 The carrying capacity of an ecosystem is the largest number of individuals that an ecosystem can support, which is determined by the distribution and abundance of energy and matter in an ecosystem. Often we think about carrying capacity in terms of a specific limiting resource or limiting factor that defines the maximum population possible. In reality, it's often a lot more complex and there are a lot of different factors that interplay in ecosystems to determine that distribution of energy and matter and consequently the carrying capacity. We can start by considering carrying capacity at the ecosystem level, which means we need to consider two sets of factors, abiotic and biotic. On the abiotic side of things, we've got weather, we've got soil conditions, sunlight, water availability, all of these non-living conditions. On the biotic side of things, we've got all of the different types and abundances of organisms in our ecosystem and how they interact with each other. Now together, these abiotic and biotic factors create these complex transfers of energy and matter in our ecosystems and consequently define our carrying capacity. Two really extreme examples of this could be a desert and a rainforest, which differ really dramatically in their abundance and distribution of that energy and matter. In something like our desert, we don't have a lot of water. We don't necessarily have a lot of biomass but we do have a lot of thermal energy and that's going to mean that generally speaking we have a low carrying capacity, though for some of our species that may differ. In our rainforests, we have a lot of energy and we have a lot of matter. We've got a huge amount of biomass in that vegetation. We've got a lot of water availability but as a consequence there's a lot of competition for other resources like sunlight. So even though our ecosystem has a larger carrying capacity, there are different factors that determine that for individual species. As an ecologist, this concept of carrying capacity is really important because often we're interested in understanding that maximum sustainable population when we're managing either threatened species or managing ecosystems themselves.