Natural Selection Image Map

Many scientists think that the most important function in the study of ecosystems is to prevent their collapse. Populations of different species are the building blocks of ecosystems. So the question becomes, why would the failure of an ecosystem be so significant?

The first rule of species survival in an ecosystem is that the population must be large enough to withstand disease, shortages of food and water, predators, and the other hazards of living.

Food Webs: The Ties that Bind

  We can establish links between predator and prey like this:


This is an example of a food chain in which energy in the grass transfers to the gazelle and from the gazelle (if it gets eaten) to the cheetah.  It is easy to see the transfer of energy (see Exchange Cycles) involved in the food chain….the grass provides the energy for the gazelle, which, in turn, provides energy to the cheetah.  Food chains connect populations to each other.

In nature, however, cheetahs feed on a variety of prey, not just gazelles. This means that cheetahs, like other organisms in an ecosystem, are members of many different food chains.  All of these interrelated food chains create a food web.  Relationships among members of a food chain must be in balance.  Otherwise, individual populations could become too large or small and fracture the delicate connections that hold the ecosystem together.

The food web in Port Lavaca bay, along the Gulf Coast of Texas. Arrows indicate food sources for life forms higher up in the web. At the bottom of the web are nutrients, bacteria, and fungi. These are used by algae and aquatic plants, which provide food for various animals and birds. Aquatic plants do not have to be eaten directly to be a food source, because when they die and decay they can provide a nutrient source for such animals as oysters.

Why should you care about populations and relationships between populations and the ecosystem?  To answer this question, it is useful to begin by thinking about sex.




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