What is a herbivore and what is not?
What is a herbivore?
The simple answer is an animal, vertebrate or invertebrate, that feeds on plants.
However, as in most aspects of biology, there are some grey areas. These include deciding what is or is not a plant, what are the boundaries between herbivores that mostly feed on live plants and saprovores that feed on dead plants and dead animals and are all pollinators herbivores.
The place of a species within the environment can be defined several ways: by its feeding type, by its " ecological function" or by the concept of " ecosystem services". These concepts are discussed below.
Herbivores are often regarded as pests, because they damage plants. When herbivores may be regarded as pests is discussed below.
In addition there are terms for the host range (number of species) that an organism lives on.
Feeding types
The following explanations of different "life styles" are based on information in Gordh G., Headrick D. 2001. A Dictionary of Entomology. CABI Publishing, Wallingford, UK. 1-1032 pages.
- Fungivores are animals that feed on fungi. Fungi are plants, but the Plant-SyNZ database does not, at present, include specific fungus feeding insects or mites. However, it does include insects that feed on lichens, which are combinations of a fungus and an alga. The database also includes insects that feed on dead parts of plants (saprovores), and which may be primarily consuming the fungi in the dead tissues of the plant.
- Herbivores are animals that feed on plants. At present, the Plant-SyNZ database does not include specific fungus feeding insects or mites and it does not include insects that are primarily pollinators, even though they feed on pollen and nectar.
The database does include insects that feed on lichens, which are combinations of a fungus and an alga.  The database also includes insects that feed on dead parts of plants (saprovores), and which may be primarily consuming the fungi in the dead tissues of the plant. The reason for this is that some wood boring insects may start feeding on live stems or trees, but through their feeding the whole plant or part of it dies and the insect completes its life feeding on dead plant tissue. At present it is difficult to distinguish between this kind of herbivore, one that feeds on dead wood and one that feeds on fungi in dead wood. Ideally, we should link the herbivore to the species of fungus and to the plant species that hosts the fungus. - Parasites are organisms that live on other organisms, but do not usually do not kill their host.
- Parasitoids are insects that have characteristics of parasites and predators. Usually a single larva feeds on and kills its host. It usually only lives on or inside one host. Sometimes several parasitoid larvae will live on or inside one host insect.
- Pathogens are micro-organisms that live on/in other living organisms. They include fungi, bacteria, and viruses. Sometimes the fungal pathogens are more visible than the insect host and can sometimes be used to identify the presence of the insect, especially scale insects.
- Pollinators are organisms that assist the transfer of pollen between flowers. Many insect pollinators also feed on the pollen and nectar produced by the plant and are technically herbivores. At present, insects that are primarily pollinators are not included in the Plant-SyNZ™ database.
- Predators are animals that catch, kill and consume other organisms. One predator usually catches and consumes several or many prey. The definition is usually restricted to animals that catch other animals. However, it is sometimes used for herbivores that feed on plants. For example seed eating insects may be called seed predators. Leaf feeding insects may also be called predators.
- Saprovores are organisms that feed on dead and decomposing organisms. Saprovores may be primarily feeding on the micro-organisms in the dead organism. Those that feed on dead plant tissue are important in the process of recycling and making the nutrients in dead plants and animals available to living plants for their growth.
Ecological function and ecosystem services
The ecological function and ecosystem services of pollinators are obvious from the name of this group of organisms. Pathogens, parasites, parasitoids and predators can be thought to regulate the numbers of other organisms, such as herbivores, and prevent extreme fluctuations in numbers and severe damage to their plant hosts most of the time.
Herbivores can be regarded as converting plant tissue into animal protein that is more suitable as food of other organisms. The beneficiaries of this include insect eating birds and lizards. It is noteworthy that some mainly herbivorous birds switch to insect eating when they are feeding their young. A second benefit from insects and mites feeding on plants is that they contribute to an increase in the productivity of the ecosystem by increasing the rate of nutrient recycling through making plant nutrients available through their excreta and dead bodies.
Plant host range
Three terms are commonly used to describe the host range of animals (including herbivores), monophagous, oligophagous and polyphagous.
Monophagous. Feeds on only one species (of plant).
Oligophagous. Feeds on several species (of plant). Oligophagous herbivores may be divided into two groups, those that feed on species in one genus and those feeding on species in several genera and which can be in up to two families. The first group being indicated as Oligophagous (genus).
Polyphagous. Feeds on many species (of plant).
A herbivore is sometimes defined as polyphagous if it feeds on species from three or more families of plants.
When are herbivores pests?
There is a common saying that a weed is a plant in the wrong place. Similarly, a herbivore is only a pest when it is in the wrong place.
New Zealand native herbivores are part of New Zealand's biodiversity and are not pests in native ecosystems and like native birds should, in general, be encouraged, on their native host plants outside native ecosystems. However, some species that cause unacceptable damage to valued non-native plants, such as lemon tree borer on citrus trees and grass grub on pasture grasses. In these circumstances these species are regarded as pests.
The feeding of some native herbivores on native plants in gardens and parks can also cause unacceptable damage, especially while a plant is young and at this stage the plant may need some protection. However, if people grow native plants they should expect, that once the plant is established, it will host native invertebrates. By doing so they are contributing to increasing the native biodiversity of the area, and in some cases helping monophagous herbivores to survive, e.g. Aceria clianthi Lamb 1952 (Acari: Eriophyidae) on Clianthus puniceus (G. Don) Lindley (Fabaceae) (Kakabeak).
Herbivores from overseas may also be pests or beneficial. Many crop pests are overseas herbivores, though some herbivores have been deliberately introduced into New Zealand to help control weeds. An example of a weed biological control agent is the gorse mite, Tetranychus linearius Dufour (Acari: Tetranychidae) that forms webbing on young growth of gorse (Ulex europaeus L.).
Herbivores from overseas that feed on native plants are pests, especially where they occur in native ecosystems. Many do not cause serious harm to the native host plants, but heavily infested plants are sometimes found and this may be particularly serious where it occurs in native ecosystems.
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