definition of omnivore

A first approach to the concept of omnivore is that it refers to organisms that feed on more than one type of food . Bears, piranhas, spiders, and humans, among many other organisms, are omnivores.

A more precise definition states that an omnivorous organism is one that has a feeding strategy in which it eats food from two or more trophic levels ; for example, plants and herbivorous organisms (ie, animals that eat only plants), or herbivorous organisms and their predators. This feeding strategy is employed by many organisms, and is ubiquitous in nature; Contrary to what was thought until the 1980s, omnivorous organisms are the rule, not the exception. And this entails a marked complexity in the study of natural ecosystems.

Food webs and omnivorous organisms

But what does it mean to eat food from two or more trophic levels? Natural ecosystems are structured into what is called a trophic web or web , which basically represents who eats whom in that ecosystem. It is a representation of the relationships between all the organisms in the ecosystem that allows us to study how the energy that each organism needs to live and develop is transferred, and that it obtains from its food ; how the nutrients necessary for the biological functions of each organism are transferred, and also, for example, how they are transferred between polluting organisms that have entered the ecosystem and that have accumulated in the tissues or organs of animals.The base of the trophic web is constituted by the primary producers , that is, the organisms that generate organic matter using solar energy, through photosynthesis. Plants in terrestrial ecosystems, or algae and phytoplankton in aquatic ecosystems are the primary producers. The next trophic level is made up of herbivorous organisms , those animals that only feed on primary producers. And the upper trophic levels are made up of carnivorous animals, predators, that feed on other herbivorous or carnivorous animals. The highest trophic level is made up of top predators or super predators, those animals that do not have a predator that feeds on them. So, according to the definition, an omnivorous animal can feed on two or more trophic levels, and many cases refer specifically to the fact that it can feed on both prey, in carnivorous behavior, and primary producers, plants, acting as a herbivore. .

A very simple food web could consist of grass in a meadow (the primary producer), deer that live in that meadow and feed on the grass (herbivorous organism), and a cat that only feeds on deer (the predator; in this simple example it would also be the top predator, since there would be no other animal to feed on it). We could also incorporate into our simple trophic web some deer parasite, a tick for example, to exemplify the third typical way of feeding in natural ecosystems, made up of these three types of herbivorous, carnivorous and parasitic animals. Natural food webs are much more complex, and omnivorous feeding strategies add to their complexity. Omnivorous organisms have more feeding alternatives in the face of food scarcity, and greater and better chances of acquiring nutrients and good quality food by diversifying their food source. Compared to other natural competitors, omnivorous predators are more resistant to disturbance and stabilize food webs due to their ability to feed on both plants and herbivorous prey or other animals.Omnivorous feeding strategies are considered to contribute to the stability and maturity of ecosystems , and indices that quantify the level of omnivory of an ecosystem are established to measure the complexity of an ecosystem and thus assess its development and level of maturity.

Omnivorous animals

Many mammals are omnivorous, such as several species of bears and suinos (pigs and peccaries); so are various species of rodents and birds, fish such as piranhas, and reptiles such as turtles. There are several examples of omnivorous animals among arthropods, such as some beetle species, spiders , insects, and mites, that can feed on prey as well as plant products such as leaves, pollen, and nectar.

The omnivorous feeding strategy can be of three types; opportunistic, obligatory, or adapted based on the relative importance of feeding on primary producers or prey. The relationship between the participation of primary producers and prey in omnivorous feeding depends on the moment of development of the organism and the circumstances and the place where said organism is found. An example of the adaptive capacity of omnivorous organisms, particularly omnivorous predators, is their use in pest control in greenhouse crops. They can feed on plants but when a pest enters the crop they can modify their diet and feed on the invasive species.

The evolution of carnivorous animals to omnivores

Several species of omnivorous animals have carnivorous ancestors, revealing an adaptation of eating habits. This is the case of some species of bears, raccoons, cacomixtles and coatis. In most cases they are small or medium-sized animals, which do not exceed 20 kg, considerably smaller than their ancestors. This is associated with the high energy requirement of these animals and the low digestion efficiency of plants in relation to the ingestion of animal proteins, thus limiting the size of animals in evolution. Bears are the exception, having evolved from their carnivorous ancestors to their omnivorous present with sizes exceeding 100kg. One explanation is that the energy demand of such large animals is modulated by their ability to hibernate, thus allowing the evolution of carnivores to omnivores with an increase in their size. However, the argument is not enough, since other hibernating animals that have carnivorous ancestors have not developed large sizes. Another argument is based on the evolution of the bears’ teeth, of their chewing system, which allowed a better adaptation to a combined diet of prey and plants.

Sources

Nina Xiaoning Zhang, Jorg G Stephan, Christer Bjorkman and Adriana Puentes. Global change calls for novel plant protection: reviewing the potential of omnivorous plant-inhabiting arthropods as predators and plant defense inducers. Current Opinion in Insect Science 47:103–110, 2021.

Qigao Jiangzuo,John J. FlynnThe Earliest Ursine Bear Demonstrates the Origin of Plant-Dominated Omnivory in Carnivora iScience 23,101235, 2020. http://creativecommons.org/licenses/by-nc-nd/4.0/