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Lecture 1

BIOL 215 Lecture Notes - Lecture 1: Defense In Insects, Defence Mechanisms, Herbivore

Biology (Sci)
Course Code
BIOL 215
Neil Price

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BIO-215- Lecture 17
Professor Neil Price
October 30, 2014
Assignment 2 will be posted on Friday at 17:00
Submit your answers through WEBTC, as you did for assignment 1
Due at 11.59pm, November 7
Herbivory and mutualism
Communities: assemblage of populations that live together in the same habitat
Readings: Chapter 12
Herbivory: a herbivore consumes its prey but does not always kill it
Defence mechanisms of plants:
Plants can’t escape from herbivores so why is the world green?
Herbivores are self regulating
This intrinsic property of herbivore populations causes them to slow down their
feeding and prevents them from completely exploiting their prey
Herbivores abundance is controlled by predation and disease, therefore the
number of herbivores is limited.
o Ex: woodland caribou are under severe control by bears that consume them,
this has an impact on the availability of plant material
Not all green plants (parts) are edible
Secondary Metabolites
Plants are known to produce a large variety of chemicals called secondary
metabolites; their exact function is not fully understood.
They are produced as a product of primary metabolism (sugar, amino acids).
Ecologists wonder if they serve a function acting as feeding deterrents from
herbivores, and serve to defend the plant from herbivory.

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BIO-215- Lecture 17
Professor Neil Price
October 30, 2014
A strong body of literature and experimental results supports this view.
Compounds are produced at the expense of the plant and impact the herbivore
negatively (reducing its fitness, effectively causing it to no longer eat that plant
because of the taste)
There is a fitness advantage to plants that chemically defend themselves
This view implies that no one compound is defendant against a single herbivore.
Plants produce such a variety of secondary metabolites because there are a large
variety of herbivores.
Resource availability hypothesis:
Plants naturally vary in their growth rates. The fast growing plants have rapid
turnover of their synthetic tissue (leafs), the slow growing plants have a low
turnover of these tissues.
The slow growing plants invest more resources into those leaves that they have,
should they lose them there is a greater fitness cost in growing back that slow
growing leaf.
Fast growing plants on the other hard invest less energy in maintaining there leafs
and regrow them quite quickly.
You might imagine that slow growing plants would invest in chemical deterrents
more than fast growing plants, to defend their leaves from herbivores.
Defence investment vs. annual growth rate
Alternate interpretation: Defence is actually costly, the more a plant invests into defence
the less energy it will invest into growth. It is a trade-off, allocating more energy to
defence leaves less energy for growth.
Chemical or morphological defences:
Defence can be divided into constitutive defence (always present and continuously
produced) and induced defences (produced in response).
Constitutive defence : Pountia cactus in the anza-Borrega desert, California.
Spines are constitutive defence. These plants also always produce a secondary

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BIO-215- Lecture 17
Professor Neil Price
October 30, 2014
Induced defence: ascophyllum nodosom attaches to the rock by a hold fast. It has
apical and basal shoots, which are grazed upon by snails. In response to grazing
they increase the production of a feeding deterrent called phlorotannin. Increase in
apical shoots is not significant, but increase in basal shoots is significant when
grazed upon. This shows how the plants allocates its energy but also the chemical
to different parts of the plant.
How could plant-herbivore interaction be tested?
Hermathophila produces spines.
Enclosure experiment: researchers put a fence up around the shrub, which
prevented grazers from coming and feeding on the plants.
The result was that the plants produced fewer thorns when protected from
When the plant grew fewer spines it produce more flowers and fruits, this
means that the spines are costly to make, and when removed the plant was
able to allocate this energy elsewhere.
Mutualistic interactions: can serve as a system of defence
In South America, on the acacia plant, ants live on the plants and make their nests
inside the thorny bulbous thorns. Sometimes the ant density can be greater than
100,000 ants living inside these plants.
The ants feed on the leaves for food and essential oils that they need for growth
and nutrition.
In return, the ants actively deter herbivores from eating them, which serves as a
defence mechanism. The ants will also bites other plants that approach close to
the acacia.
When the ants are removed, the plant succumbs to feeding by other herbivores
and eventually dies, because it doesn’t produce any secondary metabolites that
serve to defend the plants against herbivory.
Serengeti Plains
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