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This article first appeared in INHF's Summer 2006 magazine. c. text and photos by Dr. Thomas Rosburg
Biologists identify the consumption of animals as carnivory, while herbivory is the consumption of plants or fungi. Many people probably think carnivory is impressively gruesome, while herbivory is boring and unexciting. A closer look at plants might change their minds!
Many animals practice herbivory—from browsing elephants to tiny insect leaf miners. It’s also practiced by fungi, bacteria and even other plants. For example, Dutch elm disease is caused by a fungus (Ophiostoma species) and many herbaceous plant species are afflicted with fungal rusts (Puccinia species). Bacteria such as Pseudomonas and Xanthomonas cause plant diseases such as blight, wilt or spot. Some herbivorous plants that consume their “brethren” do so because they lack chlorophyll and are incapable of photosynthesis. Dodder (in the genus Cuscuta) has yellow or orange stems that wrap and twist around its host plant. Specialized roots called haustoria penetrate the stems of the host plant and extract sugars. Other photosynthetic plants use herbivory as supplemental nourishment. Bastard toadflax and lousewort occur on Iowa’s prairies and open woodlands. They have haustoria that originate from roots and tap into the roots of neighboring plants, particularly grasses. Happy Victims? There is also some evidence (albeit controversial) that grazing and browsing may actually help the plant by stimulating extra growth. For example, when herbivory removes the terminal bud, lateral buds may become more active—resulting in a bushier plant with more flowers (similar to human pruning). Some research with ungulate grazers suggests that both their urine (nitrogen-enriched) and saliva (containing essential vitamins) supports higher plant growth. However, opponents to this theory note that even if herbivory seems to increase above-ground growth, the plant’s underground structures may have been depleted more, thus lowering its total biomass. Plant Defenses Plants use specialized cells, called sclerenchyma, to make tough, herbivore-resistant tissues. Sclerenchyma is put into leaves, seed coats, and fruits (most notably the shells of nuts) to protect the seeds. Grasses and horsetails accumulate silica in epidermal cells to make tissues unpalatable and tough. (Who wants to chew on bits of glass or sand?) Extra thick bark can also provide physical defense. Chemical Weapons
In their ongoing battle against herbivores, many plants have become virtual chemical “plants.” This “chemical warfare” escalates when herbivores, particularly insects, evolve antidotes. When bugs develop mechanisms to detoxify “the green team,” plants must create new and improved versions. It is survival of the most chemically fit.
Other Strategies
Another strategy is gall formation. When an insect lays an egg on a leaf or in a stem, the plant responds by producing extra tissue in the area, providing the larvae with non-essential tissue for a food source. This strategy does not stop the herbivore, but it helps contain and limit its disruptive effects.
Summary
Although witnessing carnivory provides for rare, memorable moments, herbivory is much more common and is undoubtedly occurring all around you. It’s a silent battle between plants and animals (mostly insects) that keeps our natural world productive, diverse and healthy. ABOUT THE AUTHOR For more information, e-mail Cathy Engstrom, Director of Communications, or call (515) 288-1846. © Copyright
2008
Iowa Natural Heritage Foundation | |||||||||
If you spend time outdoors in nature, you will surely witness one of the most common ways that species interact. It may be quite dramatic, such as a Cooper’s hawk darting through a dim forest to snatch a sparrow. Or it may be much more subtle and serene, perhaps a rabbit nibbling on some clover. In either case, one species is consuming another. This interaction, called consumerism, always results in a positive outcome for one species and a negative effect for the other.