Biophysical Ecology
The thermal environment has a major impact on physiological performance of
ectotherms. Sprint speeds of snakes and lizards, flying abilities of insects, survival rates
of intertidal organisms, rates of water loss in plants, all are strongly influenced by body
temperature. Therefore, an understanding of the mechanisms controlling body
temperatures of ectotherms is an important aspect of population ecology.
The heat budget is an accounting of the gains and losses of heat by an organism, and can
be expressed as an equation
Change in heat content =metabolism – evaporation ± convection ± conduction ± radiation
Thus organisms gain heat, and increase in temperature if their gains are greater than their
losses.
For ectotherms, metabolic heat generation is typically very low, so we can primarily
concern ourselves with evaporation, convection, conduction and radiation.
Radiation is the absorption or emission of radiative energy. Absorption is typically of
short wave direct or indirect solar energy, and emission is typically of infra-red
reradiation by the body of the organism. In SC in the summer at noon, direct solar
radiation is approximately 1000 watts per square meter, which is why one gets so hot
standing in the sun in the summer. The rate of heat exchange by direct solar radiation is
dependent upon the intensity of the sunlight, by the cosine of the angle of incidence of
the sunlight with respect to the surface of the organism, and by the absorptivity of the
surface of the organism.
Convection is the transfer of heat to or from a moving fluid. For organisms in air, this is
“wind chill” when the air is colder than the organism. If the air is warmer than the
organism, convection will raise body temperature. The rate of heat exchange by
convection is dependent upon wind speed, upon the surface area of the organism exposed
to the wind, and upon the temperature difference between the organism and the wind.
Conduction is the transfer of heat to or from a solid surface. If the surface is colder than
the organism, the organism will drop in temperature (this is why you feel cold when
sitting on cold metal bleachers at a ball game), and if the surface is warmer than the
organism, the organism will increase in temperature (this is why you burn your feet on
the sand at the beach in the summer). The rate of heat exchange by conduction is
dependent upon the surface area of contact between the organism and the surface, and
upon the temperature difference between the organism and the surface.
Evaporation is the transfer of heat to the conversion of water from liquid to gas phase.
The latent heat of evaporation is 2.43 kilojoules per gram of water. This is a large amount
of heat. Evaporative heat loss explains why you feel warm at the beach before you get
into the water, and why you feel cold before you dry yourself off after getting out. It is
the mechanism by which we lose heat via sweating.
Today we will use small temperature data loggers to mimic small ectotherms. Your task
is to predict what kinds of microenvironments are likely to cause the ectotherm to gain
heat and warm up, and what kinds of microenvironments are likely to cause it to lose heat
and cool down. Your task is also to determine what impacts behavior and morphology
can have on heat transfer.
Your ectotherm is an “I-button” data logger, approximately the size and shape of a watch
battery. Its morphology is highly variable in both ectoderm thickness (naked or
marshmallow covered) and in ectoderm coloration (metallic, white, orange, or green).
There are also at least two behavioral types (although other behavioral traits are
suspected to occur): ground dwelling and arboreal (in clip with string attached). You have
eight (8) “I-button” data logger ectotherms to use today. Make sure to record the serial
number on each, and its associated morphological characteristics.
Your organism takes at least 20 minutes to fully equilibrate to its environment.
Your task is first to go outside and find some microhabitats where you think an ectotherm
would be warmer or and cooler. You should make your decision based on what you think
is the relative importance of radiation, conduction, and convection. Then you should
place each of your ectotherms in each of the habitats, in the appropriate posture. Leave
your ectotherms in each microhabitat for 20 minutes, keeping careful track of exactly
when you placed them in the microhabitat.
The data loggers record time and temperature once per minute. Synchronize your watch
with the computer that you use to start the data logger, so you can reconstruct what
experiments were done in which parts of the data record.
Location Environment Organism
Start End
Predicted
characteristics time time
Temperature
Change
When you finish your experiments, download your data, import into Excel and plot
graphs of temperature vs time. Did the ectotherms show the temperature changes that you
predicted? If so, why? If not, why not? What do you think is most important to their heat
budgets: radiation, convection, conduction? Why?
Data Collection and Analysis
To start a mission:
Plug the Ibutton logger into the cable socket on the computer.
Start the Ibutton software – blue circle icon.
If the “viewer” wizard does not start, press the “start viewer” button on the screen.
Press the “Wizard” tab
Press “next” in response to each prompt, and then press “finish.” The button should be
programmed to sample once per minute.
To download data for a mission:
Plug the ibutton into the cable socket.
Start the Ibutton software.
In the viewer program, press “mission status” tab.
Press the Quick Graph button to see a graph of your data.
Press the Export Data button to export the data. Select the “c:\temp” directory, and save
your file with a name that identifies your group and the kind of button (naked, furry,
arboreal), with a .txt extension on the file name.
Start Excel. From the “File” menu select open, and select your text file. The “text import
wizard” should start up. You will be prompted to select the type of data you are imported
“delimited”, or “fixed width”. Select “fixed width”. There is a bunch of junk at the
beginning of your data file, so you will also want to start the data importing at the
appropriate place. On the same page of the text import wizard, you should use the scroll
bar to the right of your data to look through it and find the line in which your data begin.
Then fill that number in the box marked “start import at row”. Select next to move to the
next step. Here you will need to establish columns for your data. The data begin about
50-100 lines down in the file so you will need to scroll down to see what columns to use.
Press the left mouse button to select the columns where fields begin and end. You will
want to make sure that the temperature data are in a column by themselves. Left click on
“finish” to complete the import. Make a line graph of your data by selecting the column
containing the temperature readings and clicking on the graph wizard.
When you are sure that your data are collected and graphed properly, return your Ibuttons
to your TA.