Spatial Unit Pattern

militaryzoologistΤεχνίτη Νοημοσύνη και Ρομποτική

1 Δεκ 2013 (πριν από 3 χρόνια και 8 μήνες)

86 εμφανίσεις


1

Spatial
Dynamics of Disturbance and Succession: Tracing the Impact of Watt’s

Unit Pattern

by

Jen Costanza

11/18/05


Introduction

Alex
S. Watt’s paper entitled “Pattern and Process in the Plant Community”
(Watt 1947)

has become a classic that continues to influence ecology today
.
In fact,
publications indexed by
the Institute for Scientific Information have cited
his paper
a tot
al of
671

times
between January
1980 and

November
18
, 2005
, including
32

citations since January 2005
.
His work has
influenced much of ecology and has led to many lines of research, including the description of
landscape pattern,
the
general
link between

p
attern
a
nd

ecological
process, the relationship
between disturbance and succession, and

the use of

neutral models in ecology.
In addition, t
he
p
attern
-
process relationship is one of the earliest precursors of the current field of landscape
ecology
(
Turner 1989)
.

In his paper, Watt was the first to explore the ubiquitous nature of heterogeneity and
pattern on the landscape, and was able to show that for a variety of systems, the phases of
succession manifest themselves in space.
Watt describe
d

how pr
ocess
causes

pattern in
ecological communities using seven case studies, followed by a synthesis and placement of his
work in a broader ecological context. In his examples, he
illustrated
ways in which

processes
such as

gap dynamics g
i
ve rise to a cyclic p
attern of change
that

manifest
s

itself as the "unit
pattern", in which all phases of succession
a
re represented in patches on th
e landscape. In this
way, Watt described

a pattern in space that represents succession in time.
In
one example
, Watt
detailed

a
beechwood community in which he
detailed the

cyclical change and regeneration due
to gap dynamics, as well as more extreme disturbance events.
Watt stated that i
n a constant

2

environment

with gap dynamics present
,

the plant community may be in equilibrium
and show a
constant proportion of the various pha
ses of succession, but mortality due to extreme disturbance
events such as fire or drought may cause a change in this equilibrium that may
have
consequences

well into the future. In addition, with high
distu
rbance frequency relative to
recovery time, this equilibrium may be reached only rarely.


The most important contribution Watt’s paper has made has been its influence on the way
in which ecologists view succession and disturbance.
Watt’s paper has become a

classic because
the spatial manifestation of disturbance and succession is a
ubiquitous

phenomenon in ecological
systems, and
because of the clarity with which he was able to portray his idea.
I will show that
Watt’s description

of the unit pattern has s
haped our current ideas regarding the spatial patterns
of succession and disturbance
, and will continue to be relevant to future research.


Disturbance,

Succession and

Patch

Dynamics


a Shifting Paradigm
:

pre
-
1947 to
1995

During the five decades following

the publication of Watt’s paper, ecologists’ ideas about
succession and disturbance underwent several major transformations

as a result of Watt’s unit
pattern concept.
Watt’s ideas about succession

were quite different from the prevailing ideas of
the tim
e. Earlier researchers had viewed succession as a progression in time toward a
homogenous, steady
-
state climax
condition. Terms such as monoclimax, polyclimax, and
postclimax were used

by some

to describe the
endpoints

of succession
(Clements 1936)
.

In
addition, Gleason
(1936)

assumed that within a climax community, spec
ies associations were
completely random

and homogenous. A

community could
therefore
be divided into
progressively smaller pieces that still represent the whole.
Watt showed that
even systems that
a
re presumably
at the climax state are heterogeneous and ex
perience small
-
scale internal

3

disturbances that reset the successional clock in portions of the landscape.
T
o Watt, t
he unit
pattern

wa
s

the
smallest division of the landscape into a representation of the whole.


Shortly following Watt’s paper, Whittaker
(1953)

described the “climax pattern” as
a
situation in which community productivity, structure, and pattern are at steady
-
state and are
determined by environmental factors
.

Acco
rding to Whittaker, all climaxes were determined by
biotic, edaphic, and climatic factors, and the many terms used to describe climax states were
irrelevant since succession is ultimately a progression toward a pattern that is adapted to be
highly producti
ve given the local environment.
In this way, Whittaker
expressed

Watt’s unit
pattern concept in terms of the physical environment.
Whittaker’s

concept has been used

previously

to determine

the historic vegetation
, or “potential natural vegetation”

of a giv
en place
using environmental characteristics as proxies
(Küchler 1964)
.

Watt’s and Whittaker’s ideas
were departures from the widely
-
held views of the climax
because they considered
small
-
scale disturbance to be an
autogenic

pr
ocess that leads to a
distinct
spatial
pattern on the landscape.
However, both continue
d

to
accept the concept of a
climax state

as the optimal condition for a site
. Watt describe
d

a central tendency toward
equilibrium, and Whittaker’s ideas impl
ied

that t
here is a climax state for any given
set of
environmental characteristics
.

The progression of these ideas began with an i
ncrease in studies of gap dynamics
, as well
as larger disturbances

during
the 196
0s and

1970s
.

For example,
Watt’s work inspired
Bray

(1956)

to
describe the gap dynamics of a maple
-
basswood forest
. He showed

that sugar maple
(
Acer

saccharum
)
wa
s able to take advantage of gap openings, while oak species (
Quercus

sp.
)
establish
ed

more often under den
se canopies
, leading

to a characteristic pattern in the landscape.
Williamson
(1975)

studied gap dynamics in an old
-
growth forest
and found that the forest was a

4

mosaic of species of different seral stages.
Gap dynamics were also incorporated into forest
growth models of succession (“gap models”)
(Botkin et al. 1972)
.

In addition to

gap dynamics, larger
disturbances such as fire and intertidal
fluctuations

were studied to a greater degree
, especially

beginning

in the 1970s.
Heinselman
(1973)

studied
fire in northern conifer forests and found that historically, fire determined the pattern
of
vegetation on the landscape.
The historic fire reg
ime

resulted in a mosaic effect

and prevented
older stands from attaining climax

and Heinselman suggest
ed

that fire “must be studied as an
integral part of the system”.
In addition, studies of rocky intertidal dynamics
(Levin and Paine
1974)

showed that such communities are spatial and temporal mosaics that are constantly
under
going disturbance events that disrupt succession to a climax state. These intertidal
communities were viewed as a heterogeneous system made up of essentially homogenous
patches
, and equilibrium composition depended on disturbance frequency.

As a result of
this
heightened interest in
the relationship between disturbance and
succession,
an increasing
number of researchers began to view disturbance as
an

essential and
ubiquitous

process in natural systems
.
Therefore, t
he concept of climax
as a stable, homogeno
us
state
was being ch
allenged according to that view. F
or example
, White
(1979)

asked,

“how much
change is allowable within the notion of a stable climax?”.
As a result of these new viewpoints,
the

par
adigm of patch dynamics emerged, emphasizing
the non
-
equilibrium nature of natural
communities
(Pickett 1980, Pickett and White 1985)
.

Patch dynamics describes the pattern of
patch creation in time and space, as well as the changes in individual patches throughout time.

The patch dynamic concept reje
cts the traditio
nal view of succession toward a
steady state and
instead incorporates disturbance as a fundamental, internal process. Patch dynamics recognizes

5

that disturbance plays a ubiquitous role in shaping most communities
, and it is the non
-
equilibr
ium nature of communities that promotes coexistence of species.

Patch dynamics
i
s a progression of Watt’s view of succession as the unit pattern

since it
focuses on the spatial dynamics of succession
. The patch dynamic concept not only incorporates
small,

gap
-
scale disturbance events, but also includes large disturbances such as fire
as part of
the natural system. This concept was

soon

applied to the design of nature reserves in
the

“minimum dynamic area”
idea
(Pickett and Thompson 1978)
. This idea states that reserve design
should be based on the
smallest area that preserves the disturbance regime and internal
colonization processes.

At about the same time as the patch dynamic concept was emerging,
t
he concept of the
“shifting mosaic steady state”

(Bormann and Likens 1979, Sprugel and Bormann 1981)

w
as also
introduced.
This concept essentially describes patch dynamics at a larger scale. Bormann and
Likens found that patterns may shift in space

due to disturbances
, but the overall proportion of
the landscape

in a given successional stage will achieve a

steady state over time. One example of
a shifting mosaic steady state is w
ave
-
regeneration of fir in boreal forests
(Sprugel and Bormann
1981)
. These are systems in which mature trees at the leading
edge of a
patch

are killed by wind,
and subsequently regenerate
. At any given point in time, a
ll phases of regeneration are present in
a “wave”
through the landscape.

The shifting mosaic steady state focuses the tendency of a
system toward equilibrium, wh
ile patch dynamics describes non
-
equilibrium

heterogeneity
.

Th
e
perceived

distinction between these two

theories
caused confusion among ecologists

and

le
d to research through the 1980s and 1990s

that
expanded on the patch dynamic and shifting
mosaic stead
y state ideas. In particular, the initial concept of the shifting mosaic steady state was
shown to be
invalid for systems experiencing

frequent or large
-
scale disturbance events. Turner

6

et al.
(1993)

showed that landscape
-
scale equilibrium occurs
only for relatively infrequent,
small
-
extent
distu
rbances.


In addition, hierarchy theory
(O'Neill et al. 1986)

b
egan to be applied to ecological
systems to show that landscapes are organized into patterns
that diffe
r with

spatial and

temporal
scales.

Thus, patterns at one scale are nested within dynamics at higher levels.

Wu and Loucks
(1995)

suggested that hierarchy theory be combined with patch dynamics to create a new
par
adigm

for describing landscape successional pattern and process

called “hierarchical patch
dynamics”
.
In their paper, Wu and Loucks stated that much of the
controversy over equilibrium
vs.

non
-
equilibrium
ideas
, and homogeneity vs. heterogeneity comes from

the scale
-
dependence
of these phenomena: a non
-
equilibrium system at
a fine

scale may be at equilibrium at
coarse

scale.

The shifting mosaic steady state idea describes spatial pattern over multiple patches in
large area, while patch dynamics focuses on d
ynamics between individual patches.

Hierarchical
patch dynamics

examines multiple scales. The theory

creates a framework in which

ecological
systems
are

nest
ed hierarchies of patch mosaics. The

dynamics a
t one scale are

the result of a
composite of patch d
ynamics at different scales.
According to the framework, d
isturbance
and
succession occur

at an intermediate scale, but the patchiness that results affects both finer
-

and
coarser
-
scale processes and patterns.

Therefore, Watt’s unit pattern idea had progr
essed and was
being applied at various scales.
Putting patch dynamic hierarchy theory to the test is
the subject
of much current research, as well as an area ripe for future advance.


Expanding

Research
: 199
6

to
2005


Since the mid
-
1990s,

the concept of d
isturbance as a natural process

that

leads to spatial
variability in
succession

has
led to an expansion in research on
the patch dynamics of

7

disturbance.

In particula
r, several themes have emerged, including

the study of succession
following large, infreq
uent disturbances (LIDs),
the application of variability in disturbance to

management
, and modeling patch dynamics across multiple scales
.

The occurrence of large
-
scale disturbances in the 1980s

such as the eruption of Mount St.
Helens in 1980 and the 198
8 fires in Yellowstone National Park

has led to an expansion of
research on large, infrequent disturbances (LIDs).
The large extent of
LIDs leaves little residuals
such as survivors and seed sources.

Therefore, they have ecological consequences that differ

from smaller, more frequent disturbances.
Turner et al.
(1998)

show
ed

that
succession following
LIDs differ
s

from smaller, infrequent disturbances.
LIDs

tend to produce novel

successional
pathways, w
hile small
er disturbances result in

succession

that is predictable
.

In addition, Foster
et al.
(1998)

showed that LIDs interact with the landscap
e and vegetation to

produce legacies that

can

last

for many years

into the future.

Modeling patch dynamics across multiple scales is another important area of current
research. Wu and Levin
(1997)

expanded on earlier gap models by providing a model that

treats
ecological systems as hierarchical dynamic mosaics of patches that interact at a higher level to
determine the overall structure and function of the system. They linked within
-
patch dynamics
and between
-
patch dynamics to simulate spatial heterogene
ity and its effects on ecological
processes. In this way, they put into practice the hierarchical patch dynamics paradigm proposed
by Wu and Loucks. In addition, Urban
(2005)

discussed the combination of gap

models with
other models, a cellular automaton and a stage
-
based transition model, to create two different
meta
-
models. The meta
-
models incorporated gap processes, along with landscape
-
level
processes such as disturbance. However,
the
use of
multiple
-
scal
e

models is just beginning
. They
show promise in assisting the study of gap and disturbance processes across spatial scales.


8

Another field of current research has been inspired by the v
ariability of disturbance
regimes and succession
.

The
concept of hist
oric or natural range of variability
(Landres et al.
1999)

states that while disturbance events may keep natural systems from achieving a
static or
quantitative equilibrium, systems should experience

a qualitative equilibrium, or variation within
bounds, over time. Accepting this variability and approximating the natural
, dynamic

variation of
disturbance and succession
systems has become a goal for many land managers and
conservationists.
Tinker et al
.
(2003)

showed tha
t the patchiness that resulted from clearcutting
the Targhee National Forest was outside the natural range of variability of nearby Yellowstone
National Park. Therefore, management in the Targhee was not in accordance with natural
conditions. Assessing nat
ural variation in systems continues to be a complicated task in ecology,
since variability depends on the spatial and temporal scale with which it is assessed.


Summary and
Future Directions


Watt’s idea that successional phases manifest themselves in spa
ce

and give

rise to
heterogeneity in almost all systems led to a
shifting paradigm

from traditional Clementsian
succession to the shifting mosaic steady state and patch dynamics, to hierarchical patch
dynamics. The current view of ecological systems as d
ynamic mosaics of patches formed by
disturbance and subsequent succession has resulted from Watt’s unit pattern

concept. However,
there are several key areas of research that need to be addressed. First, it is clear that these
dynamics vary by spatial and
temporal scale,

and hierarchy theory has provided a starting point
for scaling processes. However, there is a need for a solid framework that describes the ways in
which community
-
level patch dynamics scale up to ecosystem
-
level processes and is grounded i
n
empirical evidence.

Additionally,
the feedback between disturbance events and vegetation

9

pattern on the landscape should be investigated more explicitly. Specifically, more research is
needed on the compounded impacts of multiple disturbances on the land
scape. Similarly, while
alternate stable states and resilience have been investigated t
hus far, we still lack a good
understanding of whether and under what circumstances the concept is worthwhile. The pursuit
of these research areas and others will ensure

that Watt’s legacy
continues

well into the future.


References

Bormann, F. H., and G. E. Likens. 1979. Pattern and process in a forested ecosystem. Springer
-
Verlag, New York.

Botkin, D. B., J. F. Janak, and J. R. Wallis. 1972. Rational
e, limitations, and assumptions of a
Northeastern forest growth simulator. IBM Journal of Research and Development
16
:101
-
116.

Bray, J. R. 1956. Gap phase replacement in a maple
-
basswood forest. Ecology
37
:598
-
600.

Clements, F. E. 1936. Nature and structur
e of the climax. Journal of Ecology
24
:252
-
284.

Foster, D. R., D. H. Knight, and J. F. Franklin. 1998. Landscape patterns and legacies resulting
from large, infrequent forest disturbances. Ecosystems
1
:497
-
510.

Gleason, H. A. 1936. Is the synusia an associ
ation? Ecology
17
:444
-
451.

Heinselman, M. L. 1973. Fire in the virgin forests of the Boundary Waters Canoe Area,
Minnesota. Quaternary Research
3
:329
-
382.

Küchler, A. W. 1964. Potential natural vegetation of the conterminous United States.
in

American Geog
raphical Society, Special Publication No. 36.

Landres, P. B., P. Morgan, and F. J. Swanson. 1999. Overview of the use of natural variability
concepts in managing ecological systems. Ecological Applications
9
:1179
-
1188.

Levin, S. A., and R. T. Paine. 1974.
Disturbance, patch formation, and community structure.
Proceedings of the National Academy of Sciences
71
:2744
-
2747.

O'Neill, R. V., D. L. DeAngelis, and D. J. Waide. 1986. A hierarchical concept of ecosystems.
Princeton University Press, Princeton, NJ.

Pi
ckett, S. T. A. 1980. Non
-
equilibrium coexistence of plants. Bulletin of the Torrey Botanical
Club
107
:238
-
248.

Pickett, S. T. A., and J. N. Thompson. 1978. Patch dynamics and the design of nature reserves.
Biological Conservation
13
:27
-
37.

Pickett, S. T.
A., and P. S. White. 1985. The ecology of natural disturbance and patch dynamics.
Academic Press, San Diego.

Sprugel, D. G., and F. H. Bormann. 1981. Natural disturbance and the steady state in high
-
altitude balsam fir forests. Science
211
:390
-
393.

Tinker,

D. B., W. H. Romme, and D. G. Despain. 2003. Historic range of variability in landscape
structure in subalpine forests of the Greater Yellowstone Area, USA. Landscape Ecology
18
:427
-
439.


10

Turner, M. G. 1989. Landscape ecology: the effect of pattern on proc
ess. Annual Review of
Ecology and Systematics
20
:171
-
197.

Turner, M. G., W. L. Baker, C. J. Peterson, and R. K. Peet. 1998. Factors influencing succession:
lessons from large infrequent natural disturbances. Ecosystems
1
:511
-
523.

Turner, M. G., W. H. Romme
, R. H. Gardner, R. V. O'Neill, and T. K. Kratz. 1993. A revised
concept of landscape equilibrium: disturbance and stability on scaled landscapes.
Landscape Ecology
8
:213
-
227.

Urban, D. L. 2005. Modeling ecological processes across scales. Ecology
86
:1996
-
2006.

Watt, A. S. 1947. Pattern and process in the plant community. Journal of Ecology
35
:1
-
22.

White, P. S. 1979. Pattern, process, and natural disturbance in vegetation. The Botanical Review
45
:229
-
299.

Whittaker, R. H. 1953. A consideration of climax th
eory: the climax as a population and pattern.
Ecological Monographs
23
:41
-
78.

Williamson, G. B. 1975. Pattern and seral composition in an old
-
growth beech
-
maple forest.
Ecology
56
:727
-
731.

Wu, J., and S. A. Levin. 1997. A patch
-
based spatial modeling appro
ach: conceptual framework
and simulation scheme. Ecological Modelling
101
:325
-
346.

Wu, J., and O. L. Loucks. 1995. From balance of nature to hierarchical patch dynamics: a
paradigm shift in ecology. The Quarterly Review of Biology
70
:439
-
466.



Excellent
job putting your paper in context, tracing ideas prior to your classic through to recent
research.

26/26



Excellent work. Very much forward looking.


26