Question

what would have a greater biomass? rabbits or foxes

Answer 1

Let me re-word this question,
so that I can get at the idea of what I THINK it's asking.

"Which would have a greater biomass -- a population of rabbits, or a population of foxes?"

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"Biomass" is basically living material.

And when we're talking the biomass of a population,
we're talking about the size of the population.

So, here's the real question:
"Which animal is likely to be more abundant? Rabbits or foxes?"

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To answer this question,
we need to look at an energy pyramid,
and understand how energy moves, across the pyramid.

Take a look at this picture:
http://www.etap.org/demo/biology_files/lesson6/kep26.jpg

RABBITS are PRIMARY CONSUMERS.
They eat plants.

FOXES are SECONDARY CONSUMERS.
They eat rabbits.

Now, the amount of energy at the BOTTOM of the pyramid is GREATER.
The amount of energy at the TOP of the pyramid is LESSER.

When rabbits eat grass,
they only get about 10% of the energy that the grass had.

And when foxes eat rabbits,
they only get about 10% of the energy that the rabbit had.

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Let's pretend that there are....100 units of energy, at the PRODUCERS level. (The BOTTOM).

Grass is a producer.
And when the rabbit eats the grass,
it gets 10% of that energy.
So, the rabbit gets 10 units of energy,
because 10% of 100 = 10.

Now, when the fox eats the rabbit,
it gets 10% of the rabbit's energy.
So, it only gets 1 unit of energy,
because 10% of 10 = 1.

Do you see how the fox only gets a small, tiny amount of energy?
The rabbits get more energy.

Again, FOXES are SECONDARY CONSUMERS.

And since secondary consumers don't get as much energy as PRIMARY consumers,
there are LESS organisms at the SECONDARY CONSUMER level.

LESS energy leads to a LOWER population,
you see?

It would make more sense for there to be more rabbits than foxes.

This is because rabbits can get more energy out of their food, than foxes can.

And so, they have more energy, to support their species.