​A food chain involving Spartina (a plant), the marsh periwinkle snail, the blue crab, and an unknown fungus was identified in a Spartina-dominated salt marsh in North America. A study involving control and crab-exclusion experiments revealed:

A. Radulations (scrape marks) on the leaf surface made by the snails indicate the presence of snail faeces, fungi, and dead plant tissue.
B. The fungi were present only at the radulations.
C. The density of the radulations increased with higher snail densities.
D. Spartina density decreased with an increase in the snail density till it reached zero.
E. In control experiments, all four species were present till the end.

Select the option that correctly depicts the positive (+) and negative (-) interaction-type between fungi-snail and Spartina-crab, respectively.

Correct option is D

Explanation:

1. From the observations, the fungi are found at the radulations (scrape marks made by snails), suggesting that the fungi benefit from the snails' activity. Hence, the snail facilitates fungal growth — a positive interaction (mutualism or facilitation) for the fungi.

2. The presence of the blue crab helps control the snail population, preventing overgrazing of the Spartina plant. In the absence of the crab, the snail density increases, leading to more radulations and a drastic decrease in Spartina density (as stated in Point D: Spartina density decreased with increase in snail density till it reached zero). Thus, Spartina benefits from the crab presence, again representing a positive interaction.

Hence, fungi-snail: + (fungi benefit), and Spartina-crab: + (Spartina benefits from crab regulating snail population).

Information Booster:

1. Mutualistic or facilitative interactions occur when one species indirectly or directly benefits another in an ecosystem.
2. Snail-fungi interaction is positive because snail feeding facilitates fungal colonization.
3. Crab-Spartina interaction is positive because crab presence controls snail populations, reducing herbivory pressure on Spartina.
4. This is a classic example of a trophic cascade, where a top predator (crab) positively affects plant biomass (Spartina) by reducing herbivore pressure (snail).
5. The absence of a predator (crab) leads to top-down control failure, resulting in overgrazing by snails and a crash in plant population.
6. Such interactions highlight indirect mutualism, where the predator indirectly benefits the producer (Spartina).
7. Fungi here play a decomposer or secondary colonizer role, utilizing tissue damage created by herbivores for growth.

Additional Information:

• Option 1 (- and +): Incorrect. Fungi benefit from snails, so it’s not a negative interaction. This choice wrongly assumes a negative impact of snails on fungi.
• Option 2 (- and -): Incorrect. While Spartina is negatively affected by snails, it benefits from crabs. Also, fungi are positively affected by snails, not negatively.
• Option 3 (+ and -): Incorrect. It correctly represents fungi-snail interaction as positive, but Spartina-crab interaction is also positive (not negative), as crabs reduce snail herbivory pressure.