BLOG 7 – FOOD WEBS AND FOOD CHAINS

Food web vs. food chain

A food web is a graphical description of feeding relationships among species in an ecological community, that is, of who eats whom. It is also a means of showing how energy and materials flow through a community of species as a result of these feeding relationships. On The other hand, food chain is a succession of organisms in an ecological community that constitutes a continuation of food energy from one organism to another as each consumes a lower member and in turn is preyed upon by a higher member.

                                                           Fig. 1 A Food Web

The trophic-dynamic model of ecosystem structure

In ecology, trophic dynamics is the system of trophic levels, which describes the position that an organism occupies in a food chain: what an organism eats, and what eats the organism, for every level there is an increase in trophic level but a decrease in energy because of the absorption of biomass, thermodynamics and the law of conservation of energy in every trophic level. All trophic systems start with an autotroph that can be either photoautotroph or litoautotroph. All trophic-dynamic systems have four main parts: the abiotic environment, producers, consumers, and decomposers.

                                                  Fig. 2 An Ecological Pyramid

Top-down vs. bottom-up control of trophic levels

Both are theories of control of ecosystems but they have different functions.
Bottom-up control states that an ecosystem’s function is ultimately controlled by the nutrient supply to the primary producers and if the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will enter the cycle faster). 

Top-down control states that an ecosystem’s function is ultimately controlled by predation and grazing by higher trophic levels on lower trophic levels and if there is an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity "cascades" from the top levels of the food chain down to the bottom trophic levels.

Relationship between food web/food chains and biogeochemical cycles

            The movement or circulation of biogenetic nutrients through the living and non-living components of the biosphere or of any ecosystem is called biogeochemical cycling. Thus, it involves both biotic and abiotic components of organisms. There are two types of biogeochemical cycles, also called nutrient cycles, first is the closed system wherein nutrients such as oxygen is recycled instead of being lost and replenished constantly and is the complete opposite of the second type, the open system, where all energy comes finitely but within a long-lasting source like the sun but always lost. These two types are important in an ecosystem because they help producers and consumers live. Therefore, are needed for a productive food web/chain.

                                                                Fig. 3 Water Cycle

Applications of food web-food chain concepts

Absorption of Solar Radiation: autotrophs primarily absorb solar radiation.

Energy Transfer: By each trophic level consuming another, energy from the consumed level is transferred to the next. However, energy transfer is not efficient. In fact, only about 10 percent of the energy available at one level is actually passed onto the next. That means a massive level of life is needed at the base in order to sustain the life forms at the highest point. For example, if 100,000 calories are produced by a group of certain plants, only 1,000 calories will be transferred to the consumers that eat them. This is partly because not all plants or animals are consumed at every trophic level, nor are all the parts, such as beaks, shells, certain roots, leaves and poisonous fruits.

Interdependence: This interdependence of the populations within a food chain helps to maintain the balance of plant and animal populations within a community. For example, when there are too many carabaos; there will be insufficient grasses for all of them to eat. Many carabaos will starve and die. Fewer carabaos mean more time for grass and shrubs to grow to maturity and multiply. Fewer carabaos  also mean less food is available for the cow to eat and some carabaos will starve to death. When there are fewer cow, the carabao population will increase.

For example, many shark populations have been heavily overfished by people, further decreasing their predatory effects on otters. 

          In many places still without otters, commercial divers intensively harvest sea urchins for international seafood markets (see slide). Though the harvesting patterns and therefore the community effects of humans and otters may be quite different, to some extent, human harvests may simulate the controlling effect of otters. 

How have humans affected the food chain? 

    When we spray pesticides, we put the food chain in danger.  By breaking one link on the chain means all of the organisms above that link are in threat of extinction (like the domino effect).  By hunting animals nearly to extinction, everything above the animal in the food chain is put in danger.  A 'chain reaction' in the food chain can be perilous!  Since the food chain provides energy that all living things must have in order to survive, it is imperative that we protect it.

 IRRI is an autonomous international institute based in Los Banos, Laguna.  The Philippines is one of the foundation institutions of the CGIAR (Consultative Group on International Agricultural Research), and is dedicated to improving the lives and livelihoods of resource poor rice producers and consumers worldwide. It is a nonprofit organization doing research and training on agriculture.

IRRI has been at the forefront of rice research for almost thirty years (has been in the Philippines since 1960), delivering new rice varieties and practices to rice farmers throughout Asia and the developing world. Together, farmers and consumers find solutions to world hunger.

Through research, IRRI has been able to help almost half of the people all over the world who eat rice. It is doing research to help farmers grow MORE rice by using FEWER resources (less land; less water; less work and less chemicals. When there is more rice, there will be enough food for the 3 billion people who eat rice in the world!

Since 1960, IRRI has been able to grow rice plants and grains that grow faster; grow in different kinds of places; need for fewer chemicals; fight against harmful insects and are strong against plant diseases.

However, the use of pesticides can lead to extinction of some important species. Before learning to drive at the Jamboree route, during traffic, I take the Maahas road in going to UPLB. I remember there was a time when my daughter and I were covering our nose and breathing through our mouth when we passed the area near the railroad crossing. The foul smell? It was because of the smell coming from the pesticide that was previously applied by the IRRI people. Our respiratory system was affected since we would cough and sneeze due to the smell. Even if we did not see the species that have been killed, I knew that they were really affected. I knew that the participants in the food chain, for example, insects, and aquatic organisms (since the area is near a creek) were killed due to the pesticide. And this effect might put the food chain in chaos! If I could only have the authority to stop this application of pesticide, I know the extinction can be prevented. Perhaps, the least that I can do is to write a complaint to the Mayor so that the problem may be addressed properly.