the next disturbance,’ shows the plot filled with tall, full-canopy oak and hickory trees.

Summary

Communities include all the different species living in a given area. The variety of these species is called species richness. Many organisms have developed defenses against predation and herbivory, including mechanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with other members of the community. Two species cannot exist in the same habitat competing directly for the same resources. Species may form symbiotic relationships such as commensalism or mutualism. Community structure is described by its foundation and keystone species. Communities respond to environmental disturbances by succession (the predictable appearance of different types of plant species) until a stable community structure is established.

Key terms

• aposematic coloration — warning coloration used as a defensive mechanism against predation.
• Batesian mimicry — type of mimicry where a non-harmful species takes on the warning colorations of a harmful one.
• camouflage — avoid detection by blending in with the background.
• climax community — final stage of succession, where a stable community is formed by a characteristic assortment of plant and animal species.
• commensalism — relationship between species wherein one species benefits from the close, prolonged interaction, while the other species neither benefits nor is harmed.
• competitive exclusion principle — no two species within a habitat can coexist when they compete for the same resources at the same place and time.
• Emsleyan/Mertensian mimicry — type of mimicry where a harmful species resembles a less harmful one.
• environmental disturbance — change in the environment caused by natural disasters or human activities.
• foundation species — species which often forms the major structural portion of the habitat.
• host — organism a parasite lives on.
• island biogeography — study of life on island chains and how their geography interacts with the diversity of species found there.
• keystone species — species whose presence is key to maintaining biodiversity in an ecosystem and to upholding an ecological community’s structure.
• Müllerian mimicry — type of mimicry where species share warning coloration and all are harmful to predators.
• mutualism — symbiotic relationship between two species where both species benefit.
• parasite — organism that uses resources from another species, the host.
• pioneer species — first species to appear in primary and secondary succession.
• primary succession — succession on land that previously has had no life.
• relative species abundance — absolute population size of a particular species relative to the population sizes of other species within the community.
• secondary succession — succession in response to environmental disturbances that move a community away from its equilibrium.
• species richness — number of different species in a community.
• symbiosis — close interaction between individuals of different species over an extended period of time that impacts the abundance and distribution of the associating populations.

Practice

Discuss the predator-prey cycle

Give examples of defenses against predation and herbivory

Describe the competitive exclusion principle

Give examples of symbiotic relationships between species

Describe community structure and succession

This section is adapted from Biology 2e, Section 45.6: Community Ecology by Mary Ann Clark, Jung Choi, Matthew Douglas, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: figures re-encoded as WebP; four figures re-kinded from the manifest’s file-extension “photo” guess to “diagram” — Figure_45_06_01 (the lynx/hare population-cycle chart), Figure_45_06_07abc (the three Paramecium growth-curve graphs), Figure_45_06_12 (the mammal species-richness map), and Figure_45_06_16 (the three secondary-succession illustrations) — none of which is a captured photograph; a longdesc added to those four figures plus the tapeworm life-cycle diagram (Figure_B45_06_09ab), each counting and naming its axes, legend classes, panels, or numbered steps in reading order; alt text lightly normalized to a consistent “(a)”/"(b)" panel-lettering style across the multi-panel photographs, with two corrections in the invasive-species panel: the source alt’s “yellow flowers” on the buckthorn, where the photo shows dark berries, and “resembling a cricket” for the emerald ash borer, a beetle — the first reported as a source defect; the interactive note rendered as a Link to Learning callout with the module’s own openstax.org/l/find_the_mimic redirect URL, its anchor text expanded to name the destination; the everyday note rendered as an Everyday Connection callout with its bold name and italic title, its figure and paragraphs kept inside the box as printed; in-text pointers to printed figure numbers replaced with “shown below,” since Hugo does not number figures; all four Review Questions and both Critical Thinking Questions adapted into the closing interactive Practice block, placed under the objective each one tests; rubric checkpoints added to each self-check, decomposing its model answer (the source solution, kept verbatim) into check-off clauses with no new claims; fourteen key-term recall items added from the glossary (aposematic coloration, Batesian mimicry, camouflage, Emsleyan/Mertensian mimicry, commensalism, host, parasite, symbiosis, competitive exclusion principle, foundation species, keystone species, climax community, primary succession, secondary succession); one summary-sourced cloze text-recall item added (“the same ________” → resources); a local sortbins added, sorting five module sentences describing Batesian and Müllerian mimicry into the two mimicry types, as the biology playbook’s prose-based dispensation for this section permits; because the “Discuss the predator-prey cycle” objective has no Review Question, Critical Thinking Question, or glossary term of its own, its Practice group is built entirely from the section’s own predator-prey paragraphs — a multiple choice on the cycle’s length, a self-check on the hare/lynx relationship, and a multiple choice on the alternative density-dependent explanation — disclosed here and in the source ledger. fs-idm10859360 (the mimicry Review Question) prints both “Emsleyan/Mertensian mimicry” and “Mertensian mimicry” as separate options and fs-idp168517216 prints “communism” as a distractor; both are kept as printed, per the source. No source key departures or distractor replacements were needed on this page.