Species Interactions
The six types of interspecific interaction
Every pair of species in a community interacts in one of six ways, characterized by the effect on each species (positive +, negative -, or neutral 0). These interactions are the building blocks of community structure and shape the evolution of every species involved.
💡 Keystone Species and Ecological Succession
Keystone species: A species whose impact on community structure is disproportionately large relative to its biomass or abundance. Remove the keystone and community structure collapses. Robert Paine's classic experiment: removing sea stars (Pisaster ochraceus) from an intertidal zone allowed mussels to competitively exclude all other species, reducing the community from 15+ species to essentially one. Sea otters are keystone predators that control sea urchin populations — when otters were hunted to near-extinction, urchin populations exploded → kelp forests were grazed down → biodiversity collapsed.
Ecological succession: The directional change in community composition over time after a disturbance or on newly available substrate. Primary succession begins on bare rock with no soil (after glacier retreat, volcanic eruption) — pioneer species (lichens, mosses) colonize, weather rock, build organic matter, enabling soil formation → more complex plant communities establish → climax community (stable endpoint). Secondary succession begins where soil already exists but the community has been disturbed (forest fire, farm abandonment) — faster than primary because soil and seed bank already present. The climax community concept is debated — many ecologists view succession as a dynamic process without a fixed endpoint.
Com
Competition (-/-) — both species harmed
Interspecific competition occurs when two species use the same limiting resource. Both species experience reduced growth, survival, or reproduction when the other is present. Two outcomes are possible:
Competitive exclusion principle (Gause's law): Two species competing for exactly the same limiting resource in the same way cannot stably coexist — one will eventually outcompete and eliminate the other. The superior competitor wins; the inferior competitor goes locally extinct or is forced to change its resource use.
Resource partitioning: Species that would otherwise competitively exclude each other can coexist by specializing on different subsets of the shared resource — slightly different food sizes, different microhabitats, different foraging times. The species' realized niches (actual resource use under competition) become narrower than their fundamental niches (potential resource use without competition). This is character displacement — species competing in the same habitat often evolve to be more different from each other than populations of the same species that live in different habitats (e.g., Darwin's finches with different beak sizes on the same island vs. different islands).
Memory trick: Competition = -/- = both lose something. Competitive exclusion = only one can win long-term. Resource partitioning = they can coexist if they specialize.
Pre
Predation (+/-) — predator benefits, prey harmed
Predation is any interaction where one organism (predator) kills and consumes another (prey). This includes herbivory (consumer eats plant), carnivory (animal eats animal), and parasitoidism (parasitoid lays eggs in a host, larvae consume it). Predation drives evolution on both sides — an evolutionary arms race.
Classic predator-prey cycles: Lotka-Volterra equations model oscillating population cycles — prey population increases → predators have abundant food → predator population increases → more predation pressure → prey population decreases → predators starve → predator population decreases → prey recovers → cycle repeats. The Canadian lynx-snowshoe hare cycle (roughly 10-year oscillations) is the textbook example, though the real system is more complex (hare population also cycles with vegetation availability).
Prey defenses: camouflage, warning coloration (aposematism — bright colors signal toxicity), mimicry (Batesian: palatable species mimics toxic species; Müllerian: two toxic species converge on same warning pattern), behavioral defenses (schooling, herding, alarm calls).
Memory trick: Predation = +/- = predator wins, prey loses. Arms race: prey evolves defenses, predators evolve counter-defenses, round and round. Lynx and hare = the textbook predator-prey cycle.
Sym
Symbiosis — mutualism, commensalism, parasitism
Symbiosis ('living together') includes any close, long-term interaction between two species:
Mutualism (+/+): both species benefit. Mycorrhizal fungi and plant roots (fungi increase water/mineral absorption; plant provides carbohydrates). Nitrogen-fixing bacteria in legume root nodules. Pollination (bee gets nectar; plant gets pollinated). Cleaning stations (cleaner wrasse removes parasites from large fish; wrasse gets food). Mutualism is extraordinarily common and underlies the functioning of most ecosystems.
Commensalism (+/0): one benefits, other unaffected. Epiphytes growing on tree branches (epiphyte gets a sunny perch; tree unaffected). Remoras on sharks (remora gets transport and food scraps; shark theoretically unaffected). True commensalism may be rare — most apparent commensalisms turn out to be slight mutualisms or slight parasitisms on closer study.
Parasitism (+/-): parasite benefits, host harmed. The most common lifestyle on Earth — >50% of animal species are parasites at some point in their life cycle. Parasites reduce host fitness but rarely kill quickly (unlike predators) — a dead host is no longer a resource.
Memory trick: Mutualism = +/+ = both win. Commensalism = +/0 = one wins, one doesn't care. Parasitism = +/- = parasite wins, host loses. '+ means the species benefits, - means it's harmed, 0 means no effect.'
🔬 Applied Scenario — Community Ecology in Conservation and Invasive Species
Community ecology principles explain why ecosystems respond to disturbance the way they do:
A
Wolves in Yellowstone — keystone predators and trophic cascades. After wolves were reintroduced to Yellowstone in 1995, elk behavior changed dramatically — elk avoided open valleys and riverbanks where wolves could easily hunt them. Riparian vegetation recovered in previously overgrazed areas → stream banks stabilized → beaver populations returned → beaver dams created wetlands → fish, amphibians, and songbirds increased. The wolves changed not just elk numbers but elk behavior, triggering a cascade through the entire ecosystem — a 'landscape of fear.'
B
Invasive species and competitive exclusion. Introduced species often escape their native competitors, predators, and parasites → can competitively exclude native species. Kudzu (Japanese vine introduced to southeastern US) grows up to 30 cm/day and smothers native vegetation. The brown tree snake (introduced to Guam) has driven 9 of 12 native forest bird species to extinction. Zebra mussels in the Great Lakes outcompete native mussels and filter so much phytoplankton that food webs throughout the lakes are restructured.
C
Mutualism breakdown — fungal partners and coral bleaching. Many of the most important ecosystem relationships are mutualisms that can break down under stress. The coral-zooxanthellae mutualism fails under thermal stress (bleaching). The mycorrhizal mutualism that enables most plant nutrient uptake is disrupted by excess nitrogen fertilizer (when N is abundant, plants invest less in mycorrhizal partners). Disrupting key mutualisms can destabilize entire communities.
D
Parasites as community regulators. Parasites are among the most important community regulators — they preferentially attack dominant competitors, preventing competitive exclusion and maintaining diversity (the 'parasite-mediated coexistence' hypothesis). Rinderpest virus in African ungulates preferentially killed wildebeest and buffalo, suppressing these dominant grazers and allowing other herbivore species and vegetation to persist. When rinderpest was eradicated in 2001, wildebeest populations in the Serengeti tripled, fundamentally changing vegetation structure and fire regimes.
📌 Exam Application
Community ecology covers a lot of ground — focus on these high-yield areas:
1. Interaction types and signs: Competition (-/-), Predation (+/-), Mutualism (+/+), Commensalism (+/0), Parasitism (+/-), Amensalism (-/0). Know the effect on each species.
2. Competitive exclusion principle: Two species competing for identical resources cannot coexist — one wins. Resource partitioning allows coexistence by niche differentiation.
3. Fundamental vs realized niche: Fundamental = full potential range without competition. Realized = actual range under competition. Competition narrows the realized niche.
4. Keystone species: Disproportionate impact on community structure. Sea stars in intertidal, wolves in Yellowstone, sea otters in kelp forests.
5. Succession: Primary (bare rock, no soil) vs secondary (soil present, disturbed community). Pioneer species → intermediate communities → climax community.
⚠️ The Most Common Community Ecology Mistakes
Competitive exclusion requires the SAME limiting resource. Two species can coexist even if they seem to compete — as long as they use slightly different resources (resource partitioning). The competitive exclusion principle only applies to species competing for exactly the same limiting resource in exactly the same way at the same time. When you see two similar species coexisting, look for the subtle resource partitioning that allows it.
Fundamental niche ≠ realized niche. The fundamental niche is where a species COULD live based on its physiology alone. The realized niche is where it ACTUALLY lives, constrained by competition, predation, and other biotic factors. The realized niche is always ≤ the fundamental niche. In the absence of competitors, species often expand into their full fundamental niche (demonstrated by removing competitors experimentally).
Mutualism is not always obligate. Some mutualisms are obligate (neither partner can survive without the other — mycorrhizae for many orchid species). Many mutualisms are facultative (both partners can survive alone, but benefit from the relationship). Assuming all mutualisms are obligate is incorrect.
✓ Quick Self-Test
1. What is the competitive exclusion principle and what is resource partitioning?
2. Describe the difference between fundamental and realized niche.
3. What is a keystone species? Give two examples.
4. Compare mutualism, commensalism, and parasitism in terms of effect on each species.
5. What is the difference between primary and secondary succession?
Answers:
1. Competitive exclusion principle (Gause's law): two species competing for exactly the same limiting resource in the same way cannot stably coexist — one will eventually eliminate the other. Resource partitioning: competing species can coexist by specializing on different subsets of a shared resource (different food sizes, different microhabitats, different times), reducing direct competition.
2. Fundamental niche: the full range of resources and conditions a species can potentially use in the absence of competition and other biotic constraints. Realized niche: the actual subset of the fundamental niche that a species occupies in the presence of competitors, predators, and other biotic interactions. Competition typically narrows the realized niche below the fundamental niche.
3. A keystone species has a disproportionately large impact on community structure relative to its biomass or abundance. Examples: (1) Sea stars (Pisaster) in Pacific intertidal — removal allows mussels to monopolize the habitat, reducing community diversity from 15+ species to ~1. (2) Sea otters — control sea urchin populations; without otters, urchins overgraze kelp forests, collapsing biodiversity.
4. Mutualism (+/+): both species benefit (mycorrhizal fungi and plant roots, pollinators and flowers). Commensalism (+/0): one species benefits, the other is unaffected (epiphytes on trees). Parasitism (+/-): parasite benefits at the host's expense, reducing host fitness without usually killing it quickly (tapeworms, ticks, Plasmodium).
5. Primary succession begins on bare substrate with no soil (volcanic lava fields, glacial till) — pioneered by lichens and mosses that weather rock and build organic matter over decades to centuries. Secondary succession begins on disturbed land where soil already exists (abandoned farmland, burned forest) — faster because the soil seed bank and nutrient base are already present.