“I believe that this book will become the go-to reference for gaining a foundational understanding of how global climate change has and will continue to transform ecological systems in the face of anthropogenic impacts. An encyclopedic synthesis of the field, it provides exemplary coverage of the vast literature. This is an authoritative treatment of an important topic in ecology and conservation.”–Oswald J. Schmitz, Yale University

“To predict the responses of species to climate change, Post shows how interactions between species may be as important as density-independent responses to changes in their abiotic environment. Most ecologists have focused on the latter. Post makes a great case that the response of species to climate change is likely to unfold in the context of their interactions with other species, through competition, predation, and, in the case of humans, land-use change.”–William H. Schlesinger, president of the Cary Institute of Ecosystem Studies

Eric Post is professor of biology and ecology at Pennsylvania State University. He has published dozens of scholarly articles and book chapters on ecological responses to climate change, and is coeditor of Wildlife Conservation in a Changing Climate.

Ecology of Climate Change

The Importance of Biotic Interactions

By ERIC POST

PRINCETON UNIVERSITY PRESS

Copyright © 2013 Princeton University Press
All rights reserved.
ISBN: 978-0-691-14847-2

Contents

Preface: Purpose, Perspective, and Scope……………………………..xiiiThe Tension and Facilitation Hypotheses of Biotic Response to Climate
Change……………………………………………………………xivAcknowledgments……………………………………………………xxi1. A Brief Overview of Recent Climate Change and Its Ecological Context….12. Pleistocene Warming and Extinctions……………………………….243. Life History Variation and Phenology………………………………544. Population Dynamics and Stability…………………………………965. The Niche Concept……………………………………………….1326. Community Dynamics and Stability………………………………….1637. Biodiversity, Distributions, and Extinction………………………..2178. Ecosystem Function and Dynamics…………………………………..2499. Brief Remarks on Some Especially Important Considerations……………297References………………………………………………………..303Index…………………………………………………………….359

CHAPTER 1

A Brief Overview of Recent ClimateChange and Its Ecological Context

This chapter summarizes the most prominent abiotic components of recent climatechange to establish the environmental context from which the discussionin the rest of the book proceeds. As will become clear in subsequent chapters,the rapid pace and broad geographic extent of abiotic changes reviewed herecannot be viewed in isolation for, as important as they are as drivers of ecologicaldynamics, in many if not most cases the ecological responses resultingfrom them owe to an alteration of biotic interactions. As an example we mayconsider the recent mass abandonment of retreating ice floes by Pacific walrusalong the coast of Alaska in 2007 and 2010, which coincided with the lowestand third-lowest arctic sea ice extent recorded to date (Post and Brodie 2012).Walrus are benthic feeders: they consume mussels on the seafloor in shallowwater during dives from the ice edge, which they also rest on between foragingbouts (Oliver et al. 1985; Ray et al. 2006). As the ice melts and the edge movesfarther away from shore and the shallow-water environments that harbor themussels walrus eat, it becomes energetically unprofitable for these large marinemammals to try to remain in proximity to the ice edge, for they wouldhave to travel progressively longer distances to reach the shallow-water sea-beds(Ray et al. 2010). The rate of annual sea ice loss over the Arctic has beenon the order of 56,000 km² each year since 1979 (figure 1.1). As suggested laterin this chapter, loss of sea ice does not represent simply an abiotic response torecent warming; it also represents the loss of critical habitat necessary for thesurvival and reproduction of many species, and it should probably be consideredthe high-latitude equivalent, in terms of negative impacts on faunal species,of deforestation in the Tropics (Post and Brodie 2012).

Before I summarize the most prominent abiotic features of recent climatechange, two points of clarification are necessary. First, what is meant by recent?In the context of this book, recent refers to climatic changes that were set inmotion by the onset of the Industrial Revolution but have become most clearlymanifest, in terms of global abiotic changes, since approximately the middleof the twentieth century (Mann et al. 1998, 1999). Second, what is meant byrapid? It is perhaps more difficult to identify the parameters of this characteristicof recent climate change, but at the same time the notion of rapid changeis probably more intuitive to ecologists than is the notion of recent change. Forthe purposes of this book, and in keeping with most of the examples adducedherein, rapid refers to changes occurring on annual to decadal timescales. Itwill no doubt be tempting to argue that decadal-scale climatic variation is mostcertainly not rapid in the ecology of soil microorganisms, whose populationand community dynamics are important drivers of nutrient flow rates, primaryproductivity, and even biosphere-atmosphere exchange of trace gases, but constraintsmust be established, and these are two of the clearest used throughoutthis book. Rapid environmental change most certainly presents challenges tosome species, in terms of individuals’ ability to survive and reproduce, even asit presents distinct advantages to others, and no apparent benefit or disadvantageto yet others. Hence, the dynamics explored in this book include, in somecases, responses to climate change, and in other cases a lack of such response.But before we begin an accounting of these responses, from individual-levellife history responses through population-level dynamics, to community-scalechanges, to ecosystem dynamics, we must take a close look at climate changeand the abiotic dynamics it encompasses.

CLIMATE CHANGE VERSUS GLOBAL WARMING

It is tempting to use the term climate change to refer to increases in Earth’stemperatures or, similarly, to conflate the terms climate change and globalwarming. The term global warming is an oversimplification of the currentchanges in abiotic conditions on Earth because it implies that only changesin temperature are occurring, and that these temperature changes are unidirectional.Furthermore, the term has been applied most recently to connotesolely human-driven temperature increases, which has promoted dismissal ofthe entire notion of climate change by those unconvinced by the evidence ofa role for human beings in recent climate change. To draw an analogy thatillustrates the importance of avoiding conflation of the terms climate changeand global warming, we may recognize that evolution, like climate change, isa fact, whereas natural selection, like anthropogenic emissions of greenhousegases, is the accepted theory explaining how this factual process occurs.

From an ecological perspective, climate change is most meaningfully consideredas the suite of abiotic changes occurring across Earth coincident withthe onset of the Industrial Revolution and progressing over the past 150 years,including rising temperatures, temperature variability, changes in precipitationand snow cover, and diminishing sea and land ice. All these changes can belinked to ecological dynamics, though it is probably fair to state that mostresearch to date on the ecological consequences of climate change has focusedon temperature changes.

TEMPERATURE CHANGES

The Earth’s surface has warmed by approximately 0.4°C since 1850 (Brohan etal. 2006; Trenberth et al. 2007), and by approximately 0.2°C per decade overthe most recent three decades (Hansen et al. 2006) (figure 1.2), largely as a resultof increases in atmospheric CO₂ concentrations driven by the human use offossil fuels (Hansen, Sato, et al. 2008). This increase hardly seems remarkable,and indeed, it would be difficult for you or me to detect from one day to thenext. However, this single estimate, representing an average value calculatedover the entire surface area of our planet and encompassing over a century anda half of data, masks a great deal of detail. The biological importance of thisseemingly minute amount of change is evident in the widespread ecologicaldynamics it has elicited (Walther et al. 2002; Post, Forchhammer, et al. 2009).To understand the biological importance of climate change, we need to examinethe abiotic data through the eyes of an ecologist.

From figure 1.2, two aspects of the recent temperature trend should becomeimmediately apparent. First, there is considerable variation about the trendfrom year to year. The existence of this variation should signal to us as ecologiststhat not all organisms should be expected to respond similarly, in termsof rate and magnitude of change, to the recent and ongoing temperature trend.Short-lived or univoltine organisms, for example, may display life history andpopulation dynamics that mirror interannual temperature variability as well asthe overall temperature trend. Long-lived organisms, on the other hand, maydisplay very gradual and at first almost imperceptible life history and populationdynamical responses to the trend in temperature while displaying clearerresponses to interannual variability about the trend itself.

The second aspect of the time series in figure 1.2 we may be particularlyinterested in as ecologists is the difference in the magnitude of temperaturechange between the Northern and Southern Hemispheres. Whereas globalmean surface temperature has risen by 0.4°C over the past 150 years, thechange in the Northern Hemisphere alone has been closer to 0.6°C (figure1.2b), while that in the Southern Hemisphere has been about half that much,or 0.3°C (figure 1.2c). This indicates there has been considerable spatial heterogeneityacross the globe in the magnitude and perhaps also rate of warming.When spatial resolution is added to the time series shown in figure 1.2,this variability across Earth’s surface becomes readily apparent (figure 1.3).These data reveal that trends in annual mean surface temperatures over thepast century (figure 1.3a), and since the onset of the rapid warming trendbeginning in the late 1970s (figure 1.3b), vary considerably across the surfaceof the Earth (Trenberth et al. 2007). During the twentieth century, warmingtrends were most pronounced in Low Arctic central Canada, the southwesternUnited States, southeastern South America, and central and northern Asia(Smith and Reynolds 2005) (figure 1.3a). This pattern was largely reproducedduring the last three decades of the twentieth century, although with greaterapparent surface warming in northwestern South America, southern Africa,and southern Greenland (figure 1.3b) (Smith and Reynolds 2005). The challengesuch spatial variation in the strength of local and regional warmingposes to the science of ecology lies in the difficulty of predicting the responseof any organism with a cosmopolitan or quasi-cosmopolitan distribution toongoing and future climate change. This difficulty is compounded many timesover if our interest lies in predicting how entire communities of organismswill respond to climate change over continental to global scales. Detectinga response to climate change in a single population or even, in some cases,multiple populations of a given species may not necessarily indicate how thespecies as a whole will respond to climate change throughout its distribution,especially if this distribution encompasses locales that have undergone widelydifferent local temperature changes. Similarly, detecting responses to climatechange in the species constituting a community of interest may indicate nothingat all about how the entire community will respond to future increasesin temperature. The challenges inherent in such evaluations are explored insubsequent chapters.

Comparing recent temperature estimates with proxies of global mean surfacetemperatures derived from tree rings, sediment layers, and ice cores revealsthat temperatures during the past few decades have exceeded those overthe past two millennia (Mann et al. 1999; Mann and Jones 2003). Examiningthe recent temperature trend through a time-windowing approach reveals furtherinteresting details that are of ecological significance. When the data onaverage global temperatures are displayed as decadal time series, it becomesevident that the rate of warming has not been constant over the past 150 years(figure 1.4) (Mann and Jones 2003; Trenberth et al. 2007). In each successivelymore recent multidecadal period plotted in figure 1.4, the slope of temperaturechange over time increases. The rate of change over the past twenty-five yearsis approximately four times greater than the rate of change over the 150-yearperiod as a whole. This would seem to indicate that the rate of warming is increasing.From an ecologist’s perspective, this should make us wonder whetherorganisms will be able to keep pace with the rate of temperature change if itcontinues to increase. A more refined perspective on this question would try toidentify how responses to increasing rates of environmental change might varyamong taxonomic groups with different life history strategies and generationtimes. For instance, certain species with highly plastic life history strategies orshort generation times might be better able to match the pace of temperaturechange than might organisms with more highly conserved life history strategiesor longer generation times.

Finally, it is worth noting as well that changes in interannual variability intemperature may prove just as important to the ecology of some species as thetemperature trend itself. Here I refer not simply to heat waves or cold spellsand the frequency of their occurrence but also to the magnitude of negativeautocorrelation in temperatures between successive years. As we will examinein chapter 4 on population dynamics, abiotic fluctuations, when of sufficientmagnitude, may influence the stability of population dynamics. Furthermore,the magnitude of serial (temporal) and spatial autocorrelation in temperaturesmay change as warming continues. Both of these parameters have the potentialto influence ecological processes. For instance, increasing temporal autocorrelationin climatic conditions may lead to increases in population size insome types of systems that inevitably lead to population crashes (Wilmers etal. 2007b). Similarly, increasing spatial autocorrelation in temperatures overlarge (i.e., subcontinental to continental) scales may increase the spatial synchronyof populations whose dynamics are environmentally entrained, therebyincreasing extinction risk among those populations. For instance, temperaturesrecorded at weather stations along the west coast of Greenland became morespatially autocorrelated as Northern Hemisphere mean annual temperature increased,as did the population dynamics of caribou in the same area (Post andForchhammer 2004). If increasing short-term—that is, annual to subannual—variabilityin abiotic conditions is a likely outcome of future climate change,this may have the potential to alter the stability and thereby persistence ofsome populations of organisms. Whether and how this might affect the stabilityproperties of entire communities are questions examined in subsequentchapters.

PRECIPITATION CHANGES

Just as important in the ecology of some species as temperature changes, if notmore so, have been changes in precipitation and aridification (Schlesinger etal. 1990). Here again, when we examine the abiotic data as ecologists, severalfeatures of these changes should capture our attention. Although more difficultto discern than the temperature trends described above, there appears to havebeen an overall decline across the globe in total annual precipitation over landsince approximately 1950 (Mitchell and Jones 2005), although precipitationhas begun to increase globally since approximately 1993 (figure 1.5) (Wentz etal. 2007). Moreover, it appears that variability among years in total precipitationover land has increased since approximately 1970. Considerable variationacross the globe in trends in streamflow and surface runoff is expected to developover the next half century, with increases in runoff projected for northernNorth America and Eurasia and decreases projected for western NorthAmerica, southern Europe, southern Africa, and the Middle East (Milly et al.2005). Both trends and variability in precipitation and water availability havean enormous potential to alter global primary productivity. Over the decadefrom 2000 to 2009, for instance, global terrestrial net primary productivity declinedby approximately 0.55 petagrams of carbon annually, driven primarilyby a pronounced Southern Hemisphere drying trend (Zhao and Running 2010).

To derive ecological context from such data requires, however, an indicationof just how much of a decrease in annual precipitation over land has the potentialto constitute a biological constraint on primary productivity if evaporationover land continues to increase with temperature. The Palmer Drought SeverityIndex (PDSI) provides this quantification as an indication of water stress(Alley 1984; Cook et al. 1999). The PDSI time series reveals worsening andpersistent drought-like or drought-potential conditions since 1980 (figure 1.6)(Dai et al. 2004). Not only does this have the potential to alter primary productivityfrom global down to local scales, it also has the potential to drive shifts inbiome-wide plant community composition, transforming the vegetation typescharacteristic of the biomes we know today.

Aside from the recent trends in precipitation over land during the past severaldecades, there has also been considerable spatial heterogeneity across theglobe in the magnitude and direction of changes in moisture balance. Whereassome regions, such as north temperate and south temperate zones, have receivedgreater precipitation on average since the start of the twentieth century,other regions, most notably the Tropics, have become drier (Trenberth et al.2007). Changes in moisture balance reflect the influence of temperature increaseson the return of moisture to the atmosphere, and this may, over someregions, reflect increasing aridity despite increasing total annual precipitation.Spatial variation in the strength of the PDSI reflects this relationship, withincreasing drought severity in tropical Africa, Central America, Malaysia, andthe Amazon basin (Dai et al. 2004). Migratory species that travel along north-southgradients between breeding and winter ranges must contend with widelydivergent precipitation trends and the consequences of these for resource availabilityat different stages during their annual life history cycles. As well, thespatial variation in the PDSI trend over the twentieth century suggests thatprimary productivity may respond differently to temperature trends where precipitationhas increased, such as in Scandinavia, the U.S. Midwest, westernRussia, and southeastern South America, compared to where it has declinedalong with rising temperatures, such as in equatorial and subequatorial LatinAmerica and Africa.
(Continues…)Excerpted from Ecology of Climate Change by ERIC POST. Copyright © 2013 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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