Fire's Effect on Plant-Herbivore Interactions

There is no question that herbivores and plants have a very direct intimate relationship.  Early in elementary school we learn herbivores are animals that eat plants. What we didn't learn is how complex that seemingly simple relationship is. Plants actually have evolved in response to herbivorous pressures. Some plants have developed trichomes, tiny hairs on the plant's stem, that can impale any insect that tries to land on it or even secrete sticky compounds (Gurvitch, et al, 2006).  Other plants have developed tough, thick leaves and stems to deter herbivores from eating it. Some plants even create toxic compounds. An example of this is the wild tobacco plant. It creates a very well known toxin, nicotine, that poisons anything that tries to eat it (Cahill, 2015). However, herbivores also adapt to plants' adaptations. The hornworm caterpillar is unaffected from nicotine and can munch away on wild tobacco, but the complex relationship doesn't stop there. The plant then releases a chemical that alerts caterpillar predators of the caterpillars’ location (Cahill, 2015). Herbivory is also incredibly variable. Many herbivores will eat enough of the plant to kill it, while others browse sparingly on each individual plant. With all of this complexity occurring, what happens when fire is introduced into the relationship? 

Picture 1: Hornworm caterpillar on wild tobacco plant. Taken from smithsonian.com

            As stated in the previous post, in general, fire creates more undergrowth and early successional plant communities. This could lead to easier accessibility of plants to many herbivores. Many plants, such as Red Maples, are a favorite to whitetail deer when they are small enough for the deer to eat the buds, but once the tree reaches a certain size, the deer no longer utilize it. Could areas affected by fire have a higher rate of herbivory? In 1988 and '89, a study was conducted in Kansas to observe this very question. North American bison’s grazing patterns were observed on frequently and infrequently burned watersheds. During the growing season, bison were observed grazing in the frequently burned watersheds at 3 times higher rate. During autumn and winter, bison appeared to be more influenced by plant composition and grazed areas with the most cool season grasses. Although both areas were visited uniformly, the burned areas had larger areas grazed (Vinton et al, 1993). Another study in California showed similar results. In areas with high fire intensity, resprouting was delayed. The plants in the burned areas that resprouted later in the season had a higher rate of herbivory (Moreno & Oechel, 1990).

Picture 2: Bison grazing with fire in the background. Taken from wikimedia.com

            Although it appears many plants are more susceptible to herbivory following a fire, this is not always the case. Just like in response to any environmental pressure, plants evolve. One specific example came from a study about extrafloral nectaried shrubs and their relationship with ants. In burned areas the shrub was more susceptible to herbivory, but this shrub had a defense. Following fires, the plants stress raised, which lead to more concentrated extrafloral nectar. The new stronger nectar was much more enticing to ants (51.7% higher to be exact). With more ants on the shrub, the lower the rate of herbivory, which helps combat the usual higher rate following a fire (Alves-Silva & Del-Claro, 2013).

            Fire doesn't always just affect rate of herbivory. Often herbivores and fire have similar effects on a plant community, and sometimes even work together. Hickman's study showed that grazing reduced stem density, and woody species that would outcompete grasses in the prairie. She concluded that, "large grazer densities, fire, and annual climatic variability interact to influence patterns of plant community composition and diversity in tallgrass prairie"(Hickman et al, 2003).  In Europe heathlands, fires create disturbance and lower nutrients, while herbivores are main factor controlling the sensitive heath systems (Hobbs & Gimingham, 1987). Together, they help reduce trees and larger shrubs from overtaking the landscape. The last example of herbivores and fire working together is in the African savanna. The study showed that while both fire and grazers helped reduce tree density individually, only working together do they create such a low tree density that leads to the coexistence of trees and grasses seen in the savanna (Staver et al, 2009).

Picture 3: Heathland burring. Taken from JNCC.gov.uk

Picture 4: Antelope standing near fire in African savanna. Take from Jacques Jangoux

            Often, this dual effect of fire and grazers working together creates a large increase in invertebrate levels, which of many are also herbivores. In the prairies, in sites where bison had grazed and fire had influenced the landscape, 45% more grasshopper species were found (Joern, 2005). In Arizona, areas that had been burned with intermediate intensity and moderately grazed by elk saw a 40% in abundance of invertebrates. However, in areas with high intensity fires and heavy elk grazing, there was a 72% lower abundance (Bailey & Whitham, 2002). Although there is often increase in invertebrates following grazing and fires, this shows that sometimes there can be too much of a good thing and we are still far from understanding the complexity of the systems around us.

            While fire's effect on herbivore-plant relationships and how they often work together have been discussed, herbivores themselves can actually inversely affect fire and the plant communities. A study done by Kramer, showed that ungulates help reduce fuel load, regardless of density, and help reduce both the occurrence and intensity of fires (Kramer et al, 2003). Again, this further shows the complexity and the unknown of the interactions between fire, herbivores, and plants.

Works Cited

Alves-Silva, E., & Del-Claro, K. (2013, April 28). "Effect of post-fire resprouting on leaf fluctuating asymmetry, extrafloral nectar quality, and ant–plant–herbivore interactions." http://link.springer.com/article/10.1007/s00114-013-1048-z

Bailey, J., & Whitham, T. (2002, June 1). "INTERACTIONS AMONG FIRE, ASPEN, AND ELK AFFECT INSECT DIVESRSITY: REVERSAL OF A COMMUNITY RESPONSE." http://onlinelibrary.wiley.com/doi/10.1890/0012-9658(2002)083%5B1701:IAFAAE%5D2.0.CO;2/full

Cahill, J.C. (2015, October 15). PBS. "What Plants Talk About." https://www.youtube.com/watch?v=CkUoVyzPEak

Gurevitch, J., Scheiner, S. M., & Fox, G. A. (2006). The Ecology of Plants. [pgs. 265-269] Sunderland, MA: Sinauer Associates.

Hickman, K., Hartnett, D., Cochran, R., & Owensby, C. (2003, March 14). "Grazing management effects on plant species diversity in tallgrass prairie." http://www.bioone.org/doi/abs/10.2111/1551-5028(2004)057[0058:GMEOPS]2.0.CO;2

Hobbs, R., & Gimingham, C. (1987). "Vegetation, Fire and Herbivore Interactions in Heathland." http://www.sciencedirect.com/science/article/pii/S0065250408600884

Joern, Anthony. (2005, April 1). "DISTRUBANCE BY FIRE FREQUENCY AND BISON GRAZING MODULATE GRASSHOPPER ASSEMBLAGES IN TALLGRASS PRAIRIE." http://onlinelibrary.wiley.com/doi/10.1890/04-0135/full

Kramer, K. Groen, T., & Van Wieren, S. (2003, August). " The interacting effects of ungulates and fire on forest dynamics: an analysis using the model FORSPACE." http://www.sciencedirect.com/science/article/pii/S0378112703001348

Moreno, J., & Oechel, W. (1990, April 30). "Fire intensity and herbivory effects on postfire resprouting of Adenostoma fasciculatum in southern California chaparral." http://link.springer.com/article/10.1007/BF00320621

Staver, A., Bond, W., Stock, W., Rensburg, S., & Waldram, M. (2009, October 1). "Browsing and fire interact to suppress tree density in an African savanna." http://onlinelibrary.wiley.com/doi/10.1890/08-1907.1/full

Vinton, M., Hartnett, D., Finck, E., & Briggs, J. (1993, January). "Interactive Effects of Fire, Bison (Bison bison) Grazing and Plant Community Composition in Tallgrass Prairie." http://www.jstor.org/stable/2426430?seq=1#page_scan_tab_contents