Rivers, lakes, ponds and streams – natural freshwater ecosystems are a precious resource because of the rich biodiversity they support and the valuable ecosystem services they provide.
Freshwater ecosystems such as streams in their natural state are teeming with life forms and provide home to a variety of aquatic as well as terrestrial plants and animals that depend on each other and their surroundings for food and survival. Retaining this biodiversity is crucial not only for the health of ecosystems, but also for negating the effects of climate change.
Retaining biodiversity, for healthy freshwater bodies
Freshwater ecosystems have a food cycle of their own and plant litter, especially leaf litter forms an important part of this food chain! Leaf litter enters streams through two routes, directly by vertical litterfall or laterally from the forest soil and is transported downstream through water flow or is retained in boulders or logs.
This retained litter serves as an important source of food and energy for organisms that reside in the streams and decomposition of litter contributes significantly to the global carbon cycle while also helping in maintaining the health and ecological balance of the freshwater body. This decomposition is mainly carried out by microorganisms such as aquatic fungi and specialised invertebrate organisms- mainly litter consuming detritivores that obtain carbon and nutrients from the litter and associated fungi.
What are detritivores?
Detritivores are organisms that feed on detritus or organic waste made up of dead trees, plants and animals. They also derive nutrition using coprophagy—a nutrition strategy that involves consuming the faeces of living organisms.
Detritivores occupy the topmost position in the food chain. Organisms referred to as producers (mainly plants and trees) form the base of the food chain and make their own food through photosynthesis or chemosynthesis. Above the producers are the different types of consumers namely herbivores, carnivores, omnivores etc. Detritivores consume detritus as well as the decaying forms of producers or consumers.
On land, detritivores include insects like beetles, butterflies, dung flies, small animals like slugs and snails, and soil-dwelling millipedes, centipedes, and woodlice. In marine environments (oceans), detritivores include lobsters, crabs, and even sea cucumbers or sea stars. Some marine detritivores also live on seabed and are known as bottom-feeders. Others include barnacles, polychaete worms and corals that obtain their nutrition by feeding on floating organic decaying matter.
Detritivores in aquatic (freshwater) ecosystems
Detritivores are classified on the basis of food source, modes of feeding, and habitat preferences. For example, in aquatic ecosystems mayflies, caddisflies, chironomid midges, and elmid beetles, which graze or scrape microflora (bacteria) from mineral and organic surfaces are classified as scrapers; stoneflies, caddisflies, crane flies, crayfish, and shrimp are classified as shredders as they chew on large pieces of decomposing material; stoneflies, mayflies, crane flies, elmid beetles, and copepods are classified as gatherers as they feed on fine particles of decomposing organic matter; and mayflies, caddisflies, and black flies as filterers as they have specialised structures for sieving fine suspended organic material.
Biodiversity loss spells doom for tropical streams
Local studies show that presence of a diverse group and adequate numbers of detritivore organisms is important for health of streams. However, no global study exists on the relationship between detritivore diversity and decomposition in streams, which can greatly help to understand local and regional variations in the diversity–decomposition relationship.
This paper titled ‘Impacts of detritivore diversity loss on instream decomposition are greatest in the tropics’ published in the journal Nature Communications discusses the findings of a global study conducted on 38 streams distributed across 23 countries including Tamil Nadu in India.
The study aimed to assess if detritivore diversity helped in better litter decomposition in streams; the role of factors such as abundance, biomass and body size of detrivores in decomposition and how patterns of decomposition varied across locales by type of vegetation and climate.
The study found that:
- High diversity in detritivore organisms aided better decomposition of litter in streams thus helping to maintain the ecology and health of streams in tropical areas.
- High numbers of detritivores and presence of more organic matter were found to influence decomposition in temperate and boreal areas.
- The influence of detritivore abundance and presence of organic matter on decomposition varied with latitude, with negligible effect in the tropics and more at higher latitudes.
- A strong relationship was found between detritivore diversity and decomposition in the tropics indicating that species losses in these areas, where detritivore diversity is already lower than at higher latitudes can have the greatest impact on decomposition and consequently on freshwater ecology and health.
- Detritivores in tropical areas are vulnerable due to a number of environmental stressors. For example, excessive and uncontrolled use of harmful pesticides in agriculture can lead to deaths of a large number of detritivores. Urbanisation, land use changes, destruction of habitats such as streams due to construction activities can also spell doom for detritivores. Climate warming is also feared to cause more extinctions in the tropics because detritivore species are very sensitive to changes in temperature and can suffer greater physiological changes, even with smaller changes in temperature.
- Lower detritivore diversity and their higher susceptibility to extinction can make streams in tropical areas more vulnerable as reduced decomposition rates can lead to impaired ecosystem functioning. With overexploitation of natural resources feared to further affect tropical streams, efforts to conserve and protect tropical detritivore species need to be made a priority concern globally, argues the paper.