Correct Option
The correct option is Earthworms.
Explanation
Detritivores (also known as detritus feeders) are heterotrophic organisms that obtain nutrients by consuming detritus, which consists of decomposing plant and animal parts as well as faeces. They play a critical role in the initial stage of decomposition by performing fragmentation-breaking down large organic particles into smaller ones. This process increases the surface area available for microbial decomposers (bacteria and fungi).
Option Analysis
- Earthworms is correct: Earthworms are the quintessential example of detritivores in terrestrial ecosystems. They ingest soil mixed with decaying organic matter, break it down physically in their digestive tract, and excrete it. This activity not only recycles nutrients but also aerates the soil.
- Phytoplankton is incorrect: Phytoplankton are microscopic marine algae and cyanobacteria. They are producers (autotrophs) that synthesize their own food via photosynthesis and form the base of aquatic grazing food chains.
- Molluscs is incorrect: While some species within the phylum Mollusca feed on detritus, the group is extremely diverse and includes herbivores (e.g., snails grazing on algae) and active carnivores (e.g., octopuses, squids). Therefore, it does not exclusively or correctly identify common detritivores in a general context.
- Sparrows is incorrect: Sparrows are vertebrates that function as consumers. They are generally omnivorous, feeding on seeds (primary consumers) and insects (secondary consumers), and are part of the grazing food chain rather than the detritus food chain.
Key Takeaway
Detritivores (e.g., earthworms, termites, dung beetles) are distinct from decomposers (e.g., bacteria, fungi). Detritivores physically ingest and fragment dead organic matter, whereas decomposers secrete enzymes to chemically break down organic substances extracellularly.