Here's why:

* Ecosystem: A biological community of interacting organisms and their physical environment.

* In a certain pond: This defines the specific location of the ecosystem.

* Fish, insects, and algae interact with other living things: This describes the biotic components of the ecosystem (living things).

* Their surroundings: This describes the abiotic components of the ecosystem (non-living things like water, sunlight, temperature, etc.).

Therefore, the pond with its fish, insects, algae, and their surrounding environment together form an ecosystem.