The correct answer is option 4: Heterotrophs' survival depends directly or indirectly on autotrophs.

This statement is true because of the fundamental roles autotrophs and heterotrophs play in the ecosystem's food web. Autotrophs, also known as producers, are organisms that can produce their own food using inorganic sources like sunlight (photosynthesis) or chemical energy (chemosynthesis). Examples include plants, algae, and some bacteria. Heterotrophs, or consumers, are organisms that cannot produce their own food and must obtain energy by consuming other organisms. This includes animals, fungi, and many bacteria.

The food web is structured such that autotrophs form the base. Herbivores (primary consumers) feed on autotrophs. Carnivores (secondary, tertiary, etc., consumers) then consume herbivores or other carnivores. Even decomposers, which break down dead organisms, ultimately rely on the organic matter initially produced by autotrophs. Therefore, the survival of heterotrophs is entirely dependent on the existence and productivity of autotrophs, either directly (herbivores) or indirectly (carnivores and decomposers).

Let's examine why the other options are incorrect:

• Option 1 is false because many bacteria are heterotrophs, not autotrophs.
• Option 2 is false because autotrophs don't depend on heterotrophs for survival; they produce their own food.
• Option 3 is false because animals are heterotrophs, not autotrophs.

The interconnectedness of life within an ecosystem is clearly demonstrated by the crucial dependence of heterotrophs on autotrophs for sustenance.