Herbivory: Plant Populations

Freshta Hazim, Andre Oliveira-Shahinfar & Deenesh Dhanrajh

                                                           What is Herbivory?                                                              An animal that is anatomically and physiologically adapted to eating plant material

                                           Trophic Levels of food chain

●Level 1: Food chains start at trophic level 1 with primary producers such as plants

●Level 2: Contains primary consumers such as Herbivores

●Level 3: Contains Carnivores that eat herbivores and they are called secondary consumers

●Level 4: Contains Carnivores that eat other carnivores and they are called tertiary consumers

                                       Herbivours feeding strategies

                                Strategy                                                         Diet

                                  Xylophages                                                               Wood

                                  Mucivores                                                 Sap, Plant Fluids

                                  Palynivores                                                            Pollen

                                  Granivores                                                              Seeds

                                 Nectarivores                                                       Nectar

                                    Folivores                                                                  Leaves

                                   Frugivores                                                                  Fruit

How do Plants Defend Themselves against Herbivorous?

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counterhe effects of herbivore attack.

Plant Defences & Herbivore Offences

Plant Defences

●Constitutive: always present in the plant

●Induced: produced or translocated by the plant following damage or stress

Mechanical Defences or Chemical Defences

Types of Plant Defences

Mechanical Defences

●Sharp prickles, spines, thorns, trichomes inhibit herbivory

●Calcium Oxalate, Calcium Carbonate make digestion painful

●Cell wall and additional epidermis protection directed towards water damage,Pathogens

●Silica functions as defence against invertebrates only

Chemical Defences

●Nitrogen Compounds: Alkaloids, Cyanogenic glycosides

●Terpenoids: Pyrethrins, Isoprene

●Phenolics: Tannins, Lignins, Furanocoumarins


●Nitrogen Compounds: Alkaloids, Cyanogenic glycosides

●Terpenoids: Pyrethrins, Isoprene

●Phenolics: Tannins, Lignins, Furanocoumarins

●Derived from amino acids

●Alters fat storage by inhibiting phosphodiester bonds

●Can inhibit protein synthesis and DNA repair mechanisms

●Ex. Nicotine, Caffeine, Morphine, Ergovaline


●Produced by endophyte-tall fescue symbiosis

●Acremonium coenephialum

●Causes: fescue foot, fat necrosis and fescue toxicosis

●Minor Symptoms: Rough hair coats, heat stress, suppressed appetite, poor growth

●Major Symptoms: Lost limbs, loss of blood flow to feet, hard fat deposits in abdomen, reduced calving

●Nearly 1 billion dollars lost annually

The Optimal Defence Hypothesis

●Many grass spp. contain highest concentrations of defensive secondary compounds under abiotic stress

●Resource rich environments, replacing tissue=easy

●Resource poor environments, replacing tissue=costly. Therefore, more resources allocated towards defence

Advantages of Tall Fescue

●Cool season perennial grass

●High Yields

●Drought Resistant

●Resistant to overgrazing

●High quality nutrient source

Herbivore Offences

1.Feeding Choices



Feeding Choices

●Herbivores faced with nutrient deficient plants

●Select for plants with higher N content

● Generalist herbivores have a mixed and balanced diet

●Mixed diet allows for uptake of various nutrients

●Avoid patchy plant defences


●Arthropods and vertebrates contain cytochrome P-450 enzymes that act as plant metabolite suppressors

●Salivary enzymes such as glucose oxidase suppresses early symptoms of plant defences. Ex. lipoxygenase

●TBSP (Tannin Binding Salivary Proteins)

Moose and reindeer saliva displayed a mitigating effect on the on the expansion of the fungal endophyte. The standard colony decreased with the application of herbivore saliva. This may be a cause of enzymes similar to and including glucose oxidase (Bazely, Tanentzap, Vicari, 2014)

In measuring the ergovaline levels of clipped plants, the levels decreased when application of herbivore saliva was added. However, uncut plants showed a lower level of ergovaline than plants that were cut and brushed with distilled water. Therefore, the defence of ergovaline seems to be an inducible defence as the rate is higher in plants that were clipped. (Bazely, Tanentzap, Vicari, 2014)


●Allow herbivores to exploit food that would otherwise be toxic

●Herbivores can’t digest cellulose but house bacteria, fungi and protozoa that do

●Aid herbivores in procuring mixed plant diet


What is Overgrazing?

Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications or by free grazing in rangelands by native or non-native wild animals (Collinge 2000­).

Ecological effects of Overgrazing

  • Reduces usefulness and productivity of the land
  • Major cause of erosion and desertification
  • Leading cause of the spread of invasive species and non-native plants and weeds

Economic Impacts of Overgrazing

  • Can change Vegetative Communities through over browsing and cost forest restoration projects upwards of $750 million dollars annually
  • Impacts economies through the revenue generated by recreational uses of herbivorous organisms, such as hunting and ecotourism. Hunting industry negatively impacted as governments must use funds to maintain graze lands.
  • Ecotourism, particularly in Africa, where many large mammalian herbivores such as elephants, zebras and giraffes help to bring in millions of dollars to various nations annually (Frost 2008).


Fig showing the contribution of overgrazing to desertification

Rotational Grazing

Under improper rotational grazing, overgrazed plants do not have enough time to recover to the proper height between grazing events. The animals resume grazing before the plants have restored carbohydrates reserves and grown back roots lost after the defoliation. There are efficient ways of managing rotational grazing, by developing a system that allows sufficient regrowth (Strauss 2002).

Ways to combat Overgrazing

  • Allocate funding for modernization of rural farmers
  • Create and enforce ecological limits on companies/farmers in respect to defoliation
  • Create significant benefits and subsidies for companies that employ proper grazing techniques

What causes Overgrazing in range lands?

Overgrazed rangeland is often characterized by an increase in weeds or unpalatable plants, increased soil erosion, and a decrease in the biomass of important forage plants. Rangelands typically receive less precipitation than more productive farmland, and have few or low growing plants which shouldn't be confused with an overgrazed site. Overgrazing should not be confused with overstocking. Overstocking is when a site is heavily stocked with more animals than the site could support for a grazing season, such as is often the case with targeted grazing. However, poor management coupled with overstocking can severely degrade a site (Cepni 2012).

Overgrazing, a growing Problem

Comment Stream

3 years ago

Was very detailed but some sections were not clearly defined and didn't make sense. There was a lot of scientific terms that some people wouldn't understand, and didn't make sense due to a lack of context. food examples though :)