The Marine Ecosystem and Food Webs

The transfer of matter and energy within the marine community has resulted in a close interdependence of three major categories of marine organisms: producers, consumers, and decomposers. Plants are autotrophic, or self-nourishing, capable of absorbing readily available solar energy and building high-energy organic substances such as carbohydrates. In the process, they use inorganic nutrients (primarily nitrate and phosphate), water and dissolved gases. Plants are referred to as primary producers of the marine ecosystem and are placed in the first trophic level. The consumers and decomposers are unable to synthesize their own food from inorganic(not made by living things) substances and ultimately must depend on plants for nourishment. These are the heterotrophs or consumers. Consumers are divide into three categories: herbivores, carnivores, and omnivores. Animals adapted to feed on plants are herbivores and occupy the second trophic level, while those that prey on other animals are carnivores. The carnivores occupy the third and higher trophic levels. The decomposers, primarily bacteria and fungi, exist on detritus, the waste products of all types of organisms as well as the dead remains of the organisms themselves.

The movement of nutrients and dissolved gases is cyclic in nature, going from plants to animals, to decomposers, and eventually back to plants. In contrast, the energy flow in an ecosystem is unidirectional, from the sun through the plants to the consumers and decomposers (fig 9.1). Living organisms, like most other energy-consuming systems, are not highly efficient in their use of energy. Much of the solar energy available at the sea surface is not absorbed by marine plants. Furthermore, a portion of the energy that is captured by the plants in photosynthesis is used for cellular maintenance, growth, and reproduction. Thus only a fraction of the energy produced by photosynthesis is available to the herbivores.

A similar decrease in available energy occurs between the herbivores and carnivores. The efficiency of energy transfer in marine organisms from one trophic level to the next is between 6% to 20%. In other words, only 6%-20% of the energy available to any trophic level is usually passed on to the next. A widely accepted average efficiency is 10%.

The paths of nutrient and energy flow through the living portion of the marine ecosystem are referred to as food chains. With only a few near-shore exceptions, the first trophic level of marine food chains is occupied by widely dispersed plants of microscopic dimensions called phytoplankton. The microscopic character of most of the marine primary producers imposes a size restrictions on many of the occupants of higher marine trophic levels. Since very few animals are adapted to feed on organisms much smaller than themselves, marine herbivores also are usually quite small(zooplankton, small fish). Invariably, large marine animals are carnivores and occupy levels far up the food chain. In contrast, the plants of the terrestrial (land) ecosystems are generally quite large. As a result, most large land animals are herbivores. Food chains can be arranged in a linear fashion to illustrate the decrease in available energy and material from lower to higher trophic levels. Figure 2.9 illustrates such a food pyramid proceeding from phytoplankton to herring at the third trophic level.

Vary seldom in natural system do straight-line food chains actually occur. A more descriptive term for the complex feeding relationships of organisms is a food web. Figure 2.10 outlines in greater detail the major trophic relationships of the organisms leading to the herring in figure 2.9. Even the complex of feeding relationships outlined in figure 2.10 is an oversimplification, for it ignores other marine animals that compete with the herring for the same food sources.

Symbiosis

Many other marine organisms obtain their food by establishing highly specialized symbiotic relationships. Symbiosis is an intimate and prolonged relationship between two or more species in which at least one of the species benefits. Symbiotic relationships can be classified into three categories: commensalism, mutualism, and parasitism (Figure 2.12.). Commensalism provides an obvious benefit to one partner (the symbiont) without seriously affecting the host one way or another. Examples are the remoras, fishes that attach to sharks, whales, and turtles. The remoras obtain food from small fragments of the host’s prey. In mutualism, both participants benefit. Such a relationship exists between the large reef fish and the cleaner fish that keep them free of parasites. Another example of mutualism is the clown fish and the sea anemones. The clown fish brings food to the anemones, while the stinging tentacles of the sea anemone serve as a deterrent to predators seeking out the clown fish. Parasitism is a relationship where one participant, the parasite, benefits at the expense of the host. Many fish are host to isopods that attach to the fish and derive their nutrition from the body fluids of the fish, there by robbing the host of its energy supply.

Questions

1. a. What are the three major classes of marine organisms?

b. How does each get nutrition?

c. Give a living example of each from the marine and terrestrial environment.

2. What are three types of consumers and give a marine and terrestrial example of each?

3. Describe the flow of nutrients within the marine environment. How is the flow of energy different?

4. Describe the efficiency of energy flow in the marine environment.

5. Why are terrestrial food chains so different than marine food chains?

6. Why are food webs more informative than food chains?

7. Create a food web of at least 7 organisms in a marine and terrestrial environment.

8. Define symbiosis.

9. Give a terrestrial example of:

a. commensalism

b. mutualism

c. parasitism

10. Why don’t parasites want to kill their hosts?