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The Concept of Organisms as Ecosystem Engineers Ten Years on: Progress, Limitations, and
Challenges
Author(s): Justin P. Wright and Clive G. Jones
Source:
BioScience,
Vol. 56, No. 3 (Mar., 2006), pp. 203-209
Published by: University of California Press on behalf of the American Institute of Biological Sciences
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Articles
The
Concept
of
Organisms
as
Ecosystem
Engineers
Ten
Years
On:
Progress,
Limitations,
and
Challenges
JUSTIN
P WRIGHT AND CLIVE G. JONES
The
modification
of
the
physical
environment
by
organisms
is
a critical
interaction
in
most
ecosystems.
The
concept
of
ecosystem engineering
acknowledges
this
fact and allows
ecologists
to develop
the
conceptual
tools
for uncovering general
patterns
and building
broadly
applicable
models.
Although
the
concept
has
occasioned
some
controversy during
its development,
it is quickly
gaining
acceptance
among
ecologists.
We outline
the nature
of some
of
these
controversies
and describe
some
of the
major insights
gained
by
viewing ecological
systems through
the
lens
of ecosystem engineering.
We close
by discussing
areas
of research
where
we believe
the
concept
of organisms
as ecosystem
engineers
will be most
likely
to lead
to significant
insights
into
the
structure
and
function
of ecological
systems.
Keywords:
ecosystem
engineer,
conceptual
models,
biodiversity, ecosystem
function,
habitat
modification
Ecologists have long recognized
that organisms
can have
important
impacts
on physical
and chemical
processes
occurring
in the environment. While some influ-
ences invariably
arise from organismal
energy
and material
uptake
and
waste
production,
many
organisms
alter
physical
structure and
change
chemical
reactivity
in ways
that are
in-
dependent
of their
assimilatory
or dissimilatory
influence.
In-
deed, Darwin devoted an entire book to the effects of
earthworms
on soil formation (Darwin 1881). That such
changes
have
the potential
to influence
organismal
distribu-
tion and abundance
and ecosystem processes
is well recog-
nized.
Nevertheless,
while
scattered,
diverse
examples
of these
types
of organismal
effects on the abiotic
environment
have
steadily accumulated in the ecological literature
(Thayer
1979,
Naiman et al. 1988),
until recently
there has
been little
attempt to seek commonality or generality among them.
Furthermore,
ecological
textbooks have
rarely
included such
effects
among
the roster
of important
forces
structuring
eco-
logical populations
and
communities or influencing ecosys-
tem functioning;
instead,
they
have
traditionally
focused
on
interactions such as competition
and predation,
or empha-
sized
metabolically
regulated
nutrient and energy
flows.
It was to incorporate
this variety
of abiotic
environmen-
tal
modification
by organisms,
along
with its numerous con-
sequences,
that Jones
and colleagues
(1994) proposed
the
concept of ecosystem
engineering.
In their first article on
the topic,
they
defined
ecosystem
engineers
as
"organisms
that
directly
or indirectly
modulate the availability
of resources
(other than themselves)
to other species by causing...state
changes
in biotic or abiotic
materials.
In so doing they
mod-
ify,
maintain and/or
create
habitats"
(Jones
et al. 1994).
This
and
a subsequent
article
(Jones
et al. 1997a)
laid out the con-
cept of ecosystem
engineering,
providing
models,
initial
for-
mal definitions, illustrative
examples,
postulates, general
questions
that
needed
to be answered,
and
a challenge
to the
ecological
community
to develop
and
refine these ideas. The
primary
purposes
of these
papers
were to draw
attention
to
the ubiquity
and importance
of the process
and its conse-
quences,
to provide
an integrative general
framework,
to lay
out a provisional question-based
research
agenda,
and to
give it a name.
The concept rapidly
worked
its way into the ecological
literature.
By
late
2005,
the original
article
on ecosystem
en-
gineering
(Jones
et al. 1994) had been cited more than 470
times
in the peer-reviewed
literature.
During
this
period,
the
Justin
P. Wright
(e-mail:
justin.wright@duke.edu)
is an assistant
research
professor
in the Department
of Biology
at Duke University,
Durham,
NC
27708.
He studies
the
effects of
ecosystem engineering
on
patterns
of diversity
at different
spatial
scales
and the
relationship
between
biodiversity
and
ecosys-
tem
functioning.
Clive
G.
Jones
(e-mail:
jonesc@ecostudies.org)
is
a senior
sci-
entist
and
ecologist
at the Institute
ofEcosystem
Studies, Millbrook,
NY 12545.
His research
addresses
links between
species
and ecosystems,
focusing
on eco-
logical
complexity,
ecological
theory,
and
ecosystem
engineering.
? 2006Amer-
ican Institute
of Biological
Sciences.
www.biosciencemag.org March
2006 / Vol. 56 No. 3 * BioScience 203
Articles
concept also generated
significant
controversy;
one decade
from its inception,
we thought
it would be fruitful to exam-
ine the nature of these controversies,
evaluate the success
of the concept
in stimulating
novel ecological
research,
and
speculate on its potential to generate future scientific
advances.
Ecosystem
engineering
controversies
The introduction
of new concepts
and terminology
in ecol-
ogy is frequently
met
with
resistance,
which can
often
help
re-
fine
and
clarify
a new
concept
or illustrate
potential
weaknesses
either in the new concept or in the established
paradigm
(Pickett
et al. 1994,
Graham and Dayton
2002). Such
is cer-
tainly
the case with ecosystem
engineering.
Numerous
ex-
changes
have
helped
identify
where the concept
is likely
to be
most useful
and
when it should be applied.
In addition,
some
of the
objections
raised have
highlighted important
differences
in the ways scientists think about ecological
systems.
Irre-
spective
of whether
one considers
the concept
to have
value,
closer
examination
of these issues
should
allow
a better un-
derstanding
of the assumptions
underlying
ecological
think-
ing on this topic.
One of the first
challenges
to the concept
was
exemplified
by the comments
of Power
(1997a,
1997b, Jones
et al. 1997b),
who objected
to the use of "buzzwords"
and suggested
that
the term
"ecosystem
engineering" implied
intent.
Ecology
is
certainly
a discipline
rife with
jargon,
and
care
should
always
be taken
to avoid
generating
terminology
for terminology's
sake.
However,
coining
and
clearly
defining
the term
"ecosys-
tem
engineering"
made
it possible
to recognize
that
organisms
as diverse as beavers, trees,
and marine
benthic
worms
may
be engaged
in processes
that
share certain
common features.
Using
a single
label to encompass
the
diverse
activities
by
which
organisms
modify
the abiotic
environment
was
the first
step
in trying
to build
a concept
that
could
potentially
lead to im-
portant,
interesting,
and
perhaps
surprising
generalizations.
For
example,
on a worldwide
basis,
mollusks
were
recently
es-
timated to add
physical
structure
to the
environment
(via
shells
and
resulting
reefs)
at an annual rate
equivalent
to that found
for aboveground
temperate
forests
(Gutierrez
et al.
2003). It
could be argued
that
such
a comparison
might
never have been
made, and numerous review papers
might not have been
written,
were
it not for
the umbrella
created
by the ecosystem
engineering
concept
(Lavelle
et al. 1997,
Folgarait
1998,
van
Breemen
and Finzi 1998,
Dorn and Mittelbach
1999,
Cole-
man and
Williams
2002,
Crooks
2002,
Emmerling
et al.
2002,
Scheu 2003, Williams and McDermott 2004, Wright and
Jones
2004).
At the very
least,
"ecosystem
engineer"
is a useful term for
searching
a diverse
literature for
commonalities.
Nevertheless,
we concur
with the concern
underlying
Powers's comments
about
buzzwords,
especially
given
the rapid
growth
of inter-
est in the ecosystem
engineering
concept.
Using
the term
in-
appropriately
(i.e.,
outside
of its defined
domain)
will
lead
to
"jargon
creep;',"
and if the term
becomes too broadly
or var-
iously defined,
it will become valueless,
defeating
the origi-
nal integrative
purpose.
As to the question
of intent,
while
some dictionary
definitions of "engineer"
may imply
intent
(Power
1997a),
others do not, and the term
"ecosystem
en-
gineering"
was clearly
defined without reference to intent
(Jones
et al. 1997b).
As defined
by Jones
and colleagues
(1994,
1997a),
ecosys-
tem engineering
is a process
that
most, if not all, organisms
engage
in. Indeed,
it is difficult
to imagine
a life
strategy
that
does not in some way
lead
to a degree
of modification
of the
abiotic
environment. Given
the ubiquity
of ecosystem engi-
neering,
some have
argued
that if all
organisms
are
ecosystem
engineers,
the
concept
cannot be considered useful
(Reichman
and Seabloom
2002a,
2002b).
This complaint
equates
ubiq-
uity
with nonutility.
In contrast,
others have
argued
that
the
ubiquity
of ecosystem engineering
would
seem
to make it likely
to be an important
general
form of interaction
worthy
of in-
vestigation
(Wilby
2002),
a view with which we concur.
The
difference
in these two
viewpoints
can be highlighted
by con-
sidering
parallels
with
typically
studied
assimilatory
or
trophic-
based interactions
such as herbivory,
predation,
or direct
competition
for
resources.
All organisms
must assimilate
en-
ergy
and
materials
in order
to grow
and
reproduce.
While
true,
this statement
on its own is not particularly
useful for pre-
dicting
the behavior of organisms,
the structure
of ecologi-
cal communities, or the functioning of ecosystems.
Nevertheless,
we have
extensively
used the generality
of the
assimilatory process
to develop
models
and
theory
that
allow
us to build hypotheses
about the process
and its numerous
consequences.
Lotka-Volterra models
and food web
theory
are
only two examples
of a multitude
of fruitful avenues of eco-
logical theory and research
that are broadly applicable
in
large
part
because
all
organisms
are "consumers"
in the
broad-
est sense.
By analogy,
then,
while it is not particularly
inter-
esting to state that a particular
organism
is an ecosystem
engineer,
the fact that
ecosystem engineering
is such
a wide-
spread
process gives
us reason
to believe
that the ecosystem
engineering
models
and
principles being
developed
are
likely
to be broadly
applicable.
One of the most commonly asked
questions about the
ecosystem
engineering
concept
is some variant of "How
do
ecosystem
engineers
differ from
keystone
species?"
Although
many
of the similarities
and differences
were discussed in the
original
papers
(Jones
et al. 1994,
1997a),
the topic seems
to
be a perennial
one in seminars,
discussion
groups,
and per-
sonal communications.
Indeed, Reichman
and Seabloom
(2002b) suggested
that the term "ecosystem
engineering"
should
be restricted to cases
in which
the physical
modifica-
tion of the environment
is "large
relative
to purely
physical
processes
operating
in the system,"
a definition
analogous
to
one of the more common recent definitions
of "keystone
species"
that
requires
effects
to be disproportionate
to biomass
(Power
et al. 1996).
Many
of the most obvious examples
of
ecosystem engineers
(e.g., beavers,
elephants,
reef-forming
mollusks)
do have
large
effects.
However,
as noted above,
all organisms
modify the envi-
ronment to some extent,
and they cannot all be keystone
204 BioScience * March
2006 / Vol.
56 No. 3 www.biosciencemag.org
Articles
species.
In many cases,
the work of environmental modifi-
cation is shared
across
species
within a system
(e.g.,
diverse
species
of corals
creating
reefs),
rather
than
being
the prod-
uct of one species.
In other
situations,
the
modification is done
by numerically
dominant or
biomass-dominant
species
(e.g.,
windbreaks of forest
trees).
Furthermore,
while some
keystone
species have large
effects on communities and ecosystems
through ecosystem engineering,
others have their effects
through
trophic
interactions or other
processes,
such
as
pol-
lination.
Focusing solely
on engineers
that have
important
ef-
fects overlooks the important information contained in
"trivial"
ecosystem
engineering. Being able to understand
and predict
when and where
ecosystem
engineers
will have
large
versus
small
effects is clearly
an important,
central
goal
(Jones
et al. 1994,
1997a).
However,
the explanation
for
large
effects must
necessarily encompass
reasons
why ecosystem
en-
gineers
can also
have
small,
limited
impacts
or no impact
at
all. On a more
philosophical
level,
the key
difference
between
the ecosystem
engineering
concept
and the keystone species
concept
is that
the former is process
focused,
while the latter
is outcome
focused
(Jones
et al. 1997a,
Wilby
2002).
This distinction
between
the ecosystem
engineering
ap-
proach
and the keystone species
approach
reflects a funda-
mental difference in the epistemological
stance of scientists
with respect
to ecological
systems.
Predicting
whether or not
a species
is a keystone
requires
understanding
the net effects
of an organism
on the assemblage
in which it is present.
These net effects are
typically
difficult to predict,
because of
the
open,
multiply
causal,
and
highly contingent
nature of eco-
logical
systems
(Pickett
et al. 1994).
Although
the keystone
species
concept
is a powerful
metaphor
with important
im-
plications
for conservation,
to date it has not been particu-
larly useful in generating general theories about the
functioning
of ecological systems.
In contrast,
while ecosys-
tem
engineering theory
ultimately
seeks to predict
and
explain
net effects,
it does not try to do so on the basis of outcomes.
Rather,
it focuses
on a particular,
though
variable,
mechanistic
two-part pathway
by which organisms
interact with each
other-first, via their nonassimilatory
(and nondissimila-
tory) influence on the abiotic
environment,
and second,
via
the influence of these abiotic environmental
changes on
other organisms
or coupled biotic-abiotic processes.
The
concept
deliberately
avoids
conflation of process
and
outcome,
so that the contingencies
(i.e.,
underlying
characteristics of the
abiotic
environment,
how it is organismally
modified,
and how
other
organisms
respond
to these abiotic
changes)
can be ex-
posed and addressed.
The separation
of process
and conse-
quence helps parse
the
world into more
predictable
pieces.
So
although
the overall
ecosystem
engineering consequences
of
an organism
are
clearly
contingent,
recent work
has shown
that
by considering
first how an ecosystem
engineer
modifies
the abiotic
environment,
and then
how the other
species
will
respond
to this abiotic
change,
one can
begin
to predict
how
engineering
effects of that species
are
likely
to vary.
Such an approach
has been applied
to understanding
the
effects of ecosystem
engineers
on community assemblages
across environmental
gradients (Wright and Jones 2004,
Crain and Bertness 2006) and the variation in engineer
influence on soil processes (Jones et al. 2006) and bio-
geochemistry
(Caraco
et al.
2006,
Gutierrez
and
Jones
2006).
This
indicates
that
ecosystem
engineering,
like other
process-
focused
concepts (e.g.,
energy
flow,
nutrient
cycling,
trophic
pyramids,
predation),
may
well be more
useful in generating
general
hypotheses
about the functioning
of open, multi-
causal,
contingent
ecological
systems
than are
concepts
focused
purely
on net effects.
A final
controversy
surrounding ecosystem
engineering
re-
lates
to the evolutionary
rather
than
the ecological
realm.
The
interesting
and
potentially
important
implications
of ecosys-
tem engineering
for
the evolution
of engineers,
and of other
organisms
dependent
on engineers
for
habitats,
was
pointed
out in papers
by Jones
and
colleagues
(1994, 1997a).
A rapidly
growing
field,
often
referred
to as niche
construction
(Laland
et al.
1999),
seeks
to understand
some
of the
evolutionary
con-
sequences
of feedbacks
between
engineering
organisms
and
the changes
they cause to the abiotic environment
(Odling-
Smee
et al.
2003).
Niche
construction
theory
draws on ecosys-
tem engineering
concepts
(Odling-Smee
et al.
2003),
although
it has origins
independent
of and has developed
in parallel
with the concept
of ecosystem
engineering.
One of the more
controversial
assertions
of niche construction
theory
is that
the incidental
modifications
to the environment created
by
organisms
can constitute
powerful
evolutionary
forces
(La-
land 2004, Turner
2004)-an idea referred to by Dawkins
(2004) as "pernicious."
Dawkins
(2004) has laid out a series
of points arguing
for rigorous
thinking
about the nature
of
replicators
and selection
and about
the
evidential
requirements
necessary
for
demonstrating
extended
phenotypes.
The cur-
rent
debate
about
the importance
of the extended
phenotype
to Darwinian
evolution
is vigorous
but healthy.
There is lit-
tle doubt
that there
are
ecological
feedbacks
between
organ-
isms and
the changes
they
cause
in the abiotic
environment,
and,
as originally
pointed
out by Jones
and
colleagues
(1994,
1997a),
these
feedbacks
may
well
have
important
evolution-
ary
consequences.
However,
the degree
to which ecosystem
engineering
is a potent
evolutionary
force
remains to be seen.
The current
debate is largely
focused around evolution
re-
sulting
from
ecological
feedbacks to the engineer
of its local
engineering,
and
has
yet
to address
potentially
interesting
co-
evolutionary
or donor-controlled
evolutionary
consequences
for a community.
Furthermore,
this area of research
has
gen-
erally yet
to come
to grips
with the
potential
for
ecosystem
en-
gineering
to shape major
patterns
in the
radiation,
extinction,
and evolution
ofEarth's
organisms
(but
see
Thayer
1979).
For
example,
the oxygenation
of Earth's
atmosphere by photo-
synthesizing
organisms
clearly
had an effect
on the diversi-
fication of organisms
adapted
to oxic environments,
but to
what
degree
less obvious
examples
of ecosystem engineering
have
affected
patterns
of macroevolution
is largely
unknown.
www.biosciencemag.org March
2006 / Vol. 56 No. 3 * BioScience 205
Articles
Conceptual
progress
In the
decade
since the
introduction of the concept,
much
ink
has
flowed
discussing
ecosystem
engineers.
To
what
degree
do
the numerous
papers
represent
progress
in addressing
fun-
damental
questions
raised
in the
early
papers
that
outlined the
concept,
or in developing
and
testing general
hypotheses?
In
looking
over
the literature,
we see three
general
types
of pa-
per,
each of which has
helped
develop
the concept
to varying
degrees
and in different
ways.
The
first
category
includes
papers
that mention
ecosystem
engineering
as a potentially
important
interaction,
while
fo-
cusing
on other
processes,
interactions,
or topics
(Rietkerk
et
al. 2004, Soule et al. 2005). These
papers
are
interesting
be-
cause one rationale
for
writing
the original
paper
was that
the
process
was
largely
omitted from
textbooks. These
papers
re-
flect a growing
acceptance
and
recognition
of ecosystem
en-
gineering
as a fundamentally important,
general
ecological
process.
While
such
papers
may
not directly
contribute to the
development
of the ecosystem
engineering concept,
the reifi-
cation they espouse justifies interest in and provides en-
couragement for conceptual development by interested
practitioners.
Although
these
types
of papers
are
important
in disseminating
general
awareness of ecosystem
engineering
within
and
between the subdisciplines
of ecology,
such
papers
can also enhance dissemination of ideas about ecosystem
engineering
outside of basic
ecology,
in more applied
areas
(Fragoso
et al. 1997,
Hood 1998,
Rai
et al. 2000,
Langmaack
et al.
2001,
Tanner
2001,
Rosemond
and Anderson
2003),
with
potential
benefits
in both realms.
The
second and
most numerous
category by far is the case
study-papers that
focus on an example
of ecosystem
engi-
neering and provide data on the engineering process
and
consequent
effects
on some
aspect
of organismal,
population,
community,
or ecosystem
ecology.
While it is important
to
guard against the mere accumulation of "just
so stories"
(Jones
et al. 1994,
Berkenbusch and
Rowden
2003),
case stud-
ies on a variety
of taxa
and
their numerous
effects
in diverse
environments
serve
many
purposes.
First,
like
the first cate-
gory of papers,
they increase
awareness of ecosystem
engi-
neering as a common, general process worthy of study.
Second,
they
can
indicate
aspects
of a system
that
could sub-
sequently be found to be common engineering features
(Thomas
et al. 2000, Cardinale
et al. 2004). Third,
they can
serve as specific
tests of general
hypotheses
(Wright
et al.
2002). Fourth,
they help develop
the tools, approaches,
and
metrics
required
for
studying ecosystem
engineering (Wright
et al.
2002,
Lill
and
Marquis
2003,
Bancroft et al.
2005).
Last,
and
by no means
least,
case
studies
provide
the raw
material
for subsequent
synthesis,
integration,
and generalization.
Just
as researchers
cannot study
every
predator-prey
inter-
action,
we clearly
cannot study every
example
of ecosystem
engineering.
Nevertheless,
without
case
studies,
there is little
chance for
comparative
work that
paves
the
way
for
broad
gen-
eralizations
and tests of models and predictions (Crooks
2002,
Wright
and Jones
2004).
Since
ecosystem
engineering
involves
many
types
of species
operating
in diverse abiotic environments
with numerous
in-
fluences,
there is a risk that every
study may end up collect-
ing unique data that do not lend themselves to general
conclusions.
Unstructured data collection can only move
scientists so far toward
generalization.
There is a real
need
for
gathering
data
on some of the fundamental
parameters
that
govern
the interaction
of ecosystem engineers
with the envi-
ronment
and with
other
organisms,
and a need for some de-
gree of standardization for comparative metrics. Some
examples
we think are important include parameters
de-
scribing engineered
rates of environmental
decay
(Gurney
and
Lawton
1996,
Wright
et al.
2004),
susceptibility
of the abiotic
environment
to engineering
(i.e.,
malleability),
feedback to en-
gineers
from their
engineering
(Hui
et al.
2004),
relations
be-
tween
physical
structures and physical
and chemical
abiotic
variables,
and impacts
relating
to species
richness and other
community properties.
The third and final category
consists
of papers
that con-
tribute
to advancing
the field
by developing
and
testing gen-
eral frameworks, models, and hypotheses and seeking
underlying generalities.
This special section of BioScience
contains
several
examples,
and
there are numerous others.
A
partial
list includes
general
models of population
dynamics
for ecosystem
engineers
(Gurney
and Lawton 1996, Cud-
dington
and
Hastings
2004,
Wright
et al.
2004),
analyses
of the
community impacts of engineers (Wright
et al. 2002, Lill
and
Marquis
2003,
Castilla et al.
2004),
integration
of trophic
and
engineering
impacts (Wilby
et al.
2001),
cross-system
and
cross-taxa
reviews of engineers
and their
impacts
(Lavelle
et
al. 1997,
van Breemen and Finzi 1998,
Dorn and Mittelbach
1999,
Coleman
and
Williams
2002,
Crooks
2002,
Emmerling
et al. 2002, Scheu 2003, Williams and McDermott 2004,
Wright
and
Jones
2004),
application
of the principles
of en-
gineering science to organismal ecosystem engineering
(Thomas
et
al.
2000),
and
development
of frameworks
(Lavelle
et al.
1997,
Pickett et al.
2000).
These
papers,
and others of sim-
ilar
scope, implicitly
counter
the criticism
that the concept
of
ecosystem engineering
is largely
descriptive,
and provide
constructive
examples
of how ecologists can devise novel
empirical
methods
or gain
new insights.
Looking
forward
We believe
a number of research areas
may be particularly
fruitful.
Several
are areas where
a considerable
body of work
has been done, so the groundwork
for further
development
is in place.
Others are
underexplored
areas
that strike
us as
crit-
ical for understanding
how ecosystem
engineering
interacts
with other types of interactions
to control ecological
sys-
tems,
and how the engineering
concept
might
be applied
in
ecosystem
management.
From the
beginning,
scientists have
recognized
that the ef-
fects of ecosystem engineering
will be context dependent
(Jones
et al.
1994).
In one sense,
this
is hardly surprising, given
that
one of the defining
characteristics
of ecological
systems
is their
highly
contingent
nature.
Yet
the context
dependency
206 BioScience * March
2006 / Vol.
56 No. 3 www.biosciencemag.org
Articles
of ecosystem
engineering-arising
from
the underlying
char-
acteristics of the abiotic
environment,
from
the way it is or-
ganismally modified, and from the response of other
organisms
to these
abiotic
changes-is potentially
more
pre-
dictable than assimilatory
(e.g., trophic)
influence.
As ecol-
ogists,
we know an enormous amount about how physical
and chemical abiotic factors affect organisms,
and about
coupled biotic-abiotic processes
such as biogeochemistry
and ecophysiology.
Ecologists
can also draw
on a rich
reper-
toire of understanding
of soil, water,
and
atmospheric
physics
and
chemistry
from
other
disciplines
to understand
spatial
and
temporal
variation
in abiotic factors and the many other
abiotic influences
upon them.
Thus,
if ecologists
can under-
stand how organisms
modify these physical
and chemical
abiotic
factors,
we are
quite
likely
to be able to predict
effects
of ecosystem engineering on biogeochemical processes
(Caraco
et al.
2006,
Gutierrez
and
Jones
2006)
and
species
dis-
tributions
(Wright
and Jones
2004). Furthermore,
if we can
then understand
how the ecosystem
engineering
activities of
organisms
will vary
in different
environmental
contexts,
we
can begin to predict
how the effects of ecosystem
engineer-
ing are
likely
to vary
across environmental
gradients
(Crain
and Bertness
2006,
Moore
2006).
This
latter
question
is one
for
which further
research is likely
to be particularly
fruitful.
Indeed,
many
of the recent
findings
in studies that
investigate
shifts between
competition
and facilitation
along environ-
mental
gradients
(Callaway
et al.
2002,
Maestre et al.
2005)
are
likely
to be due to changes
in the
importance
of ecosystem
en-
gineering.
To
date,
few
studies
have examined the factors,
be
they
behavioral,
developmental,
or physiological,
that control
the degree
of ecosystem
engineering
in different environ-
ments. Nor do we
know the
extent to which
feedbacks between
engineering
organisms
and their
environments
mediate
the
extent of ecosystem
engineering.
Further
progress
in under-
standing
the
contingency
of ecosystem
engineering
will
require
a better
understanding
of such
influences.
One organism
can affect another via a number of differ-
ent pathways, encompassing ecosystem engineering,
predator-prey
interactions,
direct
resource
competition,
food
web membership, pollination, vectoring, and so forth. A
number of studies
have
begun
partitioning
the net effects of
organisms
along
axes of different
interaction
types-for ex-
ample, ecosystem
engineering
effects versus
trophic
and other
effects
(Wilby
et al. 2001,
Moore
2006). This approach rep-
resents more than a simple
attempt
to generate
a scorecard
indicating
that
ecosystem
engineering
is x times more or less
important
than
trophic
effects. The
factors that control an
or-
ganism's ecosystem
engineering
activities
may
or may
not be
the same as
those
that
affect its rate
of consumption
or its rank
in a competitive
hierarchy.
For
example,
redd
construction
by
salmon is likely
to be affected
by factors
such
as particle
size,
water
temperature,
and
current
velocity
(Moore
2006),
while
the trophic
effects
of salmon will
be largely
controlled
by fac-
tors in the marine
environment. If we can understand
how
much of an organism's
net effect is due to ecosystem
engi-
neering
and
why
this is the case,
we will have a better
chance
of being able
to predict
how such effects will change
in dif-
ferent environments.
One exciting
prospect
for the concept
of ecosystem engi-
neering
is its potential
to link across
different levels of bio-
logical
organization
and
approaches.
For
example,
the
concept
has allowed
linkages
between
population
biology and land-
scape and community ecology (Wright et al. 2004), and
between
physiology
and
ecosystems
(Caraco
et al.
2006).
We
see an opportunity
for research
linking
the behavior of eco-
system
engineers
to their effects on populations,
communi-
ties, landscapes,
and ecosystems.
As noted above,
there is
great
potential
for
using
the concept
in evolutionary
studies,
provided that researchers
understand
the need for disci-
plined thought about selection and feedback
between or-
ganisms
and the abiotic
environment.
The importance
of spatial
and temporal
scales relative to
the effects
of ecosystem
engineers
was first discussed
by Jones
and colleagues
(1997a).
There is a growing
body of work
on
the effects
of ecosystem engineering
on species
richness at dif-
ferent spatial
scales (Lill and Marquis
2003, Wright
et al.
2003,
Castilla
et al.
2004).
Studies
of how ecosystem engineers
create
heterogeneity
(Pickett
et al.
2000,
Gutidrrez
and Jones
2006) and of the patch
versus
landscape
effects of engineer-
ing on biogeochemical
functions
(Caraco
et al.
2006) extend
research
on the relevance
of spatial
scale in interpreting
the
effects
of ecosystem
engineering.
As in all
ecological
studies,
determining
how best
to incorporate
the effects of spatial
and
temporal
scale
into studies
remains a challenge.
Nevertheless,
since ecosystem
engineering
frequently
creates
patches
that
differ from surrounding
areas,
this logically
leads to com-
parisons
at three
spatial
scales:
variation
between
engineered
patches, variation between engineered and unmodified
patches,
and
variation
at spatial
scales
encompassing
both en-
gineered
and unmodified
patches.
At larger
spatial
scales,
it
is worth
considering
to what
extent variation in ecosystem
en-
gineering
activity
might
explain
variations
in diversity
across
ecosystems.
While
this approach
is certainly
not universally
applicable
(engineering
can
be spatially
diffuse,
not discrete
and distinctive),
it can serve
as a starting
point for examin-
ing the effects
of ecosystem engineering
at different
spatial
scales.
In a more
general
sense,
there is much
opportunity
for
con-
tinued
theoretical
development
of the concept.
For
example,
there
is room
for
more
models
exploring
the ramifications
of
ecosystem
engineering
(cf.
Gilad
et al.
2004).
Ecologists
need
to develop
more
explicit
approaches
to scaling
relations and
better
link ecosystem engineering
process
to pattern
at vari-
ous levels
of organization.
We need a better
understanding
of engineer
feedbacks
that can generate
complex dynamics.
We
need to develop
useful
common engineering
currencies
and
comparative
metrics,
and
identify
the best
types
of meth-
ods and
approaches
that can
be used
in the study
of nature's
engineers.
Research
in these areas
will no doubt prove
useful
in ex-
panding
and
clarifying
the scope
of the concept
of ecosystem
engineering.
However,
such research
is also necessary
to in-
www.biosciencemag.org March
2006 / Vol. 56 No. 3 * BioScience 207
Articles
form issues of management.
As Jones
and colleagues
(1994)
point out, humans are
ecosystem engineers
par excellence--
as a species
we frequently
have
environmental
impacts
that
parallel
those
of other
engineers,
and
viewing
the impacts
of
humans on the environment
through
the lens of ecosystem
engineering may lead to novel insights.
Furthermore,
nu-
merous
species
create and destroy
habitats
for other
species,
and
many
exotic
species
with
large
ecological impacts
turn
out
to have
their
effects via
ecosystem
engineering
(Crooks
2002).
Ecosystem
engineers
can be important
in controlling
local
microclimate
and could therefore
be influential in main-
taining
refuges
for
other
species
in the face
of climate
change
(Cavieres
et al. 2002). Many
ecosystem
engineers
have sig-
nificant
effects on important
ecosystem processes
of man-
agement
concern--hydrology,
nutrient
cycling
and
retention,
erosion
and sediment
retention,
for example--while at the
same time creating
habitat for other
species
that also influ-
ence biogeochemical
processes
via nutrient
uptake,
conver-
sion,
and
release
(Levinton
et al. 1995,
Lavelle
et al. 1997,
van
Breemen
and Finzi
1998).
Finally,
humans are
often
respon-
sible
for the loss or introduction of such
engineering species,
with
the
potential
for
large
secondary consequences
(Coleman
and
Williams
2002).
All of this suggests
that
there
is consid-
erable
potential
for
applying
the ecosystem
engineering
con-
cept in management.
A number of research
papers
from recent
case
studies
of
ecosystem
engineers
have
included some discussion of man-
agement
implications
(Lenihan
and
Peterson
1998,
Gilkinson
et al.
2003,
Perelman
et al.
2003,
Machicote et al.
2004).
Some
reviews (Dorn and Mittelbach 1999, Martius et al. 2001,
Coleman and Williams 2002, Crooks 2002, Piraino et al.
2002,
Crain
and
Bertness
2006) and
modeling
studies
(Cud-
dington
and
Hastings
2004,
Wright
et al.
2004) have
pointed
out some of the general
management
ramifications of the
ecosystem
engineering
concept
for
conservation,
restoration,
or amelioration.
The concept
is also
beginning
to cross over
into applied
realms
(Fragoso
et al. 1997,
Hood 1998,
Rai
et al.
2000,
Langmaack
et al.
2001,
Tanner
2001,
Rosemond and An-
derson
2003).
Formal
incorporation
of the concept
does not
pervade
applications
thinking,
and
explicit
incorporation
of
ecosystem
engineering principles
into management plans
has yet to occur,
but is a potentially
fertile
territory.
Over
the last 10
years,
what
started as a concept appears
to
have
grown
rapidly
into
a major
research
initiative.
The
many
papers,
organized
sessions,
symposia
on the topic
at national
and
international
meetings,
and
special
issues
(including
this
BioScience
issue)
attest
to this fact.
The growth
of research
in
this area
is gratifying
to those of us who are
interested
in the
topic, and will inevitably
lead to a greater
understanding
of
the
ecosystem
engineering process
and
its
many
consequences.
Nevertheless,
we end with a cautionary
comment.
A central
reason for drawing
attention to ecosystem
engineering
in
the first
place
was
that
it was
being
overlooked
as an impor-
tant
contributory process
among
those
factors
affecting
the dis-
tribution
and abundance of organisms
and the functioning
of ecosystems.
With the increased
attention the topic is now
receiving,
it would
be unfortunate
if it developed
into a spe-
cialty
area,
balkanized
from
the rest of ecology.
Any
piece of
nature
incorporates
numerous
organisms
and
nonliving
en-
tities,
with interactions
among them all. Therefore,
under-
standing
nature
requires
a balance between
knowledge
about
one particular type of process
or interaction
and the inte-
gration
of all
processes
and interactions
into a cohesive
whole.
Acknowledgments
This special
section
developed
out of an organized
oral
ses-
sion held at the annual
meeting
of the Ecological
Society
of
America.
We would
like to thank
all of the
participants
in this
session
as well as numerous
colleagues
who have
challenged
us, encouraged
us, and generally
helped shape and refine
our
thinking
about the concept
of organisms
as
ecosystem
en-
gineers.
This research
was
supported by Duke
University,
the
Andrew
W. Mellon
Foundation,
and the Institute
of Ecosys-
tem Studies.
This is a contribution
to the program
of the In-
stitute
of Ecosystem
Studies.
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www.biosciencemag.org March
2006 / Vol.
56 No. 3 * BioScience 209
