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.