Wasps have a very characteristic appearance, with a very narrow stalk-like waist and a prominent thorax (chest) and abdomen.

Therefore any organism exhibiting this appearance is given the term "wasp-waist".

But this term is not only used to describe the appearance of organisms but the workings of certain marine ecosystems.

In this article I will explain how this type of ecosystem structure works, the types of organisms that live there and how they affect the environment.

Marine ecosystems are controlled by two mechanisms; a "bottom-up" mechanism and a "top-down" mechanism. In the first mechanism, the phytoplankton acts like the grass in a terrestrial food web.

So that herbivores consume the phytoplankton, which are then eaten by carnivores that in turn are eaten by larger carnivores and so on.

However, in the "top-down" mechanism, the populations of predators controlled the population of their prey in the trophic level below them.

Then there are wasp-waist ecosystems which refer to marine ecosystems which exhibit a "wasp-waist structure".

Simply put it refers to marine ecosystems where a single species or a few species of small plankton-eating fish dominate their entire trophic level in the food web.

So that what a "wasp-waist" structure does, is that it exhibits a mixture of the two methods of population control, a top-down control for zooplankton and a bottom-up control of predators.

An example of a wasp-waist species is sardines, this is because they are fast growing and have a short life span.

However, it must be remembered that the wasp-waist structure does not govern all marine ecosystems, but this structure usually occurs in simple temperature up-dwelling shelf ecosystems.

The population of these "waist" species are important for two main reasons; firstly since they represent a low trophic level, that is mobile, they can relocate according to their own internal dynamics.

Secondly, some species may prey upon the juvenile stages of their predator species therefore forming an unstable feedback loop in the food web. For instance, with the population of that predator species reduced, it can open up the way for another predator species.

If this new predator focuses on the wasp-waist species then its usual prey species is free to multiply, thus changing the dynamics of the foodweb.

The complex life histories of these species may mean that they have an effect on both higher and lower trophic levels both directly and indirectly.

For example, the "waist" species may have more than one predator so that its control over the higher trophic level is greater as it can control the populations of more than one predator species. One must also keep in mind that the diet of most species is quite varied so that predators will not feed exclusively on one prey species.

So that with more than one predator species under control the numbers of another prey species can increase to the point that they become "waist" species as well in that area.

Therefore, a change in the population of "waist" species can alter the ecosystem structure.

Until recently the management of fisheries has been basically focused on single species of commercial value. However, our fisheries today face major threats such as over-fishing and climate change.

In response, to these threats, fisheries are gravitating towards an ecosystem-based management approach. However, since ecosystems carry out a variety of services to organisms including humans one needs to understand the impacts its can tolerate when the populations of the organisms it houses change.

Therefore an understanding of marine ecosystems and how they operate is necessary for making predictions about the state of their future and in order to develop policies to manage and protect them, which demand ecological sustainability.

Therefore it can be seen that extensive research is required to find out the mechanisms that control the ecosystems found on our planet both marine and terrestrial.

Without this knowledge the health and existence of all ecosystems which are connected in one way or another and the services they provide to all organisms including humans will cease.