Because water fills the soil air spaces, restricting respiration.
Explanation
Soil-dwelling organisms, such as earthworms, rely on cutaneous respiration, meaning they exchange gases through their moist skin. This physiological process requires oxygen to be available in the interstitial air spaces (pores) between soil particles.
Analysis:
- To escape the sudden drop in soil temperature. is incorrect: Soil acts as an insulator and has high thermal inertia. Consequently, soil temperature changes occur relatively slowly compared to air temperature. The immediate surfacing is not a thermoregulatory response.
- Because water fills the soil air spaces, restricting respiration. is correct: Under normal conditions, soil pores contain a mix of air and water. During heavy rainfall, water infiltrates the soil and displaces the air pockets, leading to saturation. Since oxygen diffuses significantly slower in water than in air, the soil environment becomes hypoxic (oxygen-deficient). Organisms migrate to the surface to access atmospheric oxygen and prevent asphyxiation.
- To feed on surface vegetation washed down by rain. is incorrect: Earthworms are detritivores that feed on decaying organic matter within the soil profile. Surfacing exposes them to UV radiation and predation, making it a maladaptive behavior for feeding purposes.
- To avoid predation by sub-surface predators. is incorrect: While predation is a constant ecological pressure, the specific timing of mass surfacing during rainfall is driven by the abiotic factor of oxygen deprivation rather than predator avoidance.
Key Takeaway:
Heavy rainfall causes waterlogging that displaces air from soil pores, restricting the oxygen supply necessary for the respiration of soil-dwelling organisms, forcing them to surface.