Roots, relevance, aims and values
35
ideas about patch dynamics were some of the fi rst to
capture the link between pattern and process in
ecology, because they made explicit the functional link
between the pattern of vegetation in a landscape and
the ongoing process of plant succession. To summarize, the HPDP is a powerful framework for describing
and understanding ecosystems because it links pattern,
process and scale in a spatial and temporal hierarchy
(O ’ Neill et al ., 1986 ; Pickett et al ., 1987, 1989 ; Urban
et al ., 1987 ).
A hierarchical (multi - scale) structure has been
described for riverine systems (e.g. Frissell et al . 1986 ),
and savannas (du Toit et al ., 2003 ). Further, spatial
hierarchies are being used extensively as frameworks
for modelling ecological complexity (e.g. Wu & David,
2002 ), landscape analysis (e.g. Burnett & Blaschke,
2003 ) and the effects of climate change and land - cover
change on species distribution (e.g. Pearson & Dawson,
2003 ). In savannas, Coughenour and Ellis (1993) proposed a hierarchical structure for ecological processes
that nested small - scale patterns of disturbance (e.g.
by fi re and herbivory) within a broader spatial framework of climate, topography, geology and hydrology.
Palaeoecological evidence from the savannas of Kenya
provides strong support for this perspective (Gillson,
2004a ; see Figure 3.2 ).
Over the past few decades, rapid progress has been
made in the development of theories that describe ecosystems in terms of their resilience, defi ned as their
capacity to absorb disturbance, and their ability to reorganize when a critical threshold is exceeded (Holling,
1973 ). Ideas of resilience and thresholds provide a
framework around which the complex, non - linear
behaviour of ecosystems can be explained. Furthermore,
resilience theory integrates human infl uences on ecosystems with the feedbacks between linked environmental and social systems (Berkes et al ., 2003 ).
Many ecosystems exhibit threshold behaviour, in
which a critical environmental or biological threshold
is crossed, causing reorganization and transition to a
new quasi - stable state or phase. Such phase transitions
(reviewed in Folke et al ., 2004 ) have been observed
in coral reefs, where overfi shing, eutrophication and
bleaching can cause a switch to an algal dominated
reef; in freshwater lakes, which can switch from a
clear oligotrophic to a turbid eutrophic state because
of agricultural run - off; in savannas, which switch
between woodland and grassland phases, depending
on changes in disturbance by fi re and herbivores; and
in forests that can switch from evergreen needle - leaved
to deciduous broad - leaved, depending on climatic
variation linked to timing of disturbances.
The principle of self - organization is one of the key
features of systems that exhibit phase and transition.
Transition to a new phase may be precipitated by
extreme or external environmental factors, but maintenance of the new state is sustained by processes
internal to this phase.
Thresholds are crossed when ecological resilience is
exceeded, either because environmental or biological
change reaches a critical point or because of an unusually intense disturbance such as a hurricane, fl ood or
severe fi re. Alternatively, or in combination with environmental or biological change, ecological resilience
may be modifi ed because of anthropogenic activities
that can affect the ability of systems to absorb disturbance. Sometimes, humans have been shown to
increase resilience of favoured landscape elements,
while in other cases over - exploitation or mismanagement has led to loss of resilience (Berkes & Folke, 1998 ;
Adger, 2000 ; Dearing, 2008 ).
Even before the widespread recognition of ecological
thresholds, many ecosystems had been described in
terms of transitions between two or more quasi - stable
‘ phases ’ . Phase and transition describes the dynamic
process by which ecosystems transform between alternative states of organization. Rather than a linear
process of successional change, ecosystem behaviour
clusters around regions of higher probability space, or
domains of attraction. These domains are not necessarily equilibrium points, which is why ‘ phase ’ is a preferable term to ‘ stable state ’ or ‘ equilibrium ’ .
In rangelands, for example, Westoby et al . (1989)
described alternate states (phases), with transitions
between these states being driven by combinations of
climatic factors, and management actions such as fi re
or changes in grazing pressure. Similarly, in savanna
ecology, two apparently stable phases – woodland and
grassland – are known. Relatively rapid transitions
occur between these phases, and have been observed
at a range of spatial scales.
In east Africa, Dublin (Dublin et al ., 1990 ) hypothesized a regional scale transition from open grassland
to woodland in response to the dramatic reduction in
herbivory that occurred due to the rinderpest pandemic
at the end of the 19th century, and a later transition
back to a more open savanna in response to growing
elephant populations and fi re, caused by increasing
biomass build - up (for comparable landscape - scale
dynamics in Australia, see, e.g. Sharp & Whittaker,
35
ideas about patch dynamics were some of the fi rst to
capture the link between pattern and process in
ecology, because they made explicit the functional link
between the pattern of vegetation in a landscape and
the ongoing process of plant succession. To summarize, the HPDP is a powerful framework for describing
and understanding ecosystems because it links pattern,
process and scale in a spatial and temporal hierarchy
(O ’ Neill et al ., 1986 ; Pickett et al ., 1987, 1989 ; Urban
et al ., 1987 ).
A hierarchical (multi - scale) structure has been
described for riverine systems (e.g. Frissell et al . 1986 ),
and savannas (du Toit et al ., 2003 ). Further, spatial
hierarchies are being used extensively as frameworks
for modelling ecological complexity (e.g. Wu & David,
2002 ), landscape analysis (e.g. Burnett & Blaschke,
2003 ) and the effects of climate change and land - cover
change on species distribution (e.g. Pearson & Dawson,
2003 ). In savannas, Coughenour and Ellis (1993) proposed a hierarchical structure for ecological processes
that nested small - scale patterns of disturbance (e.g.
by fi re and herbivory) within a broader spatial framework of climate, topography, geology and hydrology.
Palaeoecological evidence from the savannas of Kenya
provides strong support for this perspective (Gillson,
2004a ; see Figure 3.2 ).
Over the past few decades, rapid progress has been
made in the development of theories that describe ecosystems in terms of their resilience, defi ned as their
capacity to absorb disturbance, and their ability to reorganize when a critical threshold is exceeded (Holling,
1973 ). Ideas of resilience and thresholds provide a
framework around which the complex, non - linear
behaviour of ecosystems can be explained. Furthermore,
resilience theory integrates human infl uences on ecosystems with the feedbacks between linked environmental and social systems (Berkes et al ., 2003 ).
Many ecosystems exhibit threshold behaviour, in
which a critical environmental or biological threshold
is crossed, causing reorganization and transition to a
new quasi - stable state or phase. Such phase transitions
(reviewed in Folke et al ., 2004 ) have been observed
in coral reefs, where overfi shing, eutrophication and
bleaching can cause a switch to an algal dominated
reef; in freshwater lakes, which can switch from a
clear oligotrophic to a turbid eutrophic state because
of agricultural run - off; in savannas, which switch
between woodland and grassland phases, depending
on changes in disturbance by fi re and herbivores; and
in forests that can switch from evergreen needle - leaved
to deciduous broad - leaved, depending on climatic
variation linked to timing of disturbances.
The principle of self - organization is one of the key
features of systems that exhibit phase and transition.
Transition to a new phase may be precipitated by
extreme or external environmental factors, but maintenance of the new state is sustained by processes
internal to this phase.
Thresholds are crossed when ecological resilience is
exceeded, either because environmental or biological
change reaches a critical point or because of an unusually intense disturbance such as a hurricane, fl ood or
severe fi re. Alternatively, or in combination with environmental or biological change, ecological resilience
may be modifi ed because of anthropogenic activities
that can affect the ability of systems to absorb disturbance. Sometimes, humans have been shown to
increase resilience of favoured landscape elements,
while in other cases over - exploitation or mismanagement has led to loss of resilience (Berkes & Folke, 1998 ;
Adger, 2000 ; Dearing, 2008 ).
Even before the widespread recognition of ecological
thresholds, many ecosystems had been described in
terms of transitions between two or more quasi - stable
‘ phases ’ . Phase and transition describes the dynamic
process by which ecosystems transform between alternative states of organization. Rather than a linear
process of successional change, ecosystem behaviour
clusters around regions of higher probability space, or
domains of attraction. These domains are not necessarily equilibrium points, which is why ‘ phase ’ is a preferable term to ‘ stable state ’ or ‘ equilibrium ’ .
In rangelands, for example, Westoby et al . (1989)
described alternate states (phases), with transitions
between these states being driven by combinations of
climatic factors, and management actions such as fi re
or changes in grazing pressure. Similarly, in savanna
ecology, two apparently stable phases – woodland and
grassland – are known. Relatively rapid transitions
occur between these phases, and have been observed
at a range of spatial scales.
In east Africa, Dublin (Dublin et al ., 1990 ) hypothesized a regional scale transition from open grassland
to woodland in response to the dramatic reduction in
herbivory that occurred due to the rinderpest pandemic
at the end of the 19th century, and a later transition
back to a more open savanna in response to growing
elephant populations and fi re, caused by increasing
biomass build - up (for comparable landscape - scale
dynamics in Australia, see, e.g. Sharp & Whittaker,