Here we come to a very important idea.  It is the idea of limiting factors.

     Every organism needs resources to draw on, for example, an energy source (food), water, special elements in nutrients, oxygen or carbon dioxide, a comfortable temperature range, shelter, and so on.  If the organism does not have access to a particular essential resource, it will stop growing, struggle, and  die.  The abundance or scarcity of the resource is a limiting factor in terms of the number of a type of organism that can live in a specific environment.

     Let's think of this in terms of examples, which will make this idea easy to understand.

Why are there no plants on this hill?

     This hill is located in Death Valley, a desert area in California.  The limiting factor here is the availability of water.

     Infrequent rains wash down these steep slopes and flow away in a short lived stream.  Such water as soaks into the earth is quickly surrendered to the hot sun and evaporates away.
 

Why do people and air breathing animals drown?  

     When people sink into water, they drown.  Why?  There is a limiting factor -- oxygen.   Our lungs cannot absorb oxygen from water, nor get rid of the carbon dioxide in our blood in it.  Air-breathing organisms can live only a very few minutes without oxygen.

Why are there no lizards in polar regions?  

     The bodies of cold blooded (exothermic) animals are the same temperature as their environment.  In the hot deserts the snakes and lizards move slowly in the morning and lie out in the sun so that their bodies and their internal chemistry can warm up.  The limiting factor here is temperature.  I don't know if a lizard in Antarctica would ever get warm enough to feel hungry or to be able to move enough to find or capture food.  There are very few insects to eat there anyway.

Why are there no sheep in Antarctica?

     Well, sheep are warm-blooded (endotherms) but they are grazers, and the Antarctic is too cold to grow grass.  (Well, there are a few sprigs of one kind of grass that does grow on the exposed earth there.)  Also almost all of the land is covered by ice and snow, so there is no available soil to provide nutrients for the grass.  The sheep would freeze to death in Antarctica anyway (limiting factor: their bodies cannot deal with this very cold climate) and would also starve if they lived long enough.  The grass also has two problems -- low temperatures, and water that cannot be absorbed  when frozen.

Seasonal Resources

    Limiting factors can be seasonal.  In much of the world the winter is too cold to support plant growth, but the return of spring brings warmer temperatures and lush vegetation.   

     In deserts, if there has been rain during the cooler months, there may be a  brief but lavish display of flowers in the spring.   It is important for desert plants to rush to the seed making stage before the water disappears again.  Here you can see a plant that "made it".  The dry, empty seed pods tell us of its success.

     Light also varies from season to season.  In polar regions the plants, warmed somewhat by the long, sunny days, photosynthesize and grow rapidly in summer.  They have evolved to grow rapidly from seed to plant to flower to seed again in the short gtowing season.  The limiting factor here is time.  There is only a short period during which all the other important factors are available.

     Even the harvest of the seasons can be affected by such events as extensive forest fires or  large volcanic eruptions.  If enough fine ash makes its way into the atmosphere, it can block out some of the sun's heat and light, creating colder than usual weather.  The amount of light and heat available become the limiting factors for plant growth.

Invisible Resources

     Not all requirements are so obvious.  It makes sense that we do not see lush jungle growth in deserts, where the rainfall is sometimes far less than ten inches a year.  However, there are  other limiting factors that we may have to think about a bit.

     Factors like the acidity of the soil and the presence of trace elements may limit or prevent plant growth.  In the ocean, algal growth is limited by the amounts of available nitrogen, phosphorus, or iron dissolved in the water.   Scientists have experimented with scattering iron filings into the ocean and have seen rapid growth of the seaweed and phytoplankton.