Invertebrates, Freshwater, Overview Headwater streams • Closed canopy • Abundant leaf fall • Shallow channel • Swift flow • Pebbles to boulders 373 Shredders Filterers and scrapers River • Open canopy • Sunlight reaching water • Deep channel • Slow flow • Fine sediments Tributary Burrowers and plankton Figure Schematic illustrating the River Continuum Concept (RCC) that proposed a relationship between stream size (small upland streams to large downstream rivers), flow and bottom substrate, and type of macroinvertebrate groups likely to dominate In the smallest streams, large inputs of leaves and wood along with benthic algae are particularly important sources of food for invertebrates These invertebrates ‘‘shred’’ the leaf material into finer particles and thus facilitate its further breakdown by fungi and bacteria Further downstream, where sunlight penetrates the water and flow is reduced, algae are also important sources of food and many invertebrates ‘‘scrape’’ the algae from rock surfaces In the very open riverine areas, plankton may be an important food source and many of the benthic invertebrates burrow into the fine sediments Modified from Vannote RL, Minshall GW, Cummins KW, Sedell JR, and Gushing E (1980) The river continuum concept Canadian Journal of Fisheries and Aquatic Sciences 37: 130–137 typically crawling with diverse assemblages of crustaceans, mollusks, and insects Small shrimplike crustaceans (amphipods) may reach very high levels of abundance on aquatic vegetation Water beetles, odonates (dragonflies and damselflies), and waterbugs (water boatmen, waterstriders, and backswimmers) are commonly the top predators Global Patterns in Freshwater Biodiversity Knowledge of lineage histories is very important because regional patterns of distribution of species are often a reflection of distinctly different evolutionary histories Adaptations to specific climates and habitat characteristics influence speciation and the ability of an organism that evolved under one set of circumstance to move broadly among different geographic areas Terrestrial and marine biologists generally have found the highest levels of animal and plant diversity in the tropics with lower levels in temperate and polar regions This is not necessarily the case for freshwater invertebrates Species richness for freshwater invertebrates is related in very complex ways to latitude and elevation, and global patterns of diversity are not straightforward for this group There have been far fewer surveys of freshwater invertebrates in the tropics than in temperate regions; however, those to date suggest that biodiversity in fresh water is not typically higher in low latitude, tropical habitats For example, in North America, bivalves and crayfish appear to be the most diverse in temperate zones (about 260 native bivalves and 320 species of crayfish) Temperate zones have a wealth of crayfish species, but tropical fresh waters have a richer shrimp and freshwater crab fauna Insects, mayflies, and stoneflies are more diverse in temperate latitudes than in the tropics, whereas dragonflies, waterbugs, and water beetles are much more diverse in the tropics Factors Influencing Biodiversity in Fresh Waters Scientists have devoted their lifetimes to the study of what determines the number of species in a given habitat type or