Honey, I shrunk the animals!

Study warns that many species are shrinking as a result of climate change. Scientists predict damaging consequences that will impact ecosystems and human livelihoods across the globe.

It is now widely accepted that climate change presents a significant threat to the Earth’s ecosystems. Over the last 100 years, the planet has experienced a 1°C rise in temperature, and a further 7°C rise is predicted by 2100¹. A large amount of climate change research has been concentrated on distribution and behaviour shifts in species, however, more recently efforts have shifted towards the effects on growth and development².

The study from Professor Bickford and Dr Sheridan (National University of Singapore)² highlights the growing amount of evidence which describes the effect of climate change on species size. Species are exhibiting smaller sizes and slower growth rates in response to increased temperature and higher rainfall variability. 

The evidence for this shrinking effect is extensive. Fossil records have demonstrated similar effects in previous periods of global warming, where both marine and terrestrial species decreased in size². Most notably during the warming phase of the Paleocene-Eocene Thermal Maximum (PETM), evidence shows a range of soil-burrowing invertebrates decreasing in size by 50-75%³. During the PETM temperatures increased by 3-7°C and rainfall decreased by 40%, so there is scope for comparisons as this is similar to the current predictions for the next 100 years². Current climate change, however, is occurring at a much faster rate than during the PETM, so shrinkage effects may be accelerated³.

Experimental and comparative studies show that organisms across many taxa are smaller when they are exposed to conditions akin to climate change. For instance, considerable growth rate reductions have been observed in species as a result of water acidification (a consequence of increased atmospheric carbon dioxide)². Red algae mass exhibit a 250% loss under acidic conditions⁴ and calcifying organisms such as corals become smaller and can even lose their ability to form exoskeletons⁵. Researchers have discovered that for a range of plant species, each degree of temperature increase can reduce the aboveground biomass by 3-17%⁶. Further comparative studies have also demonstrated that decreased rainfall can lead to species shrinkage in tropical trees⁷, mammals⁸ and amphibians⁹.

In addition to this, an increasing number of observational studies show that anthropogenic climate change over the last century has already caused species shrinkage. This is true of several mammals (including polar bears¹⁰, red deer¹¹ and woodrats¹²) and birds (including passerines¹³ and gulls¹⁴) in response to increased temperature, and of many reptiles (Including tortoises¹⁵ and iguanas¹⁶) in response to decreased rainfall. Plant species show a decrease in size in response to both increased temperature and decreased rainfall, thus meaning that trees and plants are getting smaller in areas that are becoming drier and warmer¹⁷.

Scientists have suggested several explanations for species shrinking; however Sheridan & Bickford (2011)¹ suggest that water and nutrient limitation and changes in metabolic rate of ectotherms are the most significant factors. Soil nutrients can decrease as regions become drier (more fires lead to nitrogen loss) and wetter (nutrient leaching), and decreased rainfall leads to reduced respiration. This can limit plant growth; effects which can cascade through a food web and disrupt the ecosystem. Metabolic rate is proportional to temperature¹⁸, thus growth is limited unless an organism can meet its metabolic needs at increased temperatures. An extensive range of species will be subject to these effects if they are not able to rapidly adapt to climate change. Natural selection favours smaller individuals in these conditions, which could eventually lead to the evolution of smaller species².

Trends in the shrinking of species are variable and difficult to predict. It is thought that the biggest dilemma will be the differential responses amongst species, as this could disrupt the balance of whole ecosystems and have synergistic negative impacts on biodiversity². One concern is that if producers shrink at a quicker rate than consumers, this will put pressure on the consumers and could lead to increased mortality, disease susceptibility and reduced reproductive output. Such effects could destabilize ecosystem interactions and threaten populations². Also, organisms with shorter generation times may be more resilient to climate change, as they are able to adapt more quickly than those with longer generation times, creating further ecosystem disparity¹⁷.

A major threat to species is extinction and thus loss of biodiversity. Species with narrow temperature tolerance and/or low population levels are the most susceptible². Amphibians and other ectotherms which become smaller to offset the effects of increased metabolism might become more susceptible to dessication².

Marine systems are also likely to be severely affected, particularly because of varied rates of response to climate change which could destabilize ecosystems². For instance, calcifying organisms have exhibited differing responses to acidification.

Studies have shown that clams and oysters decrease in size, whilst lobsters and crabs increase¹⁹.  Furthermore, warmer waters cannot hold as much dissolved oxygen, so organisms will encounter metabolic challenges².

The impact of shrinkage on human livelihoods across the globe is arguably the most worrying consequence of species shrinkage. Fish stocks are expected to decrease in size and number owing to climate change, and this will impact the 1 billion people who rely on fish as their primary food resource²⁰. Crops on land will also be affected as areas become drier or as rainfall variability increases, potentially reducing the length of growth seasons²¹. This will place a huge strain on countries which depend on crops to feed the population. Water limitation is also expected to be a severe issue in areas such as South Asia, where population growth rates are exploding²¹.

The consequences of species shrinkage are potentially widespread and devastating, and the publication of this study has rightfully attracted the attention of scientists, media and the public. We must now work together to fully understand this issue and put in place steps to reduce the potential damage to humans and animals alike. 

1.     IPCC Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) (Cambridge Univ. Press, 2007).

2.     Sheridan, J.A. & Bickford, D. (2011). Shrinking body size as an ecological response to climate change. Nature. DOI: 10.1038/nclimate1259

3.     Smith, J. J., Hasiotis, S. T., Kraus, M. J. & Woody, D. T. (2009). Transient dwarfism of soil fauna during the Paleocene–Eocene Thermal Maximum. Proc. Natl Acad. Sci. USA. 106, 17655–17660.

4.     Jokiel, P. L. et al. (2008). Ocean acidification and calcifying reef organisms: a mesocosm investigation. Coral Reefs. 27, 473–483.

5.     Ries, J. B., Cohen, A. L. & McCorkle, D. C. (2009). Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37, 1131–1134.

6.     Hovenden, M. J. et al. (2008). Warming and elevated CO2 affect the relationship between seed mass, germinability and seedling growth in Austrodanthonia caespitosa, a dominant      Australian grass. Glob. Change Biol. 14, 1633–1641.

7.     Parolin, P., Lucas, C., Piedade, M. T. F. & Wittmann, F. (2010). Drought responses of flood-tolerant trees in Amazonian floodplains. Ann. Bot. 105, 129–139.

8.     Yom-Tov, Y. & Geffen, E. (2006). Geographic variation in body size: the effects of ambient temperature and precipitation. Oecologia 148, 213–218.

9.     Brady, L. D. & Griffiths, R. A. (2000). Developmental responses to pond desiccation in tadpoles of the British anuran amphibians (Bufo bufo, B. calamita and Rana temporaria). J. Zool. 252, 61–69.

10.  Regehr, E. V., Amstrup, S. C. & Stirling, I. Polar Bear Population Status in the Southern Beaufort Sea Open-File Report 2006–1337 (US Geological Survey, 2006).

11.  Post, E., Stenseth, N. C., Langvatn, R. & Fromentin, J. M. (1997). Global climate change and phenotypic variation among red deer cohorts. Proc. R. Soc. Lond. B. 264, 1317–1324.

12.  Smith, F. A., Browning, H. & Shepherd, U. L. (1998). The influence of climate change on the body mass of woodrats Neotoma in an arid region of New Mexico, USA. Ecography 21, 140–148.

13.  Gardner, J. L., Heinsohn, R. & Joseph, L. Shifting latitudinal clines in avian body size correlate with global warming in Australian passerines. (2009). Proc. R. Soc. B 276, 3845–3852.

14.  Teplitsky, C., Mills, J. A., Alho, J. S., Yarrall, J. W. & Merila, J. (2008). Bergmann’s rule and climate change revisited: Disentangling environmental and genetic responses in a wild bird population. Proc. Natl Acad. Sci. USA 105, 13492–13496.

15.   Loehr, V. J. T., Hofmeyr, M. D. & Henen, B. T. (2007). Growing and shrinking in the smallest tortoise, Homopus signatus signatus: the importance of rain. Oecologia 153, 479–488.

16.  Wikelski, M. & Thom, C. (2000). Marine iguanas shrink to survive El Niño. Nature 403, 37–38.

17.  Franks, S. J. & Weis, A. E. (2008). A change in climate causes rapid evolution of multiple life-history traits and their interactions in an annual plant. J. Evol. Biol. 21, 1321–1334.

18.  Gillooly, J. F., Brown, J. H., West, G. B., Savage, V. M. & Charnov, E. L. (2001). Effects of size and temperature on metabolic rate. Science 293, 2248–2251.

19.  Vincent, G., de Foresta, H. & Mulia, R. (2009). Co-occurring tree species show contrasting sensitivity to ENSO-related droughts in planted dipterocarp forests.Forest Ecol. Manage. 258, 1316–1322 .

20.  Daufresne, M., Lengfellner, K. & Sommer, U. (2009). Global warming benefits the small in aquatic ecosystems. Proc. Natl Acad. Sci. USA. 106, 12788–12793.

21.  United Nations. (2009). Food and Agriculture Organization How to Feed the World in 2050. UN. http://go.nature.com/WFBRBm [date accessed: 26/10/11].

22.  Bryant, D. (2011). Marine Wildlife Photography: Fish Schools. Seapics.  http://www.seapics.com [date accessed: 26/10/11]

23.  Seaggreen, D. (2010). A field of dried crops due to drought in Monduli District in Tanzania. Michigan State University. http://news.msu.edu/story/7277/ [date accessed: 29/10/11]