Ch 55 — Community Ecology

55.1

Biological Communities: Species Living Together

pp. 1241–1243

Learning outcomes: define community, and describe how community composition changes across a geographic landscape.
A community is the set of species that occur at any particular locality — plants, animals, fungi, protists, and prokaryotes together. Watch Communities on YouTube, from Bozeman Science
Almost every place on Earth is occupied. Some places hold enormous numbers of species (Amazon rainforest); some hold only a few (the near-boiling water of Yellowstone geysers, where a handful of microbial species live).
Two ways to characterize a community: by its constituent species, or by its properties — chiefly species richness (the number of species present) and primary productivity (the amount of energy produced).
Interactions among community members — predation, competition, mutualism — govern ecological and evolutionary processes at once. Watch Community Ecology: Feel the Love - Crash Course Ecology #4 on YouTube, from CrashCourse
Those interactions affect population biology (whether a population grows or shrinks), the cycling of energy and nutrients through the ecosystem, and the patterns of natural selection a species faces.
The last point is the evolutionary payoff: the community context shapes the selective pressures on a species and therefore the evolutionary course it takes.
Communities are studied at many scales — detailed field observation, large-scale manipulative experiments, whole-community surveys, or a focused look at just the subset of species likely to interact. Watch Community Ecology and Landscape Ecology on YouTube, from Professor Dave Explains
The individualistic view is the one the data support: for the most part, species respond independently to changing environmental conditions.
Because responses are independent, community composition usually changes gradually across a landscape — some species appear and grow more abundant while others thin out and disappear.
Classic evidence: tree species abundance in the Santa Catalina Mountains of southeastern Arizona along a gradient from very dry to very moist. Each species has its own abundance curve, largely independent of the others.
The consequence: tree communities at different points on that gradient form a continuum, one merging into the next, rather than a set of discretely different species assemblages.
The same pattern shows up through time. North America has an excellent fossil record for trees and small mammals over the past 20,000 years.
Prehistoric communities show little similarity to today's: many species that occur together now were never found together in the past, and species that once shared a community may not even overlap in range today.
Interpretation: as climate changed with the waxing and waning of the ice ages, species shifted their distributions independently rather than moving together as intact communities.
Sometimes abundance does change synchronously across many species. That happens at an ecotone — a place where the environment changes abruptly.
Textbook ecotone: serpentine soils in the western United States. Serpentine soil has high nickel, chromium, and iron and low copper and calcium compared with normal soil.
Distinct plant communities sit on each soil type, and the transition between them happens over a very short distance — a few meters along a transect (figure 55.3 samples 16 plant species from 5 to 28 m).
Similar abrupt transitions occur wherever greatly different habitats meet: land against water, or grassland against forest.
So both patterns are real. Gradual, individualistic change is the norm; abrupt, synchronous change occurs where the physical environment itself changes abruptly.
The alternative holistic view treats a community as an integrated superorganism whose species arrive and leave together. The chapter's synthesis question asks whether some species could behave holistically while others at the same site behave individualistically.
Exam framing: if a question says species "change independently of one another in space and time," the reason is that their realized niches are regulated by different aspects of the environment — not that they arrived at different times.
A community always has a locality. It is defined by where you draw the boundary, which is why the same forest can be described as one community or as several depending on the question.
Figure 55.1 is the reference image: an African savanna community at Etosha National Park, Namibia — giraffes, zebras, kudu, and birds at one waterhole, all part of one community.
Practice question from this section: over time, would you expect greater variation in the abundance of animal life or of plant life? Animals move and reproduce faster, so their numbers usually swing harder.
Do not confuse species richness (a count) with species composition (which species). Two communities can have identical richness and share no species at all.
Do not confuse primary productivity (energy produced, a rate) with biomass (standing stock). The chapter defines productivity as the amount of energy produced.
Bottom line for 55.1: a community comprises all species that occur at one site; abundances mostly vary independently across space and time; composition changes gradually with environmental factors such as a dry-to-moist gradient.

55.2

The Ecological Niche Concept

pp. 1243–1248

Learning outcomes: define niche and resource partitioning; differentiate fundamental from realized niches; explain how other species affect a species' realized niche.
The niche an organism occupies is the total of all the ways it uses the resources of its environment — space utilization, food consumption, temperature range, moisture requirements, and conditions appropriate for mating. Watch Niche on YouTube, from Bozeman Science
Interspecific competition occurs when two species use the same resource and there is not enough to satisfy both.
Interference competition is physical: fighting to defend a territory, or displacing an individual from a location.
Exploitative competition is indirect: simply consuming the same resources, faster or more efficiently.
The fundamental niche is the entire niche a species is capable of using, based on its physiological tolerance limits and resource needs — the potential.
The realized niche is the actual set of environmental conditions, including the presence or absence of other species, in which the species can establish a stable population. Because of interspecific interactions it is usually considerably smaller than the fundamental niche. Watch Niche on YouTube, from Bozeman Science
Connell's barnacle experiment (Scotland) is the classic demonstration. Chthamalus stellatus lives in shallow water, often exposed to air by the tide; Semibalanus balanoides lives lower, rarely exposed. Watch Interspecific Competition: Connell’s Barnacle Experiment | EcolClips on YouTube, from EcolClips
Where the two meet, space is at a premium. S. balanoides always outcompeted C. stellatus by crowding it off the rocks, undercutting it, and replacing it even where it had already begun to grow — interference competition.
Remove S. balanoides and C. stellatus readily occupies the deeper zone. So no physiological barrier keeps it out; competition does.
S. balanoides, by contrast, cannot survive in the shallow zone — it lacks the tolerance of warmer, drier conditions that C. stellatus has.
Result to memorize: C. stellatus has a fundamental niche spanning both zones and a much narrower realized niche; S. balanoides has a fundamental and realized niche that are identical. Watch Interspecific Competition: Connell’s Barnacle Experiment | EcolClips on YouTube, from EcolClips
Competition is not the only thing that shrinks a realized niche. Predation can. St. John's wort (Hypericum perforatum) spread over open California rangeland until a specialized beetle was introduced; now it survives only in shady sites where the beetle cannot thrive.
The absence of a beneficial species also shrinks a realized niche. Many North American plants depend on insect pollinators — worth more than $9 billion per year to American agriculture — and pollinator populations are declining. If they vanish from a habitat, the realized niche of many plants shrinks or disappears entirely, though nothing about the plant's own tolerances changed.
Gause's Paramecium experiments (1934 and 1935) are the foundation of competition theory. Three species were grown on bacteria and yeasts feeding on oatmeal in culture tubes, and each grew well alone, reaching its own plateau density. Watch Niche and Competitive Exclusion on YouTube, from Mr. P Science
Grown together, P. aurelia and P. caudatum could not coexist: P. caudatum always declined to extinction. P. aurelia could grow about six times faster because it used the limited resource better — exploitative competition.
Principle of competitive exclusion: if two species compete for a limited resource, the species that uses it more efficiently will eventually eliminate the other locally. No two species with the same niche can coexist when resources are limiting. Watch Niche and Competitive Exclusion on YouTube, from Mr. P Science
Gause then paired the loser, P. caudatum, with a third species, P. bursaria, expecting another extinction. Instead both survived, dividing the resources.
Mechanism of that coexistence: in the upper tube, oxygen and bacterial density were high and P. caudatum fed better on bacteria; in the lower tube, low oxygen favored yeast, which P. bursaria ate better. Each had the whole tube as its fundamental niche, but their realized niches were different portions of it. Watch Resource Partitioning on YouTube, from DeBacco University
Competition was still costly: grown without a competitor, both species reached much higher densities than when grown together. Coexistence does not mean competition is absent.
Refined statement: two species can coexist long term only if resources are not limiting, or if their niches differ in one or more features. Otherwise one outcompetes the other and the loser goes extinct locally — or the two evolve differences that reduce competition between them.
MacArthur's warblers: five species of small insect-eating forest songbirds appeared to compete for the same resources, but each fed in a different part of spruce trees — near branch tips, within dense foliage, on lower branches, high in the tree, and at the very apex.
Each warbler had evolved to use a different portion of the same resource. That subdivision is resource partitioning, and it is common among sympatric species (species that occur together), which avoid competition by using different parts of the habitat, different foods, or different microclimates. Watch Resource Partitioning on YouTube, from DeBacco University
Figure 55.6 example: Caribbean Anolis lizards partition a single tree — canopy leaves and branches, peripheral twigs, the base of the trunk, and open grassy areas. Species sharing a zone take different-sized insects or split the thermal microhabitat (one in shade, one basking in the open).
Character displacement is the evolutionary process behind partitioning: natural selection causes initially similar species to diverge in resource use where they co-occur, because individuals most different from the other species escape competition.
How to detect it: compare species where their ranges partially overlap. Sympatric populations should show greater differences in morphology (form and structure) and resource use than allopatric populations of the same species.
Darwin's finches are the case study (figure 55.7). Geospiza fuliginosa and G. fortis have similar beak depths where each is allopatric (Los Hermanos Islets and Daphne Major), but clearly different beak depths where sympatric (San Cristóbal and Floreana) — one adapted to large seeds, the other to small.
Character displacement can drive adaptive radiation, letting new species specialize on different parts of the environment. But caution: shared resource use does not prove competition — the resource may not be limiting.
Even a negative correlation in abundance is not proof. Both species might respond independently to the same environmental factor, one thriving in warm conditions and the other in cool. The strongest evidence is experimental — plots with each species alone and together.
Kangaroo rat experiment (figure 55.8): 50 m × 50 m desert enclosures, kangaroo rats removed from half. Over the next three years, the number of smaller seed-eating rodents was substantially higher where kangaroo rats were absent.
Note the control detail: population sizes rose and fell in synchrony in both treatments, probably tracking weather — the treatment effect is the gap between the lines, not their shape.
Competition also shows up in traits other than population size. On St. Maarten, Anolis gingivinus in 12 m × 12 m enclosures grows faster and perches lower when the ground-dwelling A. pogus is absent.
Limits of field experiments: a negative effect is not automatically competition. Adults of two fish species may each prey on the juveniles of the other, and one species may attract shared predators or parasites that suppress the second. In the kangaroo rat study the enclosures were adjacent, arguing the same predators and parasites were present in all of them.
Some systems cannot be manipulated at all. Coyotes increased across the U.S. over three centuries as grey wolves declined, but fenced replicated experiments on animals with huge home ranges are impractical, and slow-growing trees would take centuries. The fallback is a detailed study of each species' ecological requirements and of the mechanism behind any observed effect.
Bottom line for 55.2: realized niches are smaller than fundamental niches because of interspecific interactions; resource partitioning lets sympatric species coexist while further reducing each one's realized niche.

55.3

Predator–Prey Relationships

pp. 1248–1251

Learning outcomes: define predation, and describe the effects predation can have on a population.
Predation is the consuming of one organism by another. It spans a leopard capturing an antelope and a deer grazing on spring grass — herbivory counts as predation in this chapter. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy
In simple laboratory conditions the predatory protist Didinium exterminates its Paramecium prey and then goes extinct itself, having nothing left to eat (figure 55.9). Prey fall from about 1250 individuals to near zero within 4–5 days.
Add refuges for the Paramecium and the story changes: prey drop to low levels but not to extinction, the starving predator population declines, and then the prey population recovers — the seed of predator–prey cycling. The interaction is a two-way street: predators shape prey dynamics and prey shape predator dynamics. Watch Predator-Prey Interactions, Episode 1: Understanding the Lotka-Volterra Equations on YouTube, from Explorations in Biology, Evolution, and More
Removing predators lets prey explode. Eliminating large carnivores from much of the eastern United States produced white-tailed deer explosions that strip all edible plant life within reach, and hunting sea otters to near extinction on the west coast let their prey, sea urchins, explode.
Adding predators wipes prey out. Introduced rats, pigs, dogs, and cats eat the eggs and young of Galápagos tortoises, endangering island populations.
In New Zealand, rat predation eradicated several bird and reptile species, which now survive only on offshore islands the rats never reached. On one island, a single lighthouse keeper's cat killed every individual of the Stephens Island wren. Predation is such a strong selective force that any feature reducing the chance of capture is strongly favored — which then selects for predator counteradaptations.
The prickly pear in Australia is the biological-control classic: introduced in the 19th century with no predators, it covered 12 million hectares of rangeland by 1925 as an impenetrable morass of spines.
The moth Cactoblastis cactorum, a predator from the cactus's native Argentina, was introduced starting in 1926; by 1940 cactus populations had crashed, and the cactus now persists only in small populations.
Coevolution is the process by which adaptations are selected in lockstep in two or more interacting species; the result is a coevolutionary arms race. Watch Coevolution on YouTube, from Bozeman Science
Plant morphological defenses: thorns, spines, and prickles discourage large plant eaters; glandular, sticky-tipped hairs deter insects; and grasses deposit silica in their leaves, which strengthens them and, in quantity, makes them simply too tough to eat. Watch The amazing ways plants defend themselves - Valentin Hammoudi on YouTube, from TED-Ed
Chemical defenses are even more widespread than morphological ones. Attacked wild tobacco plants emit an airborne chemical that attracts a bug that eats the attacking caterpillar — defense by recruitment.
Secondary chemical compounds are the headline defense. They are distinguished from primary compounds, which are components of major metabolic pathways such as respiration.
Secondary compounds are structurally diverse and are either toxic to most herbivores or disturb their metabolism — for example, preventing normal development of larval insects. Most herbivores simply avoid plants that contain them.
Named example 1: the mustard family (Brassicaceae) makes mustard oils, the pungent aroma and taste of mustard, cabbage, watercress, radish, and horseradish. Humans enjoy them; many insect groups are poisoned by them.
Named example 2: the milkweed family (Asclepiadaceae) and related dogbane family (Apocynaceae) produce a milky sap and usually contain cardiac glycosides, molecules with drastic deleterious effects on vertebrate heart function.
Herbivores counterattack. Some evolve ways of circumventing plant defenses and then feed on those plants exclusively — a resource nobody else can touch.
Cabbage butterfly caterpillars (Pieris rapae) feed almost exclusively on mustard-family and caper-family plants because they can break mustard oils down. Monarch caterpillars and relatives specialize on milkweeds and dogbanes.
The evolutionary story: mustard oils evolved and protected those plants for a time; later, certain insect groups evolved the ability to break them down and gained a resource with no competitors — a setup for evolutionary diversification and adaptive radiation.
Monarchs go one better: they do NOT break the cardiac glycosides down. They concentrate and store them in fat bodies, retaining them through the chrysalis, into the adult, and even into the eggs of the next generation.
So all stages of the monarch life cycle are protected. A blue jay that eats a monarch quickly regurgitates it and thereafter avoids the orange-and-black pattern entirely (figure 55.11). Some bird species have since evolved tolerance and do eat monarchs.
Animals also manufacture their own defensive chemicals: bees, wasps, predatory bugs, scorpions, spiders and many other arthropods, plus many marine invertebrates, frogs, snakes, lizards, fishes, and even some birds.
Poison-dart frogs (family Dendrobatidae) secrete toxic alkaloids in the mucus over their brightly colored skin. Some toxins kill a person from a few micrograms injected into the blood; more than 200 alkaloids have been isolated, several now used in neuromuscular research.
Warning (aposematic) coloration: brightly colored animals advertise their poisonous nature. Showy coloration characterizes animals that use poisons and stings; organisms lacking chemical defenses are seldom brightly colored. Watch Aposematic Coloration on YouTube, from Bozeman Science
Cryptic coloration is the opposite strategy — color that blends with the surroundings and hides the individual, as in the waved umber moth caterpillar that looks like the twig it hangs from.
Follow-on rule worth remembering: camouflaged animals usually do not live in groups, because a predator that finds one individual gains a clue to the presence of the others.
Mimicry lets one species capitalize on another's defense: the mimic evolves to resemble a distasteful model that already carries warning coloration. Watch Batesian vs Mullerian Mimicry | Animal Behaviour on YouTube, from sci-ology
Batesian mimicry — named for Henry Bates, who publicized it in 1857 after finding palatable Amazonian insects that resembled brightly colored distasteful species. Predators fooled by the disguise avoid the mimic.
Diet explains who is model and who is mimic: model butterflies' caterpillars feed on one or a few closely related, chemically protected plant families and incorporate the poison; mimic butterflies' caterpillars feed on many unprotected families.
North American case: the tiger swallowtail (Papilio glaucus) is the mimic. Where the poisonous pipevine swallowtail (Battus philenor) occurs, female tiger swallowtails are polymorphic and one color form closely resembles the pipevine swallowtail.
Tiger swallowtail caterpillars eat tulip, aspen, and cherry, and neither caterpillars nor adults are distasteful. The mimicry does not extend to the caterpillar stage — the tiger swallowtail caterpillar is camouflaged as a bird dropping, while the protected pipevine caterpillar is conspicuous.
Mullerian mimicry — named for Fritz Muller, 1878. Several unrelated but genuinely protected species converge on one appearance. Because all are poisonous or dangerous, predators learn a single pattern to avoid, and every species benefits.
Predator populations may even evolve innate avoidance of such patterns, and that can happen faster when several dangerous prey look alike. Sympatric Heliconius pairs (H. erato with H. melpomene, H. sapho with H. cydno) are the standard example.
Requirement for both kinds of mimicry: mimic and model must look alike AND act alike. Insects from several families that resemble wasps also fly frequently and actively from place to place, the way wasps do.
Bottom line for 55.3: high predation can drive prey to extinction; absent predators, prey explode and exhaust their resources; prey evolve defensive structures, chemistry, appearance, and behavior, and predators evolve counters.

55.4

The Many Types of Species Interactions

pp. 1252–1257

Learning outcomes: explain the forms of symbiosis; describe coevolution between mutualistic partners; explain how one ecological process can change the outcome of another happening at the same time.
In symbiosis, two or more kinds of organisms interact in often elaborate and more-or-less permanent relationships. Every symbiosis carries the potential for coevolution. Watch Symbiosis: A surprising tale of species cooperation - David Gonzales on YouTube, from TED-Ed
Three major kinds of symbiosis: mutualism (both benefit), parasitism (one benefits, the other is harmed), and commensalism (one benefits, the other is neither helped nor harmed). Watch Ecological Relationships on YouTube, from Amoeba Sisters
Standing examples of symbiosis: lichens (certain fungi with green algae or cyanobacteria), mycorrhizae (fungi with the roots of most plants — the fungus speeds nutrient absorption, the plant supplies carbohydrates), and root nodules in legumes housing nitrogen-fixing bacteria.
Leaf-cutter ants can strip a quarter or more of the total leaf surface in a tropical area in one year — but they do not eat the leaves. They chew them up underground, inoculate them with fungal spores, and farm the fungus, which feeds the ants and their larvae. Molecular clock work dates these ant–fungus symbioses to perhaps more than 50 million years ago.
Mutualism is fundamental to community structure. The showiest examples are flowering plants and their animal visitors — insects, birds, bats, and even small marsupials and lizards.
Coevolution in pollination runs both ways: flower characteristics evolved in relation to the animals that visit them, while animal characteristics changed to increase specialization for feeding at particular flowers (figure 55.15 shows a bat with pollen on its snout). The visitors carry pollen to the next plant, or eat fruits and drop the seeds far from the parent. Watch Coevolution on YouTube, from Bozeman Science
Ants and aphids: aphids pierce phloem, extract sucrose and other nutrients, and excrete the rest. Certain ants effectively domesticate them, carrying aphids to fresh plants like ranchers moving cattle and harvesting the honeydew they excrete.
Ants and acacias is the chapter's flagship mutualism. In certain Latin American Acacia species, leaf parts called stipules are modified into paired, hollow thorns inhabited by stinging ants of the genus Pseudomyrmex, which nest nowhere else. Watch Symbiosis: A surprising tale of species cooperation - David Gonzales on YouTube, from TED-Ed
The acacia also produces Beltian bodies — unique, protein-rich structures at the leaflet tips, named for the 19th-century naturalist Thomas Belt. They are absent from acacia species that lack ants, and they are the ants' primary food. The tree further secretes nectar from glands near the leaf bases, which the ants eat and feed to their larvae.
What the ants get: protected nesting space inside the swollen thorns, plus a balanced diet of sugar-rich nectar and protein-rich Beltian bodies.
What the acacia gets: ants patrol the branches and immediately attack and devour any herbivorous insect that lands; ants cut away encroaching branches that touch the tree, opening a tunnel of light the acacia can grow through even in lowland Central American rainforest; and ant excretions plus imported organic material supply abundant nitrogen. Remove the ant colony experimentally and the acacia can no longer compete successfully.
When mutualism is not mutualism. In Kenya several acacia-ant species occur, but only one species per tree. Crematogaster nigriceps is competitively inferior to two of the others.
To block invasion, C. nigriceps prunes the acacia's branches so they cannot touch neighboring trees that would serve as bridges. The pruning destroys the tissue that produces flowers, effectively sterilizing the tree — a mutualism that has become parasitic.
Parasitism harms the host and benefits the parasite. Many parasites kill the host, so the ecological effects can resemble predation.
Ectoparasites (external parasites) feed on the exterior surface. Figure 55.17 shows dodder (Cuscuta), a flowering plant that lost its chlorophyll and its leaves and now takes food from the host plant it grows on.
Parasitoids are insects — many are wasps — that lay eggs in or on a living host; the larvae feed on the host's body and often kill it.
Endoparasites (internal parasites) live inside the host and occur across many animal and protist phyla. Internal parasitism involves far more extreme specialization than external parasitism: conditions inside a host are very different and much more constant, so structure is often simplified and unneeded parts lost — tapeworms are the standard case.
Case study: the flatworm Dicrocoelium dendriticum uses ants as an intermediate host but matures in large grazing mammals such as cattle and deer. Deer do not eat insects, so transmission looks impossible. Its solution: one worm migrates to the ant's brain and makes the ant climb a grass blade and lock its mandibles there at the end of the day — exactly when herbivores graze.
Commensalism: one species benefits, the other is neither hurt nor helped. Often one organism is simply physically attached to another.
Epiphytes are plants that grow on the branches of other plants; the host is generally unharmed and the epiphyte gains access to sunlight. Spanish moss (Tillandsia usneoides, pineapple family) is the standard example. The marine version is barnacles riding on whales — carried to new food, protected, and given increased water circulation, with no cost to the host unless the passengers get too numerous.
When commensalism is not commensalism. Clownfish live among sea anemone tentacles that would paralyze other fish, feeding on scraps from the anemone's meals. If removing food particles helps the anemone catch more prey, the relationship is really mutualism.
On land, oxpeckers cling to cattle and antelopes, picking off parasites and insects and living their whole life cycle on the host. That is usually called commensalism but is really mutualism — until the birds pick scabs and drink the blood, which can greatly weaken a herbivore during a drought and makes the relationship parasitic. There is no clear-cut boundary between these categories.
Ecological processes have interactive effects. More than one type of interaction usually operates at once, and one interaction can modify or even reverse the outcome of another.
Predation reduces competition. When resources are limiting, a superior competitor can competitively exclude others — but a predator that lowers competitor numbers can prevent or greatly reduce that exclusion. The mechanism is prey switching: a predator eats species A when A is abundant and switches to B when A becomes rare, and abundant prey attract more predator species.
Paine's sea star experiment (figure 55.21). Robert Paine removed the predatory sea star Pisaster ochraceus from areas of rocky intertidal shoreline and monitored its prey. Watch How Starfish Changed Modern Ecology on YouTube, from Nature on PBS
The control detail worth knowing: in control plots the sea stars were picked up and put back where they were found, so that the effect of people walking through the study site was controlled for.
Result: without sea stars, the mussel Mytilus californianus exploded, occupied all available space, and crowded out seven other indigenous species. Because predation reduces competition, eliminating a major predator such as wolves or mountain lions usually decreases biological diversity.
Parasitism may also counter competition. Two sympatric flour beetles: without an intracellular parasite, Tribolium castaneum is dominant and T. confusum goes extinct. With the parasite present the outcome reverses. Same logic in the field — the inferior St. Maarten Anolis is resistant to lizard malaria, so the two species coexist only where that parasite occurs.
Indirect effects: species that never interact directly can still affect one another through a third species.
The rodent–ant study is the worked example. Desert rodents and ants both eat seeds, so removing rodents from enclosures should help ants. Ant colonies did rise at first — then declined back toward control levels.
The chain that explains the reversal: rodents prefer large seeds, ants prefer small ones, and large-seeded plants outcompete small-seeded plants. Removing rodents → more large-seeded plants → fewer small seeds → fewer ants. So rodents have a direct negative and an indirect positive effect on ants, and the two point in opposite directions.
Keystone species have effects on community composition that are greater than their abundance would predict. Predators like Pisaster qualify because they stop one species from outcompeting the rest, maintaining high species richness. Watch How Starfish Changed Modern Ecology on YouTube, from Nature on PBS
Keystone species need not be predators. Some are habitat engineers: beavers convert running streams into small impoundments, flooding new areas (figure 55.23), and alligators dig deep lake-bottom holes that hold the only water in a drought, letting aquatic species survive until the lake refills.
Bottom line for 55.4: mutualism, commensalism, and parasitism are the three symbioses; mutualists often coevolve; and simultaneous processes interact, so predation and parasitism can lessen resource competition.

55.5

Ecological Succession, Disturbance, and Species Richness

pp. 1258–1261

Learning outcomes: define succession and distinguish primary from secondary; describe how early colonizers affect which species come later; explain how disturbance can raise or lower species richness.
Even when the climate of an area stays stable year after year, communities tend to change from simple to complex. That process is succession. Watch Ecological Succession: Change is Good - Crash Course Ecology #6 on YouTube, from CrashCourse
It is a familiar sight: a vacant lot or a cleared woodlot is slowly occupied by more and more species until the clearing disappears.
Secondary succession occurs where an existing community has been disturbed but organisms still remain — a cleared or burned woods, an abandoned field. Soil, seeds, and roots are already there. Watch Ecological Succession: Nature's Great Grit on YouTube, from Amoeba Sisters
Primary succession occurs on bare, lifeless substrate — new rock, or open water — where organisms gradually move in and change the site's nature.
The two settings that define primary succession: land exposed by retreating glaciers, and volcanic islands rising from the sea.
Glacier Bay, Alaska is the case study (figure 55.24). Freshly deglaciated ground is mineral-poor, basic in pH because of carbonates in the rock, and very low in nitrogen.
Lichens are the first vegetation able to grow there. Their acidic secretions help break down the substrate, lower the pH, and add to the accumulation of soil.
Mosses colonize those first pockets of soil, building up enough nutrients for alder shrubs to take hold.
Over about 100 years, alders — which carry symbiotic nitrogen-fixing bacteria — raise soil nitrogen sharply, and their acidic leaves lower soil pH further.
Finally spruce trees grow up above the alders, shade them out, and form a dense spruce forest by roughly year 200. Nitrogen in the forest floor peaks near 300 g/m² of surface.
Aquatic parallel: an oligotrophic (nutrient-poor) lake gradually becomes eutrophic (nutrient-rich) as organic matter accumulates. Community composition changes and species richness first increases, then declines.
The engine of succession: species alter the habitat and its available resources in ways that favor different species than themselves.
Three dynamic concepts organize the process: establishment, facilitation, and inhibition.
Establishment: early successional stages are dominated by weedy, r-selected species that tolerate the harsh abiotic conditions of barren areas. They are colonizers, not competitors.
Facilitation: early arrivals change the environment in ways that let other species colonize — lichens making soil, alders adding nitrogen. Later species are literally enabled by the earlier ones.
Inhibition: new colonizers can have negative effects on species already present. The spruce that shade out the alders that made their soil are the same chapter's example.
Exam framing: pioneer species in primary succession are not the fiercest competitors and do not keep the habitat constant for themselves — they change the habitat in ways that favor invasion by other species.
Disturbance resets succession. Disturbances may be local — a single tree falling, an animal digging a hole and uprooting vegetation — or larger and more widespread. Watch Disturbance and Species Diversity || Intermediate Disturbance Hypothesis . on YouTube, from Biology Lectures with Sonam
Intermediate disturbance hypothesis: communities experiencing moderate amounts of disturbance have higher species richness than communities experiencing either very little or a great deal.
First explanation: moderate disturbance leaves patches of habitat at different successional stages at the same time, so the area as a whole contains the full range of species characteristic of every stage.
The worked example is a rainforest light gap (figure 55.26). A fallen tree opens a gap that other species invade; the gap runs through its own successional sequence until a canopy species reoccupies it.
If a forest holds many gaps of different ages, many species coexist — some in young gaps, others in older ones. That is how gap dynamics maintains rainforest diversity.
Second explanation: moderate disturbance prevents the community from reaching the late successional stage in which a few dominant competitors eliminate most other species.
Both ends of the axis lose richness. Too little disturbance → competitive dominants take over. Too much disturbance → the community is stuck permanently in the earliest, species-poor stage.
Ecologists increasingly find that disturbance is common rather than exceptional, which undercuts the old picture of succession.
Consequence: the idea that communities inevitably move along a successional trajectory to a predictable end-state — a climax community — is no longer widely accepted.
Predicting a community's future state is genuinely hard, because unpredictable disturbances keep countering successional change. Working out the role of disturbance is an active research area.
Applied version of the same idea: clear-cutting versus selective harvest is a disturbance-scale question — how much disturbance, at what patch size, leaves the most species.
Bottom line for 55.5: succession runs primary (bare substrate) or secondary (disturbed community); early species alter the environment for later ones and later ones may suppress earlier ones; and moderate disturbance can maximize species richness.