What determines carrying capacity in an ecosystem?
If we're discussing carrying capacity in the context of a species, the number of available mates, predation, disease, or competition with another species could determine the species' carrying capacity. For example, a species of bird may have plenty of space and but if it needs to compete with another species for food, the degree and extent of competition will limit the population.
Or there may be plenty of available habitat, food, and water for tigers in India, but if there are only a few tigers breeding and producing offspring, the carrying capacity of the species will be determined by the number of females producing young.
Many of these same limiting factors apply when we refer to the carrying capacity of an ecosystem. For example, in a desert ecosystem, there may be plenty of available space, but the number of organisms the system can hold may depend on the amount of water.
Another ecosystem may be limited by nutrient-poor soil. If the soil in an area is low in nutrients, fewer photosynthetic plants will be able to grow, which means less food for organisms consuming the plants, which means less food for organisms that eat those organisms, and so forth. The nutrient concentration of the soil affects the carrying capacity of the entire ecosystem.