Herbivore

Herbivore
n., plural: herbivores
[ˈ(h)ɜːɹbɪvɔːɹ]
Definition: An organism that consumes herbaceous vegetation

Herbivore Definition

A herbivore is the primary consumer of the food chain that usually occupies the second trophic level after the primary producers, i.e. plants. Every form of living organism needs fuel to run its basic life mechanism and metabolism.

In order to ensure that none of the living beings run out of their respective energy bars, nature has chalked out a definite food chain and food cycle. All living organisms are interlinked and interconnected to each other by these food chains and webs. There are predetermined levels called trophic levels.

Now that you know how to define herbivores, let’s have a brief overview of the trophic levels to fully grasp the definition of herbivores in biology and ecology.

A trophic level refers to a level or a position in a food chain, food web, or ecological pyramid. All trophic levels are occupied by a group of organisms that share more or less similar feeding mode, choice of nutrition supply, source of food, etc.

There are three fundamental groups of living beings classified on the basis of their feeding modes. In ecology, they are assigned specific terminologies as follows, producers, consumers, and decomposers.

• Producers~Autotrophs~1st Trophic Level
  The producers are the ones that obtain nourishment directly from inorganic sources in nature. They primarily conduct processes like photosynthesis (e.g. plants) and chemosynthesis. With the aid of these processes, they are able to convert the inorganic sources of nutrition to organic form. They usually occupy the most primary trophic level or the first trophic level in food chains and webs.
• Primary Consumers~Heterotrophs~2nd Trophic Level= HERBIVORES
  Next in the order are the primary consumers. Consumers are the ones that feed on the organic matter that the organisms at the preceding trophic level synthesize. Here, the preceding trophic level is of primary producers; hence the consumers here are called as the primary consumers. The animals that eat plants are called herbivores. Additionally, since they become the source of food for the carnivores, they are also called secondary producers.

  

• Secondary Consumers~Heterotrophs~3rd Trophic Level= CARNIVORES
  Here the preceding trophic level is of herbivores; hence the consumers here are called as secondary consumers. They consume and rely upon herbivores for their nourishment. They occupy the 3rd trophic level in food chains and webs.
• Tertiary Consumers~Heterotrophs~4th Trophic Level= Also CARNIVORES
  In many food chains and webs, there are some more carnivores sitting at higher levels than other carnivores. They are commonly called tertiary consumers.
• Topmost Trophic Level~Decomposers
  Last in the order are decomposers. Decomposers are those that break down dead organic material and wastes lying in the ecosystem. They make sure that the ecosystem is effectively cleaned upon their feeding action.

Watch this vid about herbivores and other types of consumers:

How does the cycle go on?

Usually, the primary producers are plants, algae, and chemosynthetic bacteria. Primary consumers are the “herbivorous” organisms or herbivores animals that feed directly on the primary producers. So, when questioned about why are some animals herbivores, we can explain the fact that since this subset of animals depends only upon ‘primary producers’ for their food and eat only plant material, grass, and algae.

The secondary consumer is a carnivore. They feed directly upon the herbivores to fulfill their nutritional requirements. And then there are tertiary consumers who feed on secondary consumers. These are those carnivores who feed on other carnivores.

Now let’s delve deeper and try to dissect this topic in order to gather more knowledge about herbivores. By the end of this article, you will be in a position to answer all the vital questions like ‘What is a herbivore?’ … ‘Are herbivores vegan?” …

Evolution of Herbivory

When we try to understand the subject of herbivores and the concept of herbivory in the light of evolutionary biology, we need to pay attention to the characteristic features that make an organism herbivore.

By doing so, drawing correlations between those characteristic features and the geological time scales where those features might have evolved becomes easier to understand. There are some time-tested sources from which we draw these correlations. They are:

1. Fossilized plants: These are the physical and anatomical evidence like spines, thorns, prickles, and spikes preserved in the course of time. These contribute as a major evidence of herbivory as they serve as plant defense mechanisms against plant-eating animals (herbivores). Trying molecular dating with these fossilized plant parts can help leap and bound in dating the years of herbivore evolution.
2. Looking for damages related to herbivory in animal fossils: When studying animal fossils, one can be proactive in looking for herbivory-related damages. These could range from the presence of some plant parts or debris in the animal fecal residues, some stuck spines or thorns in fossilized animal body parts, etc.
3. Studying the intricacies of herbivore mouthparts and digestive system: If you look closely at how nature has designed the mouth, teeth, and digestive system of the herbivores, it may help a lot in understanding its evolutionary history. These are the physiological evidence of herbivory development on the evolutionary time scale. Nature has designed their mouthparts in a way that supports their “solely plant-based diet” that requires a lot of rasping or grinding action. That’s the reason for the often observed flat teeth in herbivores that support this action. Also, their digestive systems have been optimized with a healthy balance of mutualistic gut flora (bacterial species and protozoa species) that are proficient in digesting tough plant material. Cell walls are made up of cellulose-rich content.

Food Chain

The importance of herbivores in the food chain has been well elaborated on in the overview section. These interrelationships between plants, herbivores, carnivores, and decomposers play essential roles in the continuity of life in the ecosystem.

Feeding Strategies

There are essentially two two feeding strategies amongst all herbivores. They are:

• Grazing

Grazing is a type of feeding strategy where a herbivorous organism feeds solely on the low-growing plants like grass (in a terrestrial ecosystem) or algae (in an aquatic ecosystem). Grazers serve several essential roles in the functioning of an ecosystem like redistribution of nutrients in the ecosystem, regular pruning of the grasslands and keeping them open, etc. An organism is placed in the category of grazers when approximately 90% of the forage available is only grass. There are further subtypes of grazing feeding strategies:

• • Graminivory: Strategy of “eating grasses”. Examples: Hippopotamuses, horses, cows, and white-tailed deer.
  • Coprophagy: Strategy of “eating feces or re-ingesting the partly-digested fecal remains of other organisms”.
  • Pseudorumination: Strategy of “mimicking ruminant organisms in possessing a multi-chambered stomach for rumination action but not chewing the cud thereafter, hence pseudo-rumination”.
  • Algivory: Grazing on different types of algae. Examples- crabs, sea urchins, fish, flamingos, arrow crabs, etc.

• Browsing

Browsing is a type of feeding strategy where a herbivorous organism feeds on leaves and twigs. An organism is placed in the category of browsers when approximately 90% of the forage available is only leaves and twigs. There are further subtypes of browsing feeding:

• • Frugivory: Strategy of “eating fruits”. The host plant benefits from the seed dispersal via herbivores. Example– fruit bats, parrots, orangutans, many birds, and ruffed lemurs.
  • Folivory: Strategy of “eating only leaves”. The host plants benefit from regular pruning via leaf and twig consumption. Example– pandas (eat only bamboo shoots), red colobuses.

*Note: Sometimes in nature, a 3rd feeding strategy is observed. It is called the “intermediate feeding strategy” or the “mixed-feeding strategy”.

• Other strategies

There are some more feeding strategies in herbivores, namely:

• • Nectarivory: Strategy of “eating nectar of flowers”. Example- butterflies, hummingbirds, bumblebees, honey possum, some species of fox, etc.
  • Granivory: Strategy of “eating seeds”. Example- squirrels, Hawaiian honeycreepers, rodents, ants, etc.
  • Palynivory: Strategy of “eating pollen from flowers”. Example- honeybee, beetles, pollen wasp, moths, bumblebees, flies, etc.
  • Xylophagy: Strategy of “eating the wood of shrubs and trees”. Example- termites, woodlice, bark beetles, wood-boring beetles, and many other species of Arthropods.
  • Mucivory: Strategy of “ingesting mucus of plant parts”. Example- aphids.

Theories for correlating animals and their feeding strategies

There have been several theories proposed to explain the correlation between animal type and their choice of food (feeding strategy). Some of them are:

1. Kleiber’s Law (correlates animal’s size and its preference for some particular food type)
2. Holling’s Disk Equation
3. Marginal Value Theorem

Plant-Herbivore Interactions

Some important points to understand when we talk about plant and herbivore interactions are:

• Herbivory is an essential biological mechanism that controls the diversity, spread, and distribution of various plant species in an area. This is although quite variable when studied in depth.
• Herbivory determines ecosystem dynamics in a big way by defining community structure and regulating functional processes of various species.
• Herbivory essentially shapes the plant defense mechanisms to a certain extent by determining the level of defensiveness a plant species needs in order to escape herbivory over evolutionary times.
• It is not just that the herbivores control and regulate the plant diversity, but the plant diversity also determines the extent to which a herbivore’s population can flourish in an ecosystem.
• Basically, these plant-herbivore interactions are co-evolving.
• Plant-herbivore interactions are subject to many abiotic factors like temperature, water availability, levels of irradiations, climate, availability of inorganic nutrients to producers (plants), biogeographical features, latitude, longitude, altitude, etc.

Herbivore offense

As we have learned now that plants and herbivores co-evolve, there must be some strategies that herbivores have adapted to optimally fit in the ecosystems. This set of skills equips the herbivores to better utilize the available plant sources as their food and maximize the food intake for their own nourishment. We can categorize these skills to counter plant defense mechanisms under 3 subheadings:

1. Feeding Choice: This strategy is very important for a herbivore to select its feeding source, i.e., the type of plant. Usually, herbivores aren’t very specific about the plants in their habitat. But developing an “avoidance mechanism” towards some plants via specific cues (sensing toxins) can help them in avoiding serious negative implications on their health.
2. Herbivore Modification: This strategy is significant for herbivores in managing the plant constitution once it enters into their system. Since plants are home to several chemical entities, secondary metabolites, fluids, saps, toxins, salts, etc, herbivores, too, have developed their mouth parts and digestive systems in a similar way to counter all this. These adaptations allow herbivores in overcoming plant defenses and toxicities.

   

   Some of these modifications are:

   • Ability to detoxify secondary metabolites and chemicals.
   • Ability to sequester toxins.
   • Ability to alter the chemical structure of toxins to render them ineffective.
   • Ability to sense environmental cues to avoid toxins.
   • Ability to produce large amounts of saliva (a mechanism to reduce the negative effect of toxins).
   • Ability to digest robust plant structures by providing optimum conditions for gut flora (symbionts like bacteria and protozoa) to flourish.
3. Plant Modification: This strategy is an exceptional one as some herbivores possess the ability to modify their plants of choice to maximize their feeding on them. This strategy minimizes the plant’s defense action and impact and aids easy feeding by the herbivore.

Plant defense

Herbivores have to be pre-equipped to deal with an array of plant defenses. Usually, biologists prefer to categorize these defense strategies based on different characteristic features:

• Based on the extent of the plant’s response to the herbivory:
  • Tolerance
  • Resistance
• Based on the spontaneity of the plant’s response:
  • Constitutive
  • Induced
• Based on the nature of the plant’s response:
  • Physical/Mechanical
  • Chemical
  • Physiological

Predator-Prey Theory

According to this theory, the population sizes of predators (herbivores) and prey (plants) are interrelated. Their interactions are cyclic. The increase in one’s number (plants) is countered by the other (herbivores) in a given ecosystem. Likewise, the decrease in one’s number (herbivores) provides an opportunity for the other (plants) to replenish their numbers in the ecosystem. Eventually, in the long run, co-evolution takes care of both populations.

Mutualism

When observed from the plant’s perspective in a cursory view, this interaction between plants and their predators (herbivores) doesn’t sound like a mutualistic relationship. But when studied intensively, you get to learn how many herbivores sustain the essential life processes of a plant’s life. Some of these processes are pollination and seed dispersal. Without the completion of these processes, the continuity of many plant species may not be possible.

Impacts

There are many environmental and anthropological effects on herbivores and their relations with plants. Not only in science, herbivores also hold an essential position in environmental and economy-related discussions.

• Trophic cascades and environmental degradation

Trophic cascades are reported when some member of a food chain is wiped off from its trophic level. This leaves the members of the immediate lower trophic level to reproduce and increase in their numbers ‘unabatedly’. In the case of the plant-herbivore relationship, the vanishing of the carnivores sitting up on the higher trophic level than herbivores can cause a lot of chaos. Herbivores increase in their numbers beyond expectations in such a case. This puts the populations of plants in that ecosystem at risk of wiping out.

There is a well-noted example of environmental degradation linked to trophic cascades- “coral reef ecosystems”. When the herbivorous fish populations decline in a coral reef ecosystem, the seaweeds outcompete the corals and grow unabatedly. This ultimately deprives the corals of sunlight availability. The presence of herbivorous fish becomes vital in such a case to control and regulate the populations of seaweeds in this ecosystem.

• Economic impacts

Herbivores contribute a big bounty to the ecotourism industries. Wildlife safaris to have encounters with elephants, hippos, deer, horses, elks, antelopes, etc fascinate the human population. This brings in a lot of share of the economic transactions to the presence of herbivores in this world.

Frequently Asked Questions

Let’s check your knowledge of herbivores now!

• Is deer herbivore? Answer: Yes
• Are horses herbivores? Answer: Yes
• Are elephants herbivores? Answer: Yes
• Is a hippo herbivore? Answer: Yes
• Give examples of herbivore insects. Answer: Butterfly, Bumblebee, …
• Give examples of ocean herbivores. Answer: Some varieties of fish, arrow crab, etc…
• Give examples of tundra herbivores. Answer: Rodents
• Is a mouse herbivore? Answer: May or may not be, it depends on the species
• Are rhinos herbivores? Answer: Yes
• Give an example of a desert herbivore. Answer: Camel
• Do monkeys eat meat? Answer: Yes, they are omnivorous

Good To Know!

Where will you place parasitic plants amongst the different trophic levels?

Are they plants or herbivores or carnivores? Scientifically, we place parasitic plants under the category of herbivores as they feed upon other plant sources for their nutritional requirements. A parasitic plant is equipped with the skill to derive nourishing nutrients by sticking and sucking the sap from the host plant. An interesting example is the Dodder Plant!

A vine native to the tropics around the world, dodder has the ability to efficiently wrap around its host and extend its haustoria (similar to roots in plants). Haustoria help in both, the attachment of the parasitic plant to the host and nutrition derivation. Dodder is so efficient in sucking off the nutrients that it eventually renders the hosts empty of its own nutrients. Sometimes it also leads to the death of the host plant.

Dodder — a herbivore yet a deadly parasite!

Answer the quiz below to check what you have learned so far about herbivores.

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References

• Sagers, C.L. (1992). “Manipulation of host plant quality: herbivores keep leaves in the dark”. Functional Ecology. 6 (6): 741–743. doi:10.2307/2389971
• Milchunas, D.G.; Noy-Meir, I. (October 2002). “Grazing refuges, external avoidance of herbivory and plant diversity”. Oikos. 99 (1): 113–130. doi:10.1034/j.1600-0706.2002.990112.x
• Huntly, N. (1991). Herbivores and the Dynamics of Communities and Ecosystems. Annual Review of Ecology and Systematics, 22, 477–503. http://www.jstor.org/stable/2097271
• S. J. McNaughton (1986). On Plants and Herbivores. The American Naturalist 128:5,765-770
• S. W. Pacala and M. J. Crawley. (1992) Herbivores and Plant Diversity. The American Naturalist 140:2, 243-260