This is not the first indication that global warming impact on tropical species is greater than expected. Small increases in temperature in tropical ecosystems can have large impacts as many species are already near their thermal physiological maximum. Many species also have small dispersal ranges which increases their risk of extinction through small changes in habitat or environment.
Direct impact of climate warming on insect fitness
Research by Curtis A. Deutsch et al from University of Washington and Colorado State University studied Impacts of climate warming on terrestrial ectotherms across latitude (Full PDF) (PNAS on 8 May 2008 ). They examined the direct impact of warming on insect fitness at various latitudes, from the tropics to temperate and arctic environments. Here is what they reported in their article abstract:
The results show that warming in the tropics, although relatively small in magnitude, is likely to have the most deleterious consequences because tropical insects are relatively sensitive to temperature change and are currently living very close to their optimal temperature. In contrast, species at higher latitudes have broader thermal tolerance and are living in climates that are currently cooler than their physiological optima, so that warming may even enhance their fitness. Available thermal tolerance data for several vertebrate taxa exhibit similar patterns, suggesting that these results are general for terrestrial ectotherms.
Caption: Fitness curves for representative insect taxa from temperate (A) and tropical (B) locations, and (C) the change in fitness because of climate warming for all insect species studied, as a function of latitude. (A and B) Fitness curves are derived from measured intrinsic population growth rates versus temperature for 38 species. Source: Deutsch et al (2008)
Deutsch and his colleagues end the discussion section of their paper with a disturbing conclusion:
Ultimately, organisms with the greatest risk of species extinction from rapid climate change are those with a low tolerance for warming, limited acclimation ability, and reduced dispersal. Most terrestrial organisms having these characteristics are tropical and many of these organisms are occupying disappearing climate regimes. This conclusion is troubling because it places the greatest biological risks of climate change in the tropics where biodiversity is greatest.
More recently, a paper was published in October 2010 on Global metabolic impacts of recent climate warmingby Michael Dillon et al in Nature (see abstract) which argued that the metabolic rate "is a fundamental measure of physiological activity and ecological impact, increases exponentially rather than linearly with temperature in ectotherm.....Because of temperature's nonlinear effects on metabolism, tropical organisms, which constitute much of Earth's biodiversity, should be profoundly affected by recent and projected climate warming."
Caption: Figure 1: Global changes in temperature and in metabolic rates since 1980. a, Changes in mean temperature (5-year averages) for Arctic, North temperate, South temperate and tropical regions. b, Predicted absolute changes in mass-normalized metabolic rates by geographical region. Source: Michael E. Dillon et al (2010) via Skeptical Science
Latest science says reproduction may be the Achilles' heel
This new paper published in January 2012 - Degrees of disruption: projected temperature increase has catastrophic consequences for reproduction in a tropical ectotherm (abstract) - by Jeanne A. Zeh and colleagues from the University of Nevada, Reno, investigated the effect of elevated temperature on the life cycle of the neotropical pseudoscorpion (Cordylochernes scorpioides). It was published online in Global Change Biology.
In the introductory sentences of the abstract the authors stress that if the metabolic theory of climate warming is proved correct as per the Michael Dillon et al paper in Nature, it could have profound implications for global biodiversity. A large portion of the planet's species are found in the tropics.
Although climate change models predict relatively modest increases in temperature in the tropics by the end of the century, recent analyses identify tropical ectotherms as the organisms most at risk from climate warming. Because metabolic rate in ectotherms increases exponentially with temperature, even a small rise in temperature poses a physiological threat to tropical ectotherms inhabiting an already hot environment. If correct, the metabolic theory of climate warming has profound implications for global biodiversity, since tropical insects and arachnids constitute the vast majority of animal species. Predicting how climate change will translate into fitness consequences for tropical arthropods requires an understanding of the effects of temperature increase on the entire life history of the species.
The researchers conducted a split brood experiment on one species. Here is how they describe the experiment in the article abstract:
Here, in a comprehensive case study of the fitness consequences of the projected temperature increase for the tropics, we conducted a split-brood experiment on the neotropical pseudoscorpion, Cordylochernes scorpioides, in which 792 offspring from 33 females were randomly assigned at birth to control- and high-temperature treatments for rearing through the adult stage. The diurnally-varying, control-treatment temperature was determined from long-term, average daily temperature minima and maxima in the pseudoscorpion's native habitat. In the high-temperature treatment, increasing temperature by the 3.5 °C predicted for the tropics significantly reduced survival and accelerated development at the cost of reduced adult size and a dramatic decrease in level of sexual dimorphism. The most striking effects, however, involved reproductive traits. Reared at high temperature, males produced 45% as many sperm as control males, and females failed to reproduce. Sequencing of the mitochondrial ND2 gene revealed two highly divergent haplogroups that differed substantially in developmental rate and survivorship but not in reproductive response to high temperature.
The abstract concludes with:
"Our findings suggest that reproduction may be the Achilles' heel of tropical ectotherms, as climate warming subjects them to an increasingly adverse thermal environment."
Lizard species already suffering global decline
A 2010 study - Erosion of Lizard Diversity by Climate Change and Altered Thermal Niches (abstract) - on how increasing temperatures affect lizards, found that up to 20 percent of all lizard species are predicted to go extinct by 2080 and that ongoing extinction of populations is directly related to climate change. From the abstract:
Since 1975, we estimate that 4% of local populations have gone extinct worldwide, but by 2080 local extinctions are projected to reach 39% worldwide, and species extinctions may reach 20%. Global extinction projections were validated with local extinctions observed from 1975 to 2009 for regional biotas on four other continents, suggesting that lizards have already crossed a threshold for extinctions caused by climate change.
Migration and adaptation strategies limited
Many tropical fauna species will attempt to migrate or use adaptation techniques in dealing with higher temperatures. The velocity of climate change envelopes has been measured at 27.3 km/decade on land. But habitat loss through human development and land use changes will in many cases impede potential migration. Active preservation of plant biodiversity is one important way to provide a crucial buffer to negative effects of climate change on biodiversity.
A recent study by Mark Urban et al - On a collision course: competition and dispersal differences create no-analogue communities and cause extinctions during climate change (Full paper) - argues that extinction rates are being underestimated due to species interactions and dispersal differences, and these factors need to be included into future predictions of biodiversity under climate change.
What we can be certain of is that there is currently a Biodiversity crisis with Habitat loss and climate change causing 6th mass extinction. Acting on climate mitigation will determine the extent of the mass extinction but for many species it is already too late given the thermal inertia in the climate system.
- Jeanne A. Zeh, Melvin M. Bonilla, Eleanor J. Su, Michael V. Padua, Rachel V. Anderson, Dilpreet Kaur, Dou-Shuan Yang, David W. Zeh, Global Change Biology, 13 January 2012 - Degrees of disruption: projected temperature increase has catastrophic consequences for reproduction in a tropical ectotherm (abstract) DOI: 10.1111/j.1365-2486.2012.02640.x
- Michael E. Dillon, George Wang, & Raymond B. Huey, Nature, 6 October 2010 - Global metabolic impacts of recent climate warming (abstract) doi:10.1038/nature09407
- Curtis A. Deutsch, Joshua J. Tewksbury, Raymond B. Huey, Kimberly S. Sheldon, Cameron K. Ghalambor, David C. Haak, and Paul R. Martin, PNAS 8 May 2008 - Impacts of climate warming on terrestrial ectotherms across latitude (Full PDF) doi?10.1073?pnas.0709472105
- Climate Citizen, January 6, 2012 - Climate change and habitat loss threaten biodiversity, extinction rate underestimated
- Climate Citizen, January 4, 2012 - Species biodiversity under threat from the velocity of climate change
- Graphic from Deutsch et al (2008).
- Graphic of Metabolic Rates reproduced from Michael E. Dillon et al (2010) via Skeptical Science.