The mesopredator release hypothesis is a relatively new hypothesis from 1988 which describes the phenomenon of trophic cascade in certain terrestrial communities. It states that as top predators 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.
Mesopredator release is a fairly new concept that is gaining approval, although it is still being debated and studied. The mesopredator release hypothesis states that if an apex predator is taken out of an ecosystem, the number of mesopredators (defined as medium-sized predators, such as raccoons, skunks, snakes, cats, and foxes) often increase in abundance when larger predators are eliminated. Mesopredator populations will surge and the predation of smaller, more vulnerable prey species will increase. As a result, the shared prey may suffer more from predation than when the apex predators (or top-predators) were controlling the mesopredators. This may lead to dramatic prey population decline, or even extinction, especially on islands. This process arises when mammalian top predators are considered to be the most influential factor on trophic structure and biodiversity in terrestrial ecosystems. Top predators may feed on herbivores and kill predators in lower trophic levels as well. Thus, reduction in the abundance of top predators may cause the medium-sized predator population to increase, therefore having a negative effect on the underlying prey community. The mesopredator release hypothesis offers an explanation for the abnormally high numbers of mesopredators and the decline in prey abundance and diversity. The hypothesis supports the argument for conservation of top predators because they protect smaller prey species that are in danger of extinction. This argument has been a subject of interest within conservation biology for years, but few studies have adequately documented the phenomenon.
One of the main criticisms of the mesopredator release hypothesis is that it argues in favor of the top-down control concept and excludes the possible impacts that bottom-up control could have on higher trophic levels. This means that it supports the argument that top predators control the structure and population dynamics of an ecosystem, but it does not take into account that prey species and primary producers also have an effect on the ecosystem’s structure. Another major problem is that the hypothesis is offered as an explanation after large predators have already become rare or extinct in an ecosystem. Consequently, there is no data on the past ecosystem structure and the hypothesis cannot be tested. As a result, information on the past conditions has been inferred from studies of the present conditions. However, contemporary examples of mesopredator release exist, such as the culling of cats on Macquarie Island.
The hypothesis is sometimes also applied to humans as apex predators that produce top-down effects on lower trophic levels. However, it fails to recognize bottom-up effects that anthropogenic land transformations can have on landscapes on which primary producers, prey species, and mesopredators dwell. Possible bottom-up effects on an ecosystem can be from bioclimatic impacts on ecosystem productivity and from anthropogenic habitat alterations. Examples of anthropogenic habitat change include agriculture, grazing land, and urbanization. More importantly, the hypothesis does not take into account that higher trophic levels are affected by primary productivity. It also does not mention that trophic interactions operate at different strengths according to the ecosystem. Therefore, the roles of predation and food/nutrient processes in influencing ecosystem structures remain open to controversy and further testing.
See also 
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