Download Herbivory

Lab 5: Species interactions - Herbivory
I. Introduction to Herbivory
Herbivory is a term used to describe a particular species interaction in which a plant is consumed (whole
or in part) by some consumer organism. The consumer (Herbivore) may range greatly in size from
organisms that are smaller than the plant on which they feed (e.g. insect larvae, moths), to large grazing
animals (e.g. deer, elephants).
There are numerous types of herbivores which have adopted methods for feeding on different parts of
their host plant. As such, on any given plant, the leaves, buds, stems, bark, flowers, fruits, seeds or roots
may be consumed. Accordingly, herbivores have developed numerous feeding methods which allow them
to effectively exploit certain plant resources. Herbivore feeding may include techniques such as
removing and chewing fruits, leaves and stems, stripping and consuming bark, boring into buds, seed and
roots or drinking nectar and eating pollen from flowers.
The damage inflicted upon plants by herbivores can affect these plants in negative and/or positive ways
depending on the type, frequency and intensity of herbivory. For example, low levels of grazing may
actually stimulate a plant to grow more quickly and produce more biomass. Once grazing levels become
intense, however, plants are often unable to compensate for the loss of biomass and therefore either
maintain very low biomass or die. Insects that feed on the nectar or pollen of plant flowers may also be of
benefit to the plant, as they often facilitate cross pollination as they move from one plant to another. On
the contrary, many insects (e.g. aphids, locusts and several insect larvae) are considered serious pests as
they cause significant damage to vegetative and reproductive structures on plants.
Many plants have evolved strategies that allow them to compensate for, or completely avoid losses due to
herbivory. Several plants possess physical and mechanical adaptations that reduce or prevent the
occurrence of herbivory. For example, plants may have hairy or waxy leaves, thorny stems or thick bark,
all of which reduce the ability of a herbivore to damage the plant. Another strategy involves the
production of toxins or resins by the plant, which serves to reduce digestibility of the plant, repel or (in
severe cases) kill the herbivore.
The development of effective strategies for avoiding or deterring herbivores can significantly enhance the
survivorship of a plant species. There are several examples, however, of herbivores which have
developed counter mechanisms that allow them to avoid or, in some unique cases, exploit the plant’s
defenses. A good example of this demonstrated by the Monarch butterfly (Danaus plexippus) which
feeds exclusively on Milkweed (Asclepias curassavica) (on the leaves as a larva and on the nectar as an
adult). Milkweed is a herbaceous plant that produces a milky sap which acts as a skin irritant, as well as
toxic glycosides which are poisonous if ingested. By consuming this plant, Monarch butterflies are able
to sequester the plant toxins and are therefore become poisonous to birds and other animals which may
prey upon them.
II. Objectives
In this lab, we will compare herbivory damage on two species of plants in the FIU preserve, one of which
uses chemical defenses to deter herbivory, the other of which does not. The objectives are:
1) To estimate level of herbivory on two species of plants in the FIU preserve
2) To compare levels of herbivory between these two plants
3) To examine the effectiveness of chemical defenses against herbivory
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III. Instructions
Before you leave the lab to go to your field site for today’s exercise, be sure to do the following.
1. Generate several hypotheses as a class that you can test with today’s exercise (Hint: the
objectives, above, may help with this).
2. Discuss how to keep track of the data that you record at your field site and set up data sheets.
3. Gather all the equipment that you will need for the lab exercise(s).
We will be going to the FIU preserve on campus and collecting leaves from two species of plants and
then determining the level of herbivory (plant damage by insects).
Field Instructions:
1.
2.
3.
4.
Organize yourselves into groups of 4 to 5 people.
Make sure you have: 6 paper bags and 1 pair of clippers or a blade for cutting leaves.
Trek out to the FIU preserve.
Randomly identify three replicate plants of the first species (this will be done by searching for the
plant and selecting the 2nd, 4th and 6th plant found).
5. On each plant randomly select 10 leaves (this will be done by selecting 2 leaves from each of 5
branches on the plant).
6. Place the 10 leaves in a paper bag labeled as follows:
Date:
Group #:
Species name:
Replicate #:
7. Randomly identify three plants of the second species using the same method as before.
8. On each plant randomly select 10 leaves
9. Place the 10 leaves in a labeled paper bag
10. You should have 3 bags each with 10 leaves of species 1 and 3 bags each with 10 leaves of
species 2.
11. Take all collected leaves back to the lab
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Lab Instructions:
1. In the lab, we are going to calculate the approximate percentage of each leaf that has been
removed by herbivory.
2. You will do one bag at a time, so as not to mix up the samples
3. First remove a leaf from the bag, place the leaf on a piece of graph paper and trace the outline of
the entire leaf
4. Next, trace the outline of any portions of the leaf that were removed by herbivory
5. You should end up with a drawing that looks like this:
6. Next you will calculate the total area of the leaf (outlined in black) and the area that has been
removed by herbivory (outlined in red).
7. The total area will be calculated by first counting the approximate number of squares on the graph
paper that are included in your drawing. If only part of a square is included within the drawing,
then record the approximate proportion of the square.
8. Each square is equal to 1 mm2, therefore by adding the total number of squares, you can estimate
the total area in mm2.
1 square = 1mm2
Approx 80%square = 1mm2
The above example shows a total of: 23 squares + 0.75 + 0.75 + 0.25 + 0.1 + 0.1 + 0.5 + 0.9 + 0.8
+ 0.25 + 0.1 + 0.01 + 0.4 + 0.8 + 0.1 + 0.4 + 0.4 + 0.01 = 29.62 mm2.
9. The total area that has been removed by herbivory will be calculated in a similar manner by
simply counting the number of squares included in the damaged portion of the leaf.
The above example shows a total removed area of: 0.4 + 0.2 + 0.7 + 0.02 + 0.5 + 0.2= 2.02 mm2.
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10. Next you will calculate the percentage of the leaf removed by herbivory. This is equal to:
The area removed by herbivory
The total area of the leaf
x 100
The above example shows the percentage of the leaf removed by herbivory as (2.02/29.62)x 100
= 6.82%
11. These calculations will be done for each of the 10 leaves collected from each of the 3 replicates
for each of the 2 species of plants.
12. You will need to record your data in tables similar to the ones below:
Percentage herbivory for leaves of species 1
Plant 1 %
herbivory
Replicate
1
2
3
4
5
6
7
8
9
10
Plant 2
%
herbivory
Percentage herbivory for leaves of species 2
Plant 3
%
herbivory
Plant 1
%
herbivory
Plant 2
%
herbivory
Plant 3
%
herbivory
Replicate
1
2
3
4
5
6
7
8
9
10
Information sources
Ricklefs R. E. and Miller G. L. 2000. Ecology. 4th Edition. W. H. Freeman and Company. 822pp.
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