Ecological Communities (Chapter 41)
Overview – Communities in Motion
Biological community = set of populations of different species living close enough to interact.
Case study: Bluestreak cleaner wrasse inside giant moray eel → both benefit (parasite removal vs food). Illustrates mutualism but also introduces range of interspecific interactions.
Chapter framework:
41.1 Interspecific interactions (help, harm, neutral)
41.2 Community diversity & trophic structure
41.3 Role of disturbance
41.4 Biogeographic controls
41.5 Pathogens & disease ecology
41.1 Interactions Between Species
Interspecific interactions can be classified based on their effect on the per-capita survival and reproduction (fitness) of the interacting species:
Competition (-/-): Occurs when both species are negatively affected.
Exploitation (+/-): One species benefits at the expense of the other (predation, herbivory, parasitism).
Positive interactions: mutualism (+/+), where both species benefit; and commensalism (+/0), where one species benefits and the other is neither harmed nor helped.
The "0" designation signifies no measurable fitness effect on a species.
Competition (-/-)
Competition arises when two or more species require the same limiting resource that is in short supply. Common limiting resources include water, nutrients, prey, light, and space.
Competitive Exclusion Principle (G. F. Gause, 1934): This principle states that two species that utilize identical ecological niches cannot coexist indefinitely in the same community. Even a slight reproductive advantage by one species will inevitably lead to the local extinction of the inferior competitor. A classic laboratory demonstration involved extit{Paramecium aurelia} out-competing extit{P. caudatum} under stable conditions, with extit{P. caudatum} declining to extinction when grown together.
Ecological niche: An organism's ecological niche encompasses its complete "role" in the ecosystem, including all biotic (living) and abiotic (non-living) resources it utilizes or influences.
Fundamental niche: Represents the potential range of conditions and resources a species can use in the absence of biotic interactions (like competition).
Realized niche: The portion of the fundamental niche that a species actually occupies, often restricted by biotic factors such as competition, predation, or disease.
Connell's barnacle experiment illustrated this: when the larger barnacle extit{Balanus} was removed from intertidal zones, the smaller barnacle extit{Chthamalus} was able to expand its distribution downward, indicating that competition with extit{Balanus} limited its realized niche.
Resource partitioning: Over evolutionary time, sympatric (geographically overlapping) species can evolve such that they differentiate their ecological niches, allowing them to coexist. An example is the seven species of extit{Anolis} lizards found in the Caribbean, where each species perches at different heights or on different parts of trees, thus partitioning the habitat resource.
Character displacement: This phenomenon occurs when traits that enable species to exploit different resources or reduce competition diverge more in sympatric populations (where they coexist) than in allopatric populations (where they live separately). A classic example is the Galápagos finch beak depths; in areas where extit{Geospiza fortis} and extit{G. fuliginosa} coexist, extit{G. fortis} has a noticeably larger beak, while extit{G. fuliginosa} has a smaller beak, allowing them to specialize on different seed sizes.
Exploitation (+/-)
Exploitation is an overarching term for interactions where one species benefits by consuming or harming another species. It includes predation, herbivory, and parasitism.
Predation
Predation involves a predator killing and eating its prey. Predators have evolved various adaptations to successfully capture prey, including acute senses (sight, smell, hearing), specialized physical structures like claws, fangs, or venom, and speed or ambush tactics like camouflage.
Prey defenses: Organisms have developed diverse strategies to avoid being eaten:
Behavioral defenses: These include hiding (e.g., burrowing), fleeing, forming groups for protection (herding), or displaying alarm calls.
Morphological defenses: Physical structures such as quills (porcupines), shells (turtles), and spines (cactuses).
Physiological/chemical defenses: Producing defensive chemicals or toxins that can be harmful, distasteful, or deterrent to predators.
Coloration: Visual adaptations play a crucial role:
Cryptic coloration (camouflage): Blending with the environment to avoid detection, as seen in the canyon tree frog.
Aposematic coloration (warning coloration): Bright, conspicuous colors (e.g., reds, yellows, blacks) that advertise to predators that the prey is toxic or dangerous, as exemplified by poison dart frogs.
Batesian mimicry: A harmless species mimics the warning coloration of a venomous or unpalatable species, such as the hawkmoth larva imitating a venomous snake.
Aggressive mimicry: A predator mimics something harmless or attractive to lure prey, like the alligator snapping turtle's tongue, which resembles a worm.
Herbivory
Herbivory involves an herbivore consuming parts of a plant or alga. Herbivores possess adaptations for efficient plant consumption, such as chemical sensors to identify edible plants, a keen sense of smell, specialized dentition (teeth) for grinding plant material, modified gut microbiomes, and selective feeding behaviors.
Plant defenses: Plants have evolved various defenses against herbivores:
Physical defenses: Spines, thorns, and tough leaves deter feeding.
Chemical defenses: Producing toxic or distasteful compounds (e.g., strychnine, nicotine, peppermint oils) that can deter or harm herbivores.
Parasitism
Parasitism is an interaction where one organism, the parasite (+), derives nourishment from another organism, the host (- ext{ or } 0), which is harmed in the process (or at least loses fitness).
Endoparasites: Parasites that live inside the host's body (e.g., tapeworms, malaria parasites).
Ectoparasites: Parasites that feed on the external surface of the host (e.g., ticks, fleas, lice).
Parasitoids: A unique group of insects (mostly wasps and flies) that lay their eggs on or in a living host. The larvae then hatch and consume the host, eventually leading to its death (e.g., a wasp parasitoid on a hornworm).
Many parasites have complex life cycles involving multiple hosts and can manipulate the host's behavior to facilitate their transmission. For instance, acanthocephalan worms can alter the behavior of their crustacean hosts, forcing them into open water, which increases their likelihood of being eaten by birds, the worms' definitive hosts.
Ecological impact: Parasites can significantly reduce host fitness, impacting their survival and reproduction. They can also alter host population dynamics (e.g., moose weakened by heavy tick loads become more susceptible to wolf predation).
Positive Interactions
Positive interactions are where at least one species benefits, and neither is harmed.
Mutualism (+/+)
Mutualism is an interspecific interaction where both interacting species benefit from the relationship; the benefits for each partner must outweigh the costs.
Examples: Mycorrhizae (fungi provide plants with enhanced water and nutrient absorption, particularly phosphorus (P), while receiving carbohydrates from the plant); acacia trees provide housing and food (nectar and protein-rich Beltian bodies) for ants, which in turn defend the tree from herbivores; cleaner fish and shrimp remove parasites from larger "client" fish, obtaining food while the clients benefit from parasite removal; cellulose-digesting microbes in the guts of ruminants (like cows and sheep) break down plant material, providing nutrients for the host and a stable environment for the microbes; zooxanthellae (algae) live within coral polyps, providing corals with sugars through photosynthesis while receiving shelter and nutrients from the coral.
Mutualistic relationships can be obligatory (where one or both species cannot survive without the other) or facultative (where both species can survive independently but benefit from the interaction).
Commensalism (+/0)
Commensalism is an interaction where one species benefits, and the other species is neither significantly harmed nor helped.
Examples: Shade-tolerant wildflowers growing under the canopy of large forest trees benefit from the filtered light and consistent moisture, while the trees are largely unaffected. Cattle egrets often follow grazing animals in fields, feeding on insects stirred up by the movement of the grazers. While typically considered commensal, this interaction can occasionally shift to mutualism if the egrets remove ticks from the grazers or warn them of approaching predators, providing a direct benefit to the host.
The outcomes of interactions are not always fixed and can shift with environmental context or the specific behaviors involved (e.g., a commensal relationship might become mutualistic or even slightly parasitic under different conditions).
Facilitation
Facilitation is a broad sub-category of positive interactions that describes species interactions where one species indirectly or directly modifies the environment in a way that benefits one or more other species. Often, in direct terms, these interactions appear as (+/0) or (+/+), but the overarching concept emphasizes environmental modification.
A notable example is the black rush ( extit{Juncus gerardii}) in salt marshes. During hot periods, the rush shades the soil surface, which prevents salt accumulation through evaporation (reducing salinity) and increases oxygen levels in the soil. This environmental modification allows approximately 50% more plant species to grow in the presence of the rush than would otherwise be possible in the harsh salt marsh conditions.
41.2 Community Diversity & Trophic Structure
Species Diversity Metrics
Species richness = number of species.
Relative abundance = proportion each species contributes.
Shannon diversity index:
(H = -
\sum{i=1}^{S} pi
\ln pi) where pi = relative abundance of species i.Example: Four-species forests; equal abundances H
\approx 1.39 vs skewed H
\approx 0.71.
Diversity → Function & Stability
Cedar Creek grassland experiments (MN): plots with 1–16 spp.
↑ richness ⇒ ↑ biomass productivity, ↑ year-to-year stability, ↑ drought resilience.
More diverse communities resist invasion (Connecticut tunicate study: invader 4× likelier to establish in low-diverse assemblages).
Trophic Structure
Food chain: energy flow Producer → Primary consumer → Secondary → Tertiary → Quaternary → Decomposers.
Food web: interlinked chains; organisms occupy multiple trophic levels (krill eat phytoplankton + copepods).
Antarctic pelagic web: phytoplankton → krill/copepods → fishes/squid/penguins/seals → whales.
Species With Disproportionate Impact
Foundation species: high biomass or dominance; create habitat (trees, kelp, creosote bushes).
Keystone species: low biomass but pivotal via trophic roles.
\textit{Pisaster ochraceus} sea star predation on mussels maintains intertidal diversity; removal → richness drops from ≈15 → 8.
Ecosystem engineers: alter physical environment (beavers create wetlands; trees modify microclimate).
Control Models
Bottom-up: nutrients N \rightarrow V \rightarrow H \rightarrow P; manipulating lower level (fertilizer) cascades up.
Top-down (trophic cascade): predators regulate lower levels N
\leftarrow V
\leftarrow H
\leftarrow P.Biomanipulation: remove planktivorous fish or add piscivores to clear algae; Lake Vesijärvi fish removal + pike-perch addition restored clarity.
41.3 Disturbance & Succession
Nonequilibrium Model
Communities constantly change after disturbances (storm, fire, flood, drought, human activity).
Stability = ability to maintain/return to composition; many systems seldom reach equilibrium.
Intermediate Disturbance Hypothesis (IDH)
\text{High or Low disturbance} \Rightarrow
\text{Low diversity} (stress/exclusion vs competitive dominance).
\text{Intermediate disturbance} \Rightarrow
\text{Max diversity} (opens space without wiping out tolerant spp.).NZ stream study: Invertebrate taxa peaked at mid-level flood disturbance index.
Examples
Yellowstone 1988 fires: lodgepole pine cones serotinous; rapid colonization → herb layer in 1 yr; forests adapted to periodic, landscape-scale burns.
Ecological Succession
Primary succession = on lifeless substrate (new lava, retreating glacier). Sequence at Glacier Bay:
Pioneer (mosses, liverworts, fireweed,
\textit{Dryas})
\textit{Dryas} dominantAlder thickets (N₂-fixing)
Sitka spruce → western/mountain hemlock forest; bogs form on poorly drained flats.
Secondary succession = soil intact; post-fire or abandoned ag fields (e.g., Yellowstone, prairies).
Mechanisms: facilitation, inhibition, tolerance.
Human Disturbance
Agriculture cleared N. American prairies; tropical deforestation for lumber/cattle/crops.
Marine trawling damages benthic communities (ocean floor before vs after photos).
41.4 Biogeographic Factors & Diversity
Latitudinal Gradient
Richness highest in tropics, declines toward poles.
Causes:
Evolutionary history: older, less disturbed → more speciation time.
Climate: ↑ solar energy + ↑ water → ↑ evapotranspiration.
Positive correlation between vertebrate richness and potential evapotranspiration (PET)
\text{mm yr}^{-1}.
Species–Area Relationship
Species–area curve: S = cA^{z} (larger area → more species; z ≈ 0.2–0.4 for continents, islands).
Island Equilibrium Model (MacArthur & Wilson):
Immigration ↓ and extinction ↑ as island richness ↑.
Small or distant islands → lower immigration, higher extinction → lower equilibrium S.
Florida Keys experiment: Fumigation reset arthropod richness; recolonization rate matched model; near islands recovered faster.
41.5 Pathogens & Community Structure
Local Impacts
Coral white-band disease (unknown pathogen) eliminated staghorn & elkhorn corals → algae takeover → fish community shift (surgeonfish dominate) → structural complexity & diversity ↓.
Sudden oak death (SOD) caused by
\textit{Phytophthora ramorum} protist; >1 million trees killed; cascading effects on acorn woodpeckers, oak titmouse.
Zoonotic Diseases & Community Ecology
Zoonosis = pathogen jumps from other animals → humans, often via vector (ticks, mosquitoes, lice).
Lyme disease case study: genetic assays show >50 % of infected ticks fed on masked & short-tailed shrews, not just white-footed mice → targeting key hosts improves control strategies.
Avian flu (H5N1): spread from SE Asia → Europe/Africa; wild-bird migration monitoring in Alaska to predict entry into Americas.
Swine flu (H1N1) 2009: rapid global dissemination via human travel; >18 000 confirmed deaths by 2011.
Human transport & trade accelerate pathogen dispersal (e.g., nursery plants introduced
\textit{P. ramorum} to North America).
Management Implications
Need ecosystem perspective: understand host–vector–pathogen networks, trophic interactions, and physical environment to control disease spread (schistosomiasis, rabies prevention, etc.).
Key Equations & Numbers (LaTeX format)
Shannon diversity index: (H = -
\sum{i=1}^{S} pi
\ln p_i)Bottom-up sequence: N \rightarrow V \rightarrow H \rightarrow P
Species–area relationship: (S = cA^{z})
Competitive exclusion demonstrated when P.\,caudatum \to 0 in mixed culture with P.\,aurelia.
Cedar Creek: plots with 16 spp. produced significantly more biomass than mono-cultures across a decade.
Florida Keys fumigation: islands regained pre-fumigation richness (~40 spp.) within \approx 140 days.
Ethical, Practical & Philosophical Notes
Recognizing value of keystone & foundation species informs conservation priorities (e.g., protecting sea stars or coral restoration).
Ecosystem engineering underscores role of species as environmental modifiers; human engineering can mimic or counteract these effects.
Disease ecology links wildlife conservation with public health;