OUTLINE

• Heterotrophs• Chemical composition and

nutrient requirements• Defenses of the plants• Mimicry• Evolution in defense mechanisms

and what they lead

HETEROTROPHS

Heterotrophic organisms use organic molecules both as a source of carbon and as an energy source.

Major groups of heterotrophs:HerbivoresCarnivoresDetritivores

All these three groups face problems

CHEMICAL COMPOSITION AND

NUTRIENT REQUIREMENTS

• Elemental composition of the organisms

• C:N ratio in organisms• Additional nutrients• How do plants and animals supply

their nutrient requirements?

HERBIVORES• Herbivores are the organisms that

eat plant.• Problems that the herbivores face

- defense mechanisms of the

plants:1. physical defenses2. chemical defenses

• toxins•digestion-reducing substances

1. Physical Defenses

• Thorns• Abrasive silica in the tissues• Toughened tissues with

cellulose and lignin

2.Chemical Defenses

• Increased C:N ratio due to the cellulose and lignin content results in a carbon-rich; nutrient poor plant.

• If a pine tree is considered; tree trunks occupy most of the area, creating a biomass that has a C:N ratio of 300.1.

• What is more, most of the animals cannot digest cellulose or lignin.

• These animals overcome this problem by the help of the bacteria, fungi or protists that live in their digestive tract.

• The further chemical defenses are grouped as toxins and digestion-reducing substances.

• Toxins: kill, impair, repel most of the would-be consumers.

• Digestion reducing substances: compounds that bind to plant proteins, inhibiting their breakdown by enzymes and further reducing the nitrogen availability which is already low.

• Chemists have isolated thousands of species and draw a conclusion: more tropical plants contain toxic alkaloids than do temperate species.

• Although plants have evolved higher levels of chemical defenses, herbivores can still consume approximately %11-%48 of the leaf biomass in tropical forests.

• Despite the fact that the plants in temperate forest have lower levels of chemical defenses, herbivores only consume %7 of the leaf biomass.

• Conclusion: Natural selection for chemical defense is more intense in tropical plant populations.

Experiment: Generalization about high levels of chemical defense

among plantsRobin Bolser, Mark Hay

(1996)• Hypothesis: Tropical seaweeds

have more chemical defenses than temperate seaweeds.

• They collected several species of seaweeds from:North Carolina (temperate species)Bahama Islands (tropical species)

• Same species or at least, species that belong to same genus were taken.

• Palatability of the seaweeds was tested using temperate and tropical sea urchins.

• Results:– Clear preference for temperate

species of seaweeds.– Both temperate and tropical sea

urchin species preferred temperate species of seaweeds.

– Additional tests showed that tropical seaweed species possessed more intense chemical defense.

No defense is perfect…

• Most defenses of the plants work, but not all.

• The tobacco plant uses nicotine to repel herbivorous insects, which suddenly die after ingesting nicotine.

• However, some species can excrete nicotine or convert it to non-toxic molecules.

• Spotted cucumber beetle is attracted by the toxins and repellents unlike other insects.

• Some specialized herbivores can even use toxins as a source of nutrition.

DETRITIVORES• Detritivores (detrius feeders or

saprophages) feed on nonliving organic matter.

• They obtain nutrients by consuming detrius.

• They play a crucial role in nutrient cycling.

• They consume carbon and energy rich, but nitrogen poor food.

• Plants already have low nitrogen content. When their leaves are cast off, they contain even lower amounts of nitrogen.

• Low nitrogen content of the detritus results in nitrogen poor food for detritivores.

• Average nitrogen contents of the living and nonliving leaves from tropical rain forests through deserts and temperate forests showed that living leaves contain about twice the nitrogen as dead leaves.

CARNIVORES

• Carnivores mainly feed on animal flesh.

• Their prey are nutritionally rich.• Their problems about

nourishment:– They cannot go out from their

environment– They do not choose prey at will– Prey usually have good defense

mechanisms

Defense mechanisms of prey

• Basic defense: camouflage.• Anatomical defenses: spines,

shells, repellents, poisons.• Behavioral defenses: flight,

taking refuge in burrows, banding together in groups, playing dead, fighting, flashing bright colors, spitting, hissing, screaming at predators.

Aposematic coloration: “think twice before feeding

on me”• Aposematic:

Conspicious, bright colors that warns predators about the possibility of poison.

• Aposematic coloration is usually consists of contrasting patches of orange, yellow and black.

MIMICRY

• Mimicry: similarity in appearance.– Müllerian mimicry : Among noxious

species. (stinging bees, wasps, poisonous snakes, butterflies)

– Batesian mimicry : Harmless species mimic noxious species, taking them as a model.

The monarch butterfly (the model) contains noxious chemicals in its tissues. 

The viceroy butterfly (harmless species) mimics monarch butterfly.

How have prey populations evolved their defenses?

• H. Kettlewell (1959) found that predation by birds favors camouflage.

• Birds choose conspicuous members, leave better camouflaged ones. Namely, predators eliminate poorly defended prey and leave the well defended ones.

• Conclusion: average level of defense increases with time.

White and black peppered moths.

Birds eat conspicuous members of the pepper moths.

• As a consequence of good prey defenses, prey capture rate by predators is low.

• Observations about bald-faced hornets showed that they rapidly pounce on inanimate objects as they may be prey. Their prey capture rate was found to be less than %1. (Bernd Heinrich, 1984)

Carnivor preys usually similar in nutrient content

• Carnivores are widely distributed geographically; their diets can vary from one place to other.

• Variety in prey is related with the availability of the prey.

Size-Selective Predation

• Predators must catch and subdue their prey.

• It’s most common in solitary predators; like Puma concolor.

• Puma size changes depending on the latitudinal gradient; and prey size is directly proportional with the predator size.

Size selective predation has two reasons:

1. Large prey is hard to subdue and may even injure the predator; small prey is difficult to find or catch.

2. Energetic basis.

Results of predator-prey relationships

• Predators consume nutritionally rich prey but face with well defended and strong prey.

• Predators eliminate poorly defended members, leading average prey defenses improve.

• Improvement in average prey defenses make poor predators be eliminated and leave fewer offspring.

• Improvement in hunting skills lead further natural selection.