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• Interactions 1 in Ecosystems Habitat and Niche 14.2 Community Interactions 14.3 Population Density and Distribution 14.4 Population Growth Patterns Data Analysis READING COMBINATION GRAPHS 14.5 Ecological Succession HMDScience.com ONLINE Labs • Modeling Predation • QuickLab Survivorship Curves • Limiting Nutrients for Algae • Making a Local Field Guide • Predator-Prey Interactions • Monitoring Bird Populations • Using GPS in Ecological Surveys • Population Dynamics • Video Lab Yeast Population Growth PREMIUM CONTENT Environmental Stress Explore how healthy parrot populations have weathered hurricanes in the past. Then determine whether an endangered parrot population can recover. ° Why are these zebras fighting? For the zebra, life on the African savannah is about survival. Whether escaping th e ambush o f a pride o f lions, walking vast distances t o drink fresh water, or competing f o r the right t o mate with females, only the best adapted zebras will survive and pass on their genes. The interactions among organisms, and between organisms and their environment, make ecosystems function. READING TOOLBOX This reading tool can help you learn the material in the following pages. USING LANGUAGE YOURTURN Predictions Some predictions are conditional: In t h e following sentences, identify t h e condition and t h e Something might happen, but only i f something else prediction. happens first. For example, i f the temperature drops below 1. freezing, snow might fall. The prediction is that snow might fall tonight. But snow might fall under one condition. First, the temperature has t o drop below freezing. A f t e r t h e deer population reaches 600 individuals o n t h e island, t h e deer will eat most o f t h e vegetation, and t h e number o f deer will decrease. 2. If t h e otters are removed f r o m t h e ecosystem, t h e sea urchins will eat all o f t h e kelp. Chapter 14: Interactions in Ecosystems 401 Habitat and Niche VOCABULARY KEY C O N C E P T Every organism has a habitat and a niche. habitat M A I N IDEAS ecological niche O A habitat differs from a niche. competitive exclusion O Resource availability gives structure t o a community. ecological equivalent Connect to Your World — — T h e ways in which a zebra interacts w i t h its e n v i r o n m e n t a n d o t h e r organisms are o n ly a small p a r t o f t h e e c o l o g y o f t h e African plains. To u n d e r s t a n d w h a t individuals, populations, a n d c o m m u n i t i e s n e e d t o survive, ecologists s t u d y t h e interactions a m o n g species a n d b e t w e e n species a n d t h e i r environment. W h y d o e s a zebra fit so well into t h e African savannah? Q M A I N IDEA A habitat differs from a niche. FIGURE 1.1 A lion must hunt and kill its prey in order t o survive on the African savannah. Its role as a t o p predator is part o f t h e lion's niche. On the vast plains of Africa, tall grasses grow among trees and shrubs, and small pools of water surrounded by thirsty animals dot the landscape. This challenging environment is the home of the African lion, shown in FIGURE 1.1. Here, lions stalk through tall grass to hunt zebras and antelope, find places to rest in the shade of trees, and never stray far from valuable pools of water. These are just a few of the environmental features that make up the lions habitat. A habitat can be described as all of the biotic and abiotic factors in the area where an organism lives. These factors include all aspects of the environ ment, including the grass, the trees, and the watering holes. Each species interacts with its environment in a different way. Within an ecosystem, each species has an ecological niche. An ecological niche (nihch) is composed of all of the physical, chemical, and biological factors that a species needs to survive, stay healthy, and reproduce. You can think of a habitat as where a species lives and a niche as how it lives within its habitat. A niche includes • Food The type of food a species eats, how a species competes with others for food, and where it fits in the food web are all part of its niche. • Abiotic conditions A niche includes the range of conditions, such as air temperature and amount of water, that a species can tolerate. • Behavior The time of day a species is active as well as where and when it reproduces are factors in the niche of a species. 402 Unit 5: Ecology Looking closely at all of these factors, we can see that while an antelope may use the tall grasses of the African plains as a food resource, a lion may use the s a m e grasses as camouflage for hunting. A lion uses the antelope as a food resource and hunts primarily during low-light times like dawn or dusk. In order to avoid the intense heat of the savannah, lions often spend afternoons in the shade. These examples are only a few parts of the lions ecological niche, but they help to give a picture of how a lion fits into the African savannah. Connect W h a t are some o f the abiotic and biotic factors o f your habitat? a READING TOOLBOX TAKING NOTES Define ecological niche by organizing your notes into a chart. Ecological N \ c h e food Abiotic condition^ Q M A I N IDEA Resource availability gives structure to a community. You have learned that the ability of an individual to survive and reproduce is the driving force behind natural selection. A species needs resources such as food, water, and shelter to be successful in its habitat. The organism that is best suited to obtain these resources is most likely to survive and reproduce. But what if two species are competing over limited resources? fyeJnavior Virtuai _.-Aki Population Niches and Competition Competitive Exclusion We have already seen that many species can share similar habitats and that they may use some of the same resources, as shown in FIGURE 1.2. But when two species use the same resources in the same ways, one species will always be better adapted to the environment. The principle of competitive exclusion states that when two species are competing for the same resources, one species will be better suited to the niche, and the other species will be pushed into another niche or become extinct. The North American gray squirrel was introduced to Great Britain in the late 1800s. The native European red squirrel was forced to compete with the newcomer for the same food resources, habitat, and space. In this case the gray squirrel was better adapted to the niche and pushed out its smaller competitor. Currently, the red squirrel population is declining due to competition with its larger, more aggressive cousin. But competitive exclusion can also result in other outcomes. • Niche partitioning The two squirrel species could have naturally divided different resources based on competitive advantages. If one type of squirrel ate nuts from the tops of trees while others ate nuts from the ground, the niche would have been divided. FIGURE 1.2 Even though bees and butterflies both use these flowers f o r food, they occupy different niches. Many species with similar niches can coexist. • Evolutionary response The two species of squirrel could have experienced divergent evolution. Selection for larger teeth might have allowed one type of squirrel to become better at cracking large nuts, while selection for smaller teeth might have allowed the other to eat small seeds. Chapter 14: Interactions in Ecosystems 403 FIGURE 1.3 E c o l o g i c a l Equivalents Ecological equivalents are two species that occupy similar niches in geographically separate areas. Survive within a Niche South America Madagascar The mantella frog (left) and the poison dart frog (right) have evolved similar defense mechanisms. The bright coloration o f each is a warning t o predators. Each frog secretes a highly poisonous toxin through its skin that makes i t an unpleasant meal f o r a predator. Synthesize Explain how natural selection resulted in t h e evolution o f t w o similar frog species in t w o similar niches. Ecological Equivalents The competitive exclusion principle involves species competing for resources in the same community. In different communities, ecological equivalents occur in very similar niches. In mathematics, numbers that are equal are called equivalents. Similarly, ecological equivalents are species that occupy similar niches but live in different geographical regions. Pictured in FIGURE 1.3, the mantella frog of Madagascar and the poison dart frog of South America have much the same niche in similar habitats. They both have brightly colored skin that secretes a highly poisonous toxin to ward off predators. Both prey on similar insects and live in a similar habitat, but because they live in different regions of the world, they never compete for the same resources. !X£2SaXZ£I~ AMPHIBIANS Amphibians were the first vertebrates t o move out o f the water and onto land. In Vertebrate Diversity, you will learn more about amphibians. Apply Are these frogs experiencing competitive exclusion? Explain. Formative Assessment REVIEWING O M A I N IDEAS 1. W h a t are t h e t h r e e parts o f an organism's ecological niche? 2. W h a t d o e s t h e principle of competitive exclusion say will happen when t w o species c o m p e t e for t h e same resource? CRITICAL T H I N K I N G 3. Predict If a group o f mantella frogs were transported t o t h e ecosystem o f t h e poison dart frogs, what might happen t o t h e t w o species' populations? 4. Analyze A bison and an elk live in t h e same habitat and feed o n t h e same grasses. Does this mean t h a t t h e competitive exclusion principle d o e s CONNECT TO EXOTIC SPECIES 5. Considering t h e c o m p e t i t i v e exclusion principle, why may | it be harmful t o t r a n s p o r t a species, such as a rabbit, t o another habitat where it currently does n o t exist? 14.2 Community Interactions ^VOCABULARY competition Organisms interact as individuals and as populations. KEY C O N C E P T predation M A I N IDEAS symbiosis O Competition and predation are t w o important ways in which organisms interact. mutualism commensalism O Symbiosis is a close relationship between species. parasitism Connect to Your World Each day, t w o h o t d o g v e n d o r s sell virtually identical p r o d u c t s t o a n y o n e w h o is hungry. They may b e o n different sides o f t h e street, b u t t h e y are still trying t o sell h o t d o g s t o t h e s a m e hungry consumers. A v e n d o r selling h o t pretzels may also b e trying t o sell t o t h e s a m e customers, b u t w i t h a slightly varied product. Just like t h e s e vendors, organisms c o n s t a n t l y c o m p e t e w i t h o n e another. Q M A I N IDEA Competition and predation are two important ways in which organisms interact. Two birds may fight over territories. A fish may prey on insects floating on the water. These are just two examples of the many interactions between species in an ecosystem. FIGURE 2.1 Snakes are predators that swallow their prey whole. The hollow fangs o f this timber rattlesnake inject venom t o para lyze and kill its prey. Competition C o m p e t i t i o n occurs when two organisms fight for the same limited resources. There are two different types of competition: interspecific competition and intraspecific competition. Even though they may have different niches, two species may still use similar resources. Interspecific competition occurs when two different species compete for a limited resource, such as space. In a lawn, for example, grass, dandelions, and many other plants all compete for nutrients and water. Competition also occurs among members of the same species. This is known as intraspecific competition. Individuals of a particular species struggle against one another for limited resources. You can observe intraspecific competition during the spring breeding season of birds. A typical male will share a particular territory with males of different bird species but will not tolerate another male of its own species in the same area. Predation Another way species interact with one another is through predation. Predation is the process by which one organism captures and feeds upon another organism. Many organisms, such as the snake in FIGURE 2.1, have become highly adapted to hunting and killing their prey. Chapter 14: Interactions in Ecosystems 405 Predator/Prey Relationship The timber rattlesnake, for example, is a predator that preys on small animals such as mice, voles, rabbits, and squirrels. Lying silent, hidden among leaf litter on the forest floor, the rattlesnake has found a niche as an ambush predator. A swift bite from the snakes fangs injects its venom. The venom attacks the nervous system and eventually paralyzes the prey. The snake swallows the paralyzed animal whole. Herbivores can also be considered predators. The deer that eats grass in fields and leaves from trees is preying on the plants. Evaluate How does natural selection shape p r e d a t o r - p r e y relationships? © M A I N IDEA Symbiosis is a close relationship between species. a READING TOOLBOX VOCABULARY The word symbiosis comes from the Greek word sumbios, which means "living together." A honeybee buzzes away from a flower with its reward of nectar. Small pollen grains have become attached to the bee's back. When the bee arrives at the next flower, the pollen fertilizes the egg of the next plant. In this way, a relationship, or symbiosis, between the bee and the flower has evolved. Symbiosis is a close ecological relationship between two or more organisms of different species that live in direct contact with one another. There are three major types of symbiosis: mutualism, commensalism, and parasitism. Mutualism 3 READING TOOLBOX TAKING NOTES Mutualism, commensalism, and parasitism are distinct types o f symbiosis. Add details with examples o f your own. Symbiosis N u t u a lifm Cory)rY)er)Sa\lSm Pof-oyitifm Mutualism is an interspecies interaction in which both organisms benefit from one another. The relationship between the lesser long-nosed bat and the saguaro cactus is another example of mutualism. During the spring, bats help pollinate the cacti through the indirect transfer of pollen as they fly from one cactus to another to feed on flower nectar. When the fruit ripens in the sum mer, bats become fruit eaters, as shown in FIGURE 2.2. The cactus benefits when bats spread its indigestible seeds across the desert. Commensalism Another type of symbiotic relationship is commensalism. Commensalism is a relationship between two organisms in which one receives an ecological benefit from another, while the other neither benefits nor is harmed. Right now you may be a part of a commensal relationship. Buried deep in the hair follicles of your eyelashes are microscopic mites that feed on the secretions and dead skin cells of your body. These harmless organisms are called demodicids, and they have found their highly specialized niche in your hair follicles. Parasitism Parasitism is a symbiotic relationship involving a species that directly harms its host. Parasitism is a relationship similar to predation in that one organism benefits while the other is harmed. But unlike a predator, which quickly kills and eats its prey, a parasite benefits by keeping its host alive for days or years. For example, the braconid wasp lays its eggs inside a caterpillar. When the larvae hatch, they eat the caterpillar from the inside out, consuming the nutrients they need to grow into adults. 406 Unit 5: Ecology FIGURE 2.2 S y m b i o t i c R e l a t i o n s h i p s l he interactions between species in an ecosystem can t a k e many d i f f e r e n t forms- A symbiotic relationship involves interactions between organisms o f different species t h a t live in direct contact. Parasitism 0 Organism is harmed Organism is n o t affected Organism benefits i Braconid wasp I Hornworm caterpillar Braconid larvae feed on their host and release themselves shortly before reach ing the pupae stage o f development. The host hornworm will eventually die as its organs are con sumed by wasp larvae. Commensalism | Human Our eyelashes Demodicids Eyelash mites f i n d all they need t o survive in t h e t i n y follicles o f eyelashes. Magnified here 225 times, these creatures measure 0.4 m m in length and can be seen only w i t h a microscope. are home t o t i n y mites t h a t feast on oil secre tions and dead skin. W i t h o u t harming us, up t o 20 mites may be living in one eyelash follicle. : colored SEM; magnificatio Mutualism Lesser long-nosed bat The bat _) depends on night-blooming cacti as its primary source o f food. Cacti are a rich source o f f r u i t and nectar, staples o f t h e bat's diet. • a L'ITICA R C VIEWING ( Saguaro cactus As t h e bat feeds on the cactus' fruit, i t also ingests t h e seeds. These indigestible seeds are dispersed t o new locations as the bat flies across t h e desert. How might the symbiotic relationship chai i f eyelash mites desti^yed hair follicles? 14: Interactions in Ecpsystems 407 FIGURE 2.3 Human Parasites: Inside a n d O u t Humans can get parasites in many ways. Leeches attach to the exposed skin of humans. By penetrating human skin, hookworms find their home in the digestive tract. Many leeches feed on the blood o f a host organism. Freshwater leeches such as this one can grow t o lengths o f 12 cm o r more. Hookworms are endoparasites with sharp teeth that attach t o the intestinal wall o f a host organism and absorb nutrients f o r food, (colored SEM; magnification 200x) Hypothesize Why is it important for ectoparasites to stay undetected by their hosts? >,s- CONNECT T O INVERTEBRATES Leeches a n d hookworms are c l a s s i f i e d a s i n v e r t e b r a t e s . In Invertebrate Diversity, y o u will l e a r n m o r e a b o u t t h e diversity o f invertebrates. The needs of a parasite are met by a host—the victim of the parasite. There are two different ways that parasites can use their host. An ectoparasite makes its home on the exterior of an organism, attaching itself to the outside of the host and usually feeding on its fluids. Common ectoparasites include fleas, ticks, and leeches, such as the one seen in FIGURE 2.3. Many types of ectoparasites are also known to carry a wide variety of diseases that can affect their host. Parasites can also be found inside of living organisms. Endoparasites live in the tissues and organs of a host where, safely hidden, they feed on the nutrients ingested by their host. Large endoparasites, such as tapeworms and hook worms, and smaller protozoan endoparasites can kill their host if not treated. Connect What type of symbiosis is the relationship between a dog and its owner? SELF-CHECK Online Formative Assessment R E V I E W I N G O M A I N IDEAS CRITICAL T H I N K I N G 1. During t h e fall spawning o f salmon, grizzly bears fight over space o n t h e banks o f a river. W h a t t y p e of >n is this? Compare and Contrast How are predation and parasitism similar? 2. Describe and give examples o f t h e t h r e e types o f symbiosis. a kill, birds will sometimes arrive t o pick a t t h e leftover carcass. Which are t h e predators: t h e birds, t h e lion, o r both? Why? How d o they differ? Synthesize After a lion has m a d e HMD5cience.com PREMIUM CONTENT r m v m w E A N I M A L BEHAVIOR 5. You have probably heard the j saying "There is safety in numbers." Why might traveling in a large group be beneficial t o prey s p e c i e s ? u 14.3 Population Density and Distribution VOCABULARY population density Each population has a density, a dispersion, and a reproductive strategy. KEY C O N C E P T population dispersion M A I N IDEAS survivorship curve O Population density is the number o f individuals that live in a defined area. O Geographic dispersion o f a population shows how individuals in a population are spaced. O Survivorship curves help t o describe the reproductive strategy o f a species. •>!<• Connect to Your World If y o u have ever traveled f r o m a rural area into a city, y o u may have n o t i c e d a change in p o p u l a t i o n density. Cities have m o r e d e n s e populations, while rural areas have m o r e widely dispersed populations. Scientists measure species p o p u l a t i o n s in a similar way. W h a t c a n w e learn f r o m p o p u l a t i o n d a t a ? Q M A I N IDEA Population density is the number o f individuals that live in a defined area. The wandering albatross may fly over open ocean waters for days or weeks at a time without ever encountering another bird. In contrast to this solitary lifestyle, elephant seals may gather in groups of a thousand or more on Cali fornia beaches. By collecting data about a population in a particular area, scientists can calculate the density of a population. Population density is a measurement of the number of individuals living in a defined space. ,c CONNECT T O GENE FLOW Recall that in The Evolution o f Populations you learned about gene f l o w and geographic isolation. Population dispersion patterns influence the rate o f gene flow among and between species. Calculating an accurate population density can tell scientists a great deal about a species. When scientists notice changes in population densities over time, they work to determine whether the changes are the result of environ mental factors or are simply due to normal variation in the life history of a species. In this way a wildlife biologist can work to make changes that will help to keep the population healthy. One way to calculate population density is to create a ratio of the number of individuals that live in a particular area to the size of the area. This formula is simplified as follows: # of individuals 2 area (units ) = population density For example, if scientists sampling a population of deer counted 200 individuals in an area of 10 square kilometers, the density of this deer popula tion would be 20 deer per square kilometer. Connect W h a t might a decrease in the density o f a deer population over a specif'' t i m e period tell scientists about the habitat in t h e area? 410 Unit 5: Ecology FIGURE 3.1 D i s p e r s i o n P a t t e r n s pispersion patterns help us understand species interactions by showing h o w populations group together. Many species o f f i s h swim together in large groups called schools. By moving as a large mass, individuals have an advantage in avoiding predators. Nesting sites o f the gannet show uniform distances f o r protection o f eggs from other males. Territorial organisms generally display uniform dispersion. The three-toed tree sloth, a solitary animal, spends most o f its life in the canopy o f tropical forests. The sloth has almost no competitors and has few natural predators. Q M A I N IDEA Geographic dispersion o f a population shows how individuals in a population are spaced. Other information can be gained from population density measurements. Patterns of geographical dispersion give us ideas of how individuals of the same species interact and how different species interact with one another. Population dispersion is the way in VISUAL VOCAB which individuals of a population are spread in an area or a volume. Population dispersion is t h e way in which individuals o f a p o p u l a t i o n FIGURE 3.1 shows the three types of are spread in an area o r a volume. population dispersion. • • • Individuals may live close together in groups in order to facilitate mating, gain protection, or access food resources. C l u m p e d dispersion Territoriality and intraspecies competition for limited resources lead to individuals living at specific distances from one another. Uniform dispersion Clumped dispersion Uniform dispersion Random dispersion Individuals are spread randomly within an area or a volume. Random dispersion Infer W h a t t y p e o f intraspecies interaction might cause uniform dispersion? Chapter 14: Interactions in Ecosystems 411 I N T E R P R E T I N G DATA Survivorship Curves MATERIALS In this lab, you will make a type 1 survivorship curve using data from the obituary section o f a newspaper. • obituary section o f a newspaper • graph paper PROBLEM What is t h e trend in data for type 1 survivorship curves? PROCEDURE TABLE 1. SURVIVORSHIP DATA Survivors % Surviving 0-5 35 - 1 = 34 97 6-10 34 - 1 = 33 94 11-15 33 - 0 = 33 94 16-20 33 - 4 = 29 83 21-25 29 - 1 = 28 80 Age (years) 1. Obtain the obituary section of the newspaper. 2. Create a data table like the one at right that extends t o include five-year age groups up t o 91-95 years. 3. For 35 obituaries, place a tally next t o the age group in which t h e individual died. 4. Subtract the number of individuals that died from Deaths the number o f remaining survivors, and record the answer in the third column of your data table. Calculate the percent surviving in each age group by dividing the number o f survivors by 35 and multiplying by 100. Repeat this step for all age groups. ANALYZE AND CONCLUDE 1. Graph Data Draw a survivorship curve by plotting t h e age group on t h e x-axis, and the percent survivors on they-axis. 2. Analyze Explain t h e trend in the data. © M A I N IDEA Survivorship curves help to describe the reproductive strategy o f a species. The California red-legged frog of the western United States is an amphibian that reproduces by laying 2000 to 5000 eggs in late winter and early spring. In one to two weeks, these eggs hatch, and over the next four to seven months, the tadpoles grow into frogs. If so many eggs are laid, why is this frog a threat ened species in much of the western United States? Many predators feed on the eggs of the red-legged frog, so of the thousands of eggs laid, only a small number of offspring will survive to adulthood. This type of reproductive strategy is to produce a lot of offspring. Species use many other reproductive strategies as well. Survivorship curves illustrate how offspring survival from birth to death fits in with the survival strategies of a particular species. A survivorship curve is a generalized diagram showing the number of surviving members over time from a measured set of births. By measuring the number of offspring born in a year and following those offspring through until death, survivorship curves give information about the life history of a species. For example, we will begin with 100 coyotes born in year zero. After one year, 10 of those baby coyotes died from disease or predation. Of the original 100, 90 are left. During year two, 4 more coyotes die, leaving 86 of 412 Unit 5: Ecology the original 100. In year three, 3 more die, leaving 83 of FIGURE 3.2 SURVIVORSHIP CURVES the original 100. The number of individuals surviving from year to year decreases, but a substantial portion of the group will live a full life and reproduce. In FIGURE 3.2, you can see the three basic patterns of animal survivor ship curves. Type I The graph shows a type I survivorship curve in orange. Type I survivorship represents a life history that is common among large mammals, including humans. The curve shows a low level of infant mortality and a popula tion that generally will survive until old age. A behavior 10 20 30 40 50 60 70 80 9 0 Percentage o f maximum life span that most organisms showing type I survivorship share is parental care for the young. Most infant organisms are unable to care for themselves. By protecting their young, parents are better able to ensure that their offspring stay alive until they can survive on their own. Organisms such as birds, small mammals, and some reptiles show a survivorship rate that is roughly equal at all ages of an organisms life. At all times, these species have equal chances of living and dying, whether from disease or as a result of predation. A type II survivorship curve is shown in green on the graph. Type II Organisms with type III survivorship (shown in blue) have a very high birth rate and also a very high infant mortality rate. Species with type III survivorship are generally invertebrates, fish, amphibians, and plants. Many of their offspring will die from predation, but inevitably a few will survive to adulthood and be able to pass their genes on to the next generation. Though the California red-legged frogs are threatened largely because of habitat loss and pollution, the frogs are also targets of high levels of predation at an early age, making recovery for this species especially difficult. Type III Synthesize Is there any connection between survivorship curves and reproductive strategies? Explain. 14.3 Formative Assessment REVIEWING O M A I N IDEAS CRITICAL T H I N K I N G 1- A shoreline mussel species has a population density o f o n e organism per square meter. Will all mussels b e found o n e m e t e r apart? Explain. 2. Draw and label a diagram showing t h e three population dispersion patterns. r How d o survivorship curves show three types o f reproductive strategies? •'JJ 4. Analyze W h a t might be t h e advantages o f having a clumped dispersal pattern? 5. Infer An organism has t e n offspring. Two o f t h e s e offspring die each year over a five-year period. Is t h e organism more likely t o be a bird or an insect? Explain. @ SELF-CHECK Online HMD5cience.com PREMIUM CONTENT CONNECT TO ABIOTIC FACTORS 6. O n t h e African savannah, what types o f abiotic factors may lead t o high population density and clumped dispersion patterns? Chapter 14: Interactions in Ecosystems 413 Population Growth Patterns I KEY C O N C E P T VOCABULARY Populations grow in predictable patterns. immigration M A I N IDEAS emigration O Changes in a population's size are determined by immigration, births, emigration, and deaths. exponential growth logistic growth O Population growth is based on available resources. carrying capacity O Ecological factors limit population growth. population crash limiting factor density-dependent limiting factor - Connect to Your World T h at ba na na y o u left in y o u r backpack did n o t g o unnoticed. After o n e week, you density-independent limiting factor o p e n y o u r bag a n d d o z e n s o f tiny insects swarm out. T h e smell o f rotting fruit follows c l o s e behind. O n l y a w e e k ago, t h e p o p u l a t i o n o f fruit flies in y o u r backpad was zero. Just b e f o r e y o u o p e n e d it, t h e p o p u l a t i o n h a d grown t o several dozen. How did this p o p u l a t i o n grow s o quickly? © M A I N IDEA Changes in a population's size are determined by immigration, births, emigration, and deaths. The size of a population is usually changing. If resources such as food and water are abundant, or plentiful, a population may grow. On the other hand, i: resources are in short supply, the population may decrease in size. Hopefully, the normal fruit fly population in your backpack is zero. But if an abundance of resources, such as an overripe banana, becomes available, the population will increase dramatically. However, when the resources are removed, the fruit fly population in your backpack will once again return to zero. Four factors affect the size of a population. Immigration When one or two fruit flies found the banana, they immi grated into your backpack. Immigration is the movement of individuals into a population from another population. • Births Additional fruit flies were born in your backpack. Births increase the number of individuals in a population. • Emigration After you opened your backpack, some fruit flies flew out and left to find other rotting fruit. Emigration is the movement of individuals out of a population and into another population. • Deaths You might have squashed a couple of unlucky fruit flies as you were opening your backpack. The size of a population decreases when individuals die. • 1 R E A D I N G TOOLBOX VOCABULARY The word immigrate c o m e s immigrare, f r o m t h e Latin w o r d m e a n i n g " t o g o into," a n d t h e word emigrate c o m e s f r o m emigrare, t h e Latin w o r d meaning "to move." ~ \ Apply When a population is declining, what t w o factors are likely outpacing what other t w o factors? q M A I N IDEA population growth is based on available resources. population growth is a function of the environment. The rate of growth for a population is directly determined by the amount of resources available. A population may grow very rapidly, or it may take a bit of time to grow. There are two distinct types of population growth. Exponential Growth When resources are abundant, a population has the opportunity to grow rapidly. This type of growth, called exponential growth, occurs when a population size increases dramatically over a period of time. In FIGURE 4.1, you can see that exponen tial growth appears as a J-shaped curve. FIGURE 4.1 EXPONENTIAL GROWTH 1000 J i 800 j2 6 0 0 j5 2 0 i. JO 400 Q > E 200 3 v / z Exponential growth may occur o Time when a species moves to a previously uninhabited area. For example, in 1859 an Australian landowner returning home from England brought 24 European rabbits to the country for the purpose of sport hunting. The rabbits were introduced into an environment that had abundant space and food and no predators fast enough to catch them. The initial population of 24 rabbits grew exponentially and spread across the country. After many attempts to control the population, today there are between 200 million and 300 million rabbits in Australia. y / FIGURE 4.2 In Australia during the early 1900s, the introduced European rabbit population exhibited exponential growth. J 1 Logistic Growth FIGURE 4.3 LOGISTIC GROWTH Most populations face limited resources and thus show a logistic growth rate. During logistic growth, carr /ing a population begins with a period capa city of slow growth followed by a brief C o period of exponential growth before JS3 X*/ leveling off at a stable size. A graph of £L Jo /y O < Q. logistic growth takes the form of an y S-shaped curve and can be seen in FIGURE 4.3, which models a populations change in size over time. During initial Time growth, resources are abundant, and the population is able to grow Over time, resources begin to deplete, and growth starts to slow. As resources become limited, the population levels off at a size the environment can support. / Chapter 14: Interactions in Ecosystems 415 What Limits Population Growth? Carrying Capacity The environment determines how many individuals of the species can be supported based on natural cycles and species diversity. An environment, therefore, has a carrying capacity for each species living in it. The carrying capacity of an environment is the maximum number of individuals of a particular species that the environment can normally and consistently support. In nature, a carrying capacity can change when the environment changes. Consider a population of grasshoppers that feed on meadow grasses. If a fire burns part of the meadow, the insects' food resources diminish, and the carry ing capacity declines. But during years with plentiful rain, the meadow grasses flourish, and the carrying capacity rises. The actual size of the population usually is higher or lower than the carrying capacity. Populations will rise and fall as a result of natural changes in the supply of resources. In this way, the environment naturally controls the size of a population. Population Crash When the carrying capacity for a population suddenly drops, the population experiences a crash. A population crash is a dramatic decline in the size of a population over a short period of time. There are many reasons why a popula tion might experience a crash. DATA ANALYSIS READING C O M B I N A T I O N GRAPHS Combination graphs s h o w t w o sets o f d a t a o n t h e s a m e graph. O n e s e t o f d a t a may b e s how n as a bar graph, while t h e o t h e r s e t may b e s how n as a line graph. T h e t w o d a t a sets m u s t share t h e s a m e i n d e p e n d e n t variable o n t h e x-axis. Scientists can t h e n interpret t h e d a t a t o d e t e r m i n e if a relationship exists b e t w e e n t h e variables. GRAPH 1. FLORIDA FISH KILLS 1991-2001 12.5 20 Fish kill e v e n t s Rainfall ( i n c h e s ) _ 15 >Q V) c01 s =• 10 7.5-5 c This combination graph displays d a t a a b o u t fish kill events, during which many fish died a t once, a n d average m o n t h l y rainfall in Florida from 1991-2001. • T h e y - a x i s o n t h e left side represents t h e t o t a l n u m b e r o f fish kill events. • T h e y - a x i s o n t h e right side represents average m o n t h l y rainfall during t h a t t i m e . 5.0*: 2.5 Jan Feb Mar Apr May Jun Jul Aug Sep O c t Nov Dec Months • T h e x-axis s h o w s t h e m o n t h o f d a t a collection. S o u r c e : T h e U n i v e r s i t y o f F l o r i d a Extension 1. Analyze An increase in fish kills and a decrease in rainfall occurs in w h a t months?: 2. Analyze Describe t h e t r e n d in t h e fish kill e v e n t s t h r o u g h o u t t h e year. Describe t h e t r e n d in rainfall d a t a t h r o u g h o u t t h e year. 3. Hypothesize W h a t relationship might exist b e t w e e n fish kill e v e n t s a n d rainfall? Unit 5: Ecology 0.0 I n f o r m a t i o n C i r c u l a r 107. U s e d b y permission. T h e graph s h o w s t h a t in January t h e r e w e r e four fish kill e v e n t s a n d an average o f 2.7 inches o f rain. 416 10.0 For example, in 1944, 29 reindeer were introduced to St. Matthew Island off the coast of Alaska. At the time of the introduction, the entire island was c o v e r e d with a rich mat of lichens. Plenty of good food allowed the reindeer herd to grow at an exponential rate. By the summer of 1963, the island popula tion had grown to 6000 reindeer. However, over the winter, large amounts of snow fell on food resources that had already become greatly depleted by the large herd. By the spring of 1964, only 50 reindeer remained. The population crash on St. Matthew Island came as the result of two factors that limited resources: the harsh winter and the scarcity of food. P r e d i c t W h a t would have eventually happened t o the reindeer herd i f the winter had not made foraging so difficult? Explain. Q M A I N IDEA Ecological factors limit population growth. Many factors can affect the carrying capacity of an environment for a popula tion of organisms. The factor that has the greatest effect in keeping down the size of a population is called the limiting factor. There are two categories of limiting factors—density dependent and density independent. FIGURE 4.4 Taking down prey as large as a moose requires that the members o f a pack work together. As many as ten wolves may take hours o r even days t o wear down this moose. Density-Dependent Limiting Factors Density-dependent limiting factors are limiting factors that are affected by the number of individuals in a given area. Density-dependent limiting factors include many different types of species interactions. Competition Members of populations compete with one another for resources such as food and shelter. As a population becomes denser, the resources are used up, limiting how large the population can grow. Predation The population of a predator can be limited by the available prey, and the population of prey can be limited by being caught for food. On Isle Royale in Michigan, changes in wolf and moose populations, shown in FIGURE 4.5, provide an example. As the moose population grows, so does the wolf population. But at a certain point, the wolves eat so many moose that there are not enough left to feed all the wolves. The result is a de crease in the wolf population. Over time, the two popula tions rise and fall in a pattern, shown in FIGURE 4.5. FIGURE 4.5 D E N S I T Y - D E P E N D E N T L I M I T I N G F A C T O R S Parasitism and disease Parasites and diseases can spread more quickly through dense populations. The more crowded an area becomes, the easier it is for parasites or diseases to spread. The parasites or diseases can then cause the size of the population to decrease. Analyze How does the w o l f population on Isle Royale affect the carrying capacity o f the moose population? S o u r c e : Isle R o y a l e R e s e a r c h D a t a Chapter 14: Interactions in Ecosystems 417 Density-Independent Limiting Factors Density-independent limiting factors are the aspects of the environment that limit a populations growth regardless of the density of the population. Unusual weather Weather can affect the size of a population regardless of its density. For example, along the western coast of the United States, a lack of southerly winds can prevent nutrientpoor warm water from being replaced, as it normally is, with nutrient-rich cold water. The lack of nutrients in the water along the coast can prevent phytoplankton, which form the base of the marine ecosystem, from growing in their usual large numbers. In turn, zooplankton, tiny organisms that feed on phytoplankton, have smaller populations. The effects are felt all the way up the food chain, with smaller populations offish and birds. Natural disasters Volcanoes, tsunamis, tornados, and hurricanes, shown in FIGURE 4.6, can wipe out populations regardless of density. For example, the FIGURE 4.6 The storm surge accompanying a hurricane can cause dangerous flooding. large wave of a tsunami can damage fragile coral reefs, knock down entire mangrove forests, and destroy sea turtle nesting beaches. Human activities Destruction of a wetland habitat along the Platte River in Nebraska has threatened an important feeding ground for the sandhill crane. Urbanization in this area is depleting the resources these migratory birds need during their trek to nesting grounds in northern Canada and in Alaska. By clearing forests, filling wetlands, and polluting the air, land, and water, humans threaten habitats and the organisms that live in them. As we will discuss in Human Impact on Ecosystems, human influence as a limiting factor has had a profound effect on populations. For example, the introduction of nonnative species has caused population crashes in many parts of the world where biodiversity is an important part of the ecosystems functioning. Apply A population of algae in a pond is limited in size by the amount of sunlight that strikes the pond's surface. Is sunlight a density-dependent or density-independent limiting factor for the algae population? •EQ Formative Assessment REVIEWING O MAIN IDEAS CRITICAL THINKING 1. W h a t four factors determine t h e growth rate o f a population? 4. Apply W h a t might cause exponential growth t o occur only 2. How d o e s carrying capacity affect t h e size o f a population? 3. What is t h e main difference between a! factor and a density-independent limiting factor? Give examples o f each. for a s hort period when a new species is introduced t o a resourcefilled environment? 5. Synthesize How might densityd e p e n d e n t limiting factors b e affected by a flood or s o m e o t h e r natural disaster? Barwsisiviaia SYMBIOSIS 6. Give an example o f how a symbiotic relationship c o u l d cause a population crash. Ecological Succession VOCABULARY I 1 succession primary succession I pioneer species Ecological succession is a process o f change in t h e species that make up a community. KEY C O N C E P T M A I N IDEAS O Succession occurs following a disturbance in an ecosystem. secondary succession , / >,<- Connect to Your World — ——— It begins with a dirty sock. T h e n a discarded h o m e w o r k assignment. But this is only t h e start. If y o u have ever s p e n t a Saturday a f t e r n o o n cleaning y o u r b e d r o o m , y o u may have w o n d e r e d h o w a perfectly clean r o o m c o u l d m a n a g e t o b e c o m e such a c l u t t e r e d mess. A clean r o o m b e c o m i n g c l u t t e r e d is a gradual process m u c h like t h e process t h a t rebuilds d a m a g e d e c os ys te ms . © M A I N IDEA Succession occurs following a disturbance in an ecosystem. After an ecosystem experiences a devastating catastrophe and begins to regrow, the space re-forms itself through a process known as succession. Succession is the sequence of biotic changes that regenerate a damaged community or create a community in a previously uninhabited area. FIGURE 5.1 The path o f a lava flow, like this one on the island o f Hawaii (left), leaves behind noth ing but solid rock. Over time, pri mary succession will turn this harsh landscape into a fertile eco system (right). The Hawaiian Islands began to form more than 70 million years ago. Over time, volcanic eruptions like the one shown in FIGURE 5.1 created these islands in the middle of the Pacific Ocean. Eventually, the bare volcanic rock began to break down into soil, which provided a place for plants to grow. As time passed, the process of succession created unique tropical ecosystems. Succession from bare rock to such highly diverse vegetation takes a great deal of time. FIGURE 5.2 Primary S u c c e s s i o n M e l t i n g glaciers, volcanic eruptions, landslides, and strip mines can all begin t h e process o f primary succession. 0 - 1 5 y e a r s Moss, 1 5 - 8 0 y e a r s Shrubs, lichens, grasses : 80-115 y e a r s Transi 1115-200 y e a r s cottonwoods, alder tion to forest, alder, Hemlock-spruce ! forest thicket ; spruce Glacier Bay National Park in Alaska has given scientists an opportunity t o witness primary succession as the glacier recedes. Apply W h a t function might t h e mosses and lichens serve in primary succession? Primary Succession One of the best ways to understand succession is to watch it progress. Primary succession is the establishment and development of an ecosystem in an area that was previously uninhabited. The first organisms that live in a previously uninhabited area are called pioneer species. Typical examples of pioneer species are lichens and some mosses, which can break down solid rock into smaller pieces. The process of primary succession, which is illustrated in FIGURE 5.2, follows this basic pattern: CONNECT TO SYMBIOSIS A lichen is actually t w o completely different species. Fungus and algae form a symbiotic relationship in which the fungi collects water, while the algae uses chlorophyll t o conduct photosynthesis and synthesize food for the lichen community. 420 Unit 5: Ecology • Bare rock is exposed by a retreating glacier or is created when lava cools. Wind, rain, and ice begin to break down the surface of the rock, forming cracks and breaking the rock into smaller pieces. • Lichen and moss spores are blown in by wind. As they grow, they break up the rock further. When they die, their remains mix with the rock pieces to form a thin layer of soil. • Over time, seeds are blown into the area or are dropped by birds. Small flowers and hardy shrubs grow from these seeds. These new plants provide a habitat for small animals, break up the rock with their roots, and add material to the soil when they die. • As the soil continues to grow thicker, small trees take root, and different animals move into the area. These trees provide shade. • Different tree species take root in the shade and eventually replace the original trees, which need direct sunlight to thrive. following a f l o o d o r a fire, is given a chance f o r n e w life. Plants r e m a i n i n g a f t e r t h e disturbance r e e s t a b l i s h t h e ecosystem. 0 - 2 y e a r s Horse weed, crabgrass, asters 2 - 1 8 y e a r s Grass, shrubs, pine seedlings 1 8 - 7 0 y e a r s Pine 7 0 - 1 0 0 y e a r s Oak- forest and young hardwood seedlings hickory forest forests return nutrients t o t h e soil. Secondary succession uses these nutrients t o grow. Analyze W h y does secondary succession take less t i m e than primary succession? Secondary Succession Succession does not always begin from bare rock. More often, a disturbance, such as a fire or hurricane, halts the progress of succession or destroys an established community. Secondary succession, which is illustrated in FIGURE 5.3, is the reestablishment of a damaged ecosystem in an area where the soil was left intact. Plants and other organisms that remain start the process of regrowth. There is no end to secondary succession. Small disturbances, such as a tree falling, start the process again and again. The dynamic processes of succession are always changing the face of an ecosystem. Connect Where might succession occur in the ocean? Formative Assessment REVIEWING O 1. How is M A I N IDEAS primary succession different from secondary succession? 2. Why are pioneer species so impor t a n t for primary succession? CRITICAL T H I N K I N G 3. Infer Does t h e process o f primary succession take longer in tropical o r arctic areas? Explain. 4. Predict During succession, w h a t might b e c o m e t h e limiting factor for sun-loving mosses as taller plants begin t o grow? ® SELF-CHECK Online HMD5cience.com PREMIUM CONTENT s , CONNECT TO BIOLOGICAL NICHE 5. At what point during primary succession does an ecosys t e m provide t h e fewest habitats for organisms? Explain your reasoning. 14 Summary Review Games • Concept Map • Section Self-Checks KEY CONCEPTS 14.4 Population Growth Patterns 14.1 Habitat and Niche Every organism has a habitat and a niche. Each organ Populations grow in predictable patterns. ism in an ecosystem has an ecological niche, which includes t h e t y p e o f food it consumes, its behavior, and its habitat—the place where it lives. Competitive exclusion prevents t w o species from sharing t h e same niche. In different geographical regions, ecologi cal equivalents may have similar ecological niches. Population growth accommodates changes in population size d u e t o births and deaths as well as immigration and emigration. Populations experiencing exponential growth increase dramatically overtime. When resources become a limiting factor, a population will grow logistically until it reaches the environmental carrying capacity, or t h e maximum population size t h e environment can support. Density-dependent limiting factors affect dense populations, but densityindependent limiting factors affect populations regardless o f density. 14.2 Community Interactions Organisms interact as individuals and as populations. Interactions between species include competition and predation. Interactions shape ecosystem dynam ics. Parasitism, commensalism, and mutualism are symbiotic relationships involving t w o species living in direct c o n t a c t with o n e another. 14.3 Population Density and Distribution Each population has a density, a dispersion, and a reproductive strategy. The distribution o f a popula tion can be measured by population density. Species can have clumped, uniform, or random dispersion patterns. Survivorship curves describe t h e reproduc tive strategies o f different species. • e® s e • ® o ® | Q © • e • e • @ Ecological succession is a process o f change in the species that make up a community. Succession refers t o t h e progression o f plants and animals that repopulate a region after an ecological disturbance. Primary succession begins in a previously uninhabited area, such as bare rock exposed by t h e receding of a glacier or created by a volcanic eruption. Secondary succession occurs in a previously inhabited area that is damaged by an ecological disturbance, such as a fire o r a flood. Random dispersion Uniform dispersion Clumped dispersion 14.5 Ecological Succession • • ® READING TOOLBOX SYNTHESIZE Y O U R N O T E S Concept Map Use a c o n c e p t map t o display t h e differences b e t w e e n exponential and logistic growth. Main Idea Chart Use t h e main idea chart t o explain and give examples o f density-independent and density-depen d e n t limiting factors. Density Independent when Density Dependent u n t i l i t reaches J J 422 Unit 5: Ecology