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Lecture 23: Predation
Reading: Economy of Nature, pp. 400-414.
Predation
This is an interaction resulting in harm
to one individual and benefit to another individual. Defined as consumption
of one organism by another. Prey is alive when predator first attacks.
Not all consumption is predation, for example decomposers and scavengers.
In these cases, there is no interaction between consumer and "prey",
and no harm to the "prey" occurs as a result of consumption..
Functional Classification
True predator: Kills prey immediately
upon attack and often consumes all of prey.
Grazer: Attacks large numbers of prey
and only removes a part of each prey individual which is rarely lethal.
Parasite: Consume only a part of the
prey, but often are closely associated with the prey for long periods
of time. In many cases the parasite lives inside the prey host.
Parasitoid: A special type of parasite,
typically insects that lay eggs on or near prey. Prey are entirely consumed
and eventually killed by the parasitoid. Prey are other insect species,
result is always lethal to the prey.
Herbivore-Plant Interactions
Consequences of Herbivory
Removal of biomass
Interactive herbivory: normal predation,
herbivore influences individual prey growth rate, and distribution
(defoliation and fluid removal)
Non-interactive herbivory: individuals
growth and distribution of the parent plants is not influenced by
this predation, at least in the short term (seed predation)
Disease spread: incidental consequences
of herbivory
Plants are not helpless prey despite being
sessile (fixed in one location).
Defense Mechanisms of Plants
Physical Defenses
Hairs, spines, hooks
Tissue texture
Chemical Defenses
Secondary compounds
Chemicals not required for "primary"
metabolism and not necessarily defensive adaptations
Toxins or other anti-herbivore substances
Temporal Defenses
Timing of critical life history events
that are subject to herbivory
Types of plant secondary compounds (Ricklefs,
1993, pp 334-335, Table 18.2 also see Ricklefs, 1996, p 413, Table 18.2).
| Class |
Number |
Distribution |
Physiological activity |
| NITROGEN COMPOUNDS |
|
|
| Alkaloids |
10,000+ |
Widely in angiosperms, expecially in root, leaf, and
fruit |
Many toxic and bitter-tasting |
| Amines |
100 |
Widely in angiosperms, often in flowers |
Many repellent-smelling, some hallucinogenic |
| Amino Acids (nonprotein) |
400 |
Especially in seeds of legumes but relatively widespread |
Many toxic |
| Cyanogenic glycosides |
30 |
Sporadic, especially in fruit and leaf |
Poisonous (as HCN) |
| Glucosinolates |
75 |
Cruciferae and 10 other families |
Acrid and bitter |
| TERPENOIDS |
|
|
|
| Monoterpenes |
1000 |
Widely, in essential oils |
Pleasant-smellng |
| Sesquiterpene lactones |
600 |
Mainly in Compositae, but increasingly found in other
angiosperms |
Some bitter and toxic, also allergenic |
| Diterpenoids |
1000 |
Widely, especially in latex and plant resins |
Some toxic |
| Saponins |
500 |
In over 70 plant species |
Haemolyse blood cells |
| Limonoids |
100 |
Mainly in Turaceae, Meliaceae, and Simaroubaceae |
Bitter tasting |
| Curcurbitacins |
50 |
Mainly in Cucurbitacaeae |
Bitter tasting and toxic |
| Cardenolides |
150 |
Especially common in Apocynaceae, Asclepiadaceae, and
Scrophularaceae |
Toxic and bitter |
| Carotenoids |
350 |
Universal in leaf, often in flower and fruit |
Colored |
| PHENOLICS |
|
|
|
| Simple phenols |
200 |
Universal in leaf, often in other tissues as well |
Antimicrobial |
| Flavonoids |
1000 |
Universal in angiosperms, gymnosperms, and ferns |
Often colored |
| Quinones |
500 |
Widely, especially in Rhamnaceae |
Colored |
| OTHER |
|
|
|
| Polyacetylenes |
650 |
Mainly in compositae and Umbelliferae |
Some toxic |
| Source: J. B. Harborne, Introduction to Ecological Biochemistry
(2nd. ed.). Academic Press, New York (1982) |
Examples of some secondary compounds
Cardenolides: cardiac glycosides (modified
monosaccharides)
Alkaloids: morphine, caffeine, nicotine,
cocaine, cocoa
(N containing ring compounds)
Monoterpenes: peppermint, catnip, cinnamon,
cloves
Polyphenols: tannins (common in leaves
and bark of many woody plants)
A clear defensive function has not been
demonstrated for every compound in each of the classes but many examples
are unequivocal. White clover, Trifolium repens, leaves release
hydrogen cyanide (HCN) when the leaves are damaged by herbivores. Hydrogen
cyanide is a potent, rapidly acting metabolic toxin which discourages
continued herbivore consumption of the white clover leaves.
Qualitative (type) defenses are toxic chemicals
that are produced and are effective in small quantities. For example,
many of the alkaloids are very effective toxins that mimic or interfere
with herbivore neurotransmitters.
Quantitative (amount) defenses are interference
substances, digestion inhibitors or resins that make herbivory more difficult,
or less beneficial to the herbivore. For example, tannins bind to proteins
and make them indigestible (however, some tannins may be toxic in small
quantities so it is possible for a given compound to be both a quantitative
and a qualitative defense). Tannins and resins may account for as much
as 60% of leaf dry weight.
Food selection by herbivores may be regulated
more by toxins than by digestion inhibitors, but the time scale of response
need not be the same for all effective defenses.
As a working hypothesis, we assume that
secondary compounds functioning specifically as chemical defense substances
are produced at some cost to a plant (cost in energy and nutrients that
could have been used for other purposes). These substances are not simply
waste products or incidental byproducts of primary metabolism.
General Predictions About Plant Chemical
Defenses (after Krebs, 1994)
1. Defenses evolve in response to selection
by predation, but defenses reduce growth rates and reproductive output
rates.
2. Defenses will be allocated to the
most valuable tissues at risk.
3. Defenses will be reduced when herbivores
are absent and increased in response to attack.
4. Defenses are costly and cannot be
maintained if plant is under environmental stress.
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