Diversity Pattern and Ecological Roles of Freshwater Invertebrates

Freshwater invertebrates consistently play decisive role in the functioning of all aquatic ecosystems through their diverse participation in energy flow, nutrient recycling, and population regulation.

Despite their importance in stream and lake ecosystems, only about 5% of all invertebrates’ species live in fresh waters and approximately half of the 40+ phyla of metazoan and heterotrophic protozoa have fresh water representatives.

This relatively low faunal diversity and phyletic representation is indicative of the strenuous environmental conditions found in freshwater, including low osmotic state, thermal extremes and variability, and both ecological and evolutionary instability.

Environmental instability, especially the tendency of habitats to become parched, has required special life-history adaptations for survival in ephemeral aquatic ecosystems. The nature of an invertebrate’s feeding, respiratory, osmotic, and reproductive systems reflect both the unique habitat conditions to which it has adapted and its evolutionary history.

Ecological Roles of Freshwater Invertebrates

Freshwater Invertebrates consistently play decisive role in the functioning of all aquatic ecosystems through diverse participation in energy flow, nutrient recycling and population regulation. They occupy all heterotrophic functional feeding groups, such as algal grazers, filter
feeders, shredders, carnivores, and detritivores.

As important components of aquatic food webs, invertebrates bridge the gap between primary producers and fish or other high trophic level consumers. Few invertebrates’ species occupy the only one functional feeding group in all ecosystem, seasons and life stages.

Meanwhile it is usually unrealistic to assign many invertebrates e.g. crayfish to one feeding guild because these omnivores eat a great variety of living and dead animals and plants and all feeding guild of freshwater invertebrates fall victim in turn to predator such as benthic and pelagic fish.

By recycling carbon and other nutrients, invertebrates reduce loss of energy and vital elements to sediments down the stream and shorten the time to recycle materials through community food webs.

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This recycle process is termed nutrient recycling in lakes and nutrient spiraling in rivers. Without invertebrates contributions to recycling process, aquatic ecosystems would support far fewer species owing to lower available productivity and many shallow ponds would soon fill with non-decomposed plants.

In addition to roles in nutrient recycling, parasitic and predaceous invertebrates may function as density-dependent regulators of population sizes of lower trophic level species. Furthermore, by preying selectively on dominant species, a carnivore or herbivore may influence relative abundance and species diversity within an entire community far out of proportion to the predators’ abundance; such species are sometimes called “keystone predators”

Diversity of Habitat

Aquatic ecosystems consist of entire drainage basins. The nutrient and organic matter content of drainage water from the catchments area is modified in each of the terrestrial, stream, and wetland-littoral components, as water moves down the gradient to and within the lake or reservoir.

Autotrophic photosynthetic productivity is generally low to intermediate in the terrestrial components, highest in the wet-land interface region between the land and water, and lowest in the open water. Autotrophic productivity in river channels is generally low, as in the pelagic regions of lakes. Most of the organic matter of running waters is imported from floodplain and terrestrial sources.

The land-water interface region of aquatic ecosystems is always the most productive per unit area along the gradient from land to open water of both lakes and reservoirs. Because most aquatic ecosystem occur in geomorphologically mature terrain of gentle slopes and are small and shallow, the wet-land littoral components usually dominate in productivity and the synthesis of organic matter. The region of greatest productivity is the emergent macrophytes zone.

Emergent aquatic plants have a number of structural and physiological adaptations that not only tolerate the hostile reducing anaerobic sediments but exploit the high nutrient and water availability of this habitat. Nutrients entering the zone of emergent aquatic macrophytes zone tend to be assimilated by the microflora of the sediments and detritus particles, and are then recycled to the emergent macrophytes.

The deep-water pelagic zone of lakes is least productive along the gradient from land to water, regardless of nutrient availability. Growth of phytoplanktonic algae of the pelagic zone is limited by sparse distribution in a dilute environment where nutrient recycling is restricted by the sinking of senescent phytoplankton below the depth of photosynthesis.

When nutrient recycling and availability are increased, greater phytoplankton cell densities attenuate underwater light and reduce the volume of water in which photosynthesis occurs.

Despite low productivity per unit area, pelagic productivity can be collectively important in large lakes and for higher trophic levels that depend on this organic matter.

A second trophic level consists of zooplankton dominated by four major groups of animals: protozoa, rotifers, the crustaceans and benthic invertebrates. In the pelagic zone these herbivorous organisms are consumed by small fishes, fry of larger fishes, and predatory zooplankton, which comprise a third trophic level. A fourth trophic level may consist of medium-sized piscivorous fishes, and the fifth level includes piscivorous fishes. Higher trophic levels are rare in fresh water.

The species composition of the higher trophic levels affects the pathways of energy utilization from lower trophic levels. For example, efficiency of consumption of primary production by zooplankton is often appreciably greater in the absence of zooplankton-feeding fishes than in their presence.

The community structure of phytoplankton responds variably to grazing impacts in concert with their available resources (light, nutrients, organic constituents) and may or may not be able to compensate for grazing losses in overall primary production.

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