Effects of Climate Change on Animal Physiology

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The study of physiology basically deals with the functions and mechanisms within a living system. These functions and mechanisms may be influenced by the external environment. Changes in temperature for example can affect the growth, reproduction, and  survival of organisms. Thus, studying animal physiology can help us determine the effect of climate change on the diversity and distribution of organisms.

Changes in temperature
Changes in environmental temperatures have a great influence on species geographical distribution, population collapse, species extinction, and on an organism's growth and reproduction[1]. 

Molecular, cellular, and systemic processes within a living system function at a limited range of temperature (thermal window). Performance in an organism's growth, reproduction, foraging, immune competence, behaviors and competitiveness is directly affected by climatic warming. Such performance is supported by increase in oxygen consumption by aerobic scope. At temperatures below and beyond the optimal, aerobic process is negatively affected resulting to hypoxemia[1].

In fishes, thermal windows can vary between species and between populations within a species. These thermal windows are usually narrow to minimize maintenance cost. The different life stages of fishes are affected by the changes in temperature. Fish species with narrow thermal windows will be negatively affected by climatic warming.  For example, the growth and abundance of the nonmigratory eelpout in the German Wadden Sea has decreased with increase in temperature beyond the optimum. In the Japan Sea, the increase in temperature has caused a regime shift in spawning activity to anchovies where previously sardines was favored. In the Fraser and Columbia River systems, global warming has caused delayed spawning migrations of nonfeeding Pacific salmon, which potentially caused loss of fitness leading to swimming failure and mortality[1].

In zooplankton, thermal windows also vary. It has been observed that climatic warming have favored smaller zooplankton prey than larger ones in the Southern North Sea reducing the food availability for juvenile cod. This would greatly affect abundance of this species[1].

Ocean acidification
Ocean acidification is when anthropogenic CO2 emitted to the atmosphere is absorbed by the oceans causing progressive increase in ocean inorganic carbon concentrations and resulting in decreased water pH and calcium carbonate saturation. Ocean acidification is also projected to be accompanied by a rise in global mean sea surface temperature[4].

The marine ecosystem hosts a wide variety of species  ranging from organisms with simple structures to highly complex ones[4]. As such studying their physiology and their capacity to adapt to changing temperatures will aid in determining what species will be able to survive and what the community structure will look like as global warming continues.

In complex organisms, shifts in the acid-base chemistry in various body compartments are mediated by the weak acid distribution characteristics of CO2. These shifts develop in parallel to the changing chemistry of sea water to ensure proper physiological functioning in marine organisms[4]. A slight decrease in the ocean pH can result to negative effects on their reproduction and growth.

Ocean acidification prevent proper calcification of bones during development. Development of skeleton (i.e. shells) for marine organisms is pH sensitive. The absorption of CO2 by the ocean causes decrease in pH. This is because when CO2 reacts with seawater, hydrogen ion is released. The increase of hydrogen ions in the water can hinder growth and development of marine skeletons like shells.

Shells are mainly composed of calcium carbonate. The formation of calcium carbonate shells involves the binding of calcium ion with carbonate from the surrounding seawater. Carbonate also binds with hydrogen ion. The increase of hydrogen ion in seawater creates competition for carbonate resources. And because hydrogen ions have greater attraction to carbonate than calcium ions, development of calcium carbonate shells are hampered. This prevent marine organisms from growing new shells, and even shells that already exist would start to dissolve. Such situation would drive marine organisms to exert more energy into developing shells than other physiological functions like feeding and reproduction leading to decrease in size and population[6].

As ocean acidification continues, species with less expressed calcified structures like crustaceans and fishes will be more favored than organisms with heavier skeletons like corals, echinoderms, and molluscs[4].

Accumulation of hydrogen ions in the water decreases pH[6]. Decrease in water pH is harmful to marine organisms especially in saltwater fish as they are adapted to to higher pH. At lower pH fish are susceptible to fungal infections and other physical damage. Reproduction in fish is also negatively affected when pH decreases. When this happens, some fish will leave the area. In some fishes, lower pH have lethal effects[7]. 


References
[1]Portner H.O. and Farrell A.P. 2008. Physiology and Climate Change. Ecology. Vol. 322.; 690-691.
[4]Wittman A.C. and Portner H.O. 2013. Sensitivities of extant animal taxa to ocean acidification. Nature Climate Change. Vol.3; 995-1001.

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