SUCCESSION
Have you ever had the experience of starting a garden and having to deal with all those weeds? After much hard work clearing the space and planting the seeds or seedlings, no sooner is the garden established than all your efforts are diverted to removing weeds that seem to come from nowhere. Weeds in the garden are an indicator of a larger ecosystem. Your garden is invaded by pioneer plant species, weeds that try to reclaim the garden as an integral part of this larger ecosystem.
Weeds have evolved to grow under fairly harsh conditions. Through their growth and development, they condition the soil so it becomes possible for other plant species to grow.
These secondary plants then attract small and large animals that play a variety of roles in the life cycle of all the plants.
This is the process of succession that can be observed in every ecosystem and every biome on the planet. In some ecosystems, succession is complex and slow, taking from tens to hundreds of years, such as deciduous hardwood forests in the Northern temperate climates. In other ecosystems, succession takes only 3 to 5 years, such as some grasslands.
This same sequential process occurs in aquatic systems. If you were able to return to the edge of a pond at the same time each year, for several years, you would see a progression of plants growing on the edge of the water and moving into the submerged areas. The plant communities on the edge would increase in species diversity as they trap mud and other debris. This would invite a range of animals to make that spot their home.
Animals, ranging from microorganisms to large vertebrates, follow the process of succession. They might depend on the plants for food, or consume other animals including invertebrates or microorganisms. This interlinking relationship of organisms consuming other organisms is called a food web. Each ecosystem has a slightly different food web. As succession takes place in a body of water, an increasingly complex food web develops. There are many levels to a food web, including decomposers, producers, and consumers. For example, algae and other aquatic plants are the producers in the aquatic food web. Using the Sun's energy, they form carbohydrates during photosynthesis. |
WEBS OF FOOD
Macroinvertebrates, such as mayflies and water beetles, are consumers that eat the aquatic plants as their source of energy. These macroinvertebrates form the first trophic level of the food web. Other consumers such as fish and frogs may eat the macroinvertebrates; they form the second trophic level because they consume the organisms that eat the plants. Most food webs consist of at least two trophic levels and at most five, depending on the complexity of the system.
In this diagram of the food web, there is an arrow pointing from the fish to the kingfisher. This indicates that the bird is a consumer and eats the trout (or the trout hatchlings - a kingfisher can only catch trout that are 5 cm long, or less. Adult trout average 45 cm in length). The trout is also a consumer and eats a variety of macroinvertebrates, as shown  by the arrow. The nymphs (young insects), in turn, eat aquatic plants and algae, and possibly some other microinvertebrates. Kingfishers, trout, nymphs, and plants all have arrows pointing to the detritus icon in the diagram. This indicates the active role of decomposers in the food web. Plants use the nutrients released by the decomposers in the process of photosynthesis.
Adapted from STREAM ECOLOGY: A JOURNAL FOR ACTION by Jeffrey C. De Pew et al. (p. 59), copyright 1993 Missouri Botanical Garden. Reprinted with permission of Missouri Botanical Garden, St. Louis, Missouri.
The pyramid below shows the relative abundance of organisms at each trophic level. The base of the pyramid has the greatest abundance of the primary producers—the aquatic plants and algae. Notice that each trophic level above has fewer numbers of organisms than the one below. Why do you think this might be? The reason that there are fewer organisms at the top of the pyramid has to do with inefficient energy flow in the food web. As algae and other plants photosynthesize, they are able to utilize less than half of the available radiant energy from the Sun. Due to biological processes, the consumers that eat the plants are able to use only a small percentage of the energy contained in plants' organic matter.
At maximum, only 20% of the available energy is transferred up to higher levels. This results in a lesser abundance of organisms in the top level of the food web.
How do scientists determine the food web of an ecosystem? It is a fascinating but sometimes messy process. The most direct method is to catch some of the animals and obtain samples of their stomach contents. This used to involve killing the animal, but biologists now have new sampling methods that do not harm the animal. This is especially important for rare or endangered species. By examining the stomach contents it is possible to develop a fairly comprehensive view of what was eaten. These data are augmented by field observations of the animal hunting and eating. It is also possible to identify animal diet by examining its feces.
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Reference:
Ricklefs, Robert E., Ecology (New York: W. H. Freeman and Co., 1990).
Adapted from STREAM ECOLOGY: A JOURNAL FOR ACTION, by Jeffrey C. De Pew et al. (p. 59), copyright 1993 Missouri Botanical Garden. Reprinted with permission of Missouri Botanical Garden, St. Louis, Missouri. |
