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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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8/10/2019 1989 Kolasa

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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