Limiting Factors are abiotic or biotic factors that contribute to the carrying capacity of an ecosystem. Generally, carrying capacity refers to the maximum amount of organisms in a population that can be sustained by the ecosystem. The carrying capacity is determined by a variety of factors such as available fresh water and living space, ample food sources, amount of competition for resources, disease, parasites, and predators. Anything that serves to control the size of a population is called a limiting factor.

The best case scenario for a population is that it's numbers approach the carrying capacity and then level off, achieving a dynamic equilibrium with the limiting factors of the ecosystem (above left). A less desirable and sometimes dangerous scenario is when the population greatly surpasses the carrying capacity and the population's demand for resources ends up damaging the balance of the food web (above right). For example, an out-of-control deer population will result in a dramatic decrease in vegetation as the deer consume it. This in turn creates a problem not only for the deer, but also for the other organisms who rely on that vegetation as a food source.

The video above describes several ways in which humans have contributed to the loss of biodiversity. This not only affects the millions of species on our planet, but it also has a negative effect on our population as well. Obviously we depend on other organisms as our food sources, but we also depend on many of them for over half of our medicine. We also research the genetics of all varieties of organisms in order to gain clues about possible cures for our diseases.

One of the ways we threaten biodiversity is by introducing species into ecosystems to which they are not accustomed. Since they have not developed to fill a niche in their new ecosystem, many times (9 out of 10), the foreign species fails to survive. BUT in those rare instances that it finds what it needs to survive, the species can survive and fill a niche. BUT in rare instances (1 out of 10), the foreign, or invasive, species has a lack of limiting factors.

So what happens when a population does not have limiting factors to control its growth? See the graph of the deer above when the population overshoots the carrying capacity. Bad news for the deer and the rest of the food web. That's what happens when an invasive species survives in a new ecosystem and has a lack of limiting factors.

Two local examples you should be familiar with: Zebra Mussels and Purple Loosestrife