Linzey - Vertebrate Biology - Chapter 13 pptx

In competition, both species are affected adversely; in commensalism, one species benefits and the other is unaf-fected; in mutualism, species benefit each other; in parasitism and preda

C H A P T E R 1 3

Interspecific Interactions

Because organisms depend on each other for food and other

biotic factors, they inevitably interact with each other

Although the most intense relationships exist between

mem-bers of the same species, individuals do not live apart from

members of other species Living in close association,

dif-ferent species may compete for a shared resource such as

food, space, or moisture These interactions can be classified

into several categories: competition, symbiosis

(commensal-ism, mutual(commensal-ism, parasitism), predation, and human

interac-tions In competition, both species are affected adversely; in

commensalism, one species benefits and the other is

unaf-fected; in mutualism, species benefit each other; in parasitism

and predation, one species benefits and the other is harmed

Human interactions may benefit both species, only one

species, or possibly, either species

The concept of interspecific competition is one of the

corner-stones of evolutionary ecology Darwin based his idea of

nat-ural selection on competition, the struggle to survive

Whenever different species occupy the same place at the

same time, there will likely be competition for common

resources such as food, water, or space that are in limited

supply Such interspecific competition consumes both time

and energy Stress caused by such competition may decrease

growth and birth rates and/or increase the death rate; if

intense, competition can slow or even halt population growth

and cause the population to decline If ecological

require-ments of two species are similar but not identical, selection

pressure will tend to cause the species to diverge from each

other through morphological, physiological, and/or

behav-ioral specializations However, if two species have identical

ecological requirements, they will not be able to coexist

because of competition for limited resources Competition is

difficult to study and demonstrate in nature because it is such

an ephemeral phenomenon

The fundamental role of an organism in the community

is its niche (Elton, 1927) The niche is the occupational

sta-tus of the species in the community—what it does and its relation to its food, its competitors, and its enemies It is an abstract concept that has not yet been defined and fully mea-sured A niche should not be confused with a habitat, the physical place where an organism lives The niche is partially defined by characteristics of the habitat, but also by what the organism eats; how, when, and where it finds and captures its food; the time of the year and time of the day when it is most active; the optimal and extreme climatic factors (heat and cold, sun and shade, wet and dry) it can withstand; its parasites and predators; where, how, and when it reproduces; and so forth Every aspect of an organism’s existence helps define that organism’s niche Interspecific competition may play an important role in shaping a species’ niche

Niches of different species may overlap either temporally

or spatially Niche overlap may promote interspecific com-petition, but the special adaptations that each species has for its own specific niche should protect it from extinction For

example, downy woodpeckers (Picoides pubescens) and hairy woodpeckers (P villosus) are found in similar habitats from

Newfoundland to the Gulf of Mexico Although they often feed on the same tree at the same time, downies feed among the upper and smaller branches, while hairies locate their food on the trunk and larger branches Niche overlap may occur on the medium-sized branches, but competition is

minimized because the primary foraging microhabitat is

slightly different

If there is complete niche overlap between two or more species, intense competition for the niche will occur, and one species will outcompete the others The unsuccessful species will either be excluded from the habitat or forced to shift its niche—usually to a suboptimal habitat This concept is often

known as Gause’s Rule after the Russian G F Gause, who

published a study in 1934 showing that when cultured

together, one species of Paramecium drove a second,

com-peting species to extinction Hardin (1960) proposed the

name competitive exclusion principle for this phenomenon.

Although competitive exclusion has been demonstrated

clearly in the laboratory, it probably is rare in nature because

different species seldom compete for precisely the same niche

in the same habitat (Mares, 1993)

Fishes may adapt to different foods and to water of

different depths, temperatures, salinities, and oxygen

con-tents Terrestrial species use elevational, macrohabitat, and

microhabitat differences as well as food-size partitioning

Studies of plethodontid salamanders in the Appalachian

Mountains, for example, have revealed habitat

partition-ing based on elevation and moisture gradients (Hairston,

1949, 1980, 1983; Dumas, 1956; R Jaeger, 1971) Daily

(diel) and seasonal activity cycles, differences in

micro-habitats (terrestrial, arboreal), and characteristics of the

environment such as temperature or moisture are used by

anurans to partition the environment Within a breeding

pond, the difference in time of breeding, egg development,

and larval development of different species avoids direct

competition with other species Natural selection favors

those individuals that breed at such a time as to avoid

com-petition Adaptive modifications of salamander larvae and

tadpoles, including distinct interspecific differences in

mouth structure and whether they inhabit still or flowing

water, permit them to gather food in different macro- and

microhabitats (see Fig 6.30)

Anoles of the Greater Antilles (Cuba, Hispaniola,

Jamaica, and Puerto Rico) illustrate a classic case of

adap-tive radiation (Losos and de Queiroz, 1998) Each of six

species is adapted to its own ecological niche, in particular

to the substrate on which it lives and moves Anoles that live

in the grass have slender bodies and very long tails, whereas

a closely related species that must maintain its balance on

narrow twigs has evolved a short body and stubby legs A

shorter-limbed species with large toe pads inhabits the upper

trunk and canopy of trees, whereas a large species with large

toe pads lives high in the crowns of trees Some prefer

shade; others seek out sunny basking sites Although some

overlap occurs, each species consumes differing food items

Garter snakes and ribbon snakes (both members of the

genus Thamnophis) often inhabit the same general area

(sym-patric) Competition between them, however, is reduced by

differences in food requirements Garter snakes feed

exten-sively on earthworms; ribbon snakes usually shun

earth-worms, but are fond of salamanders, frogs, and small fish

Interspecific competition is evident among introduced

starlings (Sturnus vulgaris) and house sparrows (Passer

domes-ticus) and native species in North America Both European

species compete with native American hole-nesters such as

eastern bluebirds (Sialia sialis) and purple martins (Progne

subis) for suitable nest sites Due to their aggressiveness, both

introduced species often usurp or evict the native species

from their nest sites

BIO-NOTE 13.1

A Competitive Interaction

An interesting example of competitive interaction was

reported between bluebirds (Sialia) and chickadees (Parus) A pair of chickadees began building a nest of

green moss in a bluebird house The next day, the blue-birds entered the house and carried some of the moss away These actions were repeated Then the chickadees deposited a single egg on the nearly bare floor The male bluebird went inside the box and came out with the egg

in his beak He flew to a large tree limb, where the egg balanced momentarily, before falling to the ground below This action was repeated a second time, after which the chickadees left and did not return

Reed, 1989

MacArthur’s (1967) study of five species of warblers revealed they all fed on the same species of caterpillar prey, but they partitioned spruce trees into preferred foraging regions (Fig 13.1) Although some overlap occurred, com-petition was minimal, and all five species were able to coex-ist during the breeding season

Among mammals, interspecific competition occurs in

some areas between black bears (Ursus americanus) and griz-zly bears (U horribilus), between red squirrels (Tamiasciurus) and gray squirrels (Sciurus), and between southern flying squirrels (Glaucomys volans) and northern flying squirrels (G sabrinus) (Weigl, 1978; Flyger and Gates, 1983) This

com-petition may be a function of territorial behavior (Layne, 1954; Ackerman and Weigl, 1970; Flyger and Gates, 1983)

In the southern Appalachians, four closely related species of

mice (Peromyscus leucopus, P maniculatus, P gossypinus, and Ochrotomys nuttalli) inhabited a 6-ha study area (Linzey,

1968) The mice partitioned the habitat by means of spatial orientation (terrestrial vs arboreal) and by food preference Both native and domestic mammals may be affected by indirect competition, a more subtle type of interspecific inter-action Small mammals, such as mice, rabbits, prairie dogs, ground squirrels, gophers, and others, affect the growth of forage plants without competing directly with livestock or game animals for some of the aerial parts of plants These small mammals consume the roots and early growth of grasses and forbs, and their presence can result in lower for-age yields above ground Long-term investigations of the interactions among rodents, birds, and plants in the Chi-huahuan desert of southeastern Arizona have shown a per-sistent and steady competition among species despite the importance of climatic effects on the numbers of individuals (Brown et al., 1986) Brown et al (1986) stated: “Our exper-iments suggest a view of community organization in which virtually all species affect each other through a complex web

of direct and indirect interactions These relationships are

(a) Bay-breasted warbler (b) Cape May warbler

(c) Blackburnian warbler (d) Black-throated green warbler

(e) Myrtle warbler

Coexistence of competing species Robert MacArthur found that five species of warblers were able to coexist by partitioning spruce trees into

pre-ferred foraging regions: (a) bay-breasted warbler (Dendroica castanea); (b) Cape May warbler (D

tig-rina); (c) Blackburnian warbler (D fusca); (d)

black-throated green warbler (D virens); (e) myrtle warbler (D coronata).

FIGURE 13.1

highly asymmetrical, nonlinear, and influenced importantly

by the physical environment as well as by other species.”

Symbiosis (sym, “together,” and bios, “life”) is the term applied

to an intimate relationship between members of different

species Such interactions may be beneficial to one or more

members (commensalism, mutualism); other interactions may

be detrimental (parasitism) Participants in symbiotic

associ-ations often have coevolved with one another and continue

to do so

Commensalism

A commensal relationship exists when one member of the

association benefits while the other is neither helped nor harmed Here are some examples: Some fishes, such as

jack-fish (Caranx) and pilot jack-fish (Naucrates), seek protection in

the vicinity of larger fishes such as barracudas, sharks, and rays (Moyle and Cech, 1996) In most cases, the larger fishes derive no advantage from their companions A com-mensal relationship exists between gopher tortoises and many amphibians, reptiles, and mammals that inhabit their burrows (Lips, 1991) (Table 13.1) (Fig 13.2) Woodchuck

(Marmota monax) burrows also are used by a wide variety

of vertebrates and invertebrates Turtles often deposit their

TABLE 13.1

Summary of Captures of Amphibians, Reptiles, and Mammals in Gopher Tortoise Burrows

in Four Habitats in South-Central Florida

HABITATS

Scrubby Flatwoods

Amphibians

Greenhouse frog

Narrow-mouthed toad

Reptiles

Six-lined racerunner

Eastern indigo snake

Southeastern five-lined skink

Eastern coachwhip snake

Mammals

From K.R Lips, Journal of Herpetology, 25(4):477–481, 1991 Copyright © Society for the Study of Amphibians

and Reptiles, Oxford, OH Reprinted by permission.

eggs in alligator nests The eggs presumably benefit from

the alligator’s defense of the nest from predators Aquatic

turtles may hibernate inside a beaver lodge Small birds

sometimes construct their nests among the branches and

twigs of an eagle’s nest

A unique commensal relationship exists between a bird

and a lizard in New Zealand (Carr, 1970) Sooty

shearwa-ters (Puffinus griseus) often share their burrows with tuataras

(Sphenodon) (Fig 13.3) The diurnal shearwaters occupy their

burrows at night while the nocturnal tuatara is out foraging

The tuatara occupies the burrow during the day while the

shearwater is fishing When the bird migrates, the tuatara

hibernates in the burrow

An unusual commensal relationship exists between

fos-sorial blind snakes (Leptotyphlops dulcis), which feed on insect

larvae in screech owl (Otus asio) nests, thus potentially

reduc-ing larval parasitism on nestlreduc-ing owls (Gehlbach and Baldridge, 1987) Nestling owls in such nests had a higher survival rate, grew 19 percent faster, and fledged earlier than those in nests without snakes (Table 13.2) Owls transport live snakes to their nests and gain a benefit There is no evi-dence, however, that the snakes gain any benefit by being in the nests rather than in the soil

Squirrel monkeys (Saimiri sciureus) find it advanta-geous to associate with capuchin monkeys (Cebus) because

the latter provide a better predator warning system than squirrel monkeys possess (Terborgh, 1985) The recipro-cal benefit for the capuchins is minimal or nonexistent

Rats (Rattus rattus and R norvegicus) and house mice (Mus musculus) have benefited by using structures built by

Indigo snake

Florida (gopher) mouse

Gopher tortoise

Pallid cave cricket

Gopher frog

Beetle

Five-lined skink

FIGURE 13.2

The long, cool burrow of the gopher tortoise (Gopherus polyphemus) in Florida provides refuge for a variety of vertebrate

and invertebrate species The tortoise derives neither benefit nor harm from these commensal relationships Slender beetles

feed on tortoise dung; cave crickets eat beetle dung as well as fungus Gopher frogs (Rana capito) eat insects that

wan-der or fall into the burrow The nocturnal gopher mouse (Peromyscus floridanus) may excavate a side burrow in which it

constructs its nest Even the sandy dump pile may provide refuge for a five-lined skink (Eumeces inexpectatus) Sometimes,

the gopher tortoise defends its burrow against a predatory snake by blocking the entrance with its shell.

Source: Carr, The Reptiles, Life Nature Library.

FIGURE 13.3

Sooty shearwaters (Puffinus griseus) often share their burrows with

tuataras (Sphenodon) This unique commensal relationship allows the

diurnal shearwaters to occupy their burrows at night while the nocturnal

tuatara is out foraging The tuatara occupies the burrow during the day

while the shearwater is fishing When the bird migrates, the tuatara

hibernates in the burrow.

humans and reach their highest densities in agricultural

and urban areas Large grazing animals such as zebras,

cat-tle, buffalo, and horses stir up insects as they feed Birds,

such as cattle egrets (Caserodius albus) (see Fig 11.9) and

cowbirds (Molothrus ater), live among these mammals and

feed on the grasshoppers, leafhoppers, and other insects

disturbed by the grazing animals

Mutualism

In the type of symbiotic relationship known as mutualism,

both partners benefit from the association Clownfishes live among anemones They are not affected by the anemone’s sting, which serves to provide them a protected habitat In turn, the clownfishes defend their homesite from other species that feed on anemones, and also provide anemones with scraps of food Many other species of fishes allow them-selves to be cleaned by cleaner fishes (Fig 13.4) Some even change color, a procedure that indicates a safe time to be cleaned and also makes parasites more easily visible against

a contrasting background Some pilot fish clean the mouths

of manta rays Reef fishes have been recorded cleaning algae

or ectoparasites from sea turtles, and blacknose dace

(Rhinichthys atratulus) have been observed apparently clean-ing wood turtles (Clemmys insculpta) (Kaufmann, 1991) Kuhlmann (1966) observed a toothed carp (Gambusia) clean-ing the mouth of a crocodile (Crocodylus acutus) Cleanclean-ing

symbioses were reviewed and discussed by Feder (1966)

The small ground finch (Geospiza fuliginosa) of the

Galapagos Islands searches for ticks on marine iguanas

Oxpeckers (Buphagus africanus) remove ticks, botfly larvae,

and other parasites from zebras, rhinoceroses, and other large mammals (Fig 13.5)

The intestines of most vertebrates, including humans, provide a suitable environment for beneficial bacteria that aid in food digestion and synthesize certain vitamins Her-bivores, such as cattle, sheep, and deer, depend on bacteria and

FIGURE 13.5

Yellow-billed and red-billed oxpeckers (Buphagus sp.) perch on the rump of a plains zebra (Equus sp.) in Masai Mara National Reserve.

The birds help zebras by plucking off ticks and other pests.

protozoans to help them digest the tough cellulose cell walls

of the plant material on which they feed

Dickman (1992) noted: “Commensal and mutualistic

associations among terrestrial vertebrates are clearly dynamic,

and form and dissolve under different conditions of predator

risk, resource levels, competition, and many other factors An

important assumption is that these associations are favoured

only when the benefits to individuals exceed the costs.”

Parasitism

Parasitism is a vital interspecific interaction in which one

member—the parasite—benefits while the other member—

the host—is harmed in some way Lampreys parasitize fish

by sucking out their blood and body fluids (see Fig 4.12) Cowbirds of the New World and cuckoos of the Old World are social parasites (Milius, 1998a) (see Fig 8.72) They both lay their eggs in nests of other bird species, often removing one egg from the host’s nest prior to laying their own Female cuckoos usually lay an average of eight eggs a year Eggs are laid on alternate days and usually in two batches, separated

by several days rest (Davies and Brooke, 1991) Their

decep-TABLE 13.2

Nestling Growth Dynamics in Eastern Screech Owl Nests With One, Undisturbed (by Us), Live

Blind Snake at Fledging Time Versus Same-Season Nests Without Blind Snakes but Same-Size Broods.

From F R Gehlbach & R S Baldridge, “Live Blind Snakes (Leptotyphlops dulcis) in Eastern Screech Owl (Otus asio) Nests.” in Oecologia, 71:560–563 Copyright © 1987 Springer-Verlag, New York.

a N=10 per group in a comparison without brood-size equality; however, mean number of nestlings is no different (3.3±0.7 vs 2.8±1.0, F=1.6, NS) and results are the same; F=54.8, 7.2 (p <0.001) for growth rate and F=0.8 (NS), 2.5 (p <0.02) for fledging weight, between groups vs among broods, respectively

This Nassau grouper (Epinephelus striatus) is being cleaned by two

gobies Cleaning symbiosis is a common mutualistic relationship

between marine animals.

FIGURE 13.4

FIGURE 13.7

Vampire bat (Desmodus rotundus) feeding on the foot of a cow

Observations of vampires indicate that this stance and area of assault illustrate the most frequent method of attack The stance shows the quadrupedal relationship of bats.

White wagtail Cuckoo

Meadow bunting Cuckoo

Redstart Cuckoo

Great Reed warbler Cuckoo

FIGURE 13.6

Cuckoo eggs often mimic the appearance of the host’s eggs.

Source: Faaborg Ornithology, 1988, Prentice-Hall, Inc.

tion involves surveillance, stealth, surprise, and speed In less

than 10 seconds, the female cuckoo alights on a nest, lays her

own egg, removes one host egg, and is gone The eggs often

mimic the appearance of the host’s eggs (Fig 13.6) Cuckoo

eggs have been found in nests of at least 125 bird species in

Europe (Wyllie, 1981)

Many researchers have thought that cuckoos imprint on

their foster parents and, when adult, choose to parasitize the

same host species However, studies in which newly hatched

cuckoos were transferred into nests of other species failed to

demonstrate host imprinting (Brooke and Davies, 1991)

Besides having a shorter incubation period than their host

species, cowbirds (Molothrus ater) hatch before many hosts by

disrupting incubation of smaller eggs and, possibly, hatching

in response to stimuli from host eggs (McMaster and Sealy,

1998) In addition, young cowbirds and cuckoos are usually

larger than the natural young in the parasitized nest, and they

either take the lion’s share of the food or eject the host young

from the nest (see Fig 8.72) Friedmann and Kiff (1985)

recorded 220 species as having been parasitized by

brown-headed cowbirds, with 144 species actually rearing young

cow-birds This difference in the number of species parasitized

versus those actually rearing cowbirds is due to host

recogni-tion and counter-strategies: deserting the nest, rejecting the

cowbird egg, or depressing the egg into the bottom of the nest

In Virginia, 39 percent of dark-eyed junco (Junco

hye-malis) nests contained at least one cowbird egg (Wolf, 1987).

Cowbirds laid an average of 1.7 eggs per nest and removed

an average of 1.2 junco eggs per nest Smaller species such

as cedar waxwings (Bombycilla cedrorum), Baltimore orioles

(Icterus galbula), and warbling vireos (Vireo gilvus) remove

the cowbird egg by puncture-ejection (entire cowbird egg

removed or pieces of shell removed after egg contents are

consumed) (Sealy, 1996) Larger species generally remove

cowbird eggs by grasp-ejection

In the early 1980s, half of all nests of the least Bell’s

vireo (Vireo bellii pusillus) on the Camp Pendleton military

base in southern California were parasitized by cowbirds, and

the vireo population was near extinction (Holmes, 1993)

When a cowbird trapping program reduced parasitism to

near zero, the vireo population increased tenfold Cowbird

populations are being controlled by trapping at Camp

Pendleton as well as in the breeding grounds of several

endangered songbirds, including the Kirtland’s warbler

(Den-droica kirtlandii) in northern Michigan and the black-capped

vireo (Vireo atricapillus) in central Texas (Holmes, 1993).

American goldfinches (Spinus tristis) are regularly

para-sitized by cowbirds, but cowbirds do not survive the nestling

period because the granivorous diet of the goldfinch provides

inadequate protein (Middleton, 1991) It is rare among birds

for the diet of nestlings to be composed mainly of seeds,

because seeds are relatively low in protein

The only known parasitic mammals are vampire bats,

which feed on fresh blood of sleeping or resting birds and

mammals, including humans (Fig 13.7) Their teeth are

sharp, so that the incision is virtually painless and the victim

is not awakened or disturbed as blood is “lapped up” rather than “sucked out.” The saliva may contain an anticoagulant,

so that blood may continue to flow from the wound for sev-eral hours The bitten animal may contract virally caused diseases such as rabies, or secondary infections may develop

at the site of the wound

Male Asian elephants with longer tusks have been found

to have fewer internal parasites (Bagla, 1997) Males carrying genes for resistance to parasites are healthier and in a better

FIGURE 13.8

A black rat snake (Elaphe obsoleta) homes in on a clutch of northern

cardinal eggs Snakes strongly prefer to forage along forest edges Eggs and nestlings are normally only a minor part of the snake’s diet.

condition to develop secondary sexual characteristics

These better-fit males are then more likely to be chosen by

females as mates Since ivory hunters are likely to poach

the best males because of their larger tusks, it is feared that

poaching may weaken the elephant gene pool by removing

the most fit males and their parasite-resistant genes from

the population

Parasitism of reintroduced and captive endangered

species by external (ticks, mites, lice, fleas) and internal

(nematodes, trematodes, cestodes) parasites may be a critical

danger to their well-being and reestablishment (Phillips and

Scheck, 1991) Potential parasites need to be considered

when designing and implementing restoration projects

Predation is an interaction in which one species—the

predator—benefits from killing and eating a second

species—the prey Predators and prey coevolve, with

preda-tors becoming specialized to capture their prey, and prey

species becoming adapted to evade their predators

Each vertebrate class has a large number of predaceous

species Sharks feed on other fishes and marine mammals

Largemouth bass (Micropterus salmoides) feed primarily on

smaller fishes

Amphibians and reptiles feed on a wide variety of

inver-tebrates and verinver-tebrates, with the choice of food being

primar-ily determined by the size of the mouth-opening—amphibians

and reptiles are gape-limited predators Amphibians feed

pri-marily on invertebrates, although some larger forms such as

bullfrogs (Rana catesbeiana) will eat almost any suitably sized

animal that moves within striking range Many aquatic

tur-tles feed on invertebrates, as well as on small fish and

amphibians Snapping turtles (Chelydridae) are known to

consume amphibians, snakes, small turtles, birds, and small

mammals Most marine turtles are omnivorous Juvenile

green turtles (Chelonia mydas), however, are more carnivorous

than adults, which subsist mainly on plants Most lizards

feed on invertebrates, although some, such as the Gila

mon-ster (Heloderma suspectum) and the Komodo dragon (Varanus

komodoensis), include mammals in their diet Crocodilians

prey on fishes, reptiles, and birds and on mammals as large

as antelopes All snakes are predaceous, feeding on prey

rang-ing from larger invertebrates, birds, and bird eggs (Fig 13.8)

to mammals

Carnivores, in general, have a more difficult time

obtain-ing food than herbivores Once a carnivore captures its prey,

however, the meal is far higher in nutrition because of its

pro-tein and fat content Thus, meat eaters spend considerably

less time eating than plant eaters (Fig 13.9) In addition,

the larger the herbivore, the more time it needs each day to

obtain sufficient food

Many birds and mammals are insectivorous; others feed

on a wide variety of fishes, amphibians, reptiles, birds, and

mammals Hawks, eagles, ospreys, and owls feed on fishes,

lizards, snakes, other birds, and mammals up to the size of

skunks, monkeys, and sloths Most predatory birds consume their smaller prey whole, later regurgitating the indigestible hair, bones, feathers, scales, or insect parts as pellets (Fig 13.10) Predatory mammals include such groups as bears, raccoons,

cats, wolves, foxes, and weasels Black bears (Ursus americanus) and raccoons (Procyon lotor), for example, are major predators

on American alligator (Alligator mississippiensis) eggs and young

(Hunt and Ogden, 1991)

Predators in certain regions have preferred prey (cougar–deer; wolf–moose; fox–rabbit), but most are oppor-tunistic and will kill a variety of prey They frequently cap-ture older, weaker, debilitated animals, thus acting as a selective agent promoting the genes of those prey animals able to evade capture

Predators may differentially consume individuals based

on age or sex within populations of prey species and thus may have subtle effects on prey-population dynamics On an island off the coast of western Australia, adult house mice

(Mus musculus) foraged primarily in dense cover; juveniles,

especially females, used areas of open vegetation more than adults and were potentially most at risk of predation (Fig

13.11a) (Dickman et al., 1991) Barn owls (Tyto alba) took a

greater number of young female house mice than any other size or sex class Correlations between the hourly number of hunting owls and the overall hourly capture rates of mice

were significant for juvenile female (r=;0.84, p<0.001), almost significant for juvenile males (r=;0.57, p ~ 0.05), and not significant for adults (males: r=:0.10; females: r=;0.51) (Fig 13.11b) These data strongly support the

hypothesis that juvenile mice, especially females, that use

90

80

70

60

50

40

30

20

10

0

Body weight (lbs.)

Herbivores

100 90

80 70

60 50 40

30 20 10

0

Body weight (lbs.)

Carnivores

Elephant 5,750

Rhino 3,000

Gazelle 125

Monkey 17

Hunting dog 50

Lion 400 Cheetah 300

Polar bear 1,200

FIGURE 13.9

Although carnivores must work harder than herbivores to find a meal, a carnivore’s meal is higher in nutrition than an herbivore’s Thus, carnivores spend much less of their time eating than herbivores In addition, the larger the herbivore, the more time it needs each day simply to stay fed.

Source: Data from Shipman, “What Does it Take to be a Meateater?” in Discover Magazine, September, 1988.

more open vegetation than adults face a higher risk of pre-dation from hunting owls

Some predators and their prey have developed complex

interrelationships For example, moose (Alces alces) colonized

Isle Royale in Lake Superior, probably swimming from nearby Ontario in the early part of the 20th century (Mech, 1966) (Fig 13.12) With no effective predators and an abun-dant food supply, the population grew to very high levels by the late 1920s Murie (1934) estimated 1,000–3,000 moose present in 1929 and 1930 Significant mortalities from mal-nutrition apparently reduced the population to several hun-dred animals by the mid-1930s (Hickie, 1936) The population again increased, until direct mortality from mal-nutrition was observed in the late 1940s In 1947, a popula-tion of 600 moose was estimated by aerial strip count (Krefting, 1951) Mech (1966) estimated the 1960 popula-tion at 600 animals The moose populapopula-tion apparently increased during the 1960s (1,300 to 1,600 from 1968–1970) and leveled off, or perhaps even declined, from 1970 to 1974 Mid-winter aerial censuses in 1972 and 1974 produced esti-mates of 818±SE 234 and 875±SE 260 moose, respec-tively (Peterson, 1977)

During the winter of 1948–49, timber wolves (Canis lupus) managed to cross the ice from the mainland of Ontario

and became established on the island Their population increased and fluctuated between 20 and 50 animals during

FIGURE 13.10

A life-size regurgitated pellet from a short-eared owl (Asio flammeus).

The pellet contains the remains of a small rodent.

the period 1960–1980 Thus, predator and prey had reached

a dynamic equilibrium—a stabilization of numbers such that

each species could survive without having a detrimental

impact on the other Sufficient resources were available to

support the moose population, which was maintained at

healthy levels by selective culling of old and weak

individu-als by the wolves In 1958, wildlife biologist Durward Allen

began tracking the changing population numbers in what has

become the longest-studied system of natural predator–prey

dynamics in existence

After the wolf population on Isle Royale reached a peak

of 50 animals in 1980, it experienced a decline in the early

1980s, from which it still has not recovered (Fig 13.12)

Only four pups were born, to the same female in one wolf

pack, between 1991 and 1993 (Mlot, 1993) As of August

1993, the other two packs were down to just a pair of wolves

Night

Hour of the day

12 10 8 6 4 2 0

40

30

20

10

0

Juveniles Adults

Mus males

Tyto alba

30

20

10

0

Juveniles Adults

Mus females

(b) (a)

Adults Juveniles Females

Males

Hour of the day

70

60

50

40

30

20

10

0

40

30

20

10

0

FIGURE 13.11

(a) Hourly number of captures of Mus musculus in open

vegeta-tion, expressed as percentages of the total numbers of captures

in different sex and size categories (top) Females (bottom)

Males Results for three periods of 24 hours are combined (b)

Comparison of hourly numbers of captures of Mus musculus by

sex and size, and numbers of observations of Tyto alba Results

for three periods of 24 hours are combined

each The moose population, which has steadily increased, reached a record high of about 1,900 animals in 1993 The decline of the wolves was probably the result of two factors: an encounter with canine parvovirus in 1981, and low genetic variability (Mlot, 1993) Because the start of the wolf ’s decline coincided with a 1981 parvovirus outbreak in nearby Houghton, Michigan, it is thought that the virus could have been carried to Isle Royale on the hiking boots of visitors to the U.S national park on the island Restriction enzyme analysis of the wolves’ mitochondrial DNA revealed that they were all descended from a single female and had only about half the genetic variability of mainland wolves (Mlot, 1993) Whenever a small number of individuals manage to cross an already existing barrier and found a new geographi-cally isolated colony, they generally carry with them in their own genotypes only a small percentage of the total genetic