Carrying capacity in ecosystems refers to the maximum population size that a specific environment can sustainably support over a long period of time. It is determined by factors such as available resources, habitat suitability, and environmental conditions. When a population surpasses the carrying capacity of its ecosystem, it can lead to increased competition for resources, habitat degradation, and ultimately population decline due to scarcity of resources. This concept is crucial in understanding the balance between population size and available resources in natural systems.