II. POPULATION INTERACTIONS

II . Population Interactions
	12. Mutualism
	13. Commensalism - amensalism
	14. Predation
	15. Parasitism
	16. Competition
	17. Introductions

II. POPULATION INTERACTIONS

12. MUTUALISM

Types of interactions

Population interactions. There are three different ways that one organism may interact with another, giving 6 possible interactions between species.

Types of interactions

+ = positive or beneficial effect

0 = no effect

- = negative or harmful effect

Possible combinations

+ + = mutualism

+ 0 = commensalism

0 0 = neutralism

0 - = amensalism

+ - = predation, parasitism

- - = competition

General comments

Symbiosis. Symbiosis in its truest sense means living together and includes parasitism. Its

modern definition refers only to positive interactions. Define it before using it.

Life history. Relationships may change with life history.

Small bass eaten by sunfish, large bass eats sunfish

Fly larva is parasite, adult is nectivore

Mutualism (+ +)

Types of mutualism. In mutualistic interactions each species benefits from the other.

Facultative: each can live separately, without the other (protocooperation)

Spider crab gives food; anemone gives protection

Hollow tree gives shelter; bat gives fertilizer

Seed dispersal by birds, mammals, ants: Fruit attracts animals that distribute seeds

Obligatory: neither can live without other (true mutualism)

Often involves fungi, often in tissues of plants (mutualants)

Lichen: fungus gives support, alga gives food

Leaf cutting ant cultures fungus, fungus provides food

Termite gives habitat, flagellate digests food

Corals provide nutrients and protection; algae provide energy and precipitates CaCO3

Mycrorhyzal mutualism. Microrhyzae are filamentous fungi that take the place of root hairs on

certain plants.

Microrhyzal fungi aid adsorption, tree gives food

40% of fungi are obligate microrhyzae (bolitas, ammonites)

Some are species specific; most are specific to order

90% of green plants are facultative for microrhyzae (pine, oak, beech, etc.)

One tree may have 10 or more species of microrhyzae

Increased nutrient absorption (N, P) in nutrient poor areas

Mobilize soil nutrients that a plant could not have otherwise taken up

The correct microrhyzae can increase tree growth by 8 x

Affect plant’s chemicals

Produce growth regulators

Remove chemicals from roots that attract pathogens

Stimulate roots to produce inhibitors

Mice spread underground fruiting bodies (truffles) in feces

Nursery stock must be inoculated with mycrorhyzae

Pollination. Flowering plants and insects have been evolving together for millions of years.

Pollinators needed where plants are widely separated; wind OK in dense stands (grasses, trees)

Insects are generalists and rely on progression of flowering

Plants must specialize to attract specific pollinators

Attract with color, odor, food or oil

Specialists: long corolla for long-tongued pollinators limits dilution of pollen

Defensive mutualism. Symbiotic fungi protect grasses from herbivores.

Rye grass and tall fescue contain fungi

Produces alkaloids that are bitter and toxic: constrict blood vessels in brain u61614 convulsions

13. COMMENSALISM, NEUTRALISM AND ANEMSALISM

Commensalism (+ 0)

Commensalism. In commensalism one species benefits and the other is not affected.

Holdfast

Epiphyte: tree furnishes strata (Spanish moss, epiphytes)

Remora: shark gives food, protection

Den

Rabbit: woodchuck furnishes hole

Crab: worm furnishes tube, food

Neutralism (0 0)

Neutralism. Neutralism involves organisms that have "nothing" in common.

Snail: squirrel

Butterfly: weevil

Manta ray: shark

Amensalism (- O)

Amensalism (-+). In true amensalism one species is harmed and the other is not affected. This usually involves the secretion of a chemical which does not benefit the releasing organism. In most “amensalistic” interactions that have been well studied, one species is harmed and the other benefits.

Example: two aphids on lambs wool plant. A inhibits B, but B has no effect on A. (But does B compete with A?)

Toxic secretions (+ -)

Secretion of toxins. Many fungi, protozoans and higher plants release chemicals (acids, bases, organic toxins) into the environment to reduce competition or feeding, or even to kill for food.

Reduce competition

Inhibit bacteria (antibiotics)

Penicillium fungus: antibiotic kills bacteria

Aristida grass chemical inhibits N fixing bacteria, harming competitors that require

nitrates

Inhibit other plants (allelopaths)

Phenols: from roots, leaf wash (walnut, chaparral)

Terpenes: in air (chaparral)

Reduce feeding of herbivores

Certain chemicals from plants reduce feeding of insects (milkweed, rotenone, nicotine)

Certain algae inhibit growth of tadpoles

Fungi in grasses inhibit grazing

Kill prey

Red tide dinoflagellate: secretion kills fish.

Physteria, a protozoan releases toxins when fish are nearby, and obtains nourishment from the

dead fish

Study questions: Mutualism to amensalism

117. List the six different possible interspecies relationships based on +, -, and 0 and define each.

118. What are the two definitions of “symbiosis”?

119. Define and give examples of facultative mutualism specifying how each benefits.

120. Define and give examples of obligatory mutualism. specifying how each benefits.

120s. Why are most plants in a tropical rain forest insect pollinated, whereas trees in eastern North America are usually wind pollinated?

120b. What are microrhyzae? what is their purpose? Why are they favored by evolution?

121. What is commensalism? Give two examples.

122. What recent evidence do we have that the red tide dinoflagellate is not an amensal?

123. How do allelopaths differ from antibiotics?

14. PREDATION (+ -)

Definition. As a general rule, a predator is larger than its prey, and kills it immediately.

Usually less specialized than a parasite

Seed consumption = predation; grazing = parasitism

Importance of predation. Although predation certainly does not benefit the individual that is killed, the prey population may benefit from predation.

Removes lesser fit individuals and thus improves gene pool of prey

Keeps prey population below carrying capacity so it will not starve or badly damage its food supply

Kaibab Plateau “experiment”

1907 = 4,000 deer plus mountain lions and wolves

1925 = 100,000 deer

Now = 10,000 deer (carrying capacity = 30,000)

Reduces dominants in an area, thus decreasing competition and increasing diversity

Predator-prey cycles. If a predator kills too many of its its prey, it too will decrease, permitting the prey to increase. This results in a cyclic interaction, especially in simple communities (tundra, monoculture).

Theoretical and laboratory studies

Lotka and Volterra proposed formula which predicted a cyclic interaction of predator and prey

Prey grows exponentially

Reproduction of predator is proportional to prey eaten

Gause demonstrated this in lab using Paramecium & Didinium

Successful only if Paramecium was introduced regularly

Pimentel designed a 30-chamber compartment (fly-wasp) that cycled without introduction

Examples in nature

Prickley pear cactus and cactus moth demonstrate cyclic nature

Lynx-rabbit-plant cycle

Rabbits high: plants low, lynx recover R L P R L P

Lynx high: rabbits low, plants recover L P R L P R

Plants high: lynx low, rabbits recover P R L P R L

Survival strategies of predators, prey, and plants

Predator/prey interactions. The predator and prey both evolve to survive the other (genetic feedback), and if both are to survive, a balance must be maintained between them.

Strategy of predators

Foraging strategy. To utilize prey a predator must select a hunting area, locate the prey, pursue

it, capture it, eat it, and digest it.

Must get more energy than expended

Energy needed for maintenance, food for young, stored energy for migration or overwintering

Optimal diet: go after ideal type and size of food first

Area

Forage the most productive area (parasitic wasp did this in lab study of hosts)

Leave the area once it is below other areas

Ignore low-productive areas

Hunting strategy

Ambush: instantaneous attack (alligator, lizards, insects)

Stalking: long search time, quick attack (cats, herons)

Pursuit: long pursuit time (wolves)

Abundance of prey

Balance

In order not to seriously decrease the prey population, the predator must feed only on the

animals in excess of the number required to maintain normal density (i.e., the interest:

compensatory predation).

Threshold of security: density below which predator no longer profits from the hunt.

As prey reaches threshold predators “switch” to more abundant”buffer” prey

Threshold may be low for a highly profitable prey species (deer vs rats for cougar)

Responses to abundant prey. A predator controls abundant prey by eating more or reproducing more.

Functional response: predator eats more

Searching image may increase the % of catch as the density of prey increases

Numerical response: predator increases via natality or immigration

As prey increases, the predators become healthier and reproduce more

Types of relative predation. There are three types of relative predation.

Type 1: Increases linearly with density of prey (up to satiation)

Type 2: Slows down with density of prey. Affected by handling time (time between kills)

Type 3: Increases more rapidly than prey density. Only this type can regulate prey

Regulation occurs only if risk of individual being captured increases as density increases

Predation usually cannot stop a runaway prey natality, controls it only if prey is at low

densities

Strategy of prey. The prey that avoids capture is the one that reproduces. Chances of a prey getting caught vary with prey, predator & environment. Behavior of prey species is critical.

Avoid confrontation

Awareness of danger

Use of cover

Speed and agility

Coloration

Camouflage: coloration blends with environment

Patterns: countershading

Warning coloration: (skunk, lion fish)

Mimicry: resemble something else (thorn, leaf, stick, bark, snow)

Resemble harmful species (viceroy mimics monarch)

Defense

Chemical defenses

Alarm pheromones (fish)

Toxic or odorous secretions (amphibia, snakes, skunks)

Venoms (snakes, toads, some fish, wasps)

Physical defenses

Spines

Shells

Fight

Social behavior

Congregate (fish schools, bird & bat flocks, musk oxen herd)

Separate (caribou do not herd, to discourage lynx)

Alarm calls (birds)

Mobbing (birds)

Distraction displays (birds)

Reproduction

Reproduce faster than predators remove (aphids)

Timing of reproduction: satiates predators (sea turtles, many marine animals)

Cannibalism. Cannibalism serves to quickly reduce a population.

Increases when food is scarce (walleye), crowded conditions (guppies), stress, presence of

vulnerable individuals (herring gull eggs)

Rapidly decreases population, improves conditions of survivors

Plants. Consumption of entire plants is predation. If only part of the plant is eaten it would fit more into parasitism.

Disadvantages of consumption of leaves (roots, etc.)

Reduces photosynthesis, water absorption

Replacement growth drains stored nutrients

Reduction of stored nutrients lowers reproductive capability

Transfer of chemical defenses exposes other areas to attack (roots, etc)

New growth is often smaller, less mature, less resistant

Advantages. Moderate grazing of grasses removes taller older leaves with lower photosynthesis,

and increases light to young shoots.

Plant responses. Plants reduce herbivory with toxins or physical barriers.

Low palatability: older tissue becomes tough, indigestible

Toxins

Toxic chemicals are stored in vacuoles so won’t harm plants (wide variety of these)

Released when eaten: excreted as glandular poison, or as volatile inhibitors

Trees have tannins and resins that complex with proteins to decrease consumption by animals

or symbiotic microorganisms, but takes months to respond to damage

Annuals or perennials produce chemicals rapidly in small amounts that can be transferred to

affected areas that interferes with hosts metabolism

Grasses often have symbiotic fungi that harm herbivores

Physical defenses: thorns, tough leaves, thrichomes

Study questions: Predation

124. How do predators, parasites, and parasitoids differ?

125. What are the four problems that a predator has with its prey?

126. What was the Kaibab Plateau experiment? What did it show?

127. Under what conditions can predation regulate a prey population?

128. What did the following people demonstrate in regard to the cyclic nature of predator and prey: Lotka & Volterra, Gause, Pimentel?

129. Why was the prickly pear cactus introduced into Australia? What was the result? How does its control demonstrate the cyclic nature of predator and prey?

130. What is “searching image”? Give an example of it for people.

131. Define “genetic feedback” and give an example.

132. Why is it necessary for a fox to calculate the chances of catching a prey before chasing it?

133. Describe how musk oxen respond to attack by wolves.

15. PARASITISM (+ -)

Parasites. Parasites benefit from their hosts, which are harmed. They are smaller than their hosts and usually do not kill them; they have a higher reproductive potential and are more specialized than predators.

Types. Parasites may be internal or external to their host. Social parasites use another species’ resources.

Internal

Microparasistes: small, multiply in host, short generation time

Induce immunity, transmitted directly (no intermediate host)

Virus: rabies, encephalitis

Fungus: commonest in plants (cedar-apple rust, chestnut blight)

Bacteria: tularemia (carried by tick, deerfly, direct contact), Lyme’s disease and Rocky

Mountain spotted fever (carried by ticks)

Protozoa: malaria, African sleeping sickness, ick

Macroparasites: larger parasites

Parasitic as adult only (a few roundworms)

Parasitic as larva only

Parasitic insects: larva = parasite, adult = free-living

Attack plants, other insects, higher vertebrates

Those parasites attacking insects always kill (parasitoids)

Parasitic as adult and larva

Flukes: larva = parasite of snails, adult = parasite of vertebrates

Roundworms attack plants and animals

External

Tick, mosquito, leech, lamprey, dutch elm beetle (many of these transmit microparasistes)

A few plants: dodder, mistletoe

Herbivores that eat a few leaves fit the definition of external parasites

Social parasites: use another’s resources

Food

Gulls rob pelican’s of fish; bald eagle robs ospreys

Leaf bug robs insects caught in spiders' webs

Labor: slave ants

Rearing of young. may be facultative or obligatory, between same or different species

Brood parasites of cowbird

Ducks lay eggs in nest of same or other species

Queen ant lays eggs in another species that care for her until her eggs develop

General characteristics.

Host specificity. Because of a host’s immune system, most internal parasites are adapted to survive in

only specific organs of specific taxa (order usually)

Pig roundworm: pig intestine

Apple rust: apple and cedar leaves

Reproduction. The main problem of parasites is locating and infecting a new host

Needs high density of hosts: Density uneven: some heavily affected, others free

Produce many eggs which must leave the body

Free-swimming larva

Alternate hosts: for reproduction, survival, and entering final host

Importance of parasites. Because of their high reproductive capability, parasites may control

hosts at high densities.

Flourish when hosts are numerous, facilitating transmission

Hosts are often undernourished and have lowered resistance (rabies)

Importance: further weakens host so that it may succumb to other causes

Evolutionary trends of parasites and predators. If a parasite kills or contributes to the death of its host, it too will suffer. Therefore, the severity of the interaction tends to decrease with time.

Coevolution = genetic feedback: both species affect each other. Result = mutual tolerance

Host builds defense, parasites evoke milder symptoms

Myxomatosis virus killed 98% of rabbits, then 90%, then 50%

Effectiveness of milky spore disease on Japanese beetle is decreasing

Pimentel's wasps when introduced into chamber had 133 offspring per female

After two years, the violent fluctuations leveled off, and there were 46 offspring per female

Study questions: Parasitism

134. List a parasite from each of the major groups of organisms.

135. How can a grasshopper eating a blade of grass be classified as a parasite?

136. What is a social parasite? Give three examples

137. How would you classify man’s use of animals as an aid in hunting (falcons, hunting dogs, ferrets, cormorants)?

138. Why do internal parasites tend to be host-specific?

139. How do parasites increase the chance of locating a new host? What are three advantages of having an alternate host?

140. What causes the outbreak of rabies in Tennessee?

141. What is the evolutionary trend of parasites and their hosts?

16. COMPETITION (- -)

Definition. Competition refers to two or more individuals striving for the same thing which is in short supply. It may take several forms.

Direct vs. indirect interference. Competition may take the form of direct antagonism, or indirect

use of shared resources.

Direct interference

Direct antagonism: shoving, fussing, fighting

Sierra Nevada chipmunks: range of the four species was determined by heat tolerance and

aggression

Different periods of activity avoid direct interference

Allelopaths: produced by broom sedge, goldenrod, aster, some grasses

Indirect interference (exploitive competition). share common resource

Grasshopper and cow compete for grass (insects are man's most important interspecific

competitors)

Flowers compete for insect pollinators

Nocturnal herbivores compete with diurnal herbivores feeding on the same plant

Intraspecific competition. Competition between members of the same species forces them into

a wider range.

Forces the species into a wider range

Broadens resource base: more food types, sizes, increase genetic variability (Darwin’s

finches)

Scramble competition. In scramble competition lack of food harms all organisms, and violent

fluctuations occur

Examples: lemmings, sunfish in stocked pond, pine trees too close together

Characteristics

Share resources equally

During peak density most get less than they need for growth and reproduction

Results in chaotic oscillations in density

Average density is lower than in contest; use fewer resources

Contest competition. In contest competition, the best competitors get enough food, others

have insufficient amounts.

Example: Peck order

Characteristics

Density is relatively stable

Effects are confined to the unsuccessful

Uses a higher percentage of resources; permits a higher, more constant density

Most species are one or the other. Some change with life cycle

Water buffalo

Mortality of grazing adults = density dependent (all suffer during low food)

Mortality of juveniles = density independent (predation allows survivors to have milk)

Principle of competitive exclusion. Gause postulated that species with similar requirements will compete, and there is a tendency for one to win out over the other.

Equation: (Lotka and Volterra) logistic formula with term (aN2) for inhibitory effect of species

#2: decreases K of species #1

Same formula is also applied to species 2

dN ₌ _rN (K - N - aN2)

dt K

Principle

No two species can occupy the same niche

Complete competitors (with same requirements) can not coexist

The one least susceptible to limiting conditions will be favored

Demonstrated in lab with two species of Paramecium, and with flour beetles

Changing the environment may change the outcome of the competition

Studied in nature with impact of introduced species: starlings compete with flickers for nest

sites

Comments

A model enabling one to test a hypothesis

If conditions do not fit model, need to investigate farther, or to modify the model.

Can coexist if:

If shared resource is not in short supply (weak competition)

If numbers are kept low by predation, etc.

Coexist at edge of range

If the niches differ significantly, both can survive

Extensive niche overlap indicates low competition, abundant resources. Outcome of

competition may vary

Environmental conditions change or vary (moisture favors plant A, drought favors plant B)

Exploitation of anchovy reduced guano which decreased nutrients for plankton and permitted sardine to thrive

Overlap may occur in one gradient but not another

Niches and competition

Reduction in competition. There is an evolutionary tendency to reduce competition; there is selective pressure on both species to change so that they will survive.

Niche. A niche is that subdivision of the environment occupied by a species.

Multidimensional niche (hypervolume)

Conditions under which lives; variables to which it is adapted

Food, feeding area, shelter, temperature, rain, wind, predators, parasites

Role: relationship of food and enemies

Fundamental and realized niche

Fundamental niche: niche with no competition

Realized : where species actually occurs, with competition. Varies with developmental stage

Amount of overlap between two competing species is proportional to the degree of

competition for the resource

Sexes may occupy different niches

Male Arizona woodpecker has larger bill ® trunk. Female has smaller beak ® branches

Species divergence. Interspecific competition restricts each species to its optimal niche, and

forces competing species into new niches.

Guild: species that share a common resource

Interspecific competition restricts each species to its optimal range

The US chickadee occupies wide range; each of the four species in Europe is restricted to a

specific habitat

Encourages specialization in resource base (narrower range of resource use) for greater

efficiency

Change food, period of activity, range, habitat (broad definition of niche)

Different sized bills permit birds to utilize different seeds

Reduced variety in size of food (niche compression)

Forces different species to use different food, plants to develop different types of root

systems

Selection favors species living in areas of less overlap

Resource partitioning

Animals: coexist by using different size & kinds of food, forage in different areas, at different

times

Plants coexist under different conditions of light, soil moisture, nutrients

Terms

Niche compression: reduction in realized niche because of increased competition

Competitive release: expansion of realized niche because of reduced competition

Introductions entering new area, island introductions

Niche shift: shift in behavior to reduce competition: using new food or area

Bluegills use long rakers to become filter feeders if competition for larger food is severe

Generalists and specialists

Generalists: have broad niche: able to use wide range of food. Favored if resources vary

Specialists: highly efficient for narrow niche. Best if resources are dependable

Study questions: Competition

142. Compare direct and indirect competition and give an example of each.

143. Describe the “peck order”. List three ways that this benefits the population?

144. What is the principle of competitive exclusion? What is the result of it? How was it demonstrated in the lab?

145. What are three exceptions to this principle?

146. List three ways that interspecific competition may be reduced.

17. INTRODUCTIONS

General. Severe introductions are usually of recent origin

Few natural enemies

Little resistance in prey or host

Lag time for native species to adapt to immigrants

Harmful introductions. Unusually harmful introductions are may be found in almost every major group of organisms.

Bacteria: smallpox, syphilis

Fungi: American chestnut blight brought from China in 1904 (all dead by 1952)

Dutch elm disease has killed most elm trees

Plants: water hyacinth, 21% of N.C. vegetation

However, very few trees (mimosa, ginkgo)

Insects: Japanese beetle, cotton boll weevil, killer bees

Molluscs: Asiatic clam, zebra clam

Fish: walking catfish

Amphibia: marine toad

Birds: starling, English sparrow

Mammals: Norway rat, rabbits into Australia, mongoose, burros

Beneficial introductions. Not all introductions have been harmful:

Plants: many crop plants, ornamentals

Insects: honey bees

Fish: trout

Birds: pheasants, chickens

Mammals: most domestic mammals

Control

Quarantine to prevent entry

Otherwise use methods listed under "Biocides"

Study questions: Introductions

147. Why are most of our most serious pests often introductions?

148. Describe the “Columbian exchange”.

149. Why was the American chestnut such a fine tree? What happened to it?

150. Why have the following been particularly serious introductions in some countries? water hyacinth, cotton boll weevil, walking catfish, marine toad, mongoose in Puerto Rico, rabbit in Australia.