Final answer:
Water acts as a limiting factor in desert ecosystems, restricting plant growth and abundance due to its limited availability. Desert plants adapt by conserving water and maintaining a balance between dehydration and carbon gain, which is essential for photosynthesis.
Step-by-step explanation:
The number of plants in a desert ecosystem is, indeed, largely determined by the amount of water available, which makes water a limiting factor in this environment. As a limiting factor, water availability restricts the growth, abundance, and distribution of organisms living in the desert. In an environment where water is scarce, plants must evolve strategies for conservation, such as having smaller leaves, thicker waxy cuticles, or developing deep root systems to access groundwater. These adaptations allow plants to balance water loss with carbon dioxide uptake and overall growth.
Given these conditions, desert plants often have to compete for water, making it both a limiting factor and a source of competition among species. However, in the context of the student's question, water's role in restricting plant populations due to scarcity would best be categorized as a limiting factor, because it is the primary environmental constraint impacting plant survival and growth in arid regions.