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Tropical climates and the biodiversity associated with them have long interested natural historians. Alexander von Humboldt inspired a generation of scientists, such as Charles Darwin and Alfred Russel Wallace, to observe and study tropical ecosystems. More recently, the mid-20th century saw Theodosius Dobzhansky and Daniel Janzen lay the foundations for studying adaptation to tropical climates. Now in the 21st century, we are beginning to realize the threats posed by current and future climate change to tropical populations which, despite relatively low levels of projected warming for low-latitude regions, face potentially significant detrimental impacts. Building on the insights of researchers in decades and centuries past, improved understanding of tropical ecology, evolution and biogeography will help us to conceive how future global change will impact on biodiversity.
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The future of tropical biodiversity in the face of anthropogenic climate change is highly uncertain despite the fact that the tropics are home to the vast majority of known species globally (Bradshaw et al., 2009). This high uncertainty relative to temperate latitudes arises due to a number of factors, including a lack of data and understanding generally regarding tropical biodiversity (Bonebrake et al., 2010) and uncertainty in climate change projections in the tropics themselves (Laurance & Useche, 2009). However, recent studies have demonstrated the possibly high vulnerability of tropical ecosystems to climatic warming (e.g. Colwell et al., 2008; Deutsch et al., 2008; Tewksbury et al., 2008; Chen et al., 2009; Loarie et al., 2009; Sinervo et al., 2010; Bonebrake & Deutsch, 2012; Diamond et al., 2012).
Although the interest in climate change impacts on tropical species has only come about in recent years, I outline here a long history of thought regarding adaptation to tropical climates which has provided a strong foundation for current interest and research on tropical biodiversity and climate change. This is not a comprehensive review on climatic adaptation or latitudinal diversity gradients. Nor do I seek to review climate change impacts on tropical biodiversity. Instead, the aim of this paper is to put recent studies in context and guide future research by illuminating the informative and foundational history of study on adaptation to tropical climates. First, I discuss important and still highly relevant contributions to our understanding of climatic adaptation by 19th century scientists. Second, immediately following the Modern Synthesis, critical ideas regarding adaptation and the tropics were developed and formed the basis for our understanding of tropical ecology today. Finally, I briefly review recent studies of climate change impacts on tropical ecosystems and how they relate to earlier 19th and 20th century ideas of climatic adaptation.
All climate change impacts studies in the tropics can be traced back to early ideas relating species and biodiversity to climate. Sometimes the links between the past research and current research are explicit and sometimes they are not. My goal here is not only to clarify these links but also to encourage further investigation of ideas and works of the early pioneers who studied adaptation in tropical climates. As I hope to convince the reader in the paragraphs that follow, sometimes the transformative papers of today are essentially tests of hypotheses formed in the 1800s.
Early History: Von Humboldt, Darwin and Wallace
Others before von Humboldt knew of the latitudinal diversity gradient, but in the beginning of the 19th century he most famously and eloquently first described how species numbers (plants in particular) were greatest near the equator (Hawkins, 2001). But he didn't stop there. As Hawkins (2001, p. 470) puts it, ‘von Humboldt also provided a general hypothesis for latitudinal gradients (climate), as well as a specific causal factor (winter temperatures) and a mechanism (loss of fluidity; i.e. freezing).’ In fact, von Humboldt discussed the monthly variation in temperature and its importance for life many times throughout his writings (von Humboldt & Bonpland, 1821).
Without the aid of evolutionary explanation, von Humboldt still understood that the biogeography of plants and animals was ‘regulated’ differently in temperate and tropical regions:
These regular migrations of birds from one part of the tropics toward the other, in a zone which is during the whole year of the same temperature, are very extraordinary phenomena. […] We must presume, that the variations of drought and humidity in the equinoctial zone have the same influence, as the great changes of temperature in our climates, on the habits of animals. The heats of summer, and the pursuit of insects, call the humming birds into the northern parts of the United States, and into Canada, as far as the parallels of Paris and Berlin; in the same manner a greater facility for fishing draws the palmipede and long legged birds from the north to the south, from the Oroonoko toward the Amazon.
Thus von Humboldt realized that, in the tropics, species distributions were largely dependent upon elevation (as a proxy for temperature) as well as precipitation variability more than temperature as determined by latitude. Appropriately, von Humboldt is credited for the notion that temperature dictates global species distributions in addition to precipitation (Hawkins et al., 2003).
In a letter to Alfred Russel Wallace in 1865, Charles Darwin wrote that ‘nothing ever stimulated my zeal so much as reading Humboldt's Personal Narrative’ (Darwin, 1903, p. 263). Darwin followed in von Humboldt's footsteps through the forests of South America and was also highly influenced scientifically by von Humboldt. With the power of natural selection Darwin (1859) also looked to explain the impact of different climates across latitude on species in On the Origin of Species:
It is notorious that each species is adapted to the climate of its own home: species from an arctic or even from a temperate region cannot endure a tropical climate, or conversely […] But the degree of adaptation of species to the climates under which they live is often overrated.
He supports this statement with many examples from agriculture and domestication (Darwin, 1859). Darwin understood the complexity of climate–species relationships but could also use natural selection to explain some of the variation. In particular, it was Darwin's belief that climate acted indirectly, e.g. through the regulation of food resources, to determine the competitive environment in which natural selection would take place (Hoffmann & Parsons, 1991). It was only at high elevations and high latitudes that Darwin felt climate could act directly through ‘injurious action’ (Darwin, 1859). Nevertheless, his view was ultimately very similar to von Humboldt's as he wrote in On the Origin of Species: ‘climate plays an important role in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks’ (Darwin, 1859, p. 68).
Von Humboldt's Personal Narrative was also the key work which inspired Alfred Russel Wallace to visit the tropics where he famously and independently of Darwin formulated the theory of evolution by natural selection (Chazdon & Whitmore, 2002). Wallace too had ideas similar to Darwin and von Humboldt regarding climate and the latitudinal diversity gradient but he went further to incorporate geological and historical determinants (Wallace, 1878; Berry, 2002). Also like von Humboldt, Wallace was preoccupied with climatic uniformity at multiple temporal scales in the tropics and its impact on species diversity:
Whether we consider the differences between day and night temperatures, the variations of temperature from month to month or from year to year, or those extreme variations which we experience once perhaps in a generation or in a century […] all alike are unknown in the equatorial regions, save in a few limited and quite exceptional areas. In these more favoured portions of our earth there prevails such a general approach to uniformity of conditions (without ever reaching absolute uniformity) as seems best adapted to bring about the greatest productivity, together with extreme diversity in every department of the great world of life.
These early pioneers of ecology and evolution advanced the fields dramatically. The mechanism of heredity was a problem, however, and so set the limits of their explanation. Not until the Modern Synthesis did further advances regarding evolution in the tropics really develop beginning with Theodosius Dobzhansky's (1950) aptly titled paper ‘Evolution in the tropics’.
Evolution In The Tropics: Dobzhansky and Janzen
While Fisher, Haldane and Wright began the Modern Synthesis, it was the publication of Genetics and the Origin of Species by Dobzhansky (1937) which marked the second critical and multidisciplinary phase (Gould, 2002). Shortly afterwards, Dobzhansky turned his eye towards the tropics. In 1941 he wrote in a letter to Sewall Wright regarding an upcoming research venture in Brazil (Araújo, 2004, p. 470): ‘I think there is a possibility of much to be gained by studying population structure in species living in a climate that changes little as possible during the year.’ Over the next decade Dobzhansky spent many months doing research on Brazilian Drosophila culminating in his influential 1950 paper ‘Evolution in the tropics’ where he noted that the ‘changeable environments’ of temperate systems ‘put the highest premium on versatility rather than on perfection in adaptation’ (see Bonebrake & Mastrandrea, 2010 for further discussion). Additionally, much like von Humboldt, Dobzhansky (1950, p. 217) understood that the tropics were not so simply uniform and that ‘the limiting factor for life in the tropics is often water rather than temperature’. Reflecting on his work in Brazil, in 1962 Dobzhansky noted:
That led to a very simple idea: if the genetic drift [in temperate Drosophila populations] is due to seasonal alterations, chiefly winter resulting in destruction of the flies, then what would happen in a tropical climate where winter never comes? There season after season the population should be large enough to eliminate genetic drift. […] Now, I just as well say at this point that this proved to be wrong. It proved to be wrong because although there are no winter-summer seasons in the tropics, seasonal changes are by no means absent.
In fact, the tropics can be highly climatically variable over short distances and times or seasons, especially in the context of drought or aridity (Vanzolini & Williams, 1970). Similar to Darwin's climate-acting-indirectly idea, Dobzhansky's (1950) paper is frequently cited for his hypothesis that evolution in temperate environments is largely due to ‘physical agencies’ while in the tropics, challenges derive ‘chiefly from the intricate mutual relationships among the inhabitants’ (Dobzhansky, 1950, p. 221). However, as evident by the quotations above, to say that Dobzhansky believed climatic factors irrelevant to evolution in tropical environments overstates the point. In any case, his evolutionary insights into the origin of tropical biodiversity influenced much of the research on species latitudinal diversity gradients that followed (Brown & Sax, 2004).
In a significant advance for evolutionary research in the tropics, Daniel Janzen (1967) posited that in fact, the uniformity of thermal seasonality could subsequently serve as a ‘physiological barrier’ to tropical species given adaptation to narrow thermal ranges. He used elevational gradients to present his argument with three central themes revisited by Ghalambor et al. (2006). (1) Janzen (1967) surmised that it was the magnitude of elevational temperature gradients that acted as barriers to dispersal and not distance itself. (2) The overlap of thermal regimes would differ in temperate and tropical environments such that, a low-elevation tropical species might not overlap in climate at all with a high-elevation tropical species, whereas in temperate environments seasonality of temperature is more likely to result in some climate overlap between low- and high-elevation species. Finally, various studies (Ghalambor et al., 2006; McCain, 2009; Sunday et al., 2011) have found substantial evidence consistent with the hypothesis (3) that organisms are adapted to their encountered thermal regimes (and therefore tropical organisms have narrow thermal tolerance ranges).
Thus, while most before him saw the uniformity of temperature in tropical environments as evidence of temperature's relative inconsequence, Janzen realized that there are important evolutionary impacts of uniform temperature regimes. Multiple implications for ecological patterns have come from this observation (Kozak & Wiens, 2007; Bonebrake & Deutsch, 2012). For example, Stevens (1989) uses it as a central point in introducing Rapoport's rule, i.e. species at low latitudes exhibit more narrow ranges with respect to latitudinal extent than high-latitude species. Janzen's (1967) work did not figure prominently in climate change research until recently (Fig. 1). However, hints of its importance are apparent earlier: Stevens (1989, p. 251) writes that as a prediction of Rapoport's rule ‘a climate change that is minor to an organism from a high latitude is a major (possibly life-threatening) change to an organism from a lower latitude, even though the magnitude of the change in climate is the same’.
Conservation and Climate Change Implications for Tropical Populations
The climate change impacts literature is strongly biased towards temperate systems (Parmesan, 2006). Even for the golden toad (Bufo periglenes) of Costa Rica, reportedly the first species to have gone extinct due to climate change in the modern era (Pounds et al., 1999; Flannery, 2005), the role of anthropogenic climate change in its extinction is debatable (Rohr et al., 2008). Given such sparse evidence for contemporary climate change impacts on tropical biodiversity is there any reason to worry?
Following the revisiting of Janzen' hypothesis in Ghalambor et al. (2006), Deutsch et al. (2008) modelled the impacts of warming for terrestrial ectotherms under a future warming scenario and found relatively large negative fitness impacts for low-latitude organisms compared with those from higher latitudes. Two primary causes are responsible for this result: (1) tropical ectotherms tend to be adapted to narrow ranges in temperature (see above discussion on Dobzhansky and Janzen); and (2) tropical ectotherms frequently live closer to their critical thermal maximum than temperate organisms and therefore cannot tolerate as much warming (Deutsch et al., 2008; Angilletta, 2009). The Deutsch et al. (2008) study opened the floodgates and many high profile papers have since confirmed the relative high vulnerability of tropical populations to climate warming (e.g. Colwell et al., 2008; Tewksbury et al., 2008; Chen et al., 2009; Huey et al., 2009; Kellermann et al., 2009; Loarie et al., 2009; Dillon et al., 2010; Sinervo et al., 2010). Similar studies relating historical climatic variability to future warming have shown humans (agriculture in particular) might be especially at risk in tropical countries (Battisti & Naylor, 2009). This research has been influential in a recent paradigmatic shift in tropical conservation; only in the past few years have we really begun to acknowledge climate change as comparable to deforestation and hunting in threatening tropical biodiversity (Corlett, 2012).
Although theoretical and empirical difficulties abound in projecting future precipitation changes and how those changes might impact on species, there is also reason to believe that shifts in precipitation regimes could have large effects on tropical ecosystems and populations (Laurance & Useche, 2009; Bonebrake & Mastrandrea, 2010; Clusella-Trullas et al., 2011). Changes in aridity, for example, are likely to have strong impacts on individuals physiologically (Chown & Terblanche, 2007; Hoffmann, 2010) in addition to causing larger-scale ecosystem alterations (Brodie et al., 2012). Furthermore, Williams et al. (2007) projected high rates of disappearing climates in tropical montane ecosystems as well as high appearance of novel climates within tropical latitudes resulting from projected future seasonal changes in temperature and precipitation. Given the importance of rainfall patterns and seasonality for most tropical systems, as von Humboldt suggested, these changes to the climate are very likely to compound the detrimental warming impacts projected for tropical species.
Additional environmental changes are likely to compound the impacts of climate change on tropical biodiversity (Brook et al., 2008). The combination of land use and climate changes could imperil high numbers of tropical species (Jetz et al., 2007). Changing disturbance regimes, fire especially, could result in complex feedback loops that could amplify the impact of climate change in the tropics (Laurance & Williamson, 2001; Brodie et al., 2012). The challenge of tropical conservation in a warming world is clearly great given the consequences of these potentially interacting threats, in addition to others such as disease, hunting, and invasive species (Brook et al., 2008; Gardner et al., 2009; Corlett, 2011; Rohr et al., 2011).
The case is by no means closed, however, and some studies have questioned the apparent high vulnerability of tropical species to future warming relative to other species (Chown et al., 2010; Hoffmann, 2010). Higher projected impacts for low-latitude species are complicated by the effects of a variety of factors including extreme thermal variation (Hoffmann, 2010), biotic interactions (Gilman et al., 2010), microhabitat (Clusella-Trullas & Chown, 2011; Sears et al., 2011), acclimation (Angilletta, 2009), climate interactions (e.g. thermal load as determined by changes in cloud cover) (Chown et al., 2010), and that the relative change in temperature might not be as important as absolute changes (Corlett, 2012). What is clear from all of these recent studies is the need for continued research on climate change effects on species globally, involving the varied and often complex components that determine vulnerability (Williams et al., 2008; Huey et al., 2012).
The tropics served as an important source of inspiration to early natural historians. The tremendous scientific contributions of von Humboldt, Darwin and Wallace are difficult to overstate and their ideas arose directly from their extensive travel in tropical environments. Their early insights into climatic adaptation specifically remain relevant today and have informed recent assessments of vulnerability of tropical ecosystems to climate change. After 200 years of study many questions remain about the processes underlying latitudinal gradients in biodiversity and temperate versus tropical differences and similarities. Further theoretical investigations and continued data collection regarding tropical climatic adaptation will serve to refine our understanding of global biogeographical patterns and to improve projections of climate change impacts in the tropics. Such research will be vital if we ourselves are to adapt effectively (politically and otherwise) to climate change (Yohe et al., 2007).
Thanks to Carol Boggs, Lauren Ponisio and helpful referees/editors for input. Special thanks to Dan Janzen (in addition to the obvious reasons outlined above) and Winnie Hallwachs for hosting me at the Area de Conservación Guanacaste, Costa Rica, where much of this research was inspired.
Timothy Bonebrake studies global change impacts on biodiversity and has broad interests in natural history, ecology, evolution, conservation biology and Lepidoptera.