Population DynamicsA population is a group of individuals (all members of a single species) who live together in the same habitat and are likely to interbreed.  Each population has a unique physical distribution in time and space.  It may contain individuals of different ages and its size (density) is likely to change over time, growing or shrinking according to the reproductive success of its members.  The study of population dynamics focuses on these changes -- how, when, and why they occur.  In entomology, a good understanding of population dynamics is useful for interpreting survey data, predicting pest outbreaks, and evaluating the effectiveness of control tactics.Birth (natality), death (mortality), immigration, and emigration are the four primary ecological events that influence the size (density) of a population.  This relationship can be expressed in a simple equation:

Change inPopulation Density =   (Births + Immigration) - (Deaths + Emigration)

All other factors (both biotic and abiotic) exert their impact on population density by influencing one (or more) of the variables on the right-hand side of the above equation.  Such factors, known as secondary ecological events, may affect the frequency, extent, magnitude, or duration of a primary ecological event.  Cold winter temperatures, for example, could increase mortality and reduce population density.  On the other hand, low predation rates in the summer might increase natality and allow the population to grow.  Most secondary ecological events act as "population regulating factors".  Whenever they limit a population from reaching its maximum reproductive potential, they are regarded as "environmental resistance".Secondary ecological events can be divided into two broad categories:  density-independent factors and density-dependent factors.

Density-Independent Factors include events or conditions, often weather- or climate-related, that affect all individuals equally, regardless of the overall population density.  A hard freeze, for example, will kill the same high percentage of the potato leafhoppers in a farmer's peanut field -- no matter if the population contains a few hundred or a few million individuals.  In another species, high temperatures and/or low humidity might have a similar, non-selective impact on mortality.  Favorable climatic conditions can have a positive effect on population density just as much as unfavorable conditions can have a negative effect.  Larvae of Japanese beetles, for example, thrive in years when ample summer rainfall keeps soil conditions moist.  Other density-independent events might include wildfires, hurricanes, or hail storms.  For an aquatic species, a low concentration of dissolved oxygen or a flash flood after heavy rainfall would qualify as density-independent events because a small population would suffer the same percent mortality as a large population.

Density-Dependent Factors include events or conditions that change in severity as a population's size increases or decreases.  Common examples of density-dependent factors include predation, parasitism, and disease (one species exploiting another).  A large, dense population, for example, is usually more susceptible to the spread of parasites or contagious disease than a small, sparse population.  Predators often adapt to changes in the density of their prey populations by migrating into areas of high prey density (numerical response) or by focusing their attention primarily on the most abundant prey species (behavioral response).  As a result, large and small populations tend to suffer different rates of predation.  Competition for limited resources is also density-dependent -- each individual's share of the "pie" decreases as a population grows numerically.  In a small population, members may face competition mostly from individuals of other species who use the same resources (interspecific competition).  In large populations, however, competition may also come from other members of the same species (intraspecific competition).  In either case, competition undermines survival and reproduction.  Any physical trait or behavioral adaptation that reduces or eliminates competition is likely to be favored by natural selection.

    Intraspecific Competition     Contest Competition:   In situations where resources (food, space, etc.) are fairly stable over time, intraspecific competition may take the form of "contests" in which individuals lay claim to a "territory" and defend it from all intruders.   Each territory generally provides enough resources for the owner's survival and reproduction; failure to "win" a territory can be a competitive disadvantage.   Since only the strongest (most "fit") individuals are likely to hold a territory, they have the best chance to pass on their genes to the next generation.Scramble Competition:   In situations where resources are temporary or transient, there is little or no advantage to defending a territory.   Insects that

    Interspecific Competition     Each species occupies a unique ecological niche within its community.   The niche is a Gestalt-like concept encompassing all of the biotic and abiotic parameters that determine where a population lives (its "habitat") as well as the role it plays within the food web (its "profession").   Interspecific competition occurs whenever the niche parameters of two (or more) different species overlap.   The more the overlap, the greater the competition.Interspecific competition usually leads to one of three possible evolutionary outcomes:

1. Competitive exclusion -- one species is competitively superior and drives the other species to extinction.

2. Range restriction -- each species is confined to a subset of the range

compete for these types of resources (blow flies on a corpse, for example) "scramble" for access.   The first arrivals encounter the best conditions for survival and reproduction (first come, first served).   Latecomers encounter a depleted resource that may no longer support growth and development.

where it is able to out-compete the other species.

3. Competitive displacement -- the two species evolve in divergent directions, adapting to different resources or specializing in other ways that allow them to co-exist with little or no direct competition.

Density-dependent emigration (movement away from crowded conditions) is another important regulator of population size.  It not only reduces overcrowding in the home range, but it also increases the likelihood of establishing new populations elsewhere.  In the long term, emigration benefits the individuals who remain behind as well as the pioneers who find new places to live.Cooperative interactions may also give populations a competitive advantage, allowing them to reduce mortality, use resources more efficiently, or accomplish tasks that could not be performed by solitary individuals.  Intraspecific cooperation has certainly contributed to the evolutionary success of all social insects(ants, bees, wasps, and termites).  These species outnumber all other animals in many terrestrial habitats and, despite their small size, they usually play dominant roles in community ecology, both as consumers and as decomposers.  Cooperative interactions between different species (i.e. mutualism and commensalism) are also common in the insect world.  These symbiotic relationships occur not only between different insect species (e.g. ants and aphids) but also between insects and microorganisms, between insects and vertebrates, and between insects and plants. 

Population Growth

When food is abundant and growing conditions are favorable, a population has the potential to increase in number from generation to generation -- just as money in a bank savings account accrues interest over time.  The population's intrinsic growth rate ("r") is similar to the bank account's interest rate -- it is a measure of how quickly the increase occurs.  Growth is said to be geometric when each generation's increase is a constant percentage of the total population size.  Geometric growth is also known as exponential growthbecause the larger the population gets, the faster it grows.  With a 5% growth rate, for example, a population of 50 beetles would grow by only 31 individuals in 10 generations whereas a population of 10,000 would grow by 6,289 during the same amount of time.  The "J-shaped" curve in Figure 1 represents the typical form of an exponential growth curve.Obviously, exponential growth cannot continue indefinitely in a resource-limited environment.  Eventually a population becomes so large that it runs out of free space, outgrows its food supply, or exhausts other assets.  The upper limit on population density is called the environmental carrying capacity   (usually represented by the symbol "K").  As population density approaches the carrying capacity, competition becomes more intense, mortality increases, the birth rate drops, and any one of the following alternatives is possible:

The population may level out and stabilize below the carrying capacity.  This pattern is known as a logistic or sigmoid (S-shape) growth curve.

The population may briefly overshoot the carrying capacity and then crash, resulting in repeated cycles of "boom" and "bust".

The population may oscillate around (or below) the carrying capacity.

In reality, all of these models are gross over-simplifications.  Natural populations respond to a wide range of environmental conditions that are rarely constant over time.  Some species have complex life cycles requiring different resources or conditions at each stage of development.  Others show distinct temporal or seasonal variability in their response to environmental conditions.

r- and K-selection.   Some insects are ecological opportunists.  They exploit disturbed or unstable environments, take full advantage of transient resources, produce large numbers of offspring in short periods of time, and rapidly disperse into new habitats when conditions turn unfavorable.  This life history strategy, often called "r-selection," is typically found in species that have a short life cycle, small body size, and high mobility (ex. house flies).  Most individuals in "r-selected" populations die before reaching sexual maturity so a high reproductive potential is essential for the species to avoid extinction.  These insects often play a major role as colonizers in the early stages of ecological succession -- they are also likely to be regarded as pests if their colonial empire spreads into farms, ranches, or human habitations!On the other hand, life expectancy is usually longer for species that live in stable habitats (like mature grasslands or climax forests).  More of these individuals reach sexual maturity and populations tend to stabilize near the environmental carrying capacity.  Under these conditions, often called "K-selection," there is no particular advantage to having large numbers of offspring.  Selection pressures focus on intra-specific competition and efficient use of resources.  "K-selected" species often have longer life cycles, larger body size, and relatively low growth rates.Ecologists recognize that r- and K-selection are opposite ends in a broad spectrum of life history strategies.  Most species fall somewhere in the middle of the range with a blend of "r-selected" and "K-selected" characteristics.  Ants and termites, for example, produce large numbers of small, expendable offspring but they have long-lived colonies that are highly competitive in stable environments.

Survival StrategiesIt's not easy being a bug.There are a lot of "windshields" on the road of life!!Just try to imagine what it would be like if you were an insect and had to:

survive rainstorms, windstorms, ice storms, and hail storms,

find water in the desiccating heat of mid-summer, keep from freezing in the dead of winter, avoid flash floods, wildfires, and mud slides, locate a suitable food supply (host plant or animal), elude birds, spiders, mantids, frogs, and other predators, defend yourself against parasitoid flies and wasps, prevent infection by pathogenic fungi and

microorganisms, and escape being impaled on a pin in a student's bug collection!

Despite their small size and apparent vulnerability, insects are not entirely at the mercy of the elements.  They are equipped with high reproductive rates and numerous behavioral and physiological adaptations that assure them a fair fight in the struggle for survival.  The following sections describe a number of common adaptations that help insects survive adversity or adapt to their environment. 

Migration

Day-to-day activities of insects often involve trivial movements associated with feeding, mating, or oviposition.  These movements are "trivial" only in the sense that they are short-range, random in direction, and do not result in much dispersal of the population.  At some time in their life cycle, however, many (perhaps most) of these insects will engage in a period of directional movement that carries them beyond the range of their local habitat.  This long-distance movement, called migration, is a survival strategy with at least six potential advantages:

To escape from natural enemies To find more favorable growing conditions To reduce competition or relieve overcrowding To locate new (or unoccupied) habitats To disperse to alternate host plants To reassort the gene pool to minimize inbreeding

Although most insects migrate by flying, a few species (chinch bugs, Mormon crickets, and armyworms, for example) travel on the ground.  Migration by flight is often aided by prevailing winds.  Once airborne, small insects may be lifted by thermal convection and carried hundreds of miles on frontal air masses.  Even wingless individuals may be carried aloft by "ballooning" on silk threads or blowing off tall vegetation.  Larger insects, like dragonflies and monarch butterflies, may control the direction of their migratory flights, but most smaller insects are carried wherever the wind blows them.Migration is usually a distinct phase in the life cycle, almost always occurring before the onset of reproductive maturity.  Migrants are innately programmed to move; they are not distracted by food or mates.  Once the migratory urge is satisfied, the insect is generally in a physiological state to continue development or commence reproduction.  Migration can be a very risky venture:  in some species more than 90% of a population may die in transit.  Despite such high mortality rates, the reproductive success of individuals who survive the trek apparently makes the gamble worthwhile for the species as a whole. 

Diapause

The life cycle of many insect species may include a hormone-induced period of "dormancy" called diapause.  Like hibernation in mammals, diapause is characterized by a reduction in oxygen consumption, metabolic rate, and physical activity.  Feeding and growth are generally interrupted while the individual subsists on stored food reserves.  Diapause typically occurs during the egg stage in some species, during a nymphal or larval instar in other species, or during the pupal stage in still other species.  Even adults may enter a "reproductive diapause" which causes a significant delay in the onset of sexual maturity. In temperate climates, many species enter diapause in the fall as an overwintering adaptation.  Other species, however, have a summer diapause that helps them survive the dryness and/or heat.  In either case, the onset of diapause is triggered by an environmental cue that precedes the adverse weather conditions (short daylengths in fall, for example).  Diapause continues, even under apparently favorable conditions, until it is "broken" (terminated) by other environmental cues, such as long day lengths or exposure to a substantial period of low temperature. Diapause is not always correlated with adverse environmental conditions.  It can also regulate development within a population to ensure optimal timing of emergence or temporal synchrony with environmental resources.  Female rabbit fleas, for example, have an obligate adult diapause that is broken only by feeding on the blood of a pregnant host rabbit.  By the time the baby rabbits are old enough to be weaned, the flea's offspring will be mature and ready to accompany the rabbits when they leave the nest.  In this ecological relationship, diapause is an adaptation that keeps the flea population from exceeding the carrying capacity of its host. 

Cold-hardiness

Since insects are poikilotherms (cold-blooded animals), their body temperature is usually similar to that of the air (or water) around them.  Species that live in cold mountain streams (like mayfly naiads) or on the surface of ice and snow (like grylloblattids) are adapted for activity at low temperatures.  Most other insects, however, slow down as the temperature falls. 

They reach a dormant state, called torpor or quiescence, when they get very cold.  Physiologically, many insects prepare for winter weather by producing "antifreeze" compounds (such as glycerol, sorbitol, or trehalose) in their hemolymph and body tissues.  High concentrations of these compounds can increase cold-tolerance by lowering the freezing point of body fluids and preventing the formation of ice crystals that would cause internal injury.  In species that manage to survive in arctic and alpine environments, the overwintering stage may undergo extensive dehydration -- any ice crystals that do form will be too small to cause cellular damage.  During the long Antarctic winter, larvae of Belgica antarctica, a wingless midge, become literally frozen in place until the arrival of a spring thaw.Unlike diapause (see above), a period of quiescence lasts only as long as the weather is cold.  When temperatures rise, quiescent insects resume normal activity -- at least until the next cold front arrives!  This explains why there may be a great deal of insect activity on warm, sunny days in the middle of winter. 

Parthenogenesis

Sexual reproduction involves the union of a female's egg (1n) with a male's sperm (1n) to form a diploid zygote (2n).  Although sexual reproduction is the paradigm in most insects, there are many common species that are able to reproduce asexually (without mating), through a process known as parthenogenesis.  In these species, females are able to produce viable offspring without a contribution of sperm from a male. One type of parthenogenesis (called arrhenotoky) occurs in all members of the order Hymenoptera (ants, bees, and wasps) and also in a few species of thrips and scale insects.   In these insects, all females are diploid (2n) and all males are haploid (1n).  Mated females have voluntary control over the release of stored sperm so they can opt to lay either a fertilized egg (female) or an unfertilized egg (male).  This adaptation, which allows a female to regulate the sex of her offspring, is undoubtedly an important factor in the evolution of colony structure for social ants, bees, and wasps.

Another type of parthenogenesis (called thelytoky) is found in many aphids, scale insects, some cockroaches and stick insects, and a few weevils.  In these insects, females produce diploid (2n) eggs that develop directly into female offspring having exactly the same genetic make-up as their mother.  Since each daughter subsequently has the ability to clone herself through parthenogenesis, this form of reproduction has the potential to produce a large number of offspring in a short period of time.  Parthenogenesis is clearly an advantage for species that live in a stable environment and exploit an abundant food resource.  The lack of genetic variability within the population, however, can be a disadvantage if (when) environmental conditions change.  For this reason, many parthenogenetic species have evolved a seasonal cycle that includes at least one generation of sexual reproduction at the end of several generations of asexual reproduction. 

Polymorphism

Just as each human has a unique physical appearance, there are also individual differences among the members of a single species in the insect world.  Females are often larger than males, and one sex may have distinctive colors or markings to attract the opposite sex.  But there are also many examples of species with two or more colors, shapes, or sizes where the differences are competely unrelated to gender characteristics.  Each of these "versions" is called a morph, and therefore, species that exhibit this "split-personality" are said to be polymorphic.In social insects, polymorphism is often associated with division of labor in the nest.  Among ants, for example, large individuals with big mandibles usually serve as soldiers or foragers, while smaller individuals concentrate on care of the young or other housekeeping tasks.  In honey bees, the workers have wax glands, stings, and pollen baskets that are not present in queens or drones.  This type of specialization among individuals is an adaptation that gives social insects the ability to utilize their resources more efficiently.

In non-social species, polymorphism may be related to habitat diversity.  England's peppered moth, Biston betularia, is a well-known example of such a species.  The light-colored morph of this moth is hard to find in the daytime when it rests against a background of lichens growing on

the bark of trees.  A dark-colored morph is easy to see against the lichen, but hard to spot against the dark background of bare bark.  Depending on the background, the less-visible morph is the one most likely to survive bird predation.  During the industrial revolution, the dark morph predominated because air pollution from London's factories killed much of the lichen on trees in surrounding forests.  Now that air pollution has been reduced, the lichen are able to survive and the moth's light-colored morph is most abundant.

In Africa, the desert locust, Schistocerca gregaria, has two morphs that differ both in physical appearance and behavior.  Under low population densities, these grasshoppers develop into adults that are largely green in color, have relatively short wings, and show little or no tendency to migrate.  Under crowded conditions, however, these grasshoppers develop into brownish adults with longer wings.  These individuals eventually form huge swarms containing millions of individuals that migrate over hundreds of miles.  Crowding affects the balance of neurotransmitters, especially serotonin, and triggers a shift in development to the migratory form.Different morphs may also be associated with different generations throughout the year.  The seasonal cycle of many parthenogenetic aphids includes several generations of wingless (apterous) individuals followed by a generation of winged (alate) migrants.  This alternation of generations provides a mechanism for dispersal from one habitat to another as environmental conditions and host plant quality change throughout the year.  The rosy apple aphid, Anuraphis rosea, is typical of many such species.  In early spring it reproduces asexually, completing several wingless generations on apple trees, its primary host.  As the apple foliage matures and becomes less desirable, an alate generation develops (partheogenetically) and flies to narrow-leaved plantain, the secondary host.  Several more wingless generations develop asexually on plantain until, in late summer or early fall, another alate generation develops and flies back to the primary host.  This generation gives birth to a sexual generation (both males and females) that will mate and lay overwintering eggs on the apple trees.

Insect DefensesFor many insects, a quick escape by running or flying is the primary mode of defense.  A cockroach, for example, has mechanoreceptive hairs (setae) on the cerci that are sensitive enough to detect the change in air pressure that precedes a fast moving object (like your foot).  Nerve impulses from these receptors travel through giant neurons to thoracic ganglia at speeds up to 3 meters per second, triggering an evasive response by the legs in less than 50 milliseconds.  House flies have a similar reaction time when you try to swat them.  They leap into the air and begin flapping their wings 30-50 milliseconds after sensing a threat.Tiger moths (family Arctiidae) can detect ultrasonic echolocation by bats.  At low intensity, they fly away from the bat, but if the bat's call increases to a certain threshold they quickly drop from the air in an evasive, looping dive.  Other alarm reactions may be less dramatic, but just as effective:  Madagascar cockroaches hiss when disturbed; cuckoo wasps curl up into hard, rigid balls; tortoise beetles have strong adhesive pads on their tarsi and hold themselves tight and flat against a leaf or stem.  Other insects simply "play dead" (thanatosis) -- they release their grip on the substrate and fall to the ground where they are hard to find as long as they remain motionless.An insect's hard exoskeleton may serve as an effective defense against some predators and parasites.  Large weevils are notorious for their hard bodies - as you may discover for yourself the first time you bend an insect pin trying to push it through the thorax.   Most diving beetles are hard, slick, and streamlined; even if you can catch them, they will often squirm out of your grip.

Spines, bristles, and hairs may be effective mechanical deterrents against predators and parasites.  A mouthful of hair can be an unpleasant experience for a predator and parasitic flies or wasps may have a hard time getting close enough to the insect's body to lay their eggs.  Some caterpillars incorporate body hairs into the silk of their cocoon as an additional defense against predation.Some insects have a "fracture line" in each appendage (often between the trochanter and the femur) that allows a leg to break off easily if it is caught in the grasp of a predator.   This phenomenon, calledautotomy, is most common in crane flies, walkingsticks, grasshoppers, and

other long-legged insects.  In most cases, sacrificing a limb in this manner creates only a minor disability.  In fact, walkingsticks (especially young nymphs) may regenerate all or part of a missing appendage over the course of several molts.

Chemical Defenses

Many insects are equipped to wage chemical warfare against their enemies.  In some cases, they manufacture their own toxic or distasteful compounds.  In other cases, the chemicals are acquired from host plants and sequestered in the hemolymph or body tissues.  When threatened or disturbed, the noxious compounds may be released onto the surface of the body as a glandular ooze, into the air as a repellent volatile, or aimed as a spray directly at the offending target.

Defensive chemicals typically work in one of four ways:

1. Repellency -- a foul smell or a bad taste is often enough to discourage a potential predator.  Stink bugs, for example, have specialized exocrine glands located in the thorax or abdomen that produce foul-smelling hydrocarbons.  These chemicals accumulate in a small reservoir adjacent to the gland and are released onto the body surface only as needed.  The larvae of certain swallowtail butterflies have eversible glands, called osmeteria, located just behind the head.  When a caterpillar is disturbed, it rears up, everts the osmeteria to release a repellent volatile, and waves its body back and forth to ward off intruders.

2. Induce cleaning -- irritant compounds often induce cleaning behavior by a predator, giving the prey time to escape.  Some blister beetles (family Meloidae) produce cantharidin, a strong irritant and blistering agent that circulates in their hemolymph.  Droplets of this blood ooze from the beetle's leg joints when it is disturbed or threatened -- an adaptation known as reflex bleeding.  Irritant sprays are produced by some termites, cockroaches, earwigs, stick insects, and beetles.  The notorious bombardier beetles store chemical precursors for an explosive reaction mixture in specialized glands.  When threatened, these precursors are mixed together to produce a forceful discharge of boiling hot benzoquinone and water vapor (steam).

3. Adhesion -- sticky compounds that harden like glue to incapacitate an attacker.  Several species of cockroach guard their backsides with a slimy anal secretion that quickly cripples any worker ants that launch an attack.  Similarly, members of the soldier caste in nasute termites have nozzle-like heads equipped with a defensive gland that can shoot a cocktail of defensive chemicals at intruders.  The compounds,

which are both irritating and immobilizing, have been shown to be highly effective against ants, spiders, centipedes, and other predatory arthropods.

4. Cause pain or discomfort -- Saddleback caterpillars, larvae of the io moth, and various other Lepidopteran larvae have hollow body hairs that contain a painful irritant.  Simply brushing against these urticating hairs will cause them to break and release their contents onto your skin.  The consequence is an intense burning sensation that may last for several hours.  Many ants, bees, and wasps (the aculeate Hymenoptera) deliver venom to their enemies by means of a formidable stinger (modified ovipositor).  The venom is a complex mixture of proteins and amino acids that not only induces intense pain but may also trigger an allergic reaction in the victim.

Protective Coloration

Biologists recognize that there is usually an underlying rationale for the great diversity of shapes and colors found in the insect world.  We may not know why a particular species has parallel ridges on the pronotum or black spots on the wings, but we can be reasonably certain that this shape or color has contributed in some way, however small, to the overall fitness of the species.   It is obvious that at least some of the colors and patterns serve a defensive function by offering a degree of protection from predators and parasites.  These patterns, collectively known as protective coloration, fall into four broad categories:

1. Crypsis:

Insects that blend in with their surroundings often manage to escape detection by predators and parasites.  This tactic, called cryptic coloration, involves not only matching the colors of the background but also disrupting the outline of the body, eliminating reflective highlights from smooth body surfaces, and avoiding sudden movements that might betray location.  Obviously, this tactic loses much of its effectiveness if an insect moves from one type of habitat to another.  Well-camouflaged insects usually stay close to home or make only short trips and return quickly to the shelter of their protective cover.   Many ground-dwelling grasshoppers and katydids, for example, have colors of mottled gray and brown that help them "disappear" against a background of dried leaves or gravel.  On the other hand, closely related species that live in foliage are usually a shade of green that matches

the surrounding leaves.  The larvae of some lacewings improve their camouflage by attaching bits of moss or lichen from their environment onto the dorsal side of their body.

2. Mimesis:

Some insects "hide in plain sight" by resembling other objects in the environment.  A thorn could really be a treehopper; a small twig might be a walkingstick, an assassin bug, or the caterpillar of a geometrid moth; and sometimes a dead leaf turns out to be a katydid, a moth, or even a butterfly.  This "mimicry" of natural objects is often known asmimesis.  It goes far beyond imitation of plant parts:

o Some swallowtail larvae resemble bird droppings, others have false eyespots on the thorax that create a convincing imitation of a snake's head.

o The likeness of a caterpillar can be found on the outer edge of many lepidopteran wings, perhaps serving to fool predatory birds that may peck at the wing margin instead of the butterfly's body.

o Many butterflies and moths have eyespots on the wings that emulate the face of an owl or some other large animal.

o Slug caterpillars and hag moth larvae look like hair balls or small furry mammals.

3. Warning Colors:Insects that have an active means of defense (like a sting or a repellent spray) frequently display bright colors or contrasting patterns that tend to attract attention.  These visually conspicuous insects illustrate aposematic coloration, a term derived from the Greek words apo- (from a distance) and sema (a sign or signal) -- meaning "a signal from afar".  A predator quickly learns to associate the distinctive coloration with an "unpleasant" outcome, and one such encounter is usually enough to insure avoidance of that prey in the future.   A few individuals will die as sacrifices, but for the species as a whole, it pays to advertise!

4. Mimicry:If a distinctive visual appearance is sufficient to protect an unpalatable insect from predation, then it stands to reason that other insects might also avoid predation by adopting a similar appearance.  This ploy, essentially a form of "false advertising", was first recognized and described by Henry W. Bates in 1861.  Today, it is commonly known as Batesian mimicry.  Viceroy butterflies (mostly palatable to birds) are largely protected from predation because they resemble monarch butterflies (very distasteful).  Many species of bee flies, flower flies, robber flies, and clear-winged moths are similarly protected because they mimic the appearance (and often the behavior) of stinging bees and wasps. 

Batesian mimicry is usually a successful strategy as long as the model and mimic are found in the same location, the mimic's population size is smaller than that of the model, and predators associate the model's appearance with an unpleasant effect.In 1879, Fritz Müller recognized that two or more distasteful species often share the same aposematic color patterns.  Many species of wasps, for example, have alternating bands of black and yellow on the abdomen.  This defensive tactic, commonly known as Müllerian mimicry, benefits all members of the group because it spreads the liability for "educating the predator" over more than one species.  In fact, as the number of species in a Müllerian complex increases, there is a greater selective advantage for each individual species.Mimicry has been carried to extremes in some tropical Lepidoptera where both related and unrelated species resemble each other in size, shape, color, and wing pattern.  Collectively, these butterflies (and sometimes moths) form mimicry rings that may include both palatable and unpalatable species.  In South America, for example, longwing butterflies (Family Nymphalidae) form a mimicry ring that includes at least twelve different species (including one moth).

Although natural selection favors individuals in a population with the best camouflage or mimicry, it also favors the predator or parasite with the best prey-finding acumen.  As a result of these competing interests, coevolution between predator and prey populations inevitably leads to an ongoing escalation of offensive and defensive measures -- a scenario that Leigh Van Valen of Chicago University describes as an evolutionary "arms race". 

In order to survive in the arms race, both predator and prey must constantly evolve in response to the other's changes.  Failure to "keep up" concedes a competitive advantage to the opponent and may lead to extinction. The idea that perpetual change is necessary just to maintain the status quo has been coined the Red Queen's Hypothesis.  This name refers to a scene from

the stories of Alice in Wonderland by Lewis Carroll.  In Through the Looking Glass, Alice meets a chess piece, the Red Queen.  After running hard to follow the Queen, Alice discovers that she

has not moved from where she started.  Asked about this paradox, the Red Queen replies, "Here, you see, it takes all the running you can do to keep in the same place."

Life TablesA "life table" is a kind of bookkeeping system that ecologists often use to keep track of stage-specific mortality in the populations they study.  It is an especially useful approach in entomology where developmental stages are discrete and mortality rates may vary widely from one life stage to another.  From a pest management standpoint, it is very useful to know when (and why) a pest population suffers high mortality -- this is usually the time when it is most vulnerable.  By managing the natural environment to maximize this vulnerability, pest populations can often be suppressed without any other control methods.To create a life table, an ecologist follows the life history of many individuals in a population, keeping track of how many offspring each female produces, when each one dies, and what caused its death.  After amassing data from different populations, different years, and different environmental conditions, the ecologist summarizes this data by calculating average mortality within each developmental stage.For example, in a hypothetical insect population, an average female will lay 200 eggs before she dies.  Half of these eggs (on average) will be consumed by predators, 90% of the larvae will die from parasitization, and three-fifths of the pupae will freeze to death in the winter.  (These numbers are averages, but they are based on a large database of observations.) 

A life table can be created from the above data.  Start with a cohort of 200 eggs (the progeny of Mrs. Average Female).  This

number represents the maximum biotic potential of the species (i.e. the greatest number of offspring that could be produced in one generation under ideal conditions).  The first line of the life table lists the main cause(s) of death, the number dying, and the percent mortality during the egg stage.  In this example, an average of only 100 individuals survive the egg stage and become larvae.  The second line of the table lists the mortality experience of these 100 larvae:  only 10 of them survive to become pupae (90% mortality of the larvae).  The third line of the

table lists the mortality experience of the 10 pupae -- three-fifths die of freezing.  This leaves only 4 individuals alive in the adult stage to reproduce.  If we assume a 1:1 sex ratio, then there are 2 males and 2 females to start the next generation.If there is no mortality of these females, they will each lay an average of 200 eggs to start the next generation.  Thus there are two females in the cohort to replace the one original female -- this population is DOUBLING in size each generation!!In ecology, the symbol "R" (capital R) is known as the replacement rate.  It is a way to measure the change in reproductive capacity from generation to generation.  The value of "R" is simply the number of reproductive daughters that each female produces over her lifetime:

Number of daughters R = ------------------------------- 

      Number of mothers

If the value of "R" is less than 1, the population is decreasing -- if this situation persists for any length of time the population becomes extinct.

If the value of "R" is greater than 1, the population is increasing -- if this situation persists for any length of time the population will grow beyond the environment's carrying capacity.  (Uncontrolled population growth is usually a sign of a disturbed habitat, an introduced species, or some other type of human intervention.)

If the value of "R" is equal to 1, the population is stable -- most natural populations are very close to this value.

Practice Problem:A typical female of the bubble gum maggot (Bubblicious blowhardi Meyer) lays 250 eggs.  On average, 32 of these eggs are infertile and 64 are killed by parasites.  Of the survivors, 64 die as larvae due to habitat destruction (gum is cleared away by the janitorial staff) and 87 die as pupae because the gum gets too hard.  Construct a life table for this species and calculate a value for "R", the replacement rate (assume a 1:1 sex ratio).  Is this population increasing, decreasing, or remaining stable?

Life Tables

Practice Problem:

A typical female of the bubble gum maggot (Bubblicious blowhardi Meyer) lays 250 eggs.  On average, 32 of these eggs are infertile and 64 are killed by parasites.  Of the survivors, 64 die as larvae due to habitat destruction (gum is cleared away by the janitorial staff) and 87 die as pupae because the gum gets too hard.  Construct a life table for this species and calculate a value for "R", the replacement rate (assume a 1:1 sex ratio).  Is this population increasing, decreasing, or remaining stable?