Wednesday, 30 November 2011

New Project: Comparative test of two conflicting metabolic scaling theories

My apologies for the lack of posts recently, but unfortunately I've had so much to do that I kind of fell out of the habit of updating my blog. I will endeavour to post more frequently from now on! I began a masters in September and have since begun work on my new research project, for which my supervisor is Dr David Atkinson. Here is a brief summary of my project:

Metabolism provides a crucial foundation to all biological processes, consequently meaning that theories which describe the rate at which metabolic rate scales with body mass could have great predictive power in ecology. There are currently scientific groups trying to predict this variation and determine the scaling relationship for species across wide ranges of taxonomic groups. This has led to the emergence of two main groups of competing theories; the first focussing on surface area and the second on transportation “networks”. Both groups of theories predict differing slopes when size increase is equal in three dimensions compared to when size increase is in two dimensions only (or in some dimensions more than others such that surface area remains directly proportional to body mass). This study will attempt to resolve some of this conflict and provide a powerful test of the competing metabolic scaling theories by collating extensive data from scientific literature to examine the relationship between metabolic rate and “shape” across a wide range of marine invertebrate taxa.  

It sounds a little daunting at first, but it is actually incredibly interesting! There is huge potential for this type of theory in the field of ecology, so if my project manages to distinguish between the different theories and determine which (if any!) is a more accurate representation of the truth, then it will (I hope) be a great success. I aim to have completed the data analysis by February, so I will keep you updated on the results! For now, I should get back to the endless task of data crunching..

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. [date accessed: 26/10/11].
22.  Bryant, D. (2011). Marine Wildlife Photography: Fish Schools. Seapics. [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. [date accessed: 29/10/11]

Thursday, 24 March 2011

The hourglass dolphin

Fig. 1   The hourglass dolphin (Tidman, 2011)

Common name: Hourglass dolphin              
Latin name: Lagenorhynchus cruciger (Quoy & Gaimard, 1824)

Cruciger is Latin for “cross-bearing” and denotes the area of black and white pigmentation on the back of the dolphin which when viewed from above, resembles an hourglass (Brownell & Donahue, 1999).

Kingdom:          Animalia
Phylum:            Chordata
Class:               Mammalia
Order:              Cetacea
Family:             Delphinidae
Genus:             Lagenorhynchus

Size and Weight
Adult length:
Male: 1.63m long (Nichols, 1908)
Female: 1.66m - 1.83m long (Fraser 1966)

There is possibly sexual dimorphism in the Hourglass dolphins, as male specimens are usually slightly shorter in length. However this is disputed to some extent due to a limited number of specimens (Brownell & Donahue, 1999).

Adult weight:
90-120 kg (Miyazaki, 1986)
The small and robust hourglass dolphin has inherited the nickname the “sea cow” due to its characteristic black and white colouring (Brownell & Donahue, 1999). The common name refers to the two white patches connected by a thin white strip on each flank, which bear a resemblance to an hourglass (Brownell & Donahue, 1999). Despite the species having no known predators, the hourglass dolphin exhibits counter-shading colouration (Brownell & Donahue, 1999). Countershading is an anti-predator defence where the belly is light in colour (blending with the light sky when viewed from below) and the back is dark in colour (blending with the dark depths when viewed from above) (Oxford Dictionaries, 2010).

The fin varies significantly between individuals, but it is generally tall and curved (Brownell & Donahue, 1999). It is thought that the curve is perhaps more pronounced in older individuals (Fraser 1966). The hourglass dolphin is easily distinguished from its similarly sized neighbour, the southern right whale dolphin (Lissodelphis peronii), as unlike the right whale, it has a dorsal fin (Brownell & Donahue, 1999).

The hourglass dolphin has a circumpolar distribution, in the higher latitudes of the southern oceans (Goodall, 1997; Goodall et al, 1997; Brownell & Donahue, 1999).

Fig. 2   Hourglass dolphin distribution, dominating the cold waters of 
the Southern Hemisphere (Hammond et al, 2008).

The southern range is to the Antarctic ice-edges, with the most southerly sightings occurring near 68°S in the South Pacific (Goodall 1997; Brownell & Donahue, 1999). The distribution in the north is less well known, however they are found to at least 45°S. Some sightings have occurred as far north as 36°S in the South Atlantic Ocean and 33°S near the Chilean city of Valparaíso (Goodall, 1997).

Hourglass dolphins are most commonly seen around the Antarctic Convergence or between South America and Macquarie Island (Goodall, 1997). They can also be seen off the south coast of New Zealand, near the South Shetland Islands and around the Tierra del Fuego province (Goodall, 1997). The hourglass dolphin is the only small delphinid species which is regularly found south of the Antarctic Convergence (Goodall et al, 1997). Hourglass dolphins are usually found in southern waters during the summer and northern waters during the winter, this suggests that they migrate seasonally following the cold-water currents (Goodall, 1997).

The hourglass dolphin is found in the cold, deep waters of the Antarctic; however some sightings have occurred in relatively shallow waters near South America and the Antarctic Peninsula (Goodall, 1997). It can be found within 160km of the ice edge within the Southern range (Jefferson et al, 1993). It appears to have a preference for surface water temperatures of between 0.6°C and -13°C (Goodall, 1997), but can even inhabit waters as cold as -0.3°C (Goodall, 2002).

Hourglass dolphins are sociable and usually observed travelling in small groups of around 7 individuals (Kasamatsu et al, 1990). Group sizes, however, can range from 1 to 100 individuals (Kasamatsu et al, 1990). The ratio of males to females within these groups is unknown; however it is unusual to see a calf travelling with larger groups (Kasamatsu et al, 1990). It has been suggested that this could be due to ship evasion from females with calves, or perhaps the species undergoes synchronised breeding in the winter (Kasamatsu et al, 1990). There is a lack of information with regards to the parental care behaviour exhibited in the species, however, we do know that females nurse their young from birth and based on data from other species in the genus, it is thought that lactation lasts for 12-18 months (Brownell& Donahue, 1999).

Hourglass dolphins enjoy riding bow waves and wakes, and have been observed altering their direction of travel just so they can catch the waves created by travelling boats and ships (Bowles et al, 1994). They also like to ride the bow waves of fin whales, Balaenoptera physalus, regularly jumping out of the water as they play around the larger animals (Fraser, 1964). Whalers historically searched for this characteristic behaviour in order to locate the fin whale in their hunt. Hourglass dolphins also like to socialise with other species such as the minke whale (Balaenoptera acutorostrata), pilot whales (Globicephala melas and Globicephala macrorhynchus), beaked whales (Ziphiidae), sei whale (Balaenoptera borealis) and southern right whale dolphins (Lissodelphis peronii) (Kasamatsu et al, 1988).
Fig. 3   Hourglass dolphins porpoising (Pitman, 2011)

Other areas of uncertainty include the lifespan of the hourglass dolphin; however it is likely to be comparable to that of other species within its genus (Brownell & Donahue, 1999). The closely related Atlantic white-sided dolphin (Lagenorhynchus acutus) has a lifespan of 27 years, whereas the Pacific white-sided dolphin (Lagenorhynchus obliquidens) can live to 46 years (Klinowska, 1991).

Little is known about the feeding habits of the hourglass dolphin, but scientists have recorded small fish, cetaceans and squid (from the Onychoteuthidae and Enoploteuthidae families) from the stomach contents of several specimens (Clarke, 1986; Ash 1962). The species has also been observed feeding in plankton swarms and seabird aggregations (Clarke, 1986).

Like all toothed whales, hourglass dolphins use echolocation for orientation and prey location (Khyn et al, 2009). It is also likely that they also communicate using visual and tactile cues (Brownell & Donahue, 1999). A recent study showed that they produce very high-pitched clicks, which allow them to detect prey at more than twice the distance of other dolphin species (Khyn et al, 2009). It was suggested that this could be due to the deep and open nature of their habitat, meaning that the hourglass dolphin needs to cover larger areas to locate prey (Khyn et al, 2009).

There are currently no known threats to the hourglass dolphin (Brownell & Donahue, 1999). In 1995, the first and only study to date (March, 2011) was conducted on the population size of the species, combining data from surveys occurring from 1976 to 1988. This study estimated that there were 144,300 individuals for waters south of the Antarctic convergence (Kasamatsu and Joyce, 1995).

It is thought that the species is probably preyed upon by killer whales, Orcinus orca, but there hasn’t been any documented evidence of predation. The hourglass dolphin is not commercially hunted and accidental by catch is limited (Brownell & Donahue, 1999).

The Hourglass dolphin is classified as Least Concern (LC) on the IUCN Red List (Hammond et al, 2010) and listed on Appendix II of CITES. There have been no attempts to bring the hourglass dolphin into captivity, apart from several specimens being collected for scientific research (Brownell & Donahue, 1999). This is most likely due to their remote distribution (Brownell & Donahue, 1999). Increasing ecotourism in the Antarctic will likely lead to further knowledge of this species (Brownell & Donahue, 1999).

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