# Lecture 26: Community Ecology ## General Biology II — Organismal, Evolution & Ecology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Define a biological community and describe the types of species interactions 2. Distinguish between exploitative competition, interference competition, and apparent competition 3. Explain the competitive exclusion principle and the concept of the ecological niche 4. Describe the different forms of predation and co-evolutionary dynamics between predators and prey 5. Distinguish among mutualism, commensalism, and parasitism with examples 6. Explain the roles of keystone species, dominant species, and ecosystem engineers in communities 7. Describe ecological succession and the factors that influence species diversity --- ## Lecture Content ### I. What Is a Community? **Biological community** — all the populations of different species living and interacting in a particular area at a given time. Community ecology studies the interactions among species and the factors that determine community structure (species composition, relative abundance, diversity) **Species interactions** — the relationships between species in a community; major types: Competition (-/-) Predation (+/-) Herbivory (+/-) Parasitism (+/-) Mutualism (+/+) Commensalism (+/0). ### II. Interspecific Competition **Interspecific competition** — occurs when two or more species use the same limited resource, reducing the fitness of both. Types: **Exploitative (resource) competition** — species compete indirectly by consuming the same resource (e.g., two plant species drawing from the same soil nutrients) **Interference competition** — species directly interact to prevent access to a resource (e.g., allelopathy in plants, territorial aggression) **Competitive exclusion principle (Gause's principle)** — two species competing for the same limiting resource cannot coexist indefinitely; one will outcompete and exclude the other. Demonstrated by Gause (1934) with Paramecium aurelia and P. caudatum: grown separately both thrived; grown together, P. aurelia drove P. caudatum to extinction. **Ecological niche** — the total set of biotic and abiotic conditions in which a species can survive, grow, and reproduce: **Fundamental niche** — the full range of conditions a species can potentially occupy (in the absence of competition) **Realized niche** — the actual range occupied, which is typically smaller due to interspecific competition and other biotic interactions. **Resource partitioning** — coexisting species reduce competition by using different portions of a resource or habitat: Example: Anolis lizards in the Caribbean — different species occupy different perch heights and diameters on trees. Example: MacArthur's warblers — five warbler species forage in different zones of spruce trees. **Character displacement** — competing species evolve to be more different in sympatry (where they coexist) than in allopatry (where they occur alone) Example: beak size divergence in Darwin's finches (Geospiza fortis and G. fuliginosa) on islands where both occur together vs. islands with only one species. ### III. Predation **Predation** — an interaction where one organism (predator) kills and consumes another (prey); (+/- interaction) Predation is a major selective force driving co-evolution between predators and prey. #### Prey Defenses: **Cryptic coloration (camouflage)** — body color/pattern matches the background (e.g., leaf insects, arctic hare) **Aposematic coloration (warning coloration)** — bright, conspicuous colors advertise toxicity or unpalatability (e.g., poison dart frogs, monarch butterflies, coral snakes) **Mimicry**: **Batesian mimicry** — a harmless species resembles a harmful or unpalatable model (e.g., king snake mimics coral snake; hoverflies mimic bees) **Mullerian mimicry** — two or more unpalatable species resemble each other → reinforces predator avoidance learning (e.g., multiple species of stinging wasps/bees share yellow-black banding) Chemical defenses — toxins, venoms, noxious sprays (e.g., bombardier beetle, skunk spray, plant alkaloids) Mechanical defenses — spines, shells, armor (e.g., porcupine quills, tortoise shell) Behavioral defenses — alarm calls, mobbing, fleeing, playing dead (thanatosis), schooling/flocking (dilution effect, confusion effect). #### Predator Adaptations: Speed, stealth, ambush strategies. Acute senses (e.g., echolocation in bats, thermal pit receptors in pit vipers) Venom and toxins for subduing prey. Co-evolutionary arms race — predator and prey continuously evolve in response to each other. ### IV. Herbivory **Herbivory** — consumption of plant tissues by animals (+/- interaction) Plants have evolved extensive defenses: Physical: thorns, spines, trichomes (leaf hairs), thick bark, waxy cuticle. Chemical: secondary metabolites — alkaloids (nicotine, caffeine, morphine), terpenes, tannins, phenolics; many are toxic or reduce digestibility. Some plants recruit predators of herbivores (indirect defense): e.g., acacia trees produce extrafloral nectaries that attract ants which attack herbivores. Herbivores have counter-adaptations: detoxification enzymes, symbiotic gut microbes for cellulose digestion, behavioral avoidance of toxic plants. ### V. Parasitism **Parasitism** — one organism (parasite) lives on or in another (host), deriving nutrients at the host's expense (+/- interaction); typically does not immediately kill the host. **Ectoparasites** — live on the outside of the host (ticks, fleas, lice, leeches) **Endoparasites** — live inside the host (tapeworms, Plasmodium, nematodes) **Parasitoids** — insects that lay eggs in or on a host; larvae consume the host from within, eventually killing it (e.g., parasitoid wasps) **Brood parasitism** — a bird lays eggs in another species' nest (e.g., cuckoos, cowbirds); the host raises the parasitic offspring. Parasites can regulate host populations, alter host behavior (parasite manipulation), and drive co-evolutionary dynamics. Host defenses: immune system, behavioral avoidance, grooming. Parasite counter-adaptations: immune evasion, antigenic variation, manipulation of host behavior. ### VI. Mutualism and Commensalism #### A. Mutualism (+/+) Both species benefit from the interaction. **Obligate mutualism** — neither species can survive without the other (e.g., mycorrhizal fungi and most land plants; fig wasps and fig trees) **Facultative mutualism** — both benefit but can survive independently (e.g., cleaner fish and client fish) Examples: Mycorrhizae — fungal hyphae increase root surface area for water and mineral absorption; plant provides carbohydrates to the fungus. Nitrogen-fixing bacteria (Rhizobium) in legume root nodules — bacteria fix atmospheric N2 into ammonia; plant provides carbon compounds. Pollination mutualisms — plants provide nectar/pollen; pollinators (bees, butterflies, hummingbirds) transfer pollen. Coral and zooxanthellae (dinoflagellate algae) — algae photosynthesize and provide nutrients; coral provides shelter and CO2. Gut microbiome — bacteria in the ruminant stomach digest cellulose; bacteria receive a warm, nutrient-rich environment. #### B. Commensalism (+/0) One species benefits; the other is neither helped nor harmed. Examples: barnacles on whales (barnacles gain transport and access to food; whale unaffected), epiphytes (orchids, ferns) growing on tree branches (gain access to light; tree unaffected), birds nesting in trees. True commensalism may be rare — upon closer examination, the "unaffected" species often experiences subtle costs or benefits. A two-part figure illustrating species interactions and niche concepts. Part A (Competitive exclusion and resource partitioning): Three panels arranged vertically. The top panel shows Gause's Paramecium experiment — a graph with population size versus time, where P. aurelia and P. caudatum are grown together; P. aurelia increases while P. caudatum declines to extinction. The middle panel shows fundamental versus realized niches: two overlapping bell curves along a resource axis (e.g., food size), labeled as the fundamental niche of species A and species B, with the zone of overlap shaded; arrows show each species' realized niche shifting away from the overlap zone. The bottom panel shows Anolis lizard resource partitioning: a side view of a tree with different colored lizards at different heights and perch diameters, each species labeled and occupying a distinct microhabitat. Part B (Mimicry): A side-by-side comparison. On the left, Batesian mimicry: a toxic model species (e.g., coral snake with red-yellow-black banding) alongside a harmless mimic (king snake with similar banding), with a predator (bird) avoiding both. On the right, Mullerian mimicry: two or more unpalatable species (e.g., different wasp species) shown with similar yellow-and-black warning patterns, with an annotation that shared appearance reinforces predator avoidance learning. ### VII. Keystone Species, Dominant Species, and Ecosystem Engineers **Dominant species** — the most abundant species or those with the highest biomass in a community; exert control by their sheer abundance. Often competitively superior in exploiting resources. Removal may drastically alter community structure. Example: sugar maples in a northern hardwood forest. **Keystone species** — a species whose impact on the community is disproportionately large relative to its abundance. Removal causes dramatic changes in community structure. Example: sea otters in kelp forest ecosystems — sea otters prey on sea urchins; without otters, urchins overgraze kelp → kelp forest collapses (trophic cascade) Example: Paine's starfish removal experiment — removing Pisaster ochraceus (a sea star predator) from rocky intertidal → mussels dominated and excluded other species → diversity dropped from 15 to 8 species. **Ecosystem engineers** — organisms that physically modify the environment, creating or altering habitat: **Autogenic engineers** — modify the environment through their own structure (e.g., coral reefs, trees in a forest) **Allogenic engineers** — transform the environment through their activity (e.g., beavers building dams → create ponds and wetlands; woodpeckers creating nesting cavities used by other species). ### VIII. Ecological Succession **Ecological succession** — the gradual, directional change in community composition and structure over time following a disturbance or in a newly available habitat. **Primary succession** — colonization of a barren, lifeless area with no soil (e.g., bare rock after glacial retreat, new volcanic island, lava flow): Pioneer species — lichens and mosses colonize bare rock; begin soil formation through weathering and organic matter accumulation. Grasses and small herbaceous plants establish as thin soil forms. Shrubs and shade-intolerant trees colonize. Shade-tolerant trees eventually dominate → climax community (relatively stable endpoint) Process takes hundreds to thousands of years. **Secondary succession** — re-establishment of a community after a disturbance that destroyed existing community but left the soil intact (e.g., after a fire, flood, abandoned farmland): Proceeds faster than primary succession because soil and seed bank are present. Example: old-field succession — abandoned agricultural land → grasses and forbs → shrubs → pioneer trees → mature forest. Mechanisms driving succession: **Facilitation** — early species modify the environment in ways that make it more suitable for later species (e.g., nitrogen fixation, soil creation) **Inhibition** — early species resist displacement and make conditions less suitable for later species; succession proceeds only when early species are disturbed or die. **Tolerance** — later species are neither helped nor hindered by early species; they succeed because they are better competitors in the long term. **Disturbance** — events that remove organisms and alter resource availability (fire, storms, floods, logging) **Intermediate disturbance hypothesis** — species diversity is highest at intermediate levels of disturbance: Low disturbance → competitive exclusion → few dominant species. High disturbance → only tolerant/colonizing species survive → low diversity. Intermediate disturbance → prevents competitive exclusion while allowing many species to coexist. ### IX. Species Diversity **Species diversity** has two components: **Species richness** — the total number of species in a community. **Relative abundance (evenness)** — how evenly individuals are distributed among species. A community with high richness and high evenness has the greatest diversity. **Shannon diversity index (H')**: H' = -sum(p_i * ln(p_i)), where p_i = proportion of individuals belonging to species i. Higher H' = greater diversity. Latitudinal gradient in species diversity — species richness generally increases from the poles toward the tropics: Possible explanations: greater solar energy and productivity, longer evolutionary time without glaciation, larger tropical area, higher speciation rates. ---