Impact on prey species

To date, there are few scientific data available to assess the impact of cat predation on prey populations. Even well-fed domestic cats may hunt and kill, mainly catching small mammals, but also birds, amphibians, reptiles, fish and invertebrates.[129][165] Hunting by domestic cats may be contributing to the decline in the numbers of birds in urban areas, although the importance of this effect remains controversial.[166] In the wild, the introduction of feral cats during human settlement can threaten native species with extinction.[163] In many cases controlling or eliminating the populations of non-native cats can produce a rapid recovery in native animals.[167] However, the ecological role of introduced cats can be more complicated: for example, cats can control the numbers of rats, which also prey on birds' eggs and young, so in some cases eliminating a cat population can actually accelerate the decline of an endangered bird species in the presence of a mesopredator, controlled by cats.[168] In the Southern Hemisphere, cats are a particular problem in landmasses such as Australasia, where cat species have never been native and there were few equivalent native medium-sized mammalian predators.[169] Native species such as the New Zealand Kakapo and the Australian Bettong, for example, tend to be more ecologically vulnerable and behaviorally "naive" to predation by feral cats.[170] Feral cats have had a major impact on these native species and have played a leading role in the endangerment and extinction of many animals.[171] Cat numbers in the UK are growing and their abundance is far above the "natural" carrying capacity, because their population sizes are independent of their prey's dynamics: i.e. cats are "recreational" hunters, with other food sources.[172] Population densities can be as high as 2,000 individuals per km2[173] and the trend is an increase of 0.5 million cats annually. The mesopredator release hypothesis is a relatively new hypothesis from 1988[1] which describes the phenomenon of trophic cascade in certain terrestrial communities. It states that as top predator

decline in an ecosystem, an increase in the populations of mesopredators occurs. Mesopredators are middle trophic level predators such as raccoons, skunks, snakes, cownose rays, and small sharks. The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment. In population biology, carrying capacity is defined as the environment's maximal load,[1] which is different from the concept of population equilibrium. For the human population, more complex variables such as sanitation and medical care are sometimes considered as part of the necessary establishment. As population density increases, birth rate often decreases and death rate typically increases. The difference between the birth rate and the death rate is the "natural increase". The carrying capacity could support a positive natural increase, or could require a negative natural increase. Thus, the carrying capacity is the number of individuals an environment can support without significant negative impacts to the given organism and its environment. Below carrying capacity, populations typically increase, while above, they typically decrease. A factor that keeps population size at equilibrium is known as a regulating factor. Population size decreases above carrying capacity due to a range of factors depending on the species concerned, but can include insufficient space, food supply, or sunlight. The carrying capacity of an environment may vary for different species and may change over time due to a variety of factors, including: food availability, water supply, environmental conditions and living space. The origins of the term carrying capacity are uncertain with researchers variously stating that it was used "in the context of international shipping"[2] or that it was first used during 19th Century laboratory experiments with micro-organisms.[3] A recent review finds the first use of the term in an 1845 report by the US Secretary of State to the Senate.