Premed · Premed · General Biology 2
Lecture 26: Community Ecology
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Define a biological community and describe the types of species interactions
- Distinguish between exploitative competition, interference competition, and apparent competition
- Explain the competitive exclusion principle and the concept of the ecological niche
- Describe the different forms of predation and co-evolutionary dynamics between predators and prey
- Distinguish among mutualism, commensalism, and parasitism with examples
- Explain the roles of keystone species, dominant species, and ecosystem engineers in communities
- 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.
<image>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.</image>
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.
