intro.html

<h4>How do animals experience climate variability and change?</h4>

<p>Species and ecosystems continue to respond to climate change in unexpected ways– for example, shifting their distributions and abundances in directions opposite that expected. Such surprises highlight the need to translate our physical metrics of climates and climate change into how organisms are experiencing their environments. Differences in shape, coloration, and composition can lead two organisms to experience their shared environments very differently.</p>

<p>The body temperatures of both ectothermic (cold-blooded) and endothermic (warm-blooded) organisms can differ dramatically from air temperatures.  The body temperature of an ectotherm is determined by the exchange of heat with its environment. Endotherms additionally use endogenous heating to alter body temperatures. This means the endotherms produce heat internally for temperature regulation. </p>

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<h4>Radiation: Reflection and Absorption</h4>

<img src="albedo.jpg" width = "300px" align='left' />
<img src="ice-albedo.png" width = "250px" align='right' />

<p>A primary source of heat is the absorption of solar radiation. The physical properties of the surface of an organism are an important determinant of the amount of solar radiation absorbed. Albedo is a measure of how much light that hits a surface is reflected without being absorbed, wherein darker surfaces absorb more heat.</p>

<p>A common example used to show the great importance of albedo is the ice-albedo feedback. Ice has a high albedo (thereby absorbing little heat from solar radiation). The ice cover on poles of the Earth keeps it cooler than open ocean or vegetation (which have lower albedos). However, global warming of the polar regions has begun melting our large ice sheets -- Antarctica and Greenland -- and the Arctic sea ice. As the polar region loses ice, the polar albedo decreases due to more exposed open ocean and vegetation. A decrease in albedo means more solar radiation is absorbed, causing yet more melting of ice. And so it continues in a <i>positive feedback loop</i>.</p>

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<h4>Heat and Temperature</h4>

<p>The body temperature of an organism is determined by the balance of energy (heat) loss and gained from the environment. If an organism gains more heat from the environment that it loses, it will warm up. Think about what happens when you stand in the sunshine on a bright day. When you stand under the sunshine, you are gaining more heat from the sun than you are losing, so you warm up. There are different forms of heat flow, but the basic concept is that heat is exchanged via diffusion -- which you may recognize from chemistry classes as the movement from high to low concentration. In the case of heat, the movement is from high temperature to low temperature. Heat moves from warm to cold.  Think about what happens when you hold a chocolate bar.</p>

<p>Temperature and heat are related terms that are sometimes used interchangeably, but they have different meanings.  Here’s a thought question you should try before moving on: define temperature and heat.</p>

<p>A bit tricky?  Temperature is the average energy of molecular motion in a substance whereas heat is the total energy of molecular motion.</p>

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<img src="exchange.png" width = "250px" align='right' />

<h4>Forms of Heat Flow</h4>

<p>Four primary types of heat flow determine the body temperature of an organism:</p>

<p><b>Radiation</b>: A heat transfer between two substances that are not in contact. The largest source of heat in most cases is the absorption of solar radiation. Animals radiate energy at a rate proportional to their body temperature (thermal radiation).</p>

<p><b>Conduction</b>: A heat transfer involving two objects in physical contact with each other. Organisms can alter rates of conduction by changing their amount of contact with surfaces.</p>

<p><b>Convection</b>: A heat transfer between a solid and a liquid or a gas. Wind and water speeds influence how much heat organisms exchange with their environments via convection.</p>

<p><b>Evaporation</b>: A loss of heat due to the liquid water changing into gaseous water. Evaporation occurs when a fraction of water molecules have sufficient heat energy to escape a liquid. Organisms are cooled by evaporation until an equilibrium is reached where the air supplies the amount of heat removed by the evaporating water.</p>

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<h4>Homeostasis and the influence of behavior</h4>

<img src="pika.png" width = "200px" align='right' />

<p>Organisms are adapted to function well over a relatively narrow range of body temperatures and other physiological conditions. Thus, organisms use physiological and behavioral processes to maintain relatively stable conditions, termed <b>homeostasis</b>, despite changes in their internal and external environments. </p>

<p>Many organisms use behavior change to maintain homeostasis with their environment. Have you ever tiptoed across hot sand or pavement to avoid heating up your feet? Fanned yourself after a work-out? These (and many more!) are behaviors humans do to maintain homeostasis. </p>

<p>Moose are found in temperate forests throughout the northern hemisphere. During the summer, this large herbivore selects three kinds of food to meet its nutritional requirements: small herbaceous plants, leaves, and aquatic plants. On Isle Royale in Lake Superior, Michigan, USA, individuals are commonly found eating in ponds or deciduous stands or resting under conifers. Associated with each of these habitats is a microclimate that depends on the ground temperature, sunshine, wind speed, and humidity. Because of many factors including its large body size and its small tolerance for changes in body temperature, the moose can quickly overheat if active during midday. Belovsky (1977) has made calculations indicating that during the summer, moose carefully choose feeding times to minimize metabolic output with the constraint that the risk of overheating be small.</p>

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<h4>Net heat exchange with the environments</h4>

<p>A first step in understanding how organisms respond to their environment is to estimate how heat losses to and gains from the environment balance to determine the organismal body temperature. Such a <b>biophysical model</b> is conceptually as simple as balancing a bank account, but the estimation of heat flows can be complicated. <b>Steady-state</b> models describe the energy budget once the organismal temperature has come to equilibrium with its environment, meaning that heat flow has stabilized.</p>

<p>An energy budget at the surface of an organism can be estimated as follows:</p>

<p align = "center">
𝑄𝑛𝑒𝑡 = 𝑄𝑎𝑏𝑠 − 𝑄𝑒𝑚𝑖𝑡 − 𝑄𝑐𝑜𝑛𝑣 − 𝑄𝑐𝑜𝑛𝑑 − 𝑄𝑚𝑒𝑡 − 𝑄𝑒𝑣𝑎𝑝,
</p>

where:<br>  
𝑄𝑛𝑒𝑡 is the net energy exchange with the environment (W), <br>
𝑄𝑎𝑏𝑠 is the solar and thermal radiation absorbed (W), <br>
𝑄𝑒𝑚𝑖𝑡 is the thermal radiation emitted (W), <br>
𝑄𝑐𝑜𝑛𝑣 is energy exchange due to convection (W), <br>
𝑄𝑐𝑜𝑛𝑑 is energy exchange due to conduction (W), <br>
𝑄𝑚𝑒𝑡 is the energy emitted due to metabolism (W), and <br> 
𝑄𝑒𝑣𝑎𝑝 is the energy emitted due to evaporative water loss (W).

<p>Rates of thermal radiation, conduction, convection are determined by the difference between the temperatures of the animal and its environment, so we can solve for the body temperature that balances the energy budget. We will omit heat exchanges via metabolism and evaporation in this exercise for simplicity and because they are often only minor components of an energy budget.</p>