Global change ecologyWilliam H. Schlesinger

The Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA

Ecology has expanded from its traditional focus on

organisms to include studies of the Earth as an

integrated ecosystem. Aided by satellite technologies

and computer models of the climate of the Earth, global

change ecology now records basic parameters of our

planet, including its net primary productivity, biogeo-

chemical cycling and effects of humans on it. As I discuss

here, this new perspective shows us what must be done

to transform human behaviors to enable the persistence

of life on Earth under human stewardship.

Introduction

When I was a graduate student during the early 1970s,ecology was about organisms. The prestigious places topublish were Ecology and The American Naturalist, andthe focus was on the number of individuals or speciesfound in nature, their rates of reproduction and themarvelous adaptations that enabled them to persist intheir native habitats. The research was usually done atfield stations; each of us chose an idyllic place to spend thesummer studying how nature is put together.

Today, we see a huge emphasis on a broader view: whatis the effect of life on Earth? What changes are we, as thedominant species on the planet, forcing on the habitats ofall other species, and how are we affecting the futureprospects for life on Earth? What can we do about ourrapidly changing planet? All these questions compriseglobal change science, which fills the pages of an explosionof new journals that focus on the past and future ofour planet.

Early studies of Earth system function

There were harbingers to the birth of this new discipline.During the late 1950s, Roger Revelle commented thathumans were performing an unreplicated global exper-iment by raising the concentration of carbon dioxide (CO2)in our atmosphere with potentially serious, but unknown,consequences [1]. By 1960, working with Revelle, DaveKeeling [2] had shown that not only was the CO2

concentration increasing, but that one could also see aregular oscillation in its concentration that must be due tothe photosynthesis of land plants in the temperate zone.My colleague Dan Livingstone once said that the graphmade him ‘feel as if I had just put my finger on the beating,living heart of the world’ [3].

During the 1960s, Gene Odum showed that one couldtrace the flow of energy through ecosystems, focusing notso much on the individual species, but on an attempt tounderstand the overall process by which the products of

Corresponding author: Schlesinger, W.H. (schlesin@duke.edu).Available online 23 March 2006

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photosynthesis either move to higher trophic levels, suchas humans, or are dissipated in the environment [4]. HerbBormann and Gene Likens [5] did the same for the flow ofmaterial elements, such as calcium, potassium and otherelements that anchor the biochemistry of all organisms.Ecologists scrambled to study primary production andnutrient cycling as part of the International BiologicalProgram (IBP; http://www7.nationalacademies.org/archives/International_Biological_Program.html).

A milestone was reached when Robert Whittaker [6]and, working independently, Helmut Lieth [7] used thesedisparate field studies to estimate the net primaryproduction of the entire land surface of the Earth. Theywere the first to show just how much photosynthesis thebiosphere had to work with each year. Remarkably, usingrather crude methods, they arrived at estimates (between50 and 60 Pg C yrK1) that are similar to measurementsfrom satellites today [8]. Most satellite estimates of plantproductivity on Earth are based on the formulation of theNormalized Difference Vegetation Index (NDVI),developed by Compton Tucker and colleagues [9] andfirst used to study changes in greenness from the Sahel ofAfrica [10] to high northern latitudes [11].

I mark the beginning of global change science with thepublication of The Biosphere as a special issue of ScientificAmerican in 1970. This was the first time that I saw anintegration of the science that viewed our planet as aclosed ecosystem in which photosynthetic organismscaptured sunlight energy, enabling a profusion of otherforms of life. An array of articles outlined the globalbiogeochemical cycles and the emerging human impactson them. Not without controversy and criticism, thepublication of Limits to Growth in 1974 made manypeople realize how exponential growth in both populationand economics would collide with the resources availableon a finite plane [12].

Global views of human impacts

Documentation of human impacts on the biosphere wasnot long in coming. In 1974, Mario Molina and SherwoodRowland [13] predicted that chlorofluorocarbons (CFCs)would destroy stratospheric ozone, a forecast that isconfirmed dramatically in satellite photos from NASA ofthe ozone hole from the 1980s to today (Figure 1). Thissmall human perturbation of the global chlorine cycleposed a real threat to the survival of life on Earth, or atleast on the land surface [14]. Geochemists showed thatthe annual human production and mobilization of manyimportant elements of the periodic table (especiallynitrogen, phosphorus, sulfur and most metals of economicinterest) rivaled that of nature [15]. In 1986, Peter

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Figure 1. Total column ozone as recorded by the TOMS satellite of NASA on 5

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stratospheric ozone over Antarctica. Reproduced with permission from http://

jwocky.gsfc.nasa.gov/.

Review TRENDS in Ecology and Evolution Vol.21 No.6 June 2006 349

Vitousek and colleagues [16] estimated that humans useor dominate approximately 40% of the terrestrial netprimary production on Earth [17,18], not a pretty picturefor the future of most other species that share the planetwith us. General circulationmodels and satellite measure-ments of the climate of the Earth confirmed what SvanteArrhenius [19] had predicted nearly a century earlier:increasing levels of CO2 in the atmosphere of the Earth, aperturbation of the global carbon cycle, would lead to awarmer planet, as seen today [20,21].

These empirical studies were enriched by theorists.During the 1960s, NASA sponsored a program of scienceto examine ‘closed systems,’ with a desire to ascertain theminimum complexity that would be necessary for humansto survive in a spacecraft designed for long-distanceexplorations of the solar system [22]. Of course, then asnow, the Earth has been our planetary spaceship. The1979 publication of Gaia by James Lovelock [23] offered aprovocative view that the biosphere on Earth wasanalogous to an organism, having emergent traits suchas homeostasis that fostered its own persistence. Eachspecies on Earth was thought to contribute in some smallfashion to the stable conditions for life on Earth. AlthoughGaia has few followers now, the real impact of the bookwas that it made an entire generation of ecologists thinkbroadly about planetary ecology.

In 1969, NASA took us to the Moon, where we couldlook at our planet against the dark backdrop of space. In1976, NASA took its technology to Mars, where itmeasured and photographed what we could expect on aplanet without evidence of life. And, during the 1990s,NASA applied its technology to planet Earth, by launching

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the array of Earth Observing System (EOS) satellites thatnow monitor our planet, recording basic characteristics ofits temperature, photosynthesis, atmospheric chemistryand land cover, all beamed down to receiving stations evenas you read this (http://eospso.gsfc.nasa.gov/) [24].

Where we are today

Through early global change science, we gained a broadperspective on the impact of humans on nature. There islittle doubt that humans are now a major evolutionaryforce on Earth [25], and that our activities dominate itsecosystems, both on land and in the sea [26,27]. Satelliteviews of night-time lighting show the pervasive humanpresence on our planet [28,29]. Every ecosystem on Earthis now bathed in high CO2 from fossil fuel combustion.Chemicals of human origin are found on all continents [30]and are rapidly mixing to the deepest reaches of the sea[31]. Even the reflection or albedo of the Earth, as seen byearthshine on the Moon, has increased as a result of agreater burden of atmospheric aerosols and clouds [32].

The pristine field stations that we visited as graduatestudents are of little relevance to most of the surface of theEarth that is now managed by humans and under rapiddegradation under our stewardship. What matters mostfor the organisms that we studied so diligently 30 yearsago is not so much how they perform on Nature’s stage,but whether the stage will exist for them at all.

Priorities for global change science: the sea

There is much science left to do, especially on the 70% ofthe surface of the Earth that is covered by salt water.Althoughwe have a fairly good estimate of the net primaryproduction of the oceans [33], we have only a limited graspof the sources of nitrogen, phosphorus, iron and siliconthat fuel ocean productivity. New research on sources ofnitrogen to the oceans and its loss via anaerobic ammonia-oxidizing bacteria has recently rewritten what we knowabout nitrogen cycling in the sea [34]. Increasingatmospheric CO2 levels have already lowered ocean pHby 0.1 and are likely to lead to much greater acidificationduring the rest of this century [35]. Climatic change hasalso raised the temperature of the oceans and lowered thesalinity over broad regions [36,37]. We are also likely tochange marine net primary productivity as we alter theprovision of essential elements to marine ecosystemsthrough the dispersal of soils by wind erosion [38].

There is every indication that we have overexploitedthe oceans. Nearly 8% of oceanic productivity goes tosupport the current harvest of protein from ocean waters[39]. Increasingly, we see signs that the oceans are not aninfinite sink in which to dilute the pollutants ofindustrialized society; yet we lack a good understandingof the sources of mercury, polybrominated organiccompounds and other substances that might renderoceanic fish unfit for human consumption. Are wepolluting the last large domain on Earth, or are some ofthese compounds natural [40–42]?

Do species matter?

The original questions posed by NASA are ever morerelevant today: How much nature must be left in its

Review TRENDS in Ecology and Evolution Vol.21 No.6 June 2006350

natural state for sustainable Earth system function?Setting aside the ethics and aesthetics of preservingbiodiversity, how simple can we make an ecosystem,while still seeing it function well without massive,artificial human interventions, such as cultivation,irrigation, fertilization and pest control? Work by DavidTilman et al. [43], which showed greater losses of nitratebeneath grassland ecosystems with depauperate speciescomposition, links healthy ecosystem function to highspecies diversity. Does diversity beget stability or is theremuch redundancy of species that are pleasing to the eyebut nonessential to the sustainability of a full planet?

New questions also face us: What are the effects of thegenetically modified organisms that enable us to supplynew, artificial ‘species’ to nature, at the expense of thenatural genetic diversity that is the raw material ofevolution? What can we expect from our efforts, bothpurposeful and inadvertent, to homogenize the flora andfauna of the Earth as rapid and frequent travel and tradeaccompany our globalization of commerce and culture?How will we respond to disease pandemics?

As we anticipate and adapt to changes in the climateand chemistry of the Earth, we need large-scale and long-term experiments to understand the response of biota toincremental CO2, nitrogen and ozone, and to changes intemperature and precipitation. Experiments using Free-Air CO2 Enrichment (FACE) in forests [44], iron additionsto seawater [45] and 15N additions to streams [46] havedone much to elucidate the response of whole ecosystemsto human perturbations. The next phase of work shoulduse factorial experiments so that we can understand hownature will respond to multiple stresses.

Fortunately, we have wonderful new tools with which todo our science better. Molecular techniques will enable usto identify and understand the microbial communitiesthat dominate so much of the biogeochemical cycling onEarth. For example, molecular systematics has been usedto identify the bacteria catalyzing the anammox reactionconverting NH4 to N2 in seawater [47]. Mass spectrometryto analyze the proportion of stable isotopes in differentpools and fluxes has revolutionized how we recognize theimportance of biology in controlling the chemistry ofEarth. Eddy covariance methods enable us to measure thenet carbon exchange of large areas of the surface of theEarth (e.g. [48]) and remote-sensing technologies willenable us to monitor the function of ecosystems with muchgreater sample frequency in space and time than was everbefore possible. These measurements are crucial if we areto build models of Earth system function that effectivelycouple surface processes to changes in climate andclimatic forcings. Mercifully, each day, we see increasesin our computational abilities to synthesize all the data.

Beliefs and politics

In some corners of the globe, policy makers and politicianspay close attention to the science that shows what willhappen to a planet under inattentive stewardship. Inmuch of the undeveloped world, however, the localpopulation is perplexed about what to do to ensure asustainable future when facing the immediate question ofhow to provide enough food and clean water to survive

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each day. Sadly, in other corners, concern is shallow; manyof those who could afford to help believe that some type ofdivine intervention will carry us through a bottleneck ofan exponentially rising human population and its increas-ing demand for resources on a finite planet [49]. Theywant no personal sacrifice. Perhaps what we learned bestfrom our early field studies of ecology is that humanbehavior might not be far removed from that of otherorganisms. Each squirrel on my bird tray feeds as iftomorrow is simply another day.

Many global change scientists talk of ‘sustainability’science. Indeed, there is heated debate about the realityand meaning of the phrase ‘sustainable development.’ Inthe pre-industrial era, humans lived in concert withnature. No doubt it was a hard life, but it was sustainedfor centuries. The question we now face is whether we canlive the way we aspire to today, without degrading the lifesupport systems of the planet that would sustain ustomorrow. And now we must try our best simply topreserve the species that we studied so fervently just a fewdecades ago. Global change science has a big agendabefore it and little time in which to do it.

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