Herbivory: eating plants

What is herbivory?

Herbivory is the act of eating plants. Herbivory occurs above and below ground. Herbivores may eat any part of the plant above the soil including leaves, stems, flowers, fruit and any part of the plant below the soil including roots and tubers.

Why is herbivory important?

Herbivory can have substantial impacts on habitat health, the structure and diversity of plant and soil invertebrate communities and the productivity of economically important crops.

The positive impact of herbivory is widely debated in the scientific literature. Suggested impacts include:

• Increases in nutrient uptake and production by plants. Low level herbivory may remove aging roots and leaves, initiating new growth of young roots and shoots, which are more effective at nutrient absorption and production respectively.
• Increases in contributions to plant litter and the nutrient pool in soils. Herbivores contribute their faeces and cause plant material to fall to the ground. This facilitates decomposition.
• Increases in the quality of leaf litter and soil. Many herbivores favour young foliage, which has a higher nutrient content than old leaves. Herbivores therefore, return high concentrations of nutrient to the soil as faeces. This results in leaf litter with a higher nutrient value.
• Increase in the chances of successful seedling growth. Ants for example, may take seeds back to their nests. The soil of ants' nests is generally richer in nutrients and water than surrounding areas and is favourable for seedling growth.
• Improved conditions for plant growth. Herbivores prune plants and allow light to reach the surrounding area. This provides better conditions for seeds that fall from a parent plant.
• Increased plant protection. For example, some plants that have developed relationships with ants that protect their host plant from other herbivores and also control neighbouring plant growth through weeding and pruning.
• Control of plant populations, especially weed species. In land management any undesired plant is considered a weed, whether it is an exotic or native. Herbivory naturally controls plant numbers and the introduction of herbivores such as insects may be used as effective biological controls.
• Increased protection from further herbivory after initial plant damage. Plants that have their leaves or roots attacked release chemicals that attract predators and parasitoids which then control the herbivores.

Why are there so many types of herbivore?

The diversity of plant species is enormous and largely responsible for the diversity of the animals that feed on them.These animals have developed specialised features and feeding strategies to utilise plants as a food source.

Plants and herbivores have developed strong relationships

Plants have many features that animals have needed to overcome in order to use plants as a food source. This 'battle' between animals and plants is likened to an arms race and has led to strong relationships between many herbivores and plants. Such relationships range from animals only feeding on particular plant groups or species to animals only feeding on particular plant structures. This kind of specialisation where an animal relies on a certain plant or plant group to grow, is referred to as 'plant host specificity'.

The understanding of this relationship between animals and plants is extremely important for land management. The removal of a particular plant species or group may result in the disappearance of many animals from an area.

How do herbivores deal with plant defenses?

Complex plant physical structures

Plants have many external structures such as leaves, stems, flower parts, roots, fruits and seeds. Each of these structures may vary in form between plant species. Herbivores have characteristics that allow them to access these structures. These include:

• Appendages such as claws, spines or suckers that allow them to cling to vertical or inverted surfaces.
• Wings to reach the top of the highest plants.
• Modified legs for digging, to access roots.
• Saw-like (e.g. sawfly ovipositor), piercing (e.g. wasp ovipositor, tube-like mouthparts) or cutting tools (e.g. chewing mouthparts of beetles) that allow access to internal structures.

Complex plant chemical structures

Plants use complex compounds such as cellulose and lignin to maintain their physical structure and support. These compounds are tough, not water soluble and difficult to digest. Herbivores have employed a number of feeding strategies to overcome these chemical structures and tough plant tissues. They include:

• avoiding tough plant tissue by eating plant fluids. For example, sap-sucking bugs have a tube-like mouthpart called a rostrum, which they use to pierce the tough tissue and suck out the internal juices of the plant. Many herbivores feed on nectar and have mouthparts for lapping, sponging or sucking. Nectar feeding specialists include insects such as butterflies, moths, bees, flies, wasps and beetles, and vertebrates such as lorikeets and honey possums.
• avoiding tough plant tissue by eating soft internal tissue. For example, miners are insect larvae that eat between plant cell layers. A leaf miner may eat the entire leaf, leaving only the outer protective layers and the veins. Mining activity appears on the plant as tunnels, blotches or blisters. Common miners are the larvae of some flies, wasps, moths and sawflies.
• eating tough plant tissue using chewing mouthparts. For example, leaf chewers have mouthparts that allow them to slice through leaves. The most prolific chewers are the beetles and the larvae of moths and butterflies. Other important groups that feed directly on leaves are the grasshoppers, locusts, katydids, sawflies and stick-insects. Some groups such as leaf-cutter bees, ants, termites and wasps collect leaf fragments to construct their nests or feed their young.
• eating tough plant tissue by forming alliances with other organisms. For example, some insects have formed alliances with bacteria or fungi that are capable of breaking down complex chemicals, like the Ambrosia Beetles that distribute special fungi that liquefy woody material, which the beetle eats.

Plants defend against herbivores using complex chemicals

Plants produce many chemicals for defence against herbivores. These chemicals may kill the herbivore, or deter it from feeding or, in the case of insects, laying eggs on the plant. These chemicals can be made in large quantities and have a secondary use such as structure (e.g. tannins, phenols), or made in small quantities in response to initial plant damage by the herbivore (e.g. essential oils, alkaloids, terpenes). Herbivores have developed special ways of dealing with defence chemicals. These include:

• detoxifying plant defence chemicals. For example, caterpillars and sawflies that feed on Eucalypts are capable of breaking down some of the chemical defences. Many Australian mammals, most notably the Koala, are able to digest eucalypt leaves also.
• exploiting stressed plants. Research has shown that in the initial stages of stress, plants reallocate resources to parts of the plant important for core activities. Nutrients go into root and leaf development and not chemical defences, so that stressed plants have reduced chemical defences and are easier to eat or are more palatable.

Plants change as they grow

Plants have different stages of growth. For instance, new leaves are growing and roots are extending while elsewhere on the same plant leaves are aging and roots are decaying. At each of these stages the structural and chemical features of plant parts change. For example, younger leaves might have a higher nitrogen content than older leaves. Because of these changes herbivores, particularly insect herbivores, concentrate in different areas of the plant.

Actively growing regions of the plant such as root tips, young leaves and flower buds are nutrient rich. Galls are small swellings on plants often found in these nutrient rich areas. They are caused by insects that live and feed inside the swelling where they are protected from predators and have an ample food supply. The insect releases chemicals that disrupt the normal processes involved in plant growth resulting in a deformation of the plant tissue - the gall. Common gall formers include flies (eg. gall midges and fruit flies), wasps, bugs (namely aphids, scale insects and jumping plant lice) and thrips. There are also some non-insect gall formers such as mites and nematodes.

Which invertebrates are herbivores?

There are many invertebrate herbivores. These include snails, slugs, mites, millipedes, worms and species of insects. Insect herbivores are the most numerous and varied. It has been estimated that approximately half of all living insects are herbivores. Some large insect groups are almost exclusively plant-feeders. These include moths and butterflies, weevils, leaf beetles, gall wasps, leaf-mining flies and plant bugs.

References

• Anderson D.C. 1987. Below-ground herbivory in natural communities: A review emphasising fossorial animals. The Quarterly Review of Biology, 62 (3): 262-282.
• Australian flora and vegetation statistics, Australian National Botanic Gardens. Available online at http://www.anbg.gov.au/anbg/australian-flora-statistics.html.
• Bezemer T. M., Wagenaar R., Van Dam N. M., and Wäckers F. L. 2003. Interactions between above- and below-ground insect herbivores as mediated by the plant defence system. Oikos, 101 (3) 555.
• Bonkowski M and Scheu S 2004. Biotic interactions in the Rhizosphere: effects on plant growth and herbivore development. In Weisser W.W. and Siemann E. (eds). Insects and ecosystem function. Ecological studies, Vol 173: 71-91.
• Chapman S. K., Hart S. C., Cobb N. S., Whitham T. G., and Koch G. W. 2003. Insect herbivory increases litter quality and decomposition: an extension of the acceleration hypothesis. Ecology, 84(11): 2867-2876.
• Cooper P.D. 2001. What physiological processes permit insects to eat Eucalyptus leaves? Austral Ecology, 26 (5) pp. 556-562(7).
• Gullan P.J. and Cranston P.S. 2004. The insects: an outline of entomology. Blackwell Publ. pp: 248-288.
• Hartley S.E. and Jones T.H. 2004. Insect herbivores, nutrient cycling and plant productivity. In Weisser W.W. and Siemann E. (eds). Insects and ecosystem function. Ecological studies,Vol 173: 28-52.
• Hochuli M.D. 2001. Insect herbivory and ontogeny: how do growth and development influence feeding behaviour, morphology and host use? Austral Ecology, 26 (5): 563-570(8).
• Hunter M.D. 2001. Insect population dynamics meets ecosystem ecology: effects of herbivory on soil nutrient dynamics. Agricultural and Forest Entomology, 3: 77-84.
• Masters G.J. 2004. Belowground Herbivores and Ecosystem Processes. In Weisser W.W. and Siemann E. (eds). Insects and ecosystem function. Ecological studies, Vol 173: 94-112.
• Poveda K, Steffon-dwenter I, Scheu S, and Tschantke T 2005. Effects of decomposers and herbivores on plant performance and aboveground plant-insect interactions. Oikos, 108(3).
• Wäckers F.L. and Wunderlin R. 1999. Induction of cotton extrafloral nectar production in response to herbivory does not require a herbivore-specific elicitor. Entomologia Experimentalis et Applicata, 91 (1): 149-154.
• Weisser W.W. and Siemann E. 2004. The various effects of insects on ecosystem functioning. In Weisser W.W. and Siemann E. (eds). Insects and ecosystem function. Ecological studies, Vol 173: 3-24.
• Wardle D.A. and Bardgett R.D. 2004. Indirect effects of invertebrate herbivory on the decomposer subsystem. In Weisser W.W. and Siemann E. (eds). Insects and ecosystem function. Ecological studies, Vol 173: 54-69