O

Ocean Currents and Their Role in the BiosphereA Ganopolski, Potsdam Institute for Climate Impact Research, Potsdam, Germany

ª 2008 Elsevier B.V. All rights reserved.

Introduction

The Ocean Currents, Climate, and Biosphere

Past and Future Changes in Ocean Circulation

Impact of Ocean Circulation Changes on Biosphere

Summary

Further Reading

Introduction

This article presents an overview of the role of theocean currents in the climate systems and the waysthey affect terrestrial and marine ecosystems. The firstsection discusses how the ocean currents influence glo-bal and regional climate. The second section discussespaleoclimate evidences of the past instability of theocean circulation and future modeling projections ofthe ocean circulation changes under global warming.The third section discusses results of model simulationsand paleoclimate evidences of the impact of reorgani-zations of the ocean currents on terrestrial and marineecosystems.

The Ocean Currents, Climate, andBiosphere

The Role of the Ocean Currents in the ClimateSystem

Modern ocean circulation represents a complex three-dimensional phenomenon which is determined by theEarth’s geography and spatial patterns of surface wind,and surface heat and freshwater fluxes. Surface oceancurrents are directly driven by wind and the existence oflarge-scale oceanic gyres (Figure 1) is explained byprevailing westerlies in the mid-latitudes and tradewinds in the tropics. The divergence of surface wind-driven currents creates upward vertical movement ofwater (upwelling), which plays an important role innutrients supply to the upper ocean layer. Apart fromthat, winds and tidal energy are the primary sources ofvertical mixing in the ocean interior. Without verticalmixing provided by wind and tides, the deep ocean

would be essentially stagnant. Surface fluxes of heatand freshwater, although do not represent a directenergy source for the ocean currents, play an importantrole in controlling the ocean circulation by changing seawater temperature and salinity. The latter determinehorizontal density gradient which drives the currentsin the ocean interior.

The balance between surface heat and freshwaterfluxes also determines the areas where the deep oceanwater masses are formed. Currently, these deep watermasses are formed in several isolated locations: in theNordic Seas and the Labrador Sea in the NorthAtlantic, and around Antarctica. Although the areas ofdeep water formation occupy only a small fraction ofthe ocean, they play a fundamental role in drivingof the meridional overturning circulation, also knownas the ocean thermohaline circulation or ‘the oceanconveyor belt’. The upper branch of the ocean con-veyor is represented by the northward transport ofwarm water masses along the surface currents ofwhich Gulf Stream is the most prominent one(Figure 1). When reaching high latitudes of theNorth Atlantic, surface water is cooled down by losingenergy into the atmosphere and eventually reaches thehigh density which allows surface water to sink to thebottom of the ocean. This water then slowly movessouthward along the American continental slope andreaches the Southern Ocean, where it mixes with thedeep water masses formed around Antarctica. It isbelieved that most of deep water eventually rises tothe surface in the Southern Ocean in the areas of wind-driven upwelling, thus closing the conveyor loop. Theexistence of the thermohaline circulation is closelyrelated to the existence of deep water formationsareas. At present, there is no deep water formation in

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Figure 1 A simplified cartoon of the surface (red) and deep ocean currents (blue). The major areas of deep water formation are shown

by ovals. Yellow dots indicate the upper branch and light blue dots indicate the lower branch of the Atlantic thermohaline circulation.

2554 Global Ecology | Ocean Currents and Their Role in the Biosphere

the Pacific Ocean, and, as a result, there is no the

thermohaline circulation in this ocean. The later

explains very different climate conditions in the high

latitudes of the Atlantic and Pacific oceans.Although an average velocity associated with the

meridional overturning circulation is rather small com-

pared to typical velocities of surface ocean currents, the

meridional overturning circulation is responsible for a

large portion of the ocean meridional heat transport.

Currently, about 1 PW of energy (1 PW¼ 1015 W) is

transported northward in the North Atlantic, that is

about one-fifth of the total energy transport in the

atmosphere–ocean system in the Northern

Hemisphere. The influence of the ocean currents on

climate is illustrated by Figure 2a. It shows deviations

of local annual surface air temperature from its zonally

averaged values. It is seen that annual air temperature

over the northern North Atlantic, and, especially over

the Nordic Seas, is much higher than average tempera-

ture for the same latitudes. Thus the main reason for

mild climate conditions over most of Europe is the

existence of vigorous Atlantic thermohaline circulation.

Since the release of heat transported by the oceanic

currents into the atmosphere occurs primary during

winter, this prevents forming of the sea ice in the

high latitudes, and results in a considerable reduction

of the amplitude of seasonal temperature variations. As

shown in Figure 2b, the difference between summer

and winter temperatures over the Western Europe is

much smaller than for the same latitudes in Asia and

North America. All these factors, in combination with a

stable moisture transport from warm North Atlantic,

allow the existence of extended temperate and broad-

leaf forests over most of Europe.

Ocean Currents and Climate Change

The importance of the ocean currents for climate andclimate change has been demonstrated in a number ofmodeling studies, which showed that the Atlantic thermo-haline circulation may change rapidly in response tochange in climatological conditions, such as increasedfreshwater flux into the North Atlantic due to massiveiceberg discharge from surrounding ice sheets, as it hap-pened many times during the glacial age, or due tointensification of atmospheric hydrological cycle andmelting of the Greenland ice sheets, that may happen inthe future as a result of global warming. Changes in thethermohaline circulation, in turn, lead to dramaticchanges in the ocean heat transport and global climate.

Numerical experiments performed with climate modelsdemonstrate that a complete shutdown of the Atlanticthermohaline circulation under present-day climate condi-tions will cause surface air temperature cooling by morethan 10 �C over the Nordic Seas and northwestern Europe.The cooling is caused by cessation of the northward oceanicheat transport into high latitudes and amplified by a south-ward expansion of sea ice margin. The cooling is mostpronounced in winter when it is almost twice strongerthan in annual mean. Changes in the ocean currents notonly affect local temperature but via several oceanic andatmospheric teleconnection mechanisms spread the climatechange over the globe. In particular, the cooling caused bythe shutdown of the thermohaline circulation is simulatedover most of the Northern Hemisphere, although the mag-nitude of cooling in other areas is smaller than that over thenorthern North Atlantic. At the same time, a decrease ofinterhemispheric oceanic heat transport causes a warmingin the Southern Hemisphere, most pronounced in theSouthern Atlantic and around Antarctica. The

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Figure 2 Deviation of the annual mean surface air temperature corrected to elevation effect from the zonal average temperatures (a),

and the magnitude of seasonal variations of surface air temperature (b). The data are from Legates DR and Willmott CJ (1990) Mean

seasonal and spatial variability in global surface air temperature. Theoretical and Applied Climatology 41: 11–21.

Global Ecology | Ocean Currents and Their Role in the Biosphere 2555

reorganization of the thermohaline circulation also affectsthe hydrological cycle. In particular, cooling of surfaceNorth Atlantic reduces evaporation in this area whichcauses a drastic reduction of precipitation over most ofEurope. Another important result of the Atlantic thermoha-line circulation weakening is a southward shift in theposition of the intertropical convergency zone (ITCZ),which is associated with the rain belt around the equator.Shift of ITCZ causes a considerable redistribution of

precipitation in the tropics, with a decrease of precipitationnorth of the equator and an increase south of the equator. Italso affects the strength of subtropical monsoons, withweaker Indian and African monsoons. It is also plausiblethat regime change of the Atlantic circulation can directlyaffect tropical Pacific, in particular, El Nino/SouthernOscillation cycle, which is responsible for a large portionof climate variability in the tropics and affects climate overthe globe. Not all of the aforementioned processes are

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2556 Global Ecology | Ocean Currents and Their Role in the Biosphere

well understood and fundamental limitations of currentclimate models preclude unambiguous conclusion aboutpossibility of the abrupt changes of the oceanic circulationin the future, but a growing body of paleoclimatologicaldata indicates that, at least in the past, rapid and vigorouschanges in the ocean circulation occurred regularly and hada widespread impact on climate and biosphere. Thereby,the possibility of abrupt climate shifts caused by changes ofthe ocean circulation remains one of the concerns related toanthropogenic global warming.

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Figure 3 Paleoclimate records of Greenland temperature

anomalies compared to present-day climate reconstructed from 18O

isotope concentration (blue), atmospheric methane concentration(red), and relative abundance of woody pollen in sediments core in

the southern Italy (green). Vertical dashed lines show a probable

temporal correlation between different records. DO4, DO8, andDO12 marks the Dansgaard–Oeschger events number 4, 8, and 12,

respectively; BA refers to Bølling–Allerød warm event; and YD refers

to Younger Dryas cold event. Greenland and Antarctic data are from

Blunier T and Brook EJ (2001) Timing of millennial-scale climatechange in Antarctica and Greenland during the last glacial period.

Science 291: 109–112. Pollen data are from Watts WA, Allen JRM,

and Huntley B (1996) Vegetation history and palaeoclimate of the

last glacial period at Lago grande di Monticchio, southern Italy.Quaternary Science Reviews 15: 133–153.

Past and Future Changes in OceanCirculation

Paleoclimate Evidences for Instability ofthe Ocean Circulation

The issue of the stability of ocean circulation attracted alarge attention after discovery of abrupt climate changesin the Greenland ice cores in the early 1990s. Theserecords revealed that during the last glacial age, climatewas rather unstable and was characterized by numerousabrupt shifts between cold and relatively warm states(Figure 3). The most prominent abrupt climate changes,known as Dansgaard–Oeschger events, correspond toabrupt warmings in Greenland by 10–15 �C over justseveral years or decades. This finding was corroboratedlater by numerous marine and terrestrial paleoclimaterecords from different locations which revealed abruptclimate changes apparently synchronous with thatobserved in the Greenland ice cores.

The initial idea of W. Broecker that these abrupt cli-mate changes are related to the reorganizations of theAtlantic thermohaline circulations received in recentyears a strong support from the analysis of differentpaleoclimate records and modeling studies. It has beenshown that during the warm phases of the glacial agecorresponding to Dansgaard–Oeschger events, the Atlanticthermohaline circulation was alike its present state andwarm surface currents penetrated far into the high-latitudeNorth Atlantic. During the cold periods, known also as‘stadials’, although the Atlantic thermohaline circulationwas still active, it was less extended to the north, and amuch smaller amount of energy was transported toward theNordic Seas. This caused a substantial southward expansionof the sea ice area and a strong cooling over the NorthAtlantic realm. At last, during periods of massive icebergdischarge into the North Atlantic from the North Americanand other Northern Hemisphere ice sheets, the Atlanticthermohaline circulation was completely shut down overcenturies or even millennium causing the extreme coldclimate conditions.

There is also a growing body of paleoclimateevidences suggesting that climate impact of Dansgaard–Oeschger events was not restricted to the North Atlantic

realm, and abrupt climate changes synchronous with

Dansgaard–Oeschger events, recorded in Greenland,

have been found in many paleoclimate records in

Eurasia, tropics, and the Pacific Ocean. In the tropics,

for example, abrupt climate changes are most pronounced

in the paleoclimate proxies reflecting changes in hydro-

logical conditions (precipitation) and the strength of

summer and winter monsoons. This is fully

consistent with results of model simulations showing a

southward shift of ITCZ and weaker summer Asian mon-

soon for a weaker state of the Atlantic thermohaline

circulation.One of the most convincing arguments for the global-

scale extent of abrupt glacial climate changes is coeval

variations in methane concentration with the temperature

changes in Greenland (Figure 3). Since the major sources

of methane is the boreal and tropical wetlands, strong

excursions in methane concentration comparable in the

magnitude with the difference between glacial and mod-

ern conditions indicate large changes in temperature and

precipitation over a large part of the globe.

Global Ecology | Ocean Currents and Their Role in the Biosphere 2557

Future Changes in Ocean Circulation

It is generally recognized that an increase of atmosphericconcentration of carbon dioxide and other greenhousegases due to anthropogenic activity is the primary causeof observed global warming. While the temperature riseis the most known and well-established aspect of anthro-pogenic climate change, the rising concentration ofgreenhouse gases leads to a number of other changes,such as an intensification of hydrological cycle, changesin probability of extreme weather events, gradual meltingand retreat of the ice sheets and glaciers, shrinking of seaice area, which are already supported by the analysis ofobservational data. It is believed that the ocean circulation,as a rather sensitive and a strongly nonlinear component ofthe climate system, will also undergo considerable changesin the course of anthropogenic global warming. Based onresults of modeling experiments, the Atlantic thermohalinecirculation is considered as the most vulnerable componentof the global ocean circulation and is expected to weakenconsiderably in the future. Two major factors affect theocean circulation under global warming conditions: surfacewarming and freshening. Both factors affect the local seawater density and meridional density gradient, the primaryfactor controlling the strength of the Atlantic thermohalinecirculation. Surface freshening in the high latitude of NorthAtlantic caused by increased precipitation, enhanced riverrunoff, and melting of the Greenland ice sheet will lead to asubstantial decrease of surface density that can hinder theformation of the North Atlantic deep water masses, the keycomponent of the Atlantic thermohaline circulation.

In a number of numerical experiments with coupledclimate models, it was shown that continuous growth ofatmospheric CO2 concentration will lead to a weakening,and, in some models, to a complete shutdown of theAtlantic thermohaline circulation. Some models, however,show only a very modest reduction of the Atlantic thermo-haline circulation during the twenty-first century and donot show a complete shutdown even under a very highCO2 concentration. The models also disagree concerningthe relative role of temperature and salinity changes for thethermohaline circulation change. When assessing theresults obtained with different climate models, it is impor-tant to realize that ocean models are still relatively coarseresolution, and observational data provide no constrain onsensitivity of the oceanic circulation to temperature orsalinity changes, because these changes are still relativelysmall to be detected with confidence. Another importantuncertainty in the prediction of the future of the Atlanticthermohaline circulation is related to the changes in themass balance of the Greenland ice sheet. Model experi-ments indicate that under global warming conditions, anincreased melting of the ice sheet will overwhelm anincrease in precipitation, which implies that Greenlandmay become an important additional freshwater source

for the North Atlantic. The latter will additionally con-tribute to a freshening of the area where deep water massesare formed and to a slowdown of the thermohaline circula-tion. However, it is still unclear whether melting ofGreenland will be fast enough to cause a complete shut-down of the Atlantic thermohaline circulation.

In spite of all these uncertainties, a general consensus isthat in the course of the twenty-first century, the Atlanticthermohaline circulation will weaken, but it is unlikely thatabrupt (on the timescale of several years or decade) shut-down will occur. However, if the concentration ofgreenhouse gases will continue to rise beyond the twenty-first century, a complete shutdown of the Atalntic thermo-haline circulation will become more likely. It is importantto note that although weakening of the Atlantic thermoha-line circulation under global warming is a common featureof many climate models, even a complete shutdown of thethermohaline circulation does not imply immediate coolingor, moreover, entering of a new ice age. Modeling resultssuggest that greenhouse warming will overwhelm the effectof reduced oceanic heat transport and the warming in theNorth Atlantic will continue even in the case of substan-tially reduced thermohaline circulation. This warming,however, is expected to be smaller than in other regionsof the planet. At the same time, it is possible that if thethermohaline circulation weakens considerably, it will takecenturies for its complete recovering.

Impact of Ocean Circulation Changes onBiosphere

Impact of the Atlantic Circulation Change onTerrestrial Ecosystems

As discussed above, changes in the ocean circulation affecttemperature and precipitation worldwide. This has a directimpact on terrestrial ecosystems for which these two climatefactors exert primary control on distribution of terrestrialvegetation and their productivity. Paleoclimate recordsreveal strong and rapid reorganizations of terrestrial ecosys-tems in response to climate changes. For example, Figure 3shows a pollen record from southern Italy during the lastglacial cycle. The record reveals numerous excursions appar-ently coeval with the changes recorded in Greenland, withan abrupt increase of woody pollen during Greenland warmevents and its almost complete disappearance of during coldperiods. Detailed analysis of different pollen species in thisrecord indicates numerous transitions between forest andcold steep ecosystems during the glacial age. Similar changesin methane concentration shown in Figure 3 indicate abruptand dramatic changes in the area and climate conditions oftropical and boreal wetlands synchronous with abrupt cli-mate changes recorded in Greenland.

A strong response of terrestrial ecosystems to the reor-ganizations of the ocean circulation is also supported by

2558 Global Ecology | Ocean Currents and Their Role in the Biosphere

results of modeling experiments performed both forpresent-day and glacial climate conditions. It was shownthat a complete shutdown of the Atlantic thermohalinecirculation would cause a pronounce impact on thedistribution of ecosystems and their net primary produc-tion in different parts of the Earth. In the boreal latitudes ofthe Northern Hemisphere, the primary effect of change inthe ocean circulation is a strong winter cooling and areduction of the length of the growing season which resultsin a southward retreat of boreal forest area and shrinking ofthe area of temperate forest. In more southern locations,where the total amount of precipitation is the primarylimiting factor, a southward shift of ITCZ and a weakeningof summer monsoon lead to an expansion of Sahara desert,shrinking of the area of evergreen forest, and pronounceddecline in productivity in certain areas. It was shown that ifcollapse of the Atlantic thermohaline circulation will occurunder present-day climate conditions, it will result insubstantial reduction of natural ecosystem biomass andnet primary production. It would also have a dramaticimpact on agricultural production, especially in theWestern Europe and in some tropical areas.

Under the glacial climate conditions, in spite of the factthat a large portion of the most sensitive boreal zone wascovered by the ice sheets, changes in the Atlantic thermo-haline circulation still cause a considerable impact onecosystem, their biomass, as well as soil carbon storage.Modeling results indicate that a reduction of terrestrialbiomass might contribute to CO2 rises observed duringHeinrich events, when the coldest climate conditionsprevailed over the North Atlantic due to the collapse ofthe Atlantic thermohaline circulations.

Ocean Currents and Marine Biota

The ocean circulation plays an important role in the oceancarbon cycle and affects marine biota. The most straight-forward way of this influence is via changes in thehorizontal and vertical transport of nutrients, which arethe limiting factor for marine biota productivity over mostof the globe. The highest productivity in the ocean apartfrom shelf areas is observed in the regions of strong verticalupwelling and the areas of deep convection. Both thesemechanisms bring nutrients from the ocean interior to thesurface layer, which is usually extremely depleted withnutrients. While strong upwelling in coastal and equatorialregions is caused primarily by surface wind, the areas ofdeep mixing and upwelling in the ocean interior are closelyrelated to the thermohaline ocean circulation. This is why,changes in the ocean circulation directly, via changes inupwelling and mixing, and indirectly, via changes in sur-face wind, can affect marine ecosystems.

Paleoclimate data revealed strong variations in pro-ductivity in many locations, primarily in the NorthAtlantic, but also in Arabian Sea and coastal Pacific

Ocean areas, apparently synchronous with abrupt climatechanges recorded in Greenland and attributed to thereorganizations of the ocean circulation. At the sametime, paleoclimate data show an enhanced productivityin Iberian and North African coastal areas during coldevents associated with enhanced coastal upwelling inthese regions. Model simulations confirm that shutdownof the Atlantic thermohaline circulation can cause astrong and widespread decline of the marine ecosystemproductivity. In particular, in the northern part of theNorth Atlantic, the collapse of the Atlantic thermohalinecirculation leads to a decrease of planktonic biomass byfactor of 2 while the globally averaged export productiondecreases by 20%. The main cause for such strong reduc-tion of the North Atlantic plankton stock is a shoaling ofwintertime mixed layer which reduces supply of nutrientsfrom relatively nutrient-rich intermediate water massesto the nutrient-depleted surface ocean layer. For the restof the globe, changes in marine biota production arecaused by changes in upwelling and surface winds. Seaice extension and surface cooling can also affect oceanproductivity in the high latitudes. Modeling experimentsalso show that the response time of the marine ecosystemto changes in the Atlantic circulation is the shortest in theNorth Atlantic, while in the Pacific and Indian oceans theresponse time is order of centuries.

Even though a probability of a complete shutdown ofthe Atlantic thermohaline circulation in the futureremains uncertain, a weakening of this circulation and ashoaling of mixed layer due to surface warming andfreshening is the robust feature of majority of climatemodel simulations. This will inevitably lead to a declineof North Alantic plankton stock, which, in turn, will havea serious consequence for the fishery in these, currentlyrelatively productive areas and, eventually, affects thefood supply to the growing population of the planet.

Summary

The ocean currents play a fundamental role in the climatesystem and in a number of ways affect terrestrial andmarine ecosystems and global carbon cycle. Numerouspaleoclimate data indicate that abrupt climate changesobserved in the past were associated with changes in theocean circulation, primarily the Atlantic thermohalinecirculations. Modeling studies demonstrate that, at leaston a regional scale, this effect is very important and this issupported by numerous paleoclimate records. There is aconcern that anthropogenic global warming may cause asubstantial reorganization of the ocean circulation, whichwill not only negatively affect natural ecosystem, but maycause a considerable impact on agriculture, fishery, andcause other negative socioeconomic consequences.

Evolutionary Ecology | Optimal Foraging 2559

See also: Climate Change 1: Short-Term Dynamics;

Climate Change 2: Long-Term Dynamics; Climate Change

3: History and Current State; Climate Change Models;

Global Warming Potential and the Net Carbon Balance.

Further Reading

Blunier T and Brook EJ (2001) Timing of millennial-scale climate changein Antarctica and Greenland during the last glacial period. Science291: 109–112.

Clark PU, Pisias NG, Stocker TF, and Weaver AJ (2002) The role of thethermohaline circulation in abrupt climate change. Nature 415: 863–869.

Houghton JT (ed.) (2001) Climate Change 2001: The Scientific Basis.Cambridge: Cambridge University Press.

Kohler P, Joss F, Gerber S, and Knutti R (2005) Simulated changes invegetation distribution, land carbon storage, and atmospheric CO2 in

response to a collapse of the North Atlantic thermohaline circulation.Climate Dynamics 25: 689–708.

Legates DR and Willmott CJ (1990) Mean seasonal and spatial variabilityin global surface air temperature. Theoretical and AppliedClimatology 41: 11–21.

Rahmstorf S (2002) Ocean circulation and climate during the past120 000 years. Nature 419: 207–214.

Rahmstorf S and Ganopolski A (1999) Long-term global warmingscenarios computed with an efficient coupled climate model.Climatic Change 43: 353–367.

Schmittner A (2005) Decline of the marine ecosystem caused by areduction in the Atlantic overturning circulation Nature434: 628–633.

Watts WA, Allen JRM, and Huntley B (1996) Vegetation history andpalaeoclimate of the last glacial period at Lago grande diMonticchio, southern Italy. Quaternary Science Reviews15: 133–153.

Welinga M and Wood RA (2002) Global climatic impact of a collapseof the Atlantic thermohaline circulation. Climatic Change54: 251–267.

Optimal ForagingE R Pianka, University of Texas, Austin, TX, USA

ª 2008 Elsevier B.V. All rights reserved.

Further Reading

Foraging tactics involve ways in which animals gather

matter and energy. Matter and energy constitute profits

gained from foraging used in growth, maintenance, and

reproduction. Foraging has costs as well; a foraging ani-

mal may often expose itself to potential predators; much

of the time spent in foraging is rendered unavailable for

other activities, including reproduction.An optimal foraging tactic maximizes the difference

between foraging profits and their costs. Natural selection,

acting as an efficiency expert, has often favored such opti-

mal foraging behavior. Consider, for example, prey of

different sizes and what might be termed ‘catchability’.

How great an effort should a foraging animal make to

obtain a prey item with a given catchability and of a

particular size (and therefore matter and energy content)?

Clearly, an optimal consumer should be willing to expend

more energy to find and capture food items that return the

most energy per unit of expenditure upon them. Optimal

foragers should also take advantage of natural feeding

routes and should not waste time and energy looking

for prey either in inappropriate places or at inappropriate

times. What is optimal in one environment is seldom

optimal in another, and an animal’s particular anatomy

strongly constrains its optimal foraging tactic.

Considerable evidence suggests that animals actually do

attempt to maximize their foraging efficiencies, and a sub-

stantial body of theory on optimal foraging tactics exists.Numerous aspects of optimal foraging theory were

concisely summarized by MacArthur. He made several

preliminary assumptions: (a) Environmental structure is

repeatable, with some statistical expectation of finding a

particular resource (such as a habitat, microhabitat, and/

or prey item). (b) Food items can be arranged in a con-

tinuous and unimodal spectrum, such as size distributions

of insects. (This assumption is clearly violated by foods of

some animals, such as monophagous insects or herbivores

generally, because plant chemical defenses are typically

discrete.) (c) Similar animal phenotypes are usually

closely equivalent in their harvesting abilities; an inter-

mediate phenotype is best able to exploit foods

intermediate between those that are optimal for two

neighboring phenotypes. Conversely, similar foods are

gathered with similar efficiencies; a lizard with a jaw

length that adapts it to exploit 5-mm-long insects best is

only slightly less efficient at eating 4- and 6-mm insects.

(d) The principle of allocation applies, and no one phe-

notype can be maximally efficient on all prey types;

improving harvesting efficiency on one food type

necessitates reducing the efficiency of exploiting other

kinds of items. (e) Finally, an individual’s economic