______ is a measure of the number, variety and variability of living organisms in an ecosystem.

The term you're looking for is "biodiversity." Biodiversity refers to the variety and variability of life forms within a given ecosystem, emphasizing the range of living organisms, from the smallest bacteria to the largest mammals, and including different species, genetic variability, and the various ecosystems in which these organisms live.

Biodiversity is often quantified in terms of species richness (the number of different species in a given area), species evenness (how evenly individuals are distributed among those species), and genetic diversity (the variation of genes within species). High biodiversity is usually indicative of a healthy, resilient ecosystem, capable of withstanding environmental stressors. On the other hand, low biodiversity can lead to ecosystem instability, ultimately affecting food security, climate regulation, and economic resources.

Conservation of biodiversity is crucial for environmental sustainability, and efforts to protect it often include habitat preservation, restoration, and the management of invasive species.