Question

What would happen if there were no heterotrophs on earth?

Answer 1

Heterotrophs are defined as organisms that must consume food to obtain nutrients. These include herbivores, carnivores, etc
Thus, with a planet of only autotrophs (self-feeding organisms) will continue to reproduce and flourish. Eventually, the earth would be full of autotrophs and with out organisms to moderate their population.
However, there are other factors that can moderate population, though it may not be as effective as a heterotroph.
As a result of the nonexistence of heterotrophs, that includes organisms that help recycle the waste in the environment.
For example, there are decomposers that recycle materials. Considered as heterotrophs, without decomposers to recycle nutrients, autotrophs will lack the nutrient to undergo photosynthesis - it would just be organic waste. This will eventually lead to the death of autotrophs.
There is a possibility for mutations to occur that can allow organisms to process the waste by themselves and use the nutrients that's been processed for photosynthesis.
These are just some of the things that can happen without any heterotrophs.