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Field notes · chapter 55

Community Ecology

pp. 1241–1263 · Raven Part VIII

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33 points
  1. A community is all the species — plants, animals, fungi, protists, prokaryotes — that occur at one locality. Describe it either by its member species or by properties like species richness (number of species) and primary productivity (energy produced). Watch Communities on YouTube, from Bozeman Science Watch Community Ecology: Feel the Love - Crash Course Ecology #4 on YouTube, from CrashCourse
  2. The individualistic view wins. Species respond independently to environmental conditions, so community composition changes gradually along a gradient. Tree species in the Santa Catalina Mountains rise and fall independently along a dry-to-moist gradient — communities form a continuum, not discrete units. Watch Community Ecology and Landscape Ecology on YouTube, from Professor Dave Explains
  3. The exception is an ecotone — a place where the environment changes abruptly, so many species change abundance in synchrony. Classic case: serpentine soils (high nickel, chromium, iron; low copper and calcium) meeting normal soil, with completely different plant communities a few meters apart.
  4. A niche is the total of all the ways a species uses the resources of its environment — space, food, temperature, moisture, mating conditions. It is a job description, not an address. Watch Niche on YouTube, from Bozeman Science
  5. THE BIG ONE. Fundamental niche = potential, the full niche a species could use based on its physiological tolerances and resource needs. Realized niche = actual, the conditions where it really maintains a stable population once other species are present. Realized is almost always smaller. Watch Niche on YouTube, from Bozeman Science
  6. Connell's barnacles. On Scottish rocks, Chthamalus stellatus lives high (exposed at low tide) and Semibalanus balanoides lives low. Remove Semibalanus and Chthamalus spreads down — so its fundamental niche includes both zones and its realized niche is only the top. Semibalanus cannot survive up high at all, so its fundamental and realized niches are identical. It wins by crowding, undercutting, and replacing — that is interference competition, versus exploitative competition, which just means consuming the shared resource faster. Watch Interspecific Competition: Connell’s Barnacle Experiment | EcolClips on YouTube, from EcolClips
  7. Competition is not the only thing that shrinks a realized niche. A predator can: an introduced beetle drove St. John's wort (Hypericum perforatum) out of open rangeland and into shady refuges. So can the absence of a mutualist — losing pollinators would shrink or erase the realized niche of many plants (insect pollination is worth over $9 billion a year to U.S. agriculture).
  8. Gause, 1934–1935, three species of Paramecium. (1) Each grows fine alone. (2) P. aurelia + P. caudatum → caudatum goes extinct, because aurelia grows about six times faster on the limited resource. (3) P. caudatum + P. bursaria → both survive, dividing the tube. Watch Niche and Competitive Exclusion on YouTube, from Mr. P Science
  9. Principle of competitive exclusion: if two species compete for a limited resource, the one that uses it more efficiently eliminates 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
  10. Coexistence in tube 3 happened by resource partitioning: P. caudatum dominated the oxygen-rich top feeding on bacteria, P. bursaria took the low-oxygen bottom feeding on yeast. Fundamental niche = the whole tube for both; realized niches differed. Both still reached lower densities than when grown alone — competition was real, just not fatal. Watch Resource Partitioning on YouTube, from DeBacco University
  11. MacArthur's five warblers fed in five different parts of the same spruce trees (branch tips, dense foliage, lower branches, high in the tree, at the apex) — the textbook picture of resource partitioning by sympatric species. Watch Resource Partitioning on YouTube, from DeBacco University
  12. Character displacement = natural selection drives initially similar species to diverge where they co-occur. Test for it by comparing sympatric with allopatric populations. Darwin's finches (Geospiza fuliginosa and G. fortis) have similar beaks on islands where each lives alone, but different beak depths where they live together — one takes large seeds, the other small. Watch Resource Partitioning on YouTube, from DeBacco University
  13. Overlap in resource use is not proof of competition. The resource may not be limiting, and a negative correlation in abundance may just mean the two species respond oppositely to the same environmental factor (one likes warm, one likes cool).
  14. Kangaroo rat exclusion experiment: remove kangaroo rats from half of a set of 50 m × 50 m desert enclosures, and the number of other seed-eating rodents rises immediately and stays higher for three years — direct experimental evidence of interspecific competition. Competition also shows up as behavior and growth: Anolis gingivinus grows faster and perches lower when A. pogus is absent.
  15. Predation is the consuming of one organism by another — leopard eating antelope and deer grazing grass both count. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy
  16. In a simple tube, the predator Didinium wipes out its Paramecium prey and then starves to extinction. Add refuges and prey drop low but survive; the starving predator population then falls, letting prey recover — the basis of predator–prey cycling. Watch Predator prey cycle | Ecology | Khan Academy on YouTube, from Khan Academy Watch Predator-Prey Interactions, Episode 1: Understanding the Lotka-Volterra Equations on YouTube, from Explorations in Biology, Evolution, and More
  17. Remove predators and prey explode: no large carnivores → white-tailed deer strip the eastern U.S. understory; sea otters hunted out → sea urchins exploded. Add predators and native prey collapse: introduced rats, pigs, dogs, and cats eat Galápagos tortoise eggs and young, and a single lighthouse keeper's cat killed every Stephens Island wren.
  18. Coevolution = adaptations selected in lockstep in two or more interacting species, producing an evolutionary arms race: better prey defenses select for better predator counteradaptations, and around again. Watch Coevolution on YouTube, from Bozeman Science
  19. Plant defenses: morphological (thorns, spines, prickles, sticky glandular hairs, silica in grass leaves) and chemical. The chemical ones matter more — secondary chemical compounds are not part of primary metabolism, and they poison or derange herbivores. Attacked wild tobacco even emits a volatile that recruits a bug that eats the caterpillar. Watch The amazing ways plants defend themselves - Valentin Hammoudi on YouTube, from TED-Ed
  20. Two named chemical defenses: mustard oils in the mustard family (Brassicaceae — cabbage, radish, horseradish), toxic to many insects; cardiac glycosides in the milky sap of milkweeds (Asclepiadaceae) and dogbanes (Apocynaceae), which wreck vertebrate heart function.
  21. Herbivore counterattack: cabbage white caterpillars (Pieris rapae) break mustard oils down and specialize on that family. Monarch caterpillars do the opposite — they store cardiac glycosides in fat bodies, carrying the poison through chrysalis, adult, and even into the eggs. A blue jay that eats one monarch vomits and never touches orange-and-black again.
  22. Warning (aposematic) coloration advertises a real chemical defense — poison-dart frogs (Dendrobatidae) with over 200 alkaloids in their skin mucus. Cryptic coloration is camouflage for the undefended, and camouflaged animals usually do not group, because finding one gives the predator a clue to the rest. Watch Aposematic Coloration on YouTube, from Bozeman Science
  23. THE MIMICRY TRAP. Batesian (Henry Bates, 1857) = a palatable mimic copies a distasteful model — one is a liar (tiger swallowtail copying pipevine swallowtail). Mullerian (Fritz Muller, 1878) = several genuinely protected species converge on one pattern (Heliconius pairs) so predators only have to learn one lesson. Both require the mimic to look AND act like the model. Watch Batesian vs Mullerian Mimicry | Animal Behaviour on YouTube, from sci-ology
  24. The four interaction signs. Mutualism +/+ (ants and acacias). Commensalism +/0 (epiphytes, barnacles on whales). Parasitism +/– (dodder, tapeworms). Competition –/– (barnacles, Paramecium). Predation is +/– as well, and parasitism is really predation where the prey usually survives. Watch Ecological Relationships on YouTube, from Amoeba Sisters
  25. Ants and acacias, the model mutualism. The acacia supplies hollow stipular thorns to nest in, nectaries at the leaf bases, and protein-rich Beltian bodies on the leaflet tips. Pseudomyrmex ants pay back by devouring herbivores, pruning encroaching branches to open a tunnel of light, and hauling in nitrogen-rich organic matter. Remove the ant colony and the tree loses out. Watch Symbiosis: A surprising tale of species cooperation - David Gonzales on YouTube, from TED-Ed
  26. Interactions slide between categories. In Kenya, Crematogaster nigriceps prunes the acacia so hard that it destroys flower-producing tissue and sterilizes the tree — mutualism turned parasitic. Oxpeckers pick ticks (mutualism) but also open scabs and drink blood (parasitic). Clownfish and anemone may be commensal, or mutual if the fish cleans the tentacles.
  27. Predation reduces competition. Paine's sea stars: remove Pisaster ochraceus from rocky intertidal plots and the mussel Mytilus californianus explodes, crowding out seven other species. Species richness collapses. That is why exterminating wolves or mountain lions usually lowers diversity. Watch How Starfish Changed Modern Ecology on YouTube, from Nature on PBS
  28. Keystone species have an effect on community composition far larger than their abundance predicts. Predators like Pisaster qualify, but so do habitat engineers — beavers turning streams into ponds and alligators digging drought refuge holes. Watch How Starfish Changed Modern Ecology on YouTube, from Nature on PBS
  29. Indirect effects: rodents compete with seed-eating ants (direct –), but rodents also suppress large-seeded plants, which frees small-seeded plants, which feed ants (indirect +). Remove rodents and ant colonies first rise, then fall — the two effects pull in opposite directions.
  30. Succession = communities change from simple to complex over time even under a stable climate. Primary starts on bare, lifeless substrate (new rock, deglaciated ground, a new volcanic island, open water). Secondary starts where a community was disturbed but organisms remain (cleared or burned woods). Watch Ecological Succession: Change is Good - Crash Course Ecology #6 on YouTube, from CrashCourse Watch Ecological Succession: Nature's Great Grit on YouTube, from Amoeba Sisters
  31. Glacier Bay primary succession, in order: bare basic low-nitrogen rock → lichens (acid secretions break rock down, drop pH, build soil) → mosses → alder shrubs, whose symbiotic nitrogen-fixing bacteria load the soil with nitrogen over about 100 years → spruce forest overtopping and shading out the alders by year 200. Watch Ecological Succession: Change is Good - Crash Course Ecology #6 on YouTube, from CrashCourse
  32. Three driving concepts in succession: establishment (weedy r-selected pioneers tolerate the harsh abiotic conditions), facilitation (early species change the habitat so later species can invade), inhibition (new colonizers can suppress the species already there). Also on the aquatic side: an oligotrophic (nutrient-poor) lake accumulates organic matter and becomes eutrophic, with richness rising and then falling.
  33. Intermediate disturbance hypothesis: moderate disturbance maximizes species richness. Two reasons — (1) it leaves a patchwork of different successional stages side by side, and (2) it prevents the community from reaching the late stage where a few dominant competitors exclude everything else. Too little → dominants take over; too much → everything is stuck in the earliest stage. Rainforest light gaps from fallen trees are the example: gaps of many ages hold many species at once. Watch Disturbance and Species Diversity || Intermediate Disturbance Hypothesis . on YouTube, from Biology Lectures with Sonam