If we assign a '+' sign to represent beneficial interactions, a '-' sign to represent detrimental interactions, and a '0' sign to represent neutral interactions, then the population interaction represented by a '+' '-' refers to what?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Parasitism
Interactions between two species are named by their effect on each one: ‘+’ for benefit, ‘–’ for harm and ‘0’ for no effect. A ‘+ –’ interaction means one species gains and the other is harmed. Two interactions in the book have this sign: predation and parasitism. Of these, only parasitism is among the options.
Option (1) Mutualism — Incorrect. Mutualism is ‘+ +’: both species benefit, as in lichens and mycorrhizae.
Option (2) Amensalism — Incorrect. Amensalism is ‘– 0’: one species is harmed and the other is unaffected. No species gains.
Option (3) Commensalism — Incorrect. Commensalism is ‘+ 0’: one species benefits and the other is neither harmed nor benefited.
Option (4) Parasitism — Correct. In parasitism the parasite benefits (+) and the host is harmed (–), so its sign is ‘+ –’.
NCERT Extract:
Population Interactions – “Interspecific interactions arise from the interaction of populations of two different species. They could be beneficial, detrimental or neutral (neither harm nor benefit) to one of the species or both. Assigning a ‘+’ sign for beneficial interaction, ‘-’ sign for detrimental and 0 for neutral interaction, let us look at all the possible outcomes of interspecific interactions (Table11.1). … In both parasitism and predation only one species benefits (parasite and predator, respectively) and the interaction is detrimental to the other species (host and prey, respectively).”
Table 11.1 : Population Interactions
| Species A | Species B | Name of Interaction |
| + | + | Mutualism |
| – | – | Competition |
| + | – | Predation |
| + | – | Parasitism |
| + | 0 | Commensalism |
| – | 0 | Amensalism |