Ecology UNIT

landscape level, and microclimate, very fine, localizedpatterns, such as those encountered by the community oforganisms that live in the microhabitat beneath a fallen log.First, let’s examine Earth’s macroclimate.Ecology is a rigorous experimental science that requires abreadth of biological knowledge. Ecologists observe nature,generate hypotheses, manipulate environmental variables,and observe outcomes. In this chapter, we’ll first considerhow Earth’s climate and other factors determine the locationof major life zones on land and in the oceans. We’ll then examine how ecologists investigate what controls the distribution of species. The next four chapters focus on population,community, ecosystem, and global ecology, as we explorehow ecologists apply biological knowledge to predict theglobal consequences of human activities and to conserveEarth’s biodiversity.CO N C E P TGlobal Climate PatternsGlobal climate patterns are determined largely by the inputof solar energy and Earth’s movement in space. The sunwarms the atmosphere, land, and water. This warming establishes the temperature variations, cycles of air and watermovement, and evaporation of water that cause dramaticlatitudinal variations in climate. Figure 52.3 summarizesEarth’s climate patterns and how they are formed.52.1Regional and Local Effects on ClimateEarth’s climate varies by latitude andseason and is changing rapidlyClimate patterns include seasonal variation and can bemodified by other factors, such as large bodies of water andmountain ranges. We will examine each of these factors inmore detail.The most significant influence on the distribution of organisms on land and in the oceans is climate, the long-termprevailing weather conditions in a given area. Four physicalfactors—temperature, precipitation, sunlight, and wind—are particularly important components of climate. In thissection, we’ll describe climate patterns at two scales:macroclimate, patterns on the global, regional, andSeasonalityAs described in Figure 52.4, Earth’s tilted axis of rotationand its annual passage around the sun cause strong seasonalcycles in middle to high latitudes. In addition to these globalMarch equinox: Equator faces sun directly;neither pole tilts toward sun; all regions onEarth experience 12 hours of daylightand 12 hours of darkness.December solstice:Northern Hemisphere tiltsaway from sun and hasshortest day and longestnight; Southern Hemispheretilts toward sun and haslongest day and shortest night.60 NConstant tiltof 23.5 30 NJune solstice: Northern Hemispheretilts toward sun and has longest dayand shortest night; SouthernHemisphere tilts away from sun andhas shortest day and longest night.0 (equator)30 S Figure 52.4 Seasonal variation in sunlight intensity. Because Earth is tilted onits axis relative to its plane of orbit around the sun, the intensity of solar radiation variesseasonally. This variation is smallest in the tropics and increases toward the poles.c h a p t e r 5 2September equinox: Equator faces sundirectly; neither pole tilts toward sun; allregions on Earth experience 12 hours ofdaylight and 12 hours of darkness.An Introduction to Ecology and the Biosphere     1161

changes in day length, solar radiation, and temperature, thechanging angle of the sun over the course of the year affectslocal environments. For example, the belts of wet and dryair on either side of the equator move slightly northwardand southward with the changing angle of the sun, producing marked wet and dry seasons around 20 north and 20 south latitude, where many tropical deciduous forests grow.In addition, seasonal changes in wind patterns alter oceancurrents, sometimes causing the upwelling of cold waterfrom deep ocean layers. This nutrient-rich water stimulatesthe growth of surface-dwelling phytoplankton and the organisms that feed on them. These upwelling zones makeup only a few percent of ocean area but are responsible formore than a quarter of fish caught globally.Bodies of WaterOcean currents influence climate along the coasts of continents by heating or cooling overlying air masses that passacross the land. Coastal regions are also generally wetterthan inland areas at the same latitude. The cool, misty climate produced by the cold California Current that flowssouthward along western North America supports a coniferous rain forest ecosystem along much of the continent’sPacific coast and large redwood groves farther south. Conversely, the west coast of northern Europe has a mild climatebecause the Gulf Stream carries warm water from the equator to the North Atlantic (Figure 52.5). As a result, northwestern Europe is warmer during winter than southeasternCanada, which is farther south but is cooled by the LabradorCurrent flowing south from the coast of Greenland.Because of the high specific heat of water (see Concept 3.2),oceans and large lakes tend to moderate the climate ofnearby land. During a hot day, when land is warmer thanthe water, air over the land heats up and rises, drawing acool breeze from the water across the land (Figure 52.6).In contrast, because temperatures drop more quickly overland than over water at night, air over the now warmerwater rises, drawing cooler air from the land back out overthe water and replacing it with warmer air from offshore.This local moderation of climate can be limited to the coastitself, however. In regions such as southern California andsouthwestern Australia, cool, dry ocean breezes in summerare warmed when they contact the land, absorbing moistureand creating a hot, arid climate just a few kilometers inland(see Figure 3.5). This climate pattern also occurs around theMediterranean Sea, which gives it the name Mediterraneanclimate.LabradorCurrentCalifornia Current30 NGulf StreamNorth PacificSubtropical GyreNorth AtlanticSubtropical GyreEquatorIndianOceanSubtropicalGyre30 SSouth PacificSubtropical GyreAntarctic Circumpolar Current Figure 52.5 Global circulation of surface water in the oceans. Water is warmed at theequator and flows north and south toward the poles, where it cools. Note the similarities betweenthe direction of water circulation in the gyres and the direction of the trade winds in Figure 52.3.1162    U n i te i g h t   EcologySouthAtlanticSubtropicalGyre

Figure 52.6 How large bodies ofwater and mountains affect climate.This figure illustrates what can happen ona hot summer day.2 Air that encounters mountainsflows upward, cools at higher altitudes, andreleases water as precipitation.1 Cool air flows inland from the water,Leeward sideof mountainsmoderating temperatures near the shore.3 Less moisture is left in the airreaching the leeward side, whichtherefore has little precipitation.This rain shadow can create adesert on the back side of themountain range.MountainrangeOceanMountainsLike large bodies of water, mountains influence air flow overland. When warm, moist air approaches a mountain, the airrises and cools, releasing moisture on the windward side ofthe peak (see Figure 52.6). On the leeward side, cooler, dry airdescends, absorbing moisture and producing a “rain shadow.”This leeward rain shadow determines where many deserts arefound, including the Great Basin and the Mojave Desert ofwestern North America and the Gobi Desert of Asia.Mountains also affect the amount of sunlight reachingan area and thus the local temperature and rainfall. Southfacing slopes in the Northern Hemisphere receive more sunlight than north-facing slopes and are therefore warmer anddrier. These physical differences influence species distributions locally. In many mountains of western North America,spruce and other conifers grow on the cooler north-facingslopes, but shrubby, drought-resistant plants inhabit thesouth-facing slopes. In addition, every 1,000-m increasein elevation produces an average temperature drop of 6 C,equivalent to that produced by an 880-km increase in latitude. This is one reason that high-elevation communities atone latitude can be similar to those at lower elevations muchfarther from the equator.MicroclimateMany features in the environment influence microclimateby casting shade, altering evaporation from soil, or changing wind patterns. Forest trees often moderate the micro climate below them. Cleared areas therefore typicallyexperience greater temperature extremes than the forestinterior because of greater solar radiation and wind currentsthat arise from the rapid heating and cooling of open land.Within a forest, low-lying ground is usually wetter thanhigher ground and tends to be occupied by different treespecies. A log or large stone can shelter organisms such assalamanders, worms, and insects, buffering them from theextremes of temperature and moisture.Every environment on Earth is characterized by a mosaicof small-scale differences in chemical and physical attributes, such as temperature, light, water, and nutrients. Theseabiotic, or nonliving, factors influence the distribution andabundance of organisms. Later in this chapter, we’ll alsoexamine how all of the biotic, or living, factors—the otherorganisms that are part of an individual’s environment—similarly influence the distribution and abundance of life onEarth.Global Climate ChangeBecause climatic variables affect the geographic ranges ofmost plants and animals, any large-scale change in Earth’sclimate profoundly affects the biosphere. In fact, such alarge-scale climate “experiment” is already under way, atopic we’ll examine in more detail in Concept 56.4. Theburning of fossil fuels and deforestation are increasing theconcentrations of carbon dioxide and other greenhousegases in the atmosphere. As a result, Earth has warmed anaverage of 0.8 C (1.4 F) since 1900 and is projected to warm1–6 C (2–11 F) more by the year 2100.One way to predict the possible effects of future climatechange on geographic ranges is to look back at the changesthat have occurred in temperate regions since the last iceage ended. Until about 16,000 years ago, continental glaciers covered much of North America and Eurasia. As theclimate warmed and the glaciers retreated, tree distributionsexpanded northward. A detailed record of these changesis captured in fossil pollen deposited in lakes and ponds.If researchers can determine the climatic limits of currentdistributions of organisms, they can make predictions abouthow those distributions may change with continued climaticwarming.c h a p t e r 5 2An Introduction to Ecology and the Biosphere     1163

American beech Sugar mapleA question when applying this approach to plants iswhether seeds can disperse quickly enough as climatechanges. Fossil pollen shows that species with winged seedsthat disperse relatively far from a parent tree, such as thesugar maple (Acer saccharum), expanded rapidly northwardafter the last ice age ended. In contrast, the northward rangeexpansion of the American beech (Fagus grandifolia), whoseseeds lack wings, was delayed for thousands of years compared with the shift in suitable habitat.Will plants and other species be able to keep up with themuch more rapid warming projected for this century? Ecologists have attempted to answer this question for the Americanbeech. Their models predict that the northern limit of thebeech’s range may move 700–900 km northward in the nextcentury, and its southern range limit will shift even more. Thecurrent and predicted geographic ranges of this species undertwo different climate-change scenarios are illustrated inFigure 52.7. If these predictions are even approximately correct, the beech’s range must shift 7–9 km northward peryear to keep pace with the warming climate. However, sincethe end of the last ice age, the beech has moved at a rate ofonly 0.2 km per year. Without human help in moving to newhabitats, species such as the American beech may have muchsmaller ranges or even become extinct.Changes in the distributions of species are already evident in many well-studied groups of terrestrial, marine, andfreshwater organisms, consistent with the signature of awarmer world. For example, 22 of 35 European butterflyspecies studied have shifted their ranges farther north by35–240 km in recent decades. Other research shows thata Pacific diatom species, Neodenticula seminae, recentlyhas colonized the Atlantic Ocean for the first time in800,000 years. As Arctic sea ice has receded in the past decade, the increased flow of water from the Pacific has sweptthese diatoms around Canada and into the Atlantic, wherethey quickly became established. In the next section, we’llcontinue to examine the importance of climate in determining species distributions around the world.C o n c e p t C h e ck 5 2 . 11. Explain how the sun’s unequal heating of Earth’s surfaceleads to the development of deserts around 30 northand south of the equator.2. What are some of the differences in microclimate between an unplanted agricultural field and a nearbystream corridor with trees?3.wh a t I F ? Changes in Earth’s climate at the end ofthe last ice age happened gradually, taking centuriesto thousands of years. If the current global warminghappens very quickly, as predicted, how may this rapidclimate change affect the evolution of long-lived treescompared with that of annual plants, which have muchshorter generation times?4.m a k e c o n n e c t i o n s Focusing just on the effectsof temperature, would you expect the global distributionof C4 plants to expand or contract as Earth becomeswarmer? Why? (See Concept 10.4.)For suggested answers, see Appendix A.CO N C E P T52.2The structure and distribution ofterrestrial biomes are controlled byclimate and disturbanceThroughout this book, you have seen many examples of howclimate and other factors influence where individual speciesare found (see Figure 30.7, for instance). We turn now tothe role of climate in determining the nature and location ofEarth’s biomes, major life zones characterized by vegetationtype in terrestrial biomes or by the physical environment inaquatic biomes.Climate and Terrestrial Biomes(a) Current range 1989 AAAS(b) 4.5 C warmingover next century(c) 6.5 C warmingover next century Figure 52.7 Current range and predicted ranges for theAmerican beech under two climate-change scenarios.? The predicted range in each scenario is based on climate factorsalone. What other factors might alter the distribution of this species?1164    U n i te i g h t   EcologyBecause climate has a strong influence on the distribution of plant species, it is a major factor in determining thelocations of terrestrial biomes (Figure 52.8). One way tohighlight the importance of climate on the distribution ofbiomes is to construct a climograph, a plot of the annualmean temperature and precipitation in a particular region.

Tropical forestSavannaDesert30 NChaparralTropic ofCancerTemperate grasslandEquatorTemperate broadleafforestNorthern coniferousforestTundraTropic ofCapricorn30 SHigh mountainsPolar ice Figure 52.8The distribution of majorterrestrial biomes.Figure 52.9 is a climograph for some of the biomes foundGeneral Features of Terrestrial BiomesMost terrestrial biomes are named for major physical orclimatic features and for their predominant vegetation.Temperate grasslands, for instance, are generally found inmiddle latitudes, where the climate is more moderate thanin the tropics or polar regions, and are dominated by variousgrass species (see Figure 52.8). Each biome is also characterized by microorganisms, fungi, and animals adapted to thatparticular environment. Temperate grasslands are usuallymore likely than temperate forests to be populated by largegrazing mammals and to have arbuscular mycorrhizal fungi(see Figure orestTropical forestAnnual mean temperature ( C)in North America. Notice, for instance, that the range ofprecipitation in northern coniferous and temperate forestsis similar but that temperate forests are generally warmer.Grasslands are typically drier than either kind of forest, anddeserts are drier still.Factors other than mean temperature and precipitationalso play a role in determining where biomes exist. Someareas in North America with a particular combination oftemperature and precipitation support a temperate broadleafforest, but other areas with similar values for these variablessupport a coniferous forest (see the overlap in Figure 52.9).How might we explain this variation? One thing to remember is that the climograph is based on annual averages. Often,however, the pattern of climatic variation is as important asthe average climate. Some areas may receive regular precipitation throughout the year, whereas other areas may havedistinct wet and dry seasons.30Northernconiferousforest150Arctic andalpinetundra–150100200300Annual mean precipitation (cm)400 Figure 52.9 A climograph for some major types of biomesin North America. The areas plotted here encompass the ranges ofannual mean temperature and precipitation in the biomes.I n t e r p r e t t he D a t a Some arctic tundra ecosystems receive aslittle rainfall as deserts but have much more dense vegetation. What climatic factor might explain this difference? Explain.c h a p t e r 5 2An Introduction to Ecology and the Biosphere     1165

Although Figure 52.8 shows distinct boundaries betweenthe biomes, terrestrial biomes usually grade into neighboring biomes, sometimes over large areas. The area of intergradation, called an ecotone, may be wide or narrow.Vertical layering of vegetation is an important feature ofterrestrial biomes. In many forests, the layers from top tobottom consist of the upper canopy, the low-tree layer, theshrub understory, the ground layer of herbaceous plants, theforest floor (litter layer), and the root layer. Nonforest biomeshave similar, though usually less pronounced, layers. Layeringof vegetation provides many different habitats for animals,which sometimes exist in well-defined feeding groups, fromthe insectivorous birds and bats that feed above canopies tothe small mammals, numerous worms, and arthropods thatsearch for food in the litter and root layers below.The species composition of each kind of biome variesfrom one location to another. For instance, in the northernconiferous forest (taiga) of North America, red spruce iscommon in the east but does not occur in most other areas,where black spruce and white spruce are abundant. AsFigure 52.10 shows, cacti living in deserts of North andSouth America appear very similar to plants called euphorbsfound in African deserts. But since cacti and euphorbs belong to different evolutionary lineages, their similarities aredue to convergent evolution (see Figure 22.18).mixed swamp forests in the area shifted toward a dominanceof baldcypress (Taxodium distichum) and water tupelo(Nyssa aquatica) because these species are less susceptibleto wind damage than other tree species found there. As aresult of disturbances, biomes are often patchy, containingseveral different communities in a single area.In many biomes, even the dominant plants depend onperiodic disturbance. Natural wildfires are an integral component of grasslands, savannas, chaparral, and many coniferous forests. Before agricultural and urban development,much of the southeastern United States was dominated bya single conifer species, the longleaf pine. Without periodicburning, broadleaf trees tended to replace the pines. Forestmanagers now use fire as a tool to help maintain many coniferous forests.Figure 52.11 summarizes the major features of terrestrial biomes. As you read about the characteristics of eachbiome, remember that humans have altered much of Earth’ssurface, replacing natural communities with urban and agricultural ones. The central United States, for example, isclassified as grassland and once contained extensive areasof tallgrass prairie. Very little of the original prairie remainstoday, however, having been converted to agriculture.Disturbance and Terrestrial BiomesC o n c e p t C h e ck

amine how ecologists investigate what controls the distribu-tion of species. The next four chapters focus on population, community, ecosystem, and global ecology, as we explore how ecologists apply biological knowledge to predict the global consequences of human activities and to conserve Earth’s biodiversity. co NcEPT 52.1