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8/10/2019 1989 Kolasa
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Ecological Systems in Hierarchical Perspecitive: Breaks in Community Structure and Other
ConsequencesAuthor(s): Jerzy KolasaSource: Ecology, Vol. 70, No. 1 (Feb., 1989), pp. 36-47Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1938410
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Ecology, 70(1), 1989. pp. 36-47
?c
1989 by the Ecological Society of
America
ECOLOGICAL SYSTEMS IN HIERARCHICAL
PERSPECTIVE: BREAKS IN
COMMUNITY STRUCTURE
AND
OTHER
CONSEQUENCES'
JERZY
KOLASA2
Institute
of
Ecosystem Studies, The New York Botanical Garden, Box AB,
Millbrook,
New York 12545 USA
Abstract. Explanation
of the
patterns
of species abundances is important
because
it
may help
in
understanding mechanisms structuring communities.
A
general conceptual
model is proposed and examined
as
an alternative
to
previous propositions,
which focused
too narrowly on specific mechanisms. According
to
this model, viewing
the structure
of
an
environment
as
a nested
hierarchy
of habitat units
provides
a
general
mechanism
sufficient to account for empirically established regularities
in
species abundances. Various
biotic and abiotic factors can be considered as specific mechanisms sorting biological
components into respective levels and compartments of the hierarchical structure
of
the
environment. Because "sizes" of these compartments vary in a predictable way as a function
of their
position
in
the hierarchy,
so should
the
abundances.
The
model
may provide
a
conceptual framework that
allows
evaluation
of the relative contributions of
competition,
predation, and other biological interactions.
The operational and highly simplified version
of
the model
uses
spatial
or
temporal
measures of
ecological ranges
of
species
to facilitate
testing
of
the general
model. The
operational
model
makes three
qualitative
and
quanti-
tative
predictions: (1) although
the
species
display
a
continuous
gradation
of
properties,
the nested
hierarchy
of
habitat units should lead to
clustering
of
species
at distinct
levels;
(2) generalists should be relatively more successful
than specialists; (3) relative abundances
of
species
should be
predictable
from
their
position
in
the hierarchical structure.
An
analysis
of
eight communities, including flatworms,
aquatic insects, foraminiferans, rodents, and
birds, supports these predictions. The clusters,
or "breaks"
in
community structure, appear
to be a new
and
possibly general property.
Moreover, quantitative predictions
of relative
abundances for these communities are in surprisingly good agreement with the actual
abundances.
A
hierarchical structure of the
environment
appears promising
in
accounting
for
some
poorly explained community-level
phenomena,
such as correlation between the
species range
and
abundance,
and differences between abundance structures
of
communities
in
rigorous
and less severe environments.
The model is
compatible with commonly
ob-
served as well
as
irregular patterns
of distribution
of
species abundances,
with
high
local
abundance of some
species,
with differentiation
of
extinction
probabilities,
and with scale
dependence
of
ecological phenomena.
Keyi
words:
abundance
structure, community
structure;
habitat; heterogeneity; hierarchy;
scales.
INTRODUCTION
Recently, hierarchy
theory has made inroads into
ecology (MacMahon et al. 1978, Sugihara 1980, Allen
and Starr
1982,
Allen et
al.
1984,
Rudd et al. 1984,
Maurer 1985).
The major focus has been on devel-
opment
of a
conceptual framework
and mathematical
methods, e.g., fractals, to aid in
identification of and
differentiation
between phenomena occurring at
var-
ious temporal and spatial scales. For example, Allen
and
Wyleto (1983)
demonstrated that
the
interpreta-
tion of fire
disturbance
in
prairie communities depends
on
the
spatial extent
and
frequency
of fire. Morse et
al.
(1985)
examined
the
relationship
of leaf
structure,
scale,
and the abundance
of
leaf
animals. O'Neill et
al.
I
Manuscript
received 28 October 1986;
revised
22
May
1988; accepted
24
May
1988.
2
Presentaddress:
Departmentof Biology,McMasterUni-
versity, Hamilton,
Ontario,CanadaL8S 4K1.
(1986) have synthesized much of the recent hierarchy
and scale related work. Their
particular
focus
has
been
on ecosystems and on the implication of observation
scale for research methodology. The progress achieved
by these authors permits concentration
on further con-
sequences of the hierarchical
nature of
ecological sys-
tems.
The purpose
of this paper is to explore the possibility
for
direct applications
of
the concept
of
hierarchy to
problems traditionally assigned to community ecology.
This paper is
based on the assumption that, although
patterns
of
community structure
are
produced by
a
variety of interactions, such patterns can be considered
separately from the
detailed knowledge
about these
interactions, and the patterns themselves can be ana-
lyzed
profitably.
The
proposed perspective
has the
po-
tential for
helping
in
the
formulation of
testable hy-
potheses about the
underlying environmental structure
and
mechanisms responsible for observed patterns of
8/10/2019 1989 Kolasa
3/13
February
1989
CONSEQUENCES
OF HIERARCHY
37
abundance
of
species.
To explore
this
potential
I
focus
on the
question of species
abundance patterns.
Large taxonomic
collections normally
contain a few
very abundant species
and a long list
of rare species
(Preston 1962,
Williams 1964). Many
attempts have
been made to explain this pattern (Patrick et al. 1954,
MacArthur 1957,
Preston 1962,
Whittaker 1965,
Pie-
lou 1975, Sugihara
1980) but controversies
continue
to abound (e.g.,
Routledge 1980, Ugland
and Gray
1982, Connor et
al. 1983, Brown 1984,
May 1986).
Kolasa
and Biesiadka (1984)
and Kolasa and Strayer
(1988) have proposed
that the habitat
may be con-
ceived of
as
hierarchical
in
structure, i.e.,
that each unit
of environment may be
composed of a number
of sub-
units,
and that
this
structure
may be responsible
for
the observed patterns
of species abundances.
Here, this
conceptual model
is further developed.
Some ideas incorporated in the proposed model have
been
postulated
earlier.
Colwell
and
Futuyma (1971)
indicated advantages
of measuring niche
parameters
by
taking
into account the species'
point
of view.
McNaughton and
Wolf (1970) supported
the
idea of
using
the ecological range
as a measure
of the niche
width
but cautioned against
inconsistencies of using
orderings
based
on a
single gradient.
MacArthur
and
Levins (1964)
have discussed significance
of
recogniz-
ing grain
of the habitat.
Finally, Dueser and Shugart
(1978) and Dueser
and Porter (1986)
proposed,
and
provided
evidence,
that
species
segregate
into
micro-
habitats, and that their relative abundance is positively
correlated
with the
range
of microhabitats
used. The
latter
authors
have found competition
to be the
dom-
inating
sorting
force.
In
this paper
I
explore the advantages
of combining
attributes
of the environment
with
community
phe-
nomena. The specific purpose
is
to
create a
number of
testable statements, but,
more
important,
to
demon-
strate
how such statements
can
be
generated
and to
show the results
to which
they
can
lead.
OUTLINE
OF
THE
CONCEPT
General
assumptions
The conceptual
model developed here
entails general
assumptions
on
(1)
the
nature
of the
habitat, (2) prop-
erties of
species,
and
(3)
the relation between
the hab-
itat
and the
species.
The habitat
is considered to be
hierarchically
heterogeneous (see next
paragraph for
explanation).
Species
in
a
collection
are assumed to
display
many degrees
of
specialization
to the habitat.
The species abundances
are assumed to
reflect the rel-
ative
size as well as the
degree
of
fragmentation
of the
habitat
used by species.
Nature of the habitat. -Any habitat unit, whether a
decaying log,
a forest
stand,
a
lake,
a
mountain,
a
mountain
range,
or a system of tributaries,
is composed
of
subunits,
and these subunits
are
composed
of even
smaller subunits.
The
units,
as well as their subunits,
A
BROADRANGE
SPECIES
INTERMEDIATE
NARROW
ANGE
I
SPECIES
0
0
VARIABLE
I
FIG. 1.
Schematic
depiction
of a habitat
where homoge-
neity or heterogeneity
dependon
the resolutionwith
which
speciessee their
microhabitats.
The toplevel
may be occupied
by
a
single if
units
areconsidered
homogeneous
Smith
1972])
generalist
hat does
not
respond
numerically
o heterogeneity
at lower levels.
The next
lower level
may be used
by a few
(4
in this example)speciesspecializedat this particular caleof
resolution,
and the
lowest
level
may be used
by 16
species.
Under
the saturated
condition,
all the
21
species
share the
same physical
space,
although
not all second-level
species
overlap
all third-levelspecies.
can
be described by
a set
of variables
in the
multidi-
mensional
ecological
space
analogous
to that
of the
niche (cf.
Hutchinson
1957).
If two variables
and
two
hierarchical
decompositions
are
used
for
simplicity,
it
is
possible
to
visualize
the
structure
of
habitat as
a
hierarchy
of
subdivisions
(Fig.
1).
In this hierarchy
habitat fragments of increasingly smaller size appear
as a
function
of
increasing
resolution.
Operationally,
these fragments
may
be either
narrow
ranges
of the
most important
variable(s) (but
see
McNaughton
and
Wolf
1970)
or
integrated
sets thereof
(e.g.,
microhabitats, larger
patches,
or whole
ecosys-
tems).
Although
the
fragments
of
ecological space
are
construed
as multidimensional
sets of
variables,
they
are
likely
to
have a
spatial
expression
and to
appear
as
identifiable
patches.
In
fact,
Dueser
and
Shugart
(1978),
who studied
the relationship
between the
hab-
itat structure
and distribution
of forest rodents,
have
translated a raw, multivariable description of rodent
habitat
into spatial
patches
by using
discriminant
anal-
ysis.
At
present,
the
spatial
patches
will
be treated
as
a tentative
approximation
to the
hierarchy
of environ-
ment.
A
question
remains
whether
subdivisions
of
habitat
can be
identified
accurately.
In
my
view we
have suf-
ficient
statistical
tools
that
can be
applied
to measure-
ments
at various
spatial
and
temporal
scales
to find
out
which variables
form
integrated,
and thus
poten-
tially
relevant,
sets.
From
such
analyses
one can
con-
struct
a
model of the
habitat
and
test it
against per-
ceptions of species in the taxonomical assemblage of
interest.
Properties
of species.
-Species
differ
in
the
degree
of
specialization
and thus
in
their
resource
requirements.
A
sufficiently large
collection
of
species
is
likely
to
8/10/2019 1989 Kolasa
4/13
38 JERZY KOLASA Ecology, Vol. 70, No.
1
reveal a
continuum of
degrees of
specialization (e.g.,
Whittaker 1965,
Diamond 1986).
The meaning of
"specialization" needs to be
clarified
at this point.
Specialization,
in
the
context of the mod-
el, stands
for the
real
range
of
environment
used by a
species, regardless of ecological mechanisms that de-
termine this
range.
The
terms
specialist and
generalist
are used
in
the same, or
similar, sense that
MacArthur
(1968),
McNaughton and Wolf
(1970), and Dueser and
Shugart
(1978) applied them. More
complex measures
of
the specialization
have been proposed that
involve
both
abundance and
range
of
species
(Lane
et
al.
1975).
For the
purpose of this model
it is sufficient that a
species operate
in
a narrow
range
of
values of
temporal
and spatial variables.
The reasons for that
limited use
may involve either
morphological
or
physiological spe-
cializations and/or
restrictions
imposed by the
habitat
(predators, competitors, unfavorable physical condi-
tions, resource
availability). Specialization is thus
con-
sidered
in
this
model to
be
a
relational
property
be-
tween a
species and its
environment and does not
necessarily
imply higher
efficiency of specialists over
generalists. Indeed,
once a
species
is
restricted
to
use
only
a
smaller
part of the
ecological space, its
only
evolutionary recourse is make
the best of it: special-
ization
would then be
an unintended
consequence of
reduced opportunities.
Such an interpretation
of spe-
cialization
might be
consistent
with a view
that there
is
an advantage to
specialization, as well as
with the
claim of the concept presented here that specialists are
at a
disadvantage.
Relation
between species and
habitat.
-The
concept
of
specialization
outlined
above can now be combined
with the
hierarchical structure of
the habitat. The mod-
el
assumes that the
specialists
use small subdivisions
of
the
hierarchical structure of habitat and
the
gener-
alists
use
larger fragments
of
the
same structure.
For
the unit of
environment shown earlier
(Fig. 1)
we
can
imagine that there will
be a species using the
whole
unit,
and a number
of other
species
that
will
occupy
only fragments
of
the
ecological
space.
The more
spe-
cialized the species, the smaller the fragments to which
it responds.
The generalists may
be said to operate at
a
coarser
grain
of environment than do the
specialists.
The difference
is
of scale.
In
the
simplest
terms,
one
can conceive of
a model where one
species
is
permitted
per fragment
of
habitat
distinguished
at
any
particular
level
of
resolution.
This
is because at this level the
habitat
appears
homogeneous
and, consequently,
the
competitive
exclusion
principle
might apply (e.g.,
Mur-
ray 1986). Note also that with the
exception
of
the
top
generalist,
all
other
species
have to
cope
with
some
degree
of
fragmentation
of their
habitat.
The important assumption of the model is the link
between the
size
of
the
habitat
fragment
and the abun-
dance
of
species.
The model assumes a
strong,
although
not
necessarily
straightforward,
association
between the
overall
resource
availability
(shelter, food, breeding
territories, nest
materials, etc.)
and
the size of the hab-
itat
unit used by a
species. This relation
can be further
generalized
by saying that the
habitat type which
a
species
requires is a
resource itself, and
that number
and/or area of
suitable patches
of that habitat may
be
a measure of quantity of the resource (Whittaker 1965).
The
idealized form of the
model is not to be
taken
literally. It is
meant to
emphasize a
possible relation-
ship between
species and their
environment, and
the
implications of that
relationship. The
relationship itself
appears much
more important
than the question
of
abundance
patterns that has led to it.
In
an ideal
situation, the
abundance structure of
the
community
should thus be a
strict reflection of
the
structure of the
environment.
Other
aspects
of
the com-
munity structure (e.g.,
phenology,
reproductive strat-
egies, functional
roles) may also be related
to the
hi-
erarchy of environment in a similar way. As stated
earlier, the model ignores
specific mechanisms. Al-
though
the
role
of
deterministic vs.
stochastic
factors
has
been found to
vary among species
and commu-
nities in
determining
abundances of species, as
much
as
have
the opinions on this
matter
(Wiens 1984), these
differences do not
affect the main
propositions of the
model. It is irrelevant
that some species
may be sorted
to
their
fragments
of
the
ecological space by
stochastic
factors,
while others are
assorted
by
deterministic fac-
tors,
or
by
a
mixture of
both. Such
distinctions
may
depend
on
the scale chosen for
description
(S. A. Levin,
personal communication, 1987). The net outcome pro-
duced
by
these factors is an
association of
individual
species
with
respective habitat units at
appropriate
levels of
resolution.
Operational
and
conceptual problems.
-Related to
the model is
the
question
of
discreteness,
multidimen-
sionality,
and
identifiability
of
habitat units.
In
prin-
ciple,
whether
micro-
or macrohabitats are
considered,
the subunits of
the
ecological space
may
be
viewed as
quite
discrete from
the
species'
perspective.
Identifi-
cation of these
units
by
means
independent
of the
species
distribution
will
surely
face some
difficulties similar to
those encountered in the measurement of the niche (cf.
Colwell
and
Futuyma
1971). However,
some
impor-
tant
aspects
of the
habitat can be measured and
used
to
construct
models
of
hierarchical structure without
reference
to species. Mosaics of
patches at
various scales
are
one such
aspect.
Ecological space
is often
considered multidimen-
sional when
nongeometrical dimensions are
present
(e.g.,
Hutchinson
1957, May
1976, Harvey
and Lawton
1986). Although this
may be a necessary
consideration
at a detailed
level of
description,
such multidimen-
sionality
can be
ignored
at a coarse resolution
because
most variables have a spatial dimension and therefore
can be
mapped
into a two-
(or,
in
aquatic
habitats more
suitably
into
three-)
dimensional
space (Cohen
1978).
A
similar
argument
may
be
applied to successional
continua
in
general (cf.
Whittaker
1972),
providing
a
8/10/2019 1989 Kolasa
5/13
February
1989
CONSEQUENCES OF
HIERARCHY
39
way around the
complexity and
continuous character
of
ecological
variables.
Predictions
The
model implies a
number of consequences.
These
consequences can be tested in several ways. The model
suggests that: (1)
the number of
specialist species should
be higher than
that of the
generalists; (2) the
specialists
should, on
average, be less abundant
than the gener-
alists; (3) the
density of specialists
should be less than
that of
the
generalists; (4) the
ecological range and
abundance should
be
positively correlated; (5)
there
should be
groups of species
clustered by
similarities
in
their
ecological
range and abundance;
and (6) special-
ists should be
more
vulnerable to disturbance.
The
first two
consequences do not require
special
discussion because
they are a direct
result of the struc-
ture of habitat (Fig. 1). However, other consequences
are
not
always
obvious.
One of the
counterintuitive
consequences
of
the model is a
prediction (item
3)
of
the relative
disadvantages
of
being
a
specialist.
Con-
sider
the
generalist
in
Fig. 1.
According to the general
model, its
abundance
will
be
proportional to the con-
tinuous
area of
the
top habitat unit defined
by
the
two
variables. The situation is
different for the
specialists
of
the two lower
levels.
Although
their
abundances are
also
proportional to the area of
squares
used,
the ex-
pected abundance is
different for the
following reason.
Their
habitats appear as
single "patches"
in
the gen-
eralist's habitat. In a habitat unit even larger than one
shown
in
Fig. 1, these habitats would
multiply
but
would remain
separated from each
other by other, un-
usable
patches.
The more
specialized
the
species,
the
greater the
geometrical and ecological
distance (e.g.,
barriers of "hostile"
habitats)
between
suitable units
of
environment.
In
a
sense,
resources available to a
specialist
can be
viewed
as
being
diluted
in
the
patch-
work of
other habitat units.
Seagle and Shugart
(1985),
who
modeled effects of the mosaic of
habitat patches
on the
species
richness
and
species-area
relationships,
have found that at least two
factors
may
contribute to
extinction of a species in a habitat island: area and
disturbance-related habitat
patch dynamics.
It is not
unreasonable to assume that a
species
faces
higher
en-
ergy
and
population
costs
when it uses
patchily
dis-
tributed
habitats,
as
specialists
in
the model
do,
than
if
the
habitat
is
continuous,
as
in
the case of the
gen-
eralist
(see
also MacArthur
1968,
Fahrig
and Merriam
1985).
Theoretically,
there
may
be several
components
to these costs.
Energy
costs of
getting
from one
patch
to another
are
likely
to increase. The
mortality
due to
these costs as well as to
predation
and
exposure
to
unfavorable
physical
conditions is
likely
to
increase as
well. And finally, resource utilization may become less
efficient
if
some of these
scattered resources are not
found.
From now
on,
the sum of these costs
will
be assumed
to reduce the abundance
in
proportion
to the
degree
that the habitat
is
fragmented
at the
specialist level.
The
ratio of the
specialist's ecological
range to the
gen-
eralist's
range
is
adopted
as the measure
of this
frag-
mentation and called
the dilution factor
(D).
For ex-
ample,
if
the generalist
in
Fig.
1 had a
range
of
1,
then
each specialist at the lowest level would have
1/16
of
that
range.
The
expected
abundance of the
specialists
would thus be
reduced 16
times
if
all other factors were
the
same.
More
generally,
if
the
resource of species
Y
(Ay)
is
diluted relative to
that of
species
X
by
some
factor
(Dy),
then the
energy and population
costs to Yincrease
as a
function of this factor. The
relationship
between
abundance, decrease of the size of habitat
units,
and
dilution of
resources due to habitat
fragmentation
at
higher
levels of
resolution can be
generally
expressed
as follows:
Nx
=ftAD,).
(1)
The relative
amount of
resource available
to Y(A
y)
is
R.
Substituting
for
A
and
D in
Eq. 1,
where
D
=
Rx
Ry
one
obtains
Rx_
NA-
-
J\RX
(2a)
or,
if
one
prefers
Ni,
(2\
NY=f
R2)Y
(2b)
where R is the
ecological
range of species
X or Y and
N
is
abundance.
Eq.
2b will
be used to
predict
species
abundances
used
in
Fig. 5.
Specifically,
Ry
was
ob-
tained from the
Appendix,
column
Ecological
Range,
and
R,
was set
equal
to the
Total
Range.
Tests
of the
model.
-Because information
required
to construct a
hierarchical
model of
the
habitat struc-
ture is
rarely
available,
a
different solution
to
testing
the model is
offered.
Ecological
ranges
of
species
are
assumed to be a reflection of habitat units relevant for
the
species
in
question,
and as
such,
these
ranges
are
substituted for both
spatial
and
temporal habitat
units.
Such a
substitution is
later used
in
the
validation of
the model.
The
other
consequences of
the
model (items 4-6)
are
explained by
examining
real
community
examples.
EXAMPLES FROM REAL
COMMUNITIES
The
qualitative
and
quantitative
predictions are con-
fronted
with a
turbellarian
community
whose
species
composition and
microhabitat
preferences
have been
described (Kolasa 1983). The results (Fig. 2) show that
the
more
specialized
the
species,
the less the
total
abun-
dance
per
habitat
(Pearson
correlation,
r
=
0.61,
P
Recommended