A. Competition = both harmed (−/−) = III ✓
B. Predation = predator benefits, prey harmed (+/−) = I ✓
C. Mutualism = both benefit (+/+) = II ✓
D. Commensalism = one benefits, other unaffected (+/0) = IV ✓
Answer: A-III, B-I, C-II, D-IV
Species interactions classified by effect (+ benefit, − harm, 0 neutral): Mutualism +/+: both benefit. Commensalism +/0: one benefits, other neutral. Predation/Parasitism +/−: one benefits, one harmed. Competition −/−: both harmed. Amensalism 0/−: one neutral, one harmed. Neutralism 0/0: neither affected.
Intraspecific (same species) > interspecific competition in intensity. Lotka-Volterra competition equations predict outcomes. Competitive exclusion (Gause's principle): one species eliminates other if identical niches. Character displacement: competing species evolve different traits to reduce overlap. Ecological niche: sum of all conditions species needs.
Lotka-Volterra predator-prey model predicts oscillating population cycles: prey increases → predator increases → prey decreases → predator decreases → prey recovers → cycle repeats. Classic example: lynx-snowshoe hare cycles in Canada (10-year cycles). Prey defences: camouflage, warning colouration, mimicry, spines, toxins.
Obligate mutualism: both species cannot survive without the other. Facultative mutualism: beneficial but not essential. Key examples: Rhizobium-legume (nitrogen fixation). Mycorrhizae (80% of plants). Lichens (fungi+algae/cyanobacteria). Fig-fig wasp (obligate pollination mutualism). Cleaner fish removing parasites from larger fish. Oxpecker birds on large mammals.
Each interaction is classified by its effect on the two species. Mutualism benefits both (+/+); competition harms both (−/−); predation and parasitism benefit one and harm the other (+/−); commensalism benefits one and leaves the other unaffected (+/0); and amensalism harms one while leaving the other unaffected (−/0). Writing the sign pair before naming the interaction makes these questions mechanical rather than a memory test.
Two species competing for the same limiting resource cannot coexist indefinitely — the better competitor eliminates the other. In practice coexistence is common because species partition resources instead. MacArthur's warblers feeding in different zones of the same conifer is the standard example: behavioural resource partitioning avoids direct competition and lets several similar species share one tree.
For mutualism: lichen (fungus and alga), mycorrhiza (fungus and plant roots), and the fig and its pollinator wasp, which is the classic case of one-to-one co-evolution. For commensalism: an orchid growing on a mango branch, barnacles on a whale, and cattle egrets following grazing cattle. For amensalism: Penicillium releasing penicillin that kills nearby bacteria. Named examples carry marks that a definition alone does not.
Confusing commensalism with mutualism. In commensalism only one species gains and the other is genuinely unaffected — not slightly harmed or slightly helped.
Treating predation as purely harmful. Predators keep prey populations in check and maintain species diversity; removing them can cause competitive exclusion among the prey.