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Herbivore

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Moose on Seedskadee National Wildlife Refuge

Herbivores are animals that only eat plants. They are herbivorous animals.

Most modern herbivores (such as deer, elephants, horses) are grazers with teeth that are adapted to grind grass. The main present-day vegetation in most continents is grassland. Some herbivorous animals do eat eggs and sometimes other animal protein.

Animals who eat mainly fruit are called frugivores. Browsers eat mostly leaves and sometimes small tree branches. Animals that eat mostly grass are grazing animals.

The diet of some herbivorous animals changes with the season. In the temperate zones, some seasons are hot and some are cold, so different plants are available at different times of the year.

Digesting cellulose

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Plant cell walls are mostly made up of cellulose. No animal can digest cellulose by itself. They make use of gut flora, some of which produce an enzyme called cellulase. This is an example of symbiosis.

Herbivore-plant interactions

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According to the theory of predator-prey interactions, the relationship between herbivores and plants is cyclic.[1] When prey (plants) are numerous, herbivores increase in numbers. Reducing the plant population causes herbivore number to decline.[1] The plant population recovers, starting a new cycle. The population of herbivores fluctuates around the carrying capacity of the food source, in this case the plants.

There will always be pockets of plants not found by herbivores.[2] Eating a second plant type helps herbivores’ populations stabilize.[2] Alternating between two or more plant types provides population stability for the herbivore, while the populations of the plants oscillate.[1] When an invasive herbivore or plant enters the system, the balance is thrown off and the diversity can change or even collapse.[2]

It is easier to be an herbivorous animal than a carnivorous (meat-eating) animal. Carnivorous animals have to find and catch the animals that they eat, and sometimes the animals that they want to eat fight them. Herbivorous animals have to find the plants that they want to eat, but they do not have to catch them. Many plants have some defence against herbivores, such as spines, toxins (poisons), or a bad taste. There are many more herbivorous animals living in the world than carnivorous animals.

Herbivore effects on plant diversity

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Herbivores might increase plant diversity or decrease plant diversity.[3]

People used to think herbivores increase plant diversity by avoiding dominance.[4] Dominant species tend to exclude subordinate species. However, the effects on plant diversity caused by variation in dominance could be beneficial or negative.[5] Herbivores do increase bio-diversity by consuming dominant plant species, but they can also prefer eating subordinate species according to plants’ palatability and quality.[6] In addition to the preference of herbivores, herbivores' effects on plant diversity are also influenced by other factors, defense trade-off theory,[7] the predator-prey interaction, [8] and inner traits of the environment and herbivores.[6][9]

One way that plants could differ in their susceptibility to herbivores is through defense trade-off. Defense trade-off theory is commonly used to be seen as a fundamental theory to maintain ecological evenness.[10] Plants can make a trade-off response to resource allocation, such as between defense and growth.[11] Defenses against herbivory on plant diversity can vary in different situations. It can be neutral, detrimental or beneficial for plant fitness.[12] Even in the absence of defensive trade-offs, herbivores may still be able to increase plant diversity, such as herbivores prefer subordinate species rather than dominant species.[13]

The predator-prey interaction, especially the “top-down” regulation. The predator-prey interaction encourages the adaptation in plant species which the predator prefers. The theory of “top-down” ecological regulation disproportionately manipulates the biomass of dominant species to increase diversity.[14][15] The herbivore effect on plant is universal but still significantly distinguish on each site, can be positive or negative.

In a highly productive system, the environment provides an organism with adequate nutrition and resources to grow. The effects of herbivores competing for resources on the plant are more complicated.[16][17] The existence of herbivores can increase plant diversity by reducing the abundance of dominant species, redundant resources can then be used by subordinate species. Therefore, in a highly productive system, direct consumption of dominant plants could indirectly benefit those herbivory-resistant and unpalatable species. But the less productive system can support limited herbivores because of lack of nutrients and water. Herbivory boosts the abundance of most tolerant species and decreases the less-tolerant species’ existence which accelerates the plant extinction.[17] Mediate productive system sometimes barely has long-term effects on plant diversity. Because the environment provides a stable coexistence of different organisms. Even when herbivores create some disturbances to the community. The system is still able to recover to the original state.[3]

Light is one of the most important resources in environments for plant species. Competition for light availability and predator avoidance are equally important.[16] With the addition of the nutrients, more competition arises among plant species. But herbivores could buffer the diversity reduction. Especially large herbivores can enhance the bio-diversity by selectively excluding tall, dominant plant species, and increase light availability.[17]

Body size of herbivores is a key reason underlying the interaction between herbivores and plant diversity, and the body size explains many of the phenomena connected to herbivore-plant interaction. Small herbivores are less likely to decrease plant diversity. Because small non-digging animals may not cause many disturbances to the plant and the environment. Intermediate-sized herbivores mostly increase plant diversity by consuming or influencing the dominant plant species, such as herbivore birds, that can directly use dominant plant species.[3] While some herbivores enhance plant diversity by indirect effects on plant competition. Some digging animals at this size local community environmental fluctuations.[18] And the adaptation of plant species to avoid predators can also adjust the vegetation structure and increase diversity.[19] Larger herbivores often increase plant diversity. They use competitively dominant plant species, and disperse seeds and create disorder of the soil. Besides, their urine position also adjusts the local plant distribution, and prevent light competition.[17]

Therefore, the mechanisms of herbivores’ effects on plant diversity are complicated. Generally, the existence of herbivores increases plant diversity. But varies according to different environmental factors, multiple factors combined to affect how herbivores influence plant diversity.

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References

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  1. ↑ 1.0 1.1 1.2 Gotelli N.J. 1995. A primer of ecology. Sinauer.
  2. ↑ 2.0 2.1 2.2 Smith R.L. & T.M. 2001. Ecology and field biology. 6th ed, Cummings, New York.
  3. ↑ 3.0 3.1 3.2 Olff, Han; Ritchie, Mark E. (July 1998). "Effects of herbivores on grassland plant diversity". Trends in Ecology & Evolution. 13 (7): 261–265. doi:10.1016/s0169-5347(98)01364-0. ISSN 0169-5347. PMID 21238294.
  4. ↑ Mortensen, Brent; Danielson, Brent; Harpole, W. Stanley; Alberti, Juan; Arnillas, Carlos Alberto; Biederman, Lori; Borer, Elizabeth T.; Cadotte, Marc W.; Dwyer, John M. (2017-07-17). "Herbivores safeguard plant diversity by reducing variability in dominance". Journal of Ecology. 106 (1): 101–112. doi:10.1111/1365-2745.12821. ISSN 0022-0477. S2CID 90964494.
  5. ↑ 21074968 - Siebert, Frances Koerner, Sally E. Siebert, Frances Smith, Melinda D. Burkepile, Deron E. Hanan, Niall P. (2018). Change in dominance determines herbivore effects on plant biodiversity. Nature. OCLC 1111689725.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  6. ↑ 6.0 6.1 Bakker, Elisabeth S. (2017-04-16). "Herbivore size matters for productivity-richness relationships in African savannas: Commentary on Burkepile et al . (2017)". Journal of Ecology. 105 (3): 687–689. doi:10.1111/1365-2745.12745. ISSN 0022-0477. S2CID 90505172.
  7. ↑ Viola, D. V.; Mordecai, E. A.; Jaramillo, A.G.; Sistla, S. A.; Albertson, L.K.; Gosnell, J. S.; Cardinale, B.J.; Levine, J. M. (2010-09-20). "Competition-defense tradeoffs and the maintenance of plant diversity". Proceedings of the National Academy of Sciences. 107 (40): 17217–17222. Bibcode:2010PNAS..10717217V. doi:10.1073/pnas.1007745107. ISSN 0027-8424. PMC 2951440. PMID 20855605.
  8. ↑ Allesina, Stefano; Tang, Si (March 2012). "Stability criteria for complex ecosystems". Nature. 483 (7388): 205–208. arXiv:1105.2071. Bibcode:2012Natur.483..205A. doi:10.1038/nature10832. ISSN 0028-0836. PMID 22343894. S2CID 4367035.
  9. ↑ Olff, Han; Ritchie, Mark E.; Prins, Herbert H.T. (2002). "Global environmental controls of diversity in large herbivores". Nature. 415 (6874): 901–904. Bibcode:2002Natur.415..901O. doi:10.1038/415901a. ISSN 0028-0836. PMID 11859367. S2CID 4430221.
  10. ↑ Huot, Bethany; Yao, Jian; Montgomery, Beronda L.; He, Sheng Yang (August 2014). "Growth–Defense Tradeoffs in Plants: A Balancing Act to Optimize Fitness". Molecular Plant. 7 (8): 1267–1287. doi:10.1093/mp/ssu049. ISSN 1674-2052. PMC 4168297. PMID 24777989.
  11. ↑ Mole, Simon (October 1994). "Trade-Offs and Constraints in Plant-Herbivore Defense Theory: A Life-History Perspective". Oikos. 71 (1): 3–12. doi:10.2307/3546166. ISSN 0030-1299. JSTOR 3546166.
  12. ↑ Tuller, Juliana; Marquis, Robert J.; Andrade, Samara M. M.; Monteiro, Angelo B.; Faria, Lucas D. B. (2018-08-22). "Trade-offs between growth, reproduction and defense in response to resource availability manipulations". PLOS ONE. 13 (8): e0201873. Bibcode:2018PLoSO..1301873T. doi:10.1371/journal.pone.0201873. ISSN 1932-6203. PMC 6104975. PMID 30133458.
  13. ↑ Lind, Eric M.; Borer, Elizabeth; Seabloom, Eric; Adler, Peter; Bakker, Jonathan D.; Blumenthal, Dana M.; Crawley, Mick; Davies, Kendi; Firn, Jennifer (2013-01-24). "Life-history constraints in grassland plant species: a growth-defence trade-off is the norm". Ecology Letters. 16 (4): 513–521. doi:10.1111/ele.12078. ISSN 1461-023X. PMID 23347060. S2CID 552202.
  14. ↑ Coley, P. D.; Barone, J. A. (November 1996). "Herbivory and Plant Defenses in Tropical Forests". Annual Review of Ecology and Systematics. 27 (1): 305–335. doi:10.1146/annurev.ecolsys.27.1.305. ISSN 0066-4162.
  15. ↑ Jia, Shihong; Wang, Xugao; Yuan, Zuoqiang; Lin, Fei; Ye, Ji; Hao, Zhanqing; Luskin, Matthew Scott (2018-05-30). "Global signal of top-down control of terrestrial plant communities by herbivores". Proceedings of the National Academy of Sciences. 115 (24): 6237–6242. doi:10.1073/pnas.1707984115. ISSN 0027-8424. PMC 6004463. PMID 29848630.
  16. ↑ 16.0 16.1 Borer, Elizabeth T.; Seabloom, Eric W.; Mitchell, Charles E.; Cronin, James P. (2013-10-15). "Multiple nutrients and herbivores interact to govern diversity, productivity, composition, and infection in a successional grassland". Oikos. 123 (2): 214–224. doi:10.1111/j.1600-0706.2013.00680.x. ISSN 0030-1299.
  17. ↑ 17.0 17.1 17.2 17.3 Bakker, Elisabeth S.; Ritchie, Mark E.; Olff, Han; Milchunas, Daniel G.; Knops, Johannes M. H. (July 2006). "Herbivore impact on grassland plant diversity depends on habitat productivity and herbivore size". Ecology Letters. 9 (7): 780–788. doi:10.1111/j.1461-0248.2006.00925.x. ISSN 1461-023X. PMID 16796567.
  18. ↑ Huntly, N.; Reichman, O. J. (1994-11-18). "Effects of Subterranean Mammalian Herbivores on Vegetation". Journal of Mammalogy. 75 (4): 852–859. doi:10.2307/1382467. ISSN 1545-1542. JSTOR 1382467.
  19. ↑ Hughes, Jessica J.; Ward, David (December 1993). "Predation risk and distance to cover affect foraging behaviour in Namib Desert gerbils". Animal Behaviour. 46 (6): 1243–1245. doi:10.1006/anbe.1993.1320. ISSN 0003-3472. S2CID 53177272.
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