Why are there normally fewer consumers than producers in an ecosystem?

Question

Why are there normally fewer consumers than producers in an ecosystem?

Answer 1

Clearly it is impossible to sustain more consumers than produces. The reason being that the extra consumers have nothing to eat and so they die off. Here we are talking about the population size of both in terms not of the absolute number of individuals, but rather in terms of the amount of food produced and food consumed. When the amount of food the producers make is equal to the amount of food the consumers eat then the two populations  are considered equal in size.

What happens is that both consumer and producer populations fluctuate up and down. To simplify the analysis let's assume though that the producer population is constant. Although not realistic this analysis can give you the idea of why what you ask is true.

Starting with a large population of producers and a small population of consumers what will happen? Well the consumers are all well fed and they have a lot offspring and so their population starts to rise. After a few generations the consumer population starts to get close to the producer population. One thing to note that is very important is that it takes time for the consumer population to grow, typically more than a single generation.

As the consumer population gets very close in size to the producer population what happens? The consumers are still pretty well fed and continue to have a lot of offspring. The next generation of consumers though suddenly is bigger than the producer population. Now all of the sudden consumers are competing against each other for food and there is not enough to go around. The entire generation is suddenly facing starvation. Many die prematurely and those who are left are hungry and less healthy. So now they have far fewer offspring. The next generation therefore is quit suddenly much smaller than before and now also much smaller than the producer population. We are back to where we started with this analysis. Note that the population crash only took a single generation.

So the reason why there are normally fewer consumers than producers is because it takes quite a while for the consumer population to grow bigger than the producer population but only a short time for the consumer population to crash back down smaller again.

Answer 2

This is not really true.  It just looks that way, because we see just the larger consumers.

For example, there are more trees producing food than elephants.

But the insects and bacteria that eat trees are more numerous than the trees.

But we only think about the elephants.