Saving a Million Species

This chapter reviews the first study that provided an international assessment of the risks to biodiversity associated with climate change. Rapid acceleration of information at the end of the twentieth century showed that the distributions of terrestrial species were responding to climate change (Parmesan et al., 1999; Pounds et al., 1999; Thomas and Lennon, 1999). Combined with the extreme El Niño event of 1998 that caused major bleaching damage to coral reefs, this work confirmed that climate variation and climate change were likely to have major impacts on biodiversity (Sala et al., 2000; IPCC, 2001; Walther et al., 2002; Parmesan and Yohe, 2003).

When initial estimates of the extinction risk from climate change appeared, there was immediate public and media interest. Authors of Thomas et al. appeared on CNN, BBC, and other major national and international television networks. Newspaper headlines, often front page, appeared on the day of the report’s release. Magazine, radio, and other media treatments of the subject followed for weeks after. But to what extent was this media interest driven by policy relevance, and to what extent were the implications of the extinction risk estimates taken up in policy dialogue?

In 2004 nineteen scientists from fourteen institutions in seven countries collaborated in the landmark study described in chapter 2 (Thomas et al., 2004a). This chapter provides an overview of results of studies published subsequently and assesses how much, and why, new results differ from those of Thomas et al.

Thomas et al. (2004) pioneered the estimate of extinction risk due to climate change by coupling species range-loss simulations from species distribution models with species-loss estimates from the species-area relationships (SARs). Unfortunately, numerous conceptual and practical problems permeate this seemingly solid and straightforward approach. Chapter 4 explored developments in climate envelope modeling. Here we focus on the challenges associated with applying a SAR approach to climate-driven extinction estimates and propose a novel application of recent Maximum Entropy (MaxEnt) theory in ecology that may help to address some of them.

The golden toad (Bufo periglenes) disappeared from Costa Rica in 1989 and became the first terrestrial extinction to be linked to climate change. Like the first marine extinction attributed to climate change (see chapter 7), the extinction of the golden toad was linked to El Niño events. The marine extinction is irrefutably linked to coral bleaching, but the causes of the golden toad extinction are far more controversial. Golden toad sightings have been reported in Guatemala since the 1980s, but these sightings have never been confirmed. Although there is some hope that residual populations still survive, Bufo periglenes is currently listed as extinct in the International Union for Conservation of Nature (IUCN) Red List, and the cause of the extinction is hotly debated.

Coral reefs cover 255,000 square kilometers of the earth’s surface (Spalding and Grenfell, 1997) and likely harbor more than a million species globally, perhaps as many as 3 million (Reaka-Kudla, 1997; Small et al., 1998). Coral reefs benefit humankind in numerous ways. They provide ecosystem services and advantages to tropical human communities, including coastal protection, nurseries and sources of nutrition for fisheries, tourism, and great stores of genetic material and species (biodiversity). In addition, a less tangible benefit relates to the esthetics of coral reefs—the sheer beauty and wonders of these diverse ecosystems offer inspiration to lay persons and scientific investigators alike.

Of the abiotic changes associated with the current phase of warming occurring on Earth, the loss of sea ice, snow cover, and glaciers are among the most apparent, rapid, and potentially ecologically devastating. These physical effects make polar regions the most likely places to experience first extinctions due to climate change. Have extinctions already been recorded in high latitude species, or do population trends suggest that extinctions are imminent? This chapter answers these questions.

Deep time is geologic time, extending to the origin of the planet. For biologists in search of an understanding of extinction, the relevant portion of deep time is that in which life has existed on the planet— about the last 4 billion years (Cowen, 2000). Extinctions are first recorded when the fossil record is robust enough to offer insights into the arrival and disappearance of groups of organisms (Benton and Harper, 2009). Extinctions in deep time can therefore be identified only over about the last 600 million years, an interval of time dominated by the Phanerozoic eon (540 million years ago to present).

In this chapter we investigate the relationship between climatic change and extinction in continental ecosystems during the era of modern biotas. In contrast to the previous chapter, the time frame examined is shorter and therefore the number of major global extinction events is smaller. But at the same time, the more recent fossil record of the past 50 million years is more highly resolved, and it is possible to begin to examine causal linkages between climate and extinctions.

Millennia before the modern biodiversity crisis—a worldwide event being driven by the multiple impacts of anthropogenic global change—a mass extinction of large-bodied fauna occurred. After a million years of severe climatic fluctuations, during which the earth waxed and waned between frigid ice ages and warm interglacials, with apparently few extinctions, hundreds of species of mammals, flightless birds, and reptiles suddenly went extinct over the course of the last 50,000 years (Barnosky, 2009). Due both to our intrinsic fascination with huge prehistoric beasts and to the possible insights these widespread species losses might lend to the modern extinction problem, the mystery of the “megafaunal” (large animal) extinctions have led to much theorizing, modeling, and digging (for their fossils or environmental proxies) over the last 150 years (Martin, 2005). The topic continues to invoke strong scientific interest (Koch and Barnosky, 2006; Grayson, 2007; Gillespie, 2008; Barnosky and Lindsey, 2010; Nogues-Bravo et al., 2010; Price et al., 2011).

We have found no examples of global plant extinctions from the tropics within the Quaternary. Examples of extinctions over longer periods of time are readily documented within the fossil record, with the loss of whole families evident between Eocene and modern times (Morley, 2000, 2007). Herein lies a clue to the problem of detecting extinction of tropical plants—the taxonomic resolution of the fossil record.

Any estimate of the number of species on Earth at risk from climate change must begin with the question of how many species can be found on Earth, and because most species are insects, how many insect species in particular. The question of how many species of insects live on Earth and where they can be found is an old one. Linnaeus was aware of variation from place to place in the diversity of insects but believed that most insect species could be named in his lifetime. One of Linnaeus’s students (he called them apostles), Daniel Rolander, traveled to Surinam, however, and encountered there a diversity of insect life that he found overwhelming. Rolander began to wonder in confronting such diversity whether the species he saw would ever all be collected (the task he had been given) and named (the task Linnaeus would take for himself when Rolander returned) (Dunn, 2009c). Rolander’s experience was a hint of what was to come.

Tropical forests are biologically the richest biomes on Earth, home to half of global biodiversity and most of the insects described in the previous chapter. The prospects for the earth’s treasure of living organisms over this century are thus inevitability tied to the prospects of its greatest treasure houses, the tropical rain forest regions, whether influenced by deforestation or climate change. The extinction risk in tropical forests will have a large effect on total global extinction risk, but like that of insects is currently difficult to quantify because of several key unknowns. This chapter explores the nature of contemporary and likely future climate change in the tropics, and possible implications for the biodiversity and functioning of tropical ecosystems. It begins by reviewing the likely nature of tropical climate change, then explores the likely response of tropical organisms to such change. It highlights key uncertainties in estimation of extinction risk from climate change, including the likely pattern of precipitation change, the influence of carbon dioxide on forest persistence, the upper thermal tolerance and adaptation/acclimation ability of tropical organisms, and the relationship between habitat restriction and extinction risk.

The term “extinction” is a loaded phrase when applied to coral reefs. Often, it is applied to coral reefs generically, with the phrase “extinction of coral reefs” appearing regularly in the scientific and popular press (e.g., it appeared more than 15,000 times when typed into the Google search engine, June 4, 2011). This phrase is generally used to describe the disappearance ofcoral reefs as an ecosystem, which is very distinct from the extinction of a particular coral reef species. This distinction becomes increasingly important when considering the likely outcome for coral reefs and their biodiversity under rapid global change.

Only a handful of marine species are known to have gone globally extinct in modern times (Carlton et al., 1999; Dulvy et al., 2003). Yet it is difficult to know if this low extinction rate is due to greater resilience of marine species relative to terrestrial species, less human impact on marine species, or simply an artifact of so little of the ocean having been explored and documented. Into this already uncertain extinction picture, climate change presents additional complication.

Fresh waters—rivers, streams, lakes, ponds, wetlands—cover less than 1 percent of the earth’s surface, yet their biodiversity is unrivaled. Fully 10 percent of all known animal species and a third of all vertebrate species, including about 40 percent of the world’s fishes, live in fresh waters. Other well represented groups include insects, crustaceans, mites, and mollusks (table 17-1). Further, an estimated 20,000-200,000 freshwater animal species (mostly invertebrates, including those cryptic species inhabiting ground waters) have yet to be described (Strayer, 2006). Despite this rich diversity, extinction risk of freshwater species has been largely overlooked (Strayer and Dudgeon, 2010).

Complex networks of interacting species play important roles in the maintenance of biodiversity, the stability of food webs, and the ecosystem services that communities provide. Predicting the myriad of impacts that climate change will have on ecological interactions is a complex task. Species will respond individualistically to climatic and atmospheric changes. The geographic ranges and/or temporal coincidence of species that currently interact may therefore progressively move apart, while species that do not presently co-occur may do so in the future. Novel species combinations will result, and many present-day relationships between species may become increasingly decoupled. Changes in species interactions have enormous potential to alter community structure and composition, and these impacts may be even greater than the direct effects of a changed climate.

In a world characterized by increasing mean temperatures and a higher frequency of climatic extremes, species will reorganize their geographic distributions to track changing conditions, evolve new environmental tolerances, or risk local or global extinction. Limitations on rapid evolutionary change for most organisms of conservation concern suggest that range shifts are the most feasible mechanism for evading extinction under climate change, and migration was a common mode of species response following postglacial warming (Davis and Shaw, 2001). Changes in the geographic ranges of species will cause changes in communities, altering biological systems as we know them and affecting ecosystem services and functions. Central to these ecological changes are changes in cultural, economic, and aesthetic values.

Are a million species at risk of extinction from climate change? This book has explored the analyses required to answer that question. From estimates of the number of species on Earth, to their imperilment by climate change, we have looked across taxa, across methods, and across geographies to estimate risk.