Exploring BWC Wetlands with Dr. James Atkinson
Few people take a closer look at our waters than Dr. James Atkinson, a biologist and educator retired from Michigan State University. In this first article of a two-part series, he describes the fascinating animals from the Bengel Wildlife Center that escape the naked eye.
Advocates of the conservation of lakes, ponds and wetlands usually focus on the large, charismatic organisms found in these ecosystems. Cranes, ducks, mink, moose and fish serve as symbols of the animals needing protection. However, these ecosystems also teem with tiny creatures whose diverse roles in complex food webs make them essential for the survival of the larger animals. Many of these creatures are visible only as tiny specs with the unaided eye. The microscope can reveal their complex structures and fascinating behaviors and give us clues to the critical roles these animals play in the aquatic food web.
In recent years, the term “food web” has replaced “food chain” because we’ve come to realize that the interaction of organisms is much more varied than originally thought. The bottom level of the aquatic food web consists of two types of resources upon which succeeding levels are based. The first resource is the producers: green plants, algae, and some protists (protists are single-celled plants and animals) that convert the energy of sunlight into organic compounds through photosynthesis. The second resource involves the organic waste (animal waste, the parts of dead plants and animals) that accumulates at the bottom of wetlands. Collectively referred to as “detritus,” this material serves as the medium for a wide variety of bacteria (the so-called “bacteria of decay”) which grow by breaking down the complex organic material into simpler components. Thus begins a process of recycling critical to a healthy aquatic ecosystem.
Subsequent levels of the food web are: consumer level I that is occupied by the herbivores that feed on the producers, and detritivores that feed on the detritus and the bacteria within it; consumer level II that includes animals that feed on occupants of level I as well as other members of level II; and consumer level III consisting of the ‘top ‘carnivores among the invertebrates such as insect larvae and crayfish and the smaller vertebrates such as fish fry, frogs and salamanders. Above this last group are the larger fish, birds such as ducks, herons and eagles, and mammals such as ourselves which feed on the insects, fish and frogs.
Consumer Level I
The herbivores: There are many types of single-celled organisms, protists, that feed on algae and the photosynthesizing protists. The microscopic animal herbivores that can be found in aquatic systems include the eight-legged water bears (tardigrades) and the round worms (nematodes) that have complex mouth parts allowing them to penetrate plant cell walls and suck out the contents. Newly hatched aquatic snails feed on algae and some plants by rasping with their ribbons of ‘teeth.’ The adults of such snails are not microscopic, but nevertheless function as level I consumers.
The detritivores: The most diverse group of microscopic creatures of consumer level I feed on the bacteria within the detritus or their products (simple organic compounds). Most of the familiar protists we learned about in high school biology such as Paramecium and Amoeba are detritivores. Among the many groups of animals that occupy level I is one of the smallest but perhaps most important, the gastrotrichs. These tiny animals (smaller than some protists) feed directly upon the bacteria. There are about 800 species of gastrotriches world wide, but under certain environmental conditions a single species may be found with a million individuals per cubic meter of water. This is because gastrotrichs are among the few animals that can survive at extremely low levels of oxygen. So, in wetlands that are heavily polluted with organic wastes and thus have extremely large numbers of bacteria, the gastrotrichs thrive. Since they serve as food for level II consumers, gastrotrichs help restore such polluted wetlands to good health.
Perhaps the most diverse group of the microscopic detritivores is the rotifers. Called “wheeled animacules” by Leewenhoek, the 16th century founder of microscopy, they appear to have spinning wheels at their anterior end. This appearance is produced by the rapid beating of hair-like cilia which function to either propel the animal through the water or propel the water through the animal. In the latter case, the animal consumes the bacteria and detritus in the water, digesting the nutrients and expelling the waste. Some rotifers stop swimming in order to feed and others are able to feed while they swim.
The largest of the microscopic detritivores are representatives of two groups of worms. The flatworms are free-living relatives of important parasites (tapeworms and flukes). They have a single opening into their digestive tract that serves as both a mouth and an anus. Some of the larger flatworms such as the planaria act as scavengers, feeding on the bodies of dead animals and thus acting like vultures in the process of recycling. Many small flatworms feed directly on the detritus, apparently digesting the bacteria. Many of these worms reproduce by producing small buds that remain attached to the parent as they grow. This gives the worm a chain-like appearance. These chains should not be confused with the segmented worms, microscopic relatives of the earthworm. Unlike the flatworms, the segmented worms have separate mouth and anus, and are often so transparent that the gut contents as well as their movement reveal their presence. The active feeding of segmented worms helps to stir up the detritus, releasing some of the smaller organic molecules into the water column and making them available to swimming animals and protists.
Another group of detritivores that stirs up the detritus while feeding is the microscopic crustaceans. The water fleas (cladocerans) and copepods are relatives of crayfish and crabs with jointed legs. Some species of water fleas and copepods swim within the detritus, pulling pieces of organic material into their mouths with their many jointed legs and digesting the bacteria within.
These Level I consumers are fed on by a host of animals that many first-time pond explorers as well as scientists find fascinating. We’ll look as these incredibly adapted predators in the next issue of The Wildlife Volunteer.