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Society for American Archaeology
Earthworm Activity: A Source of Potential Disturbance of Archaeological SedimentsAuthor(s): Julie K. SteinReviewed work(s):Source: American Antiquity, Vol. 48, No. 2 (Apr., 1983), pp. 277-289Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/280451.
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2/14
EARTHWORM ACTIVITY:
A SOURCEOF POTENTIALDISTURBANCE
OF
ARCHAEOLOGICALSEDIMENTS
Julie
K. Stein
Conspicuous
disturbances
in
archaeological
sites
are
readily
detected
during
excavation.
However,
one
animal
whose
destructive
effects
are
not
often
recognized
is the earthworm. Work at the Cariston
Annis
mound
n
Kentucky,
an Archaic shell
midden,
has
resulted
in
the
identificationof
areas
of
extensive
earth-
wormdisturbance.
Archaeological
ites
most
readily
affected
are those
with
the
appropriate
vegetation
cover,
moisture and
temperature
conditions,
and available chemical elements. The
type
of
disturbance
a site
will
undergo
depends
on
the
species
of
earthworms
present. Subsurface-casting pecies
mix
matrix
only
below the
surface while surface-castingspecies bringthe fine-grainedmatrix to the surface, thus concentrating arger
objects
below
ground.
If
earthwormcasts
are
identified
n
a
profile,
one
should
proceed
cautiously
with
inter-
pretations
concerning
soil matrix.
POSTOCCUPATIONALDISTURBANCE
of
archaeological deposits
is
an
expected
phenomenon
for most
archaeologists.
Rodents
move
through
strata
accumulating
their
backdirt
at
the surface.
Tree
roots
penetrating
downward
can
be
ripped
up
if
the
tree
is blown
over.
Differential
freezing
and
thawing transports
large
objects
upward,
while
ants and some
earthworms
are
capable
of
transporting
the same
material
to
lower levels.
Wind, water,
and
gravity
scatter
material
in
every
direction,
while
catastrophic
events such as
earthquakes
can
completely
obliterate
any
original
order.
Human
activity
(plowing,
excavating)
is often the most
powerful
force
of
disturbance.
Wood
and
Johnson
(1978)
have described nine
types
of
disturbance that
affect
archaeological
deposits.
One
type
of
disturbance,
faunalturbation-the
mixing
of
sediment
by
animals
(Thorp
1949)-is
commonly
observed
in
archaeological profiles
as
networks of
abandoned
animal burrows.
A
bur-
row,
after
abandonment,
fills with
material from another soil horizon.
Differences in soil texture
and
color
allow the
feature to be
easily recognized.
But
some
animal burrows
are
so
small that
they
are
not
easily
detected;
ants,
earthworms,
spiders,
and
crickets are a few
of
the
small
animals
that disturb the
soil
(Kiihnelt
1955).
Of
all
these small
creatures,
the
earthworm
is
the
most
easily
overlooked
although
it
is often the
most
destructive.
Earthworm burrows
are
so
small
they
may
go
undetected.
Instead
of
filling
with
material
from
another
soil
horizon,
these burrows can fill
with
the
excreta
of the
earthworm.
The
excrement
is
sometimes
very
similar
macroscopically
to
the
sediment
surrounding
the burrow.
The
role of
the
earthworm
as a
maor
source
of
disturbance
in
archaeological deposits
is
here
examined. The discussion begins with a description of the life cycle of earthworms together with
the
optimal
conditions
for the
animals'
survival. The
disruptive
effects of
earthworms
are
described
and
illustrated
by
results
of
investigations
at a
shellmound
in
western
Kentucky.
Sites
most
vulnerable to
earthworm
activity
are
identified,
and
the
means for
determining
earthworm
habitation
are
presented.
The
discussion
concludes with
cautionary
notes on
the
types
of
analyses
that
are
precluded
if
earthworms
have been
churning
the
debris.
Julie
K.
Stein,
Department of
Anthropology,
DH-05,
University
of
Washington,
Seattle,
WA
98195
Copyright
?
1983 by the Society for American Archaeology
0002-7316/83/020277-1
3$1
.80/1
277
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AMERICAN
NTIQUITY
EARTHWORMS
The
earthworm
is
said
to
be the most
important
macroanimal
of
soils. Its
significance
was
recognized
by
Darwin
(1890)
as
early
as 1837 when he
began
his observations
on
the
abundance
of earthworms and their effects on soil. In 1878 the Danish soil scientist P. E. Muller (Cruickshank
1972)
identified earthworm
activity
as a crucial element
involved
in
the
genesis
of
forest soils. He
described an
A horizon
as a
layer
of
thoroughly
mixed
mineral material and well
decomposed
humus.
Both
the
mixing
of
the
two
components
and the
decomposition
of
the
organic
material is
enhanced
by
the
activities
of
earthworms
(Pitty
1978;
Wilde
1958).
Although
earthworm
species
differ
in
size and
behavior,
the
general
activities
of all
species
are similar
(Edwards
and
Lofty 1972). They
move
through
the
soil,
displacing
and
ingesting
mineral and
organic
matter. Earthworms
especially
favor
dung,
succulent
herbage
(grass),
and
tree leaves.
The leaves
of
ash,
hickory, tulip
tree,
dogwood,
and basswood are
among
the
most
favored;
foliage
of
oaks
and conifers are least favored
(Satchell
1967;
Satchell and
Lowe
1967;
Bocock
and Gilbert
1957;
Gilbert and
Bocock
1960;
Barley
1959;
Thorp 1949).
After the material has passed through their digestive tracts, earthworms eject it, as castings,
on the surface
or
in
soil crevices. Casts have far
greater stability
and
water-storage capacity
than
the soil
surrounding
the casts.
In
fact,
with continuous
activity,
earthworms can alter
a
soil's
structure
from its
original
form to
a
granular
structure
composed totally
of
castings.
Forest
soils
typically
have
a
crumb
structure
produced
from
abundant
organic
material
that has
been
pro-
cessed
by
earthworms.
The
granular-cast
structure
in
some soils
is so
resistant
that it
was able
to
withstand
rigorous
water
erosion and
air-drying
tests conducted
by
Guild
(1955).
There is a
variety
of
explanations
for
the
way
in
which
the stable
granules
are
formed.
First,
the
casts
are
simply
reinforced
mechanically
by
filaments
of
vascular
bundles
from
ingested
plant
remains.
Second,
soil
particles
are cemented
together
by
calcium humate
formed
in
the
worm's
in-
testines
by
the
calciferous
glands.
Third,
bacterial
populations,
present
either in
the
dung,
the
gut,
or
the
casts,
are
responsible
for
gluing
the soil
particles together
with
bacterial
gums. Fourth,
the development of fungal hyphae after excretion stabilizes wormcasts. And fifth, the presence of
calcium stabilizes
clay.
Earthworms
work
the soil to create extensive
burrow
networks
that are more or less vertical
for
most of
their
depth,
but branch
near the surface.
Under unfavorable
surface
conditions
(e.g.,
soil
temperature greater
than
10?C,
or
mean annual
precipitation
of less than 560
mm
[Buntley
and
Papendick
1960]),
earthworms
will
excavate
to
depths
of 6
m
(Scully
1942:504;
Edwards
and
Lofty 1972:118).
In winter,
one
or more
individuals
curl
up
at the bottom
of a burrow
and
hiber-
nate.
An
earthworm's
average
life
span
is
usually
less than
two
years
(Satchell
1967).
They
are eaten
by
toads,
frogs,
snakes,
turtles, birds,
moles,
and
shrews.
Earthworms
are
preyed
upon
by
humans.
The
giant
Australian
earthworms
(Megascolides
australisj,
which
attain
lengths
of
more
than
3 m,
are
supposedly
hunted
by
aborigines,
who
consider
them
a
great
delicacy
(Schaller
1968:61).
If
the soil becomes
saturated
with
water,
earthworms
are forced
to the surface
for
ox-
ygen
to avoid
drowning.
However,
extended
exposure
to ultraviolet
light
on
the
surface
is
also
deadly.
Of
the
more than
1,800
species
of
earthworms
in the
world,
Lumbricus
terrestris,
reddish
in
col-
or,
and
Aporrectodea
trapezoides
(formerly
called
Allolobophora
caliginosa
[Gates
1972]),
which
is
pale pink,
are
the most
common
species
both
in
Europe
and
in the
eastern
and central
United
States.
Neither
species
is
native
to the United States.
L.
terrestris,
the common
nightcrawler,
rapidly
replaced
native
species
as
forests and
prairies
were
cultivated.
Olson
(1928) reports
that
L.
terrestris became
widely
distributed
over Ohio
during
the
1920s;
and
according
to
Smith
(1929),
this
species
was
noticed near
Champaign,
Illinois
in
1896.
Most earthworm
studies
in soil
zoology
have
been
conducted
in
England
on
European species
(Darwin 1890; Edwards and Lofty 1972; Evans 1948a, 1948b; Evans and Guild 1947, 1948a,
1948b;
Guild
1948, 1951,
1955;
Nelson and
Satchell
1962;
Satchell
1955, 1958,
1967;
Satchell
and
Lowe
1967).
The
ecological requirements
discussed
here are
derived
from those studies
and
may
278
[Vol. 48,
No.
2,1983
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EARTHWORM ACTIVITY
AND
DISTURBANCE
not reflect
accurately
the
requirements
of
similar North American
species.
Until
research is
con-
ducted that
specifically
defines the behavior
of this continent's
species,
we
are forced to
apply
the
European
data.
L. terrestris is the most thoroughly studied species. It draws leaves and other materials into the
mouth
of
its
burrow,
and
produces
casts that
are
excreted
only
on the
surface around the
burrow
opening.
Thus this
species transports
great
quantities
of
sediment
to
the surface and can
conse-
quently bury
Roman walls
(Darwin
1890),
patios
(Wood
and
Johnson
1978),
and artifacts
(Atkinson
1957;
Rolfsen
1980)
within
a
few
years.
In
contrast,
A.
trapezoides,
also introduced from
Europe,
feeds below the surface
on dead
her-
bage,
bacteria,
and
fungi, excreting
casts in
nearby
soil crevices.
The results of this behavior
are
not
as
noticeable as
those of
the
common
nightcrawler
because casts
are
not
visible
on
the
sur-
face.
The
animal
ingests
and excretes
as much sediment as does L.
terrestris,
but
subsurface
casting
precludes
burial
of
objects.
Native
North
American
species
are not as
adaptable
environmentally
as these two
imported
individuals.
OPTIMAL
CONDITIONS FOR
EARTHWORMS
Earthworms can
inhabit
a wide
variety
of
environments,
but certain
conditions
are considered
optimal.
Texture, moisture,
temperature,
available food
source,
agricultural
practices,
and soil
acidity comprise
the most
important
characteristics
of
the
animal's
environment.
Texture
Soils
with
medium textures
create the best
habitats
(Evans
1948a;
Guild
1948,
1951).
Too much
sand
promotes drainage
and
lowers the
amount
of
moisture
and
organic
matter the
soil
can
re-
tain,
while
excessive
clay
restricts
burrowing
because
of
increased soil
hardness.
According
to
Guild
(1948:184),
a
light-loam (silt-loam
or
loam
[Buol
et
al.
1973])
texture is
optimal
for earth-
worms.
Moisture
Water
constitutes
75-90% of
the
earthworm's
body weight (Grant
1955)
and
to
maintain this
percentage,
moisture must
be
available all
year.
Yet too
much
moisture
will
cause
drowning.
Soil
moisture is
affected
by evapo-transpiration
(i.e.,
plant
factors such as
the
rooting, drought
tolerance, and stage and rate of plant growth; climatic variables including air temperature and
humidity,
and wind
velocity
and
turbulence;
and soil
characteristics
such
as
texture,
soil
stratification,
and
moisture suction
relations).
Most
earthworms do not
inhabit
soils
in
regions
with
mean
annual
precipitation
less
than 560 mm
(Buntley
and
Papendick
1960),
and
they
will not
occupy water-logged
locations. To
a
large
degree,
soil
porosity
and
landscape
relief control soil
moisture.
Proper drainage,
which
aerates
the
soil,
helps
keep
moisture
at a level
appropriate
for
earthworms.
Temperature
Temperature
influences
the
fecundity
of
certain earthworm
species
(Evans
and
Guild
1948a).
The
optimum
soil
temperature
is
10?C.
Earthworms are
rarely
seen
in
soils
where the
mean an-
nual air temperature is colder than 7 ?C. Earthworms can tolerate extreme seasonal
temperatures;
they respond
by moving
deeper
into the soil.
Gerard
(1967)
believes
that this
migra-
tion
occurs
when
the soil
temperature
falls below 5
?C.
Inactivity
will
also ensue
when
279
Stein]
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AMERICAN ANTIQUITY
temperatures
rise
significantly.
Gates
(1961)
reports
that earthworm
species
in
the
tropics
become inactive
(little
movement or
feeding) except
during
the monsoon
season,
although
the
movement
may
be related
to
changes
in
moisture.
Food
Source
To
thrive,
earthworms
require
an
abundant food
source;
the
type
of
food consumed varies
with
species.
Surface-feeding species
usually
prefer
tree
leaves,
grass,
and
dung (Bocock
and Gilbert
1957;
Barley
1959;
Satchell
1967;
Satchell and
Lowe
1967;
Thorp
1949).
Subsurface-feeders
prefer decomposed plant
material,
subsurface
vegetation
(roots),
humic
material,
and
microorganisms
(bacteria
and
fungi) (Barley
1959;
Tracey
1951;
Waters
1955).
The abundant
organic
content
of
archaeological
debris serves as
an excellent food
source,
especially
for
subsur-
face feeders.
Agricultural Practices
While some
agricultural techniques promote
earthworm
activity,
others are detrimental.
In re-
cent
years many
forests,
especially
on
floodplains,
have
been cleared
for
cultivation.
With
the
loss
of
leaf litter as a
food
source,
earthworm
populations
declined
sharply.
According
to
Satchell
(1958:214),
earthworm
populations
decreased
by
70%
when
grasslands
were
plowed.
If
an area
has
been cultivated in recent
years,
the size of the earthworm
population depends
not
only
on
available
food
but
also
on
the
types
of
additives
the
farmer
has
applied.
If
fertilizer
(food
source)
or lime
(a
calcium
source,
see
below)
is added
to
a
field,
the
earthworm
population
will
usually
ex-
pand (Barley
1961;
Tischler
1955).
Even some herbicides
will increase
the
population
by
providing
more
dead
herbage
on
which
earthworms
can
feed.
Pesticides,
on
the other
hand,
often
eliminate
earthworms.
Depending
on the
type, they
can
either
kill
outright
or
accumulate
in
the
earthworm's
tissues until a
toxic
level is reached.
Soil
Acidity
The
pH
of a
soil also
influences the earthworm's condition.
Tolerance
to
changes
in
acidity
varies
widely depending
on the
species
involved. Most
species
cannot
tolerate
pH
values below
4.5
(Edwards
and
Lofty 1972)
and
prefer
neutral
conditions,
a
pH
around
7.
It
is
uncertain
whether
pH
condition
is itself the factor
limiting
earthworm
survival
(Satchell
1955;
Guild
1951).
Guild
(1955)
has
suggested
that it is the
lack
of
calcium
in
acid
soils that inhibits earthworm
metabolism,
rather than the
pH.
Calcium
is
required
for
the
function of the
calciferous
gland,
an
organ
believed
by
some
to
affect
digestion
of
food
(Guild
1955).
EFFECTSOF EARTHWORMS ON ARCHAEOLOGICAL ITES
Earthworms
disrupt
archaeological
sites
in the
following ways.
1.
They
obliterate
stratification
within the midden
matrix
by
mixing
sediment.
Earthworms,
especially
subsurface-casting species,
can
mix material
from
adjacent
strata,
blending
the
colors
and
textures into
hybrid
materials
with intermediate
properties.
Archaeologists
differentiate
site
profiles by noting
differences
in color and
texture,
but earthworm
activity
can
produce
over-
lapping parameters,
thus
inhibiting
the
archaeologist's
ability
to delineate
strata.
2.
Surface-casting species
can
bury objects
by systematically
bringing fine-grained
matrix
to
the
surface.
Rolfsen
(1980)
reports
that in an
experiment
conducted
with
marked
chert and
ceramic
fragments,
earthworms
buried
the material
to
depths
as
much
as 45 cm below
the
original position
after
five
years.
3. Earthworms obscure boundaries of soil and archaeological horizons. The animals will
especially
disrupt
the boundaries
of
archaeological
features
such
as
burial
pits
and
hearth
outlines.
This
phenomenon
is
particularly
noticeable
in
areas
of
contact
between cultural
and
280
[Vol.
48,
No.
2,1983
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6/14
EARTHWORM
ACTIVITY
AND DISTURBANCE
noncultural
deposits.
If
such contact
is
located
within
the
upper
meter
of
sediment,
earthworms
will
effectively
blur
the
boundary.
4.
Earthworms
may
alter the botanical
assemblage
preserved
in
a site. Because
earthworms
can ingest anything smaller than the diameter of their mouths (approximately 2 mm), any small
carbonized
plant
remains
(i.e., seeds)
that
were
incorporated
at the site could
have been
digested
and
decomposed.
It is
unfortunate
that,
because
some
cultigens
and
wild
plants
are
usually
iden-
tified
by
the
presence
of their
seeds and
not
by
larger
plant parts,
earthworms can
selectively
remove
the evidence
for
the
presence
of such
plants.
Large
volumes
of
sediment must
be
pro-
cessed
to
insure
recovery
of
small botanical
remains.
5. Earthworms
alter the
chemistry
of
soils. Worm
casts and soil
samples
taken
adjacent
to
the
casts have
been
compared
chemically (Lunt
and
Jacobson
1944).
The results
indicate that casts
have
higher
soluble
concentrations
of
almost
all
soil
elements.
Such
properties
as
pH,
total and
exchangeable
calcium,
exchangeable
potassium
and
manganese,
available
phosphorus,
total ex-
changeable
bases,
and
organic
matter all
appear
at
higher
concentrations
in
casts than
in
the sur-
rounding
soil.
If an archaeologist is
using
the
chemistry
of soils to locate
archaeological
sites, then earthworm
activity
is not
a
significant
problem.
But if
small
chemical differences are
being mapped
within
a
signature
of
prehistoric
activity.
EARTHWORM ACTIVITY
AT
THE
CARLSTON ANNIS MOUND
The
Carlston
Annis
mound is
an Archaic shell
mound
located
on
the
Green
River,
in
west-
central
Kentucky.
The site
was
occupied
between
5149
+
300 B.P.
and 4349
+
300 B.P.
(Watson
and
Marquardt
1979).
Archaic inhabitants
subsisted
on
mussels
and fish
from
the
river,
as well
as various
nuts,
plants,
and animals
from
the
surrounding floodplain
and
uplands.
These
people
discarded
debris and so created
the
mound
that
today
rises
2
m
above the
surrounding plain,
with
dimensions of 80 by 100 m (Figure 1). Besides shells, animal bones, charred plant remains, and
abundant rocks and
sediments,
the
site
contains numerous
human
burials
(Marquardt
and
Wat-
son
1976).
At
the
Carlston Annis
mound the
dominant
earthworm now
present
is the
European species
Aporrectodea
trapezoides
(identified by
William
Fender,
personal
communication
1981).
Native
North
American
species
identified thus
far are
Diplocardia
ornata
(Gates 1942)
and D.
varivesicula
(Murchie
1966);
both
are
subsurface-casting species.
The abundance of
subsurface
casts
suggests
that these
animals
have been
active
for
some
time.
Certain
characteristics of
the
mound are
ecologically
favorable for
earthworms
in
general
and
especially
for
A.
trapezoides.
The
shell midden
has
a silt-loam to
loam texture
(mean grain
size is
.006
mm).
This
texture,
combined with
the
pore
space
created
by
the
presence
of
shells and sand-
stone
fragments, creates an ideal environment for a subsurface-casting earthworm.
The
moisture
content of
the soil is
also
ideal.
Kentucky's
mean annual
precipitation
of
1,220
mm
(Schwendeman
1958)
maintains
soil
moisture
throughout
the
year.
The
porosity
and
relief
of
the
mound
provide
the
necessary drainage
to
aerate the wet
soil,
keeping
the soil
moisture at about
30%.
Historically
recorded
temperature
extremes between
43 ?C
and
-29
?C
rarely
occur
with
a
suddenness
that
exceeds the
earthworm's
ability
to
burrow. In the
winter the
maximum frost
penetration
is
approximately
37
cm
(Wood
and
Johnson
1978:336),
leaving
sufficient room for
the
earthworms
to
hibernate.
Before
modern
vegetation
clearance,
the
food source of the
earthworm
population
at the
Carlston
Annis
mound was
most
likely
derived from the
tall
canopied
forest
and
understory plants
that
dominated the
area
(Wagner
1979),
as well
as
decomposed
plant
material,
subsurface
vegetation
(roots),
humic
residues,
and
microorganisms
(bacteria
and
fungi).
The
high
organic
content of the occupation debris was a nutritious supplement to the diet. In recent years, as the
natural
floodplain
vegetation
was
removed and
insecticides
applied,
the earthworm
population
has
probably
declined.
281
tein]
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7/14
15
Bt 5
CARLSTON
NNIS ITE
BUTLER
COUNTY,
KY
CONTOUR INTERVAL 0.25 METERS
0 10
20
30 40
50
Mag.
+
0,0
SITE
DATUM
TOP
Of
WELL
ASSUMED
TO
BE 102 00
M
Figure
1.
The
topography
of
the
Carlston
Annis
mound with
the
surrounding
plain
and
the
Green
River
(dra
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8/14
EARTHWORM ACTIVITYAND DISTURBANCE
The
chemistry
of the mound
deposits
also
represents
optimal
conditions for the animals. In the
shell
midden,
the
pH
ranges
from 7.6 to 8.3 with an
average
of 7.8. This
slightly
alkaline
pH
results
from the abundant
calcium
released
from
shells.
At the
mound,
earthworms benefit from
both ideal pH conditions and abundant available calcium, as well as from the presence of other
nutrients such as
nitrogen, phosphorus,
and
carbon.
Calculations
of
Earthworm
Activity
Close examination
of
a
profile
at
the
mound
reveals abundant earthworm casts. The
casts,
found
principally
in
large
concentrations
in
soil
crevices,
indicate
the
activity
of subsurface
casters.
The
casts
are
continuously
distributed
from
near the surface
to
the lowest
midden
deposits
2 m below the
surface,
with few cast-free
deposits
observable. Because
casts are
ex-
tremely
durable,
their
presence
does
not
only
indicate modern earthworm
activity;
like
the mid-
den,
they
could have been
accumulating
during
the
years
of
site
deposition,
or
they may
have been
deposited
in
the
4,000
years
since
deposition
ceased.
European scientists have calculated the number of earthworms that inhabit certain soils as
well
as the amount
of
material
they
rework.
Although
comparing
such
calculations
to
a
site
in
Kentucky
is
highly
speculative,
it
does serve a
purpose.
The
calculations
indicate
just
how
poten-
tially disruptive
earthworms
can
be should
an
archaeological
site be located
in
a
setting
that
is
optimal
for
the
animals'
survival. The
Carlston
Annis site
represents
such
a
site with
extraor-
dinarily
beneficial conditions.
Although
these calculations are
not
applicable
to
all forested
sites
located
in
open floodplains, they
are
presented
here
to
warn
archaeologists
of
the
potential
ef-
fects
of
earthworms.
The
number
of
earthworms
necessary
to
rework the entire
5,848
m3 of
mound material
in
4,000
years
can
be
roughly
estimated
(Table 1).
Of the total midden
volume, 1,345
m3
is
matrix. Studies
in
England estimating
the number
of
earthworms
in
soil
(reviewed
in
Satchell
[1967:273])
indicate
that the
weight
of
earthworms
in
the
top
17
cm
of
the soil was 148-162
g/m2
on
land
with
mixed
woodland
forests,
a volume of between 871 and 953
grams-of-earthworm/m3.
The
Carlston
Annis
mound was surrounded
by
mixed-woodland forests
for
most
of
its
history
(Wagner
1979).
Even
Table 1. Estimate
of
Earthworm
Population
n the
Carlston
Annis Mound.
Earthworm
Calculations
Number
of
worms
5,848
m3
(total
volume
of
mound)
23 %
1,345 m3 (volumeof matrix in mound)
871-953
g/m3 (g
of
earthworms in
1
m3,
[Satchell
1967])
1,345
m
1,172,000
g (g
of
earthworm
n
matrix)
or
2,724,000
(No.
of
whole
earthworms
n
matrix)
Rate
soil
is
ingested
4,423-43,411
g/m3/year
(weight
of soil
ingested
by
a
large
earthworm
population
n one
year,
from
Evans
[1948b],
Satchell
[1967],
and Guild
[1955])
1
m3
=
979,000
g
(weight
of
midden)
1
m3
=
225,000
g
(weight
of
matrix
only)
Time
required
to
rework matrix
equals
51
years
Time
site
exposed
to
matrix
reworking
equals
4,000
years
Stein]
283
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9/14
AMERICAN ANTIQUITY
though
the
mound
affords
the
food
and soil conditions
preferred
by
earthworms,
the
lower
estimate
of
871
g/m3
is
adopted
for this
study.
The maximum biomass
associated
with
earthworms
for the site's
1,345
m3
of matrix
is
1,172,000 g or 2,724,000 whole earthworms. This population estimate represents the number of
worms the mound's
matrix could
theoretically
support
if
each
17-cm
depth
offered
conditions
similar
to
those
found
in
English
soils.
Obviously,
more worms must be located
in
the
upper
17 cm
of the
mound than
in the lower
2 m.
But as
the
midden
material was
accumulating,
earthworms
were
present
at
each surface.
Therefore,
it
is
probable
that
throughout
its
history
the
matrix
could
support
a
total of
almost
3,000,000
earthworms.
The
weight
of soil that
earthworms
ingest
has been calculated
(Evans
1948b;
Guild
1955;
Satch-
ell
1967)
by
collecting
and
weighing
casts added
to
the
surface
in
one
year.
Although
the
weights
are based
on
casts
from
species
that excrete
on the
surface,
it
is
assumed
by
Satchell
(1958:210)
that
the
casting weights
of
species
that
void
below
the
surface must have
similar values.
The
weight
of
soil
ingested by
a
large population
in
one
year
ranges
from 752 to
7,380
g/m2
(4,423
to
43,411
g/m3).
In the
mound,
1
m3 of midden
weighs
979,000
g
(Stein
1980).
In the
midden,
1
m3
has
an
average
of 23% matrix, which therefore
weighs
225,000 g.
The results
of
these calculations
indicate
that it would take
almost
3
million earthworms
only
51
years
to
ingest
and
disrupt
all
the
matrix
in the
Carlston
Annis
site.
Obviously
this calculation
is
not a
precise
measurement
of the rate of earthworm
disturbance.
It
assumes
no
negative
feed-
back mechanisms
limiting
the
reproduction
or
ingestion
rates
of
the
earthworms. The
presence
of
moles,
birds,
and
other
predators presently
at the site
suggests
that
depletion
of
the
population
is
occurring.
However,
it
does
demonstrate
how
easily
earthworms
could
modify
the entire
matrix
of
the
midden.
Effect
of
Earthworms
at
the
Carlston
Annis
Mound
At the
Carlston
Annis
mound,
earthworm disturbance
has
been so
great
that four of the
five
disrupting effects are evident.
1. The texture
of the midden matrix
shows
a
mixing
of
two sources
of
sediment,
sandy-silt
river
deposits
and
a
clayey-silt
lake
deposit.
Figure
2
illustrates
the
grain-size
distribution
of the
two
sources
(noncultural
deposits)
and
the
resulting
distribution
of the midden
(cultural
deposits)
after
earthworms have
mixed
the matrix.
Although
the
frequency
diagrams
do
not
prove
that
earth-
worms
were the
mechanism,
they
do
suggest
that the material
has
been
mixed.
2. Boundaries
in the mound
deposits
are almost
entirely
absent.
Burials have
no
discernible
pit
boundaries.
The sediments
below the mound
grade
subtly
into the
midden
matrix.
Even
the
plow
zone is
undistinguishable
unless
sprayed
with a fine mist
of
water.
If one examines
the areas
where
boundaries
should
be
evident,
only
earthworm
casts
are
detected.
3.
In
the
project's
collection
of charred botanical
remains
we
have found
very
few seeds
(Wagner 1979). But because the site has been totally reworked by earthworms, the scarcity of
seeds need
not
imply
that
the
inhabitants
of the
mound did
not
rely
on these
plants.
However,
because
a
large
volume
of sediments
have
been
processed
by
flotation
and water
separation
(over
8,000
1),
one would
expect
to find
more seeds and
seed
fragments
than
are
now
available.
4. At
the
Carlston
Annis
mound
the
chemistry
of
the
archaeological profiles
shows little
soil
horizonation.
Profiles
commonly
contain
horizons with
well
defined soil
zones,
(e.g.,
A, B,
and
C
horizons),
but chemical
elements
in the shell
midden
are
not distributed
throughout
the archaeo-
logical profile
in
the
traditional manner.
Their distribution
reflects
the
results
of
mixing
and
dis-
rupting processes.
Many
factors can
interrupt
the
development
of soil horizons
in
a
profile
(e.g.,
intrusion
of
burials,
ground
water
fluctuations,
and
additional
sedimentation),
but
at
the Carlston
Annis mound,
which
is an
open-air
forested
landscape
composed
of
fine-grained
sediments,
the
source
of
disruption
is most
likely
earthworms.
Forest
soils
usually
have
well
defined
surface
zones (A horizons) composed of mineral and humus components mixed by earthworms. In the
mound,
earthworms
have extended
the
mixing beyond
the surface
zone
to
a
depth
of 2
m,
altering
284
[Vol. 48,
No.
2,1983
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EARTHWORMCTIVITY ND DISTURBANCE
Figure
2. The
upper
diagram
illustrates
the
grain-size
frequency
curves constructed for the noncultural
deposits
near
the
Carlston Annis
mound. These two sediment
types
are
easily
distinguished
from
each
other: the lake sediment
(L)
has more
clay
and less sand than
the
samples
collected from the river
deposits
(R).
The lower
diagram
illustrates the
grain-size
frequency
curves constructed
for
the cultural
deposits.
Their
similarity
indicates
a
random
mixing
of the two noncultural
deposits
(river
and
lake), probably pro-
duced
by
earthworm
activity. Samples
from
the shell midden
(SM),
the shell-free midden
(SfM),
and
the col-
luvium
(C)
all have
comparable proportions
of
sand, silt,
and
clay. Samples
were
analyzed
following
Folk
(1974),
including
treatment
with
H202
to remove
organics.
285
Stein]
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11/14
AMERICAN
ANTIQUITY
the
distribution
of
chemicals
within
the
profile.
Any
archaeological
interpretations
based
on
the
chemical
analysis
of
samples
from
the
matrix
of
this
mound
must
be considered
suspect.
Although
these
observations do
not
prove
that earthworms were the
dominant mechanism
disturbing the midden, the facts that the midden is rife with casts, the matrix is unstratified, the
chemical
elements
in
the soil are
mixed,
and the soil
conditions are ideal
for
earthworm
habitation,
all
strongly
suggest
that
these
creatures have indeed
played
a
major
role
in
disturbing
the
deposits.
CONCLUSIONS
The Most
Vulnerable
Kind
of
Archaeological
Sites
The
types
of
archaeological
sites most
vulnerable
to
earthworm
disturbance are
those
that
display
the
previously
described characteristics
of
food,
soil
texture,
and
soil
moisture favorable
to
the animals. These characteristics
are
most
frequently
found
in
forested
regions
in
open
set-
tings.
Favorable habitats associated
with
rivers are
levees,
dry
portions
of
the
floodplains,
and
any colluvial fans located on the margins of the floodplain. Glacial terrain with moraines, kettle
lakes
and
loess-covered
slopes
would also
provide
numerous favorable
habitats,
although
out-
wash and
gravel
features are unfavorable. Sites located
in
favorable
geomorphic
situations are
likely
to have witnessed earthworm
activity.
Sites located in areas with unusual
physical
characteristics will be
exempt
from
earthworm
disturbance.
Extremely sandy
conditions
such
as those found on some beaches and
in
dune
fields
will not
support populations.
Caves,
whether
wet or
dry, rarely
provide
suitable
habitats.
Rockshelters
are
generally
too
dry,
as are
sites in
arid
or semiarid
regions.
Sites
presently
under-
water or
waterlogged
are also
protected, although they may
be disturbed
by
other
organisms.
Sites that
do not
presently support
earthworm
populations may
have
supported
them in the
past. During
the last few
millennia,
environmental
changes
have occurred
in
many
areas,
either
on
a
regional
or
local scale.
Mean annual air
temperature
and
precipitation
has
fluctuated
since
the end
of
the
Pleistocene,
and
forest
composition
has
changed dramatically (Davis
1976;
Delcourt
and
Delcourt
1979;
Wright
1976,
1981).
Also,
the effects
of
forest
clearance,
drainage
modifica-
tions,
highway
construction,
and
expanding
urbanization have altered
local environments
significantly.
When
looking
for
evidence
of
earthworm
disturbance one
must
consider
the
possibility
of climatic and
environmental
change
for
the area
in which the site is located.
How
to Detect the Presence
of
Earthworms
To
detect
the
presence
of
earthworms
in
a site
one must
examine
the soil structure
closely.
Macroscopically,
the
presence
of
0.5-mm-sized
granules
of
matrix
or burrows
approximately
10
mm in
diameter are
indicative of earthworm
activity.
But
an
effective
study
would
have to
include
the use
of
micromorphological
techniques (Brewer
1964;
Goldberg
1979;
Rutherford
1972)
to
iden-
tify
how the
earthworms
have
altered the
archaeological deposits. By making
thin-sections
of the
soil
structure and
examining
the features
microscopically,
one
could
ascertain
the
degree
to
which the
soil
has
been altered. One should also
collect and
identify any
earthworms encountered
during
excavation.
The
species
of
earthworm
inhabiting
the
site
may
differ in the
way they
affect
the
soil,
either
casting
on the surface or in soil crevices. The difference
in
casting
will
determine
how
much
disturbance has
occurred.
Although
the earthworm
has often been
overlooked,
its
disruptive
force
is
obviously great.
Ex-
cavators should
be aware that earthworms can
be
reworking
the matrix
of
a site.
They
may
not
necessarily
affect the
position
of
larger
objects,
but
will
definitely
disturb
material
less than
2
mm
in
diameter.
If
earthworm
casts
are identified
in a
profile,
one should
proceed
cautiously
with in-
terpretations concerning
soil
chemistry.
Because
the earthworm
aerates
the soil and
processes
organic
material,
it
has
long
been
recog-
nized as a
friend
of the farmer.
But,
as
I
hope
I
have demonstrated
in
this
paper,
the earthworm
may
be
an
unsuspected
nemesis for the
archaeologist.
286
[Vol.
48,
No.
2,1983
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EARTHWORMACTIVITY
AND DISTURBANCE
Acknowledgments.
This research was a
part
of
the Shell Mound
Archaeological
Project
directed
by
Patty
Jo
Watson and William
H.
Marquardt.
It
was
supported by
a National Science Foundation Grant BNS
7808
916,
and a
University
of Minnesota Dissertation
Fellowship.
I
would like
to
thank
Stanley
E.
Chernicoff,
Herbert
E.
Wright, Jr.,
Patty
Jo
Watson,
Janet
E.
Levy,
and Karl
Butzer for their
assistance
in the
editing
of
the
manuscript, and William H. Marquardt for his contribution in drafting. Also special thanks must go to the peo-
ple
of
Logansport, Kentucky,
especially
John
L.
Thomas,
Waldemar Annis and
Ethie
Annis,
for
their
assistance
in
the collection of
earthworms
from
their
land,
and their
general
cooperation
and
enthusiasm.
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