34
Baselines, patterns and process
or ‘ ecological surprises ’ , that may be driven by environmental or biological processes (Pickett & Ostfeld,
1995 ). The need to understanding complex, dynamic
ecosystems has stimulated a wealth of new ecological
theory, and two of the most infl uential themes discussed here are the Hierarchical Patch Dynamics
Paradigm (HPDP) and the literature centred on ideas
of resilience and ecological thresholds.
The HPDP combines hierarchy theory (which proposes that scale - dependent levels of organization exist
in nature) and the patch dynamics perspective, thereby
providing a framework for structuring knowledge
about complex systems (Wu & Loucks, 1995 ). Three
main concepts are central to the HPDP (Wu, 1999 ; Wu
& David, 2002 ):
First, ecosystems may be considered to be complex
systems, because they are spatially and temporally heterogeneous and are composed of many interacting
components. Moreover, the interactions between
ecosystem components are characterized by various
feedbacks, non - linearity and threshold responses. The
complex properties and dynamics that scientists
observe in ecosystems systems emerge from these
interactions and from the exchange of energy and
materials from outside the system.
Second, complex systems can be considered as a
nested arrangement of interacting sub - systems, i.e. the
system is composed of discrete but interacting sub -
systems that are themselves composed of sub - systems.
Hierarchy theory provides scientists with a means of
organizing information about these complex systems
by identifying the systems and sub - systems and their
corresponding hierarchical levels. Sub - systems at
different levels in the hierarchy are dominated by
different processes. Higher levels are larger and are
characterized by slower processes; these higher level
processes impose constraints on lower levels, whereas
lower level processes provide the mechanism by which
higher levels emerge.
Third, ecosystems vary considerably in time and
space. HPDP provides a framework for identifying and
describing the constituent systems and sub - systems
which generate heterogeneity over time and space.
Thus, heterogeneous landscapes can be described by
identifying patches – spatially discrete entities whose
internal structure or function is signifi cantly different
from those of their surroundings. For example, in his
seminal paper, Watt described vegetation assemblages
in terms of dynamic mosaics of patches at different
successional stages (Watt, 1947 ). Crucially, Watt ’ s
Whittaker & Fern á ndez - Palacios, 2007 ). Equilibrium -
based models in ecology are therefore useful in describing the feedbacks that could theoretically lead to
stability, but in reality this stability is elusive, or at least
scale - specifi c; a patch dynamic landscape, for example,
may retain the characteristics of stability over large
spatial extents or short temporal scales, but will be
highly dynamic at fi ne spatial scales or over long time
periods.
In parallel to the dominant ideas of balance and
equilibrium, some ecologists pursued ideas of ecosystem change and landscape dynamics. As early as
1930, Charles Elton asserted that ‘ the balance of
nature does not exist and perhaps never has existed ’
(Elton, 1930 ). Elton ’ s belief arose from his understanding of the complexity and dynamism of biological and
environmental variables, leading him to believe that
change, rather than stability, was the norm for natural
systems.
In the following decade, Alexander Watt ’ s prescient
address to the British Ecological Society described
dynamic landscapes in southern England in which
patches of vegetation underwent cyclical changes of
‘ pioneer ’ , ‘ building ’ , ‘ mature ’ and ‘ degenerate ’ phases
(Watt, 1947 ). From his own observations of landscapes, he described seven ecosystems, including dwarf
heather ( Calluna ), grasslands and beech ( Fagus ) woodlands in which he had observed this pattern, leading
him to believe it was of general signifi cance to many
ecosystems.
In later years, Watt ’ s ideas proved central to the
development of our understanding of many ecosystems. The concepts of minimum dynamic area (Pickett
& Thompson, 1978 ), mosaic cycles (Remmert, 1991 ),
forest gap dynamics (Shugart, 1984 ) and Holling ’ s
adaptive cycles (Holling et al ., 2001 ) rested on Watt ’ s
thesis of patch dynamics, and his work informed many
of the infl uential papers that heralded the paradigm
shift to the ‘ new ecology ’ of nature in fl ux (Levin,
1992 ; Pickett et al ., 1992 ; Wu & Loucks, 1995 ).
3.4 UNDERSTANDING ECOSYSTEMS
IN FLUX
Under the ‘ fl ux of nature ’ paradigm, ecosystems are
understood to be heterogeneous (patchy) and dynamic
(variable over time). They are often infl uenced by multiple variables, feedbacks and non - linear responses.
Furthermore, they are prone to stochastic variations,
Baselines, patterns and process
or ‘ ecological surprises ’ , that may be driven by environmental or biological processes (Pickett & Ostfeld,
1995 ). The need to understanding complex, dynamic
ecosystems has stimulated a wealth of new ecological
theory, and two of the most infl uential themes discussed here are the Hierarchical Patch Dynamics
Paradigm (HPDP) and the literature centred on ideas
of resilience and ecological thresholds.
The HPDP combines hierarchy theory (which proposes that scale - dependent levels of organization exist
in nature) and the patch dynamics perspective, thereby
providing a framework for structuring knowledge
about complex systems (Wu & Loucks, 1995 ). Three
main concepts are central to the HPDP (Wu, 1999 ; Wu
& David, 2002 ):
First, ecosystems may be considered to be complex
systems, because they are spatially and temporally heterogeneous and are composed of many interacting
components. Moreover, the interactions between
ecosystem components are characterized by various
feedbacks, non - linearity and threshold responses. The
complex properties and dynamics that scientists
observe in ecosystems systems emerge from these
interactions and from the exchange of energy and
materials from outside the system.
Second, complex systems can be considered as a
nested arrangement of interacting sub - systems, i.e. the
system is composed of discrete but interacting sub -
systems that are themselves composed of sub - systems.
Hierarchy theory provides scientists with a means of
organizing information about these complex systems
by identifying the systems and sub - systems and their
corresponding hierarchical levels. Sub - systems at
different levels in the hierarchy are dominated by
different processes. Higher levels are larger and are
characterized by slower processes; these higher level
processes impose constraints on lower levels, whereas
lower level processes provide the mechanism by which
higher levels emerge.
Third, ecosystems vary considerably in time and
space. HPDP provides a framework for identifying and
describing the constituent systems and sub - systems
which generate heterogeneity over time and space.
Thus, heterogeneous landscapes can be described by
identifying patches – spatially discrete entities whose
internal structure or function is signifi cantly different
from those of their surroundings. For example, in his
seminal paper, Watt described vegetation assemblages
in terms of dynamic mosaics of patches at different
successional stages (Watt, 1947 ). Crucially, Watt ’ s
Whittaker & Fern á ndez - Palacios, 2007 ). Equilibrium -
based models in ecology are therefore useful in describing the feedbacks that could theoretically lead to
stability, but in reality this stability is elusive, or at least
scale - specifi c; a patch dynamic landscape, for example,
may retain the characteristics of stability over large
spatial extents or short temporal scales, but will be
highly dynamic at fi ne spatial scales or over long time
periods.
In parallel to the dominant ideas of balance and
equilibrium, some ecologists pursued ideas of ecosystem change and landscape dynamics. As early as
1930, Charles Elton asserted that ‘ the balance of
nature does not exist and perhaps never has existed ’
(Elton, 1930 ). Elton ’ s belief arose from his understanding of the complexity and dynamism of biological and
environmental variables, leading him to believe that
change, rather than stability, was the norm for natural
systems.
In the following decade, Alexander Watt ’ s prescient
address to the British Ecological Society described
dynamic landscapes in southern England in which
patches of vegetation underwent cyclical changes of
‘ pioneer ’ , ‘ building ’ , ‘ mature ’ and ‘ degenerate ’ phases
(Watt, 1947 ). From his own observations of landscapes, he described seven ecosystems, including dwarf
heather ( Calluna ), grasslands and beech ( Fagus ) woodlands in which he had observed this pattern, leading
him to believe it was of general signifi cance to many
ecosystems.
In later years, Watt ’ s ideas proved central to the
development of our understanding of many ecosystems. The concepts of minimum dynamic area (Pickett
& Thompson, 1978 ), mosaic cycles (Remmert, 1991 ),
forest gap dynamics (Shugart, 1984 ) and Holling ’ s
adaptive cycles (Holling et al ., 2001 ) rested on Watt ’ s
thesis of patch dynamics, and his work informed many
of the infl uential papers that heralded the paradigm
shift to the ‘ new ecology ’ of nature in fl ux (Levin,
1992 ; Pickett et al ., 1992 ; Wu & Loucks, 1995 ).
3.4 UNDERSTANDING ECOSYSTEMS
IN FLUX
Under the ‘ fl ux of nature ’ paradigm, ecosystems are
understood to be heterogeneous (patchy) and dynamic
(variable over time). They are often infl uenced by multiple variables, feedbacks and non - linear responses.
Furthermore, they are prone to stochastic variations,