ii. darwin's weed garden...
TRANSCRIPT
Untangling the Bank
dramaticalk Sinc many fa tors can influence local diversity (soil
characteristics, local climate, time since last site disturbance, etc.)
it would hf' difficult to say u.:hy you' res ults are sirllilar or different
frO! 1 Darw in's. In fact, ev n replications at Dowll House, where
Darwin did the exp riment. give varying results (see .vIcLauchlin
below). Such are th vagaries of ecology.
II. Darwin's Weed Garden Experiment
Darwin recognized the difficulty in conveying to the average person
me organisms' ceas less struggl for existence and the creativ role
his plays thr ugh sel ction. Comprehending this point is all the more
diffi cult becallse struggle is experienced in different ways for different
organisms, seasons, ti m s of day, life stages , location , and many other
factors. 'V"hen we observe nature we often miss the st ruggle, seeing
only peace and harmony, and mistake this for the natu ral condition of
the living world. Darwin's aim in the vVeed Carden Exp riment was to
focllS on one aroup of organisms (plants) at just one Ii£, stage (seedling),
He thought if he could remove one set of pressur s, namely crowding
by other plants , he could id ntify the other destructive forces acting on
them (e. O'., frost, drought, insects, slugs) and measure their cumulative
effects over time. H is experiment-a pioneering investigation in pop
ulation ecology-is easily replicated . See if you can measure seedling
mortality and compare yom results with his.
A. Materials
• Notebook and pencil
• Tape measure and ru ler
• String or cord
• Carden stakes or sticks (4 per plot)
• hicken wire or aluminum fl ashing, to erect a barrier around
the plot 1 ft (- 0.3 m) high
• Wood dowels 0.12.5 in . (-3 mm) in diameter, galvani zed wire,
or t'vvist ties in 4 in. (-10 cm) lengths
116 DARWIN'S BACKYA RD
• Permanent marker
• Scissors
• Paper, cut into - Yz in. (1.3 cm) squares
B. Procedure
1. In late winter measure one or more 3 x 2-ft (0.9 x 0.6-m) plots,
marking the corners with a garden stake or stick and marking
the perimeter with string or cord (attaching the cord to the
stakes or looping around them).
2. Clear the plot by removing all existing plants; d ig the soil as
necessary to remove the roots or rhizomes of any perennials.
3. Place the chicken wire around the plot, embedding it into the
ground as best you can, as a barrier to protect the area from
animals digging or trampling the emerging seedlings.
4. Check the plot on a regular schedule, at least every 2 or 3 days. As
seedlings firs t appear, mark their location
by inserting a dowel or wire bearing a num
ber into the ground adjacent to the plant,
taking care not to contact the seedling, and
record the number in the notebook. Label
each seedling individually using the paper
squares numbered with indelible marker;
use scissors or a sharp pencil to make a front backsmall slit or hole on opposite sides of each
numbered square, and insert the dO\.vel or
wire through these to affix the square as above.
5. While you continue marking the position of each seedling as it
appears, observe and note any evidence of activity or presence of
slugs, snails, insects, birds , etc. that may harm the seedlings. At
appropriate intervals, perhaps once or twice weekly, keep a log
of the climate and soil conditions, noting average temperature,
rainfall and moisture of soil. If, as the plants grow, it is possible to
identify any of them, note that, too.
6. Census days: Once or twice month ly, approximately 2-4 weeks
Untangling the Bank 11 7
apart depending on the schedule you settle upon , remove the
numbered wires without a seedling, indicating that the seedling
has been destroved. Count and record the number of numbered
wi res. Record an:-' notes about climate or other factors t hat may
explain the numbers of los t and surviving plants.
c. Analysis
1. Following Darwin's lead , for each census day record (1) the
number of seedlings killed since the previous count, (2) the
number of surviving seedlings, and (3) the total number of seed
lings that have emerged to date.
2. Use your data to produce a survivorship table, shmving the per
centage of seedlings surviving out of the cumulative number
emerging over time. For example, on March 31, 1857, Darwin
recorded that out of 5,5 seedlings total to that date, 30 survived
and 25 had died. That gave a survivorship of 301,55 =54 percent.
At his June 1, 1857, census Darwin found just 80 surviving and
277 that had died , giving 80/357 = 22 percent survivorship. Sur
vivorship in his experiment dropped to 19 percent by July 1st,
and 17 percent by Augus t 1st.
3. Graph a survivorship curve for all the seedlings in your sample,
plotti ng percent surviving on the y-axis against census date on
the x-axis.
See also.
K. E. James, "DNA Barcoding Darwin's Meadow," in Darwin-Inspired
Learning, ed. M. J. Reiss, C. J. Boulter, and D . L. Sanders (Rotterdam:
Sense Publishers, 201.5), 257-270.
J. McLauchlin, "Charles Darwin's Lawn Plot Experiment," The
London Naturalist 88 (2009), 107-113.