Though they are constantly changing, ecosystems are naturally a-balanced b-crowded c-consistent d-species-rich

A-balanced

Explanation:

An ecosystem is the community where living beings interact with their non-living environment so as to sustain and survive. The dynamics and structure of the ecosystem changes according the abiotic (non-living) and biotic (living)  factors.

The ecosystem remain balanced due to these changes. For example natural calamity such as flood being an abiotic factor will be responsible for controlling the population of overcrowded habitat. Also the population of predators will control the growth and abundance of prey species.  

i think the answer is a and b

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