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EExtenxtenssionion NoteNote
Extension Note
10 EExtenxtenssionion NoteNote
Landscape Ecology and Natural
Disturbances: Relationships to Biodiversity
Natural disturbance statistics grab natural disturbances such as wildfire,
your attention: wind, and insects (Figure , Table ).
• Hurricane-force winds flatten over To maintain a range of ecosystems and
Biodiversity ha of forest land on north- habitats and to maintain biodiversity,
Management Concepts ern Vancouver Island in the winter a new approach in forest management
in Landscape Ecology of . applies the concepts of landscape and
• Small isolated “hot spots” of moun- disturbance ecology.
tain pine beetle infestations are The field of landscape ecology
John Parminter and Patrick Daigle detected in southwestern British integrates natural disturbance regimes
B.C. Ministry of Forests Columbia in the early s. These and their effects on the distribution of
Research Program infestations irrupt rapidly a decade ecological types across a landscape,
31 Bastion Square
Victoria, BC V8W 3E7 later into massive outbreaks cover- the dispersal and movement of plant
(250) 386-6810 ing ha of lodgepole pine and animal species, and the flow of
July 1997 forests. energy and nutrients. The Forest Prac-
• Wildfire burns over ha of tices Code explicitly recognizes
British Columbia’s forest land in landscape ecology by designating
. One fire alone covers planning areas called Landscape Units,
ha—more than half of the total each with specific Landscape Unit
area burned. Objectives. The Biodiversity Guide-
These extraordinary events can book (B.C. Ministry of Forests and
“. . . if the effects of forest mean different things to different B.C. Ministry of Environment, Lands
management activities people: a reduced timber harvest, a and Parks ), a component of the
lost wilderness reserve, an unsightly Code, focuses on the significance of
closely resemble those of recreation area. Many of the feelings sustaining naturally occurring land-
natural disturbances, the generated embrace a sense of loss and scape patterns.
the belief that nature is on the ram- The guidebook recommends a
risk of losing native species page. But while these scenarios may procedure for establishing and meet-
and altering ecological appear to conflict with and impair a ing objectives to maintain biodiversity
multitude of forest resource values, at both landscape and stand levels.
processes is lowered . . .” these natural disturbances show evo- Ecological principles form the basis of
lution in action and can actually this approach, which assumes that if
maintain that increasingly precious the effects of forest management ac-
global treasure—biodiversity. tivities closely resemble those of
British Columbia’s natural ecosys- natural disturbances, the risk of losing
tems have all evolved, and are still native species and altering ecological
evolving, under the influence of processes is lowered.
January 2000. Policy direction for biodiversity is now represented by the Landscape Unit Planning
Guide. This Extension Note should be regarded as technical background only.
Ministry of Forests Research Program
Disturbance history of British Columbia’s forests for insects, wildfires, and forest
harvesting.
The other biological This extension note is the second in from human intervention, believing
concepts documented as a series designed to raise awareness of that this was the appropriate way to
landscape ecology concepts and to preserve all species.
individual extension notes provide background for the ecologi- However, scientists have increas-
include: cally based forest management ingly recognized that forest, shrub,
approach recommended in the and grassland ecosystems are dynamic
• management concepts for Biodiversity Guidebook. The emphasis entities. This view, referred to in ecol-
landscape ecology is on natural disturbance ecology ogy as the “non-equilibrium model,”
(Parminter ). We first define and considers ecosystem structure to be
(Extension Note No. 07), describe natural disturbances, the determined by interactions between
• spatial patterns, agents responsible, and the ecological the long-term forces of ecological
principles of natural disturbances, and succession, fluctuations in climate,
• connectivity, their effects on landscape patterns, and the more immediate effects of
• riparian areas, processes, and functions. We conclude natural disturbances.
by examining how the concepts of Natural disturbances are defined as
• interior habitats and natural disturbance ecology are incor- relatively distinct events in time that
edge effects, and porated into the Forest Practices Code disrupt ecosystem, community, or
and the biodiversity guidelines. population structure and that change
• seral stages across resources, the availability of suitable
landscapes. What Is Natural Disturbance habitat, and/or the physical environ-
Ecology? ment. These events occur at varying
intensities across various space and
Until relatively recently, natural re- time scales and have contributed,
source management decisions and along with climate, soils, and
activities were based on the idea that geomorphology, to producing the
ecosystems existed in a steady, self- diverse landscape patterns we see
replacing state (sometimes today.
corresponding to “old-growth” condi-
tions) and that natural disturbances Disturbance Agents and Regimes
were unimportant. Some people Wildfire, wind, drought, insects, and
sought to protect old-growth forests disease cause some of the most
Parminter’s chapter in Voller and Harrison’s Conservation Biology Principles for Forested Landscapes
provides a valuable reference for those wanting an in-depth understanding.
Area affected by some natural and cultural disturbances in British Columbia
(from Parminter 1997)
Disturbance agent Area (ha)
Insects (1921 1995) 24274990
Wildfire (1912 1995) 10577151
Forest harvesting (1913 1993) 8289096
Slash burning (1913 1993) 1744789
Landclearing (1913 1958) 438164
Wildlife habitat burning (1982 1993) 551980
Total 45876170
widespread landscape disturbance. habitats, such as riparian and wetland
They affect most ecosystems, but not areas, or to the spatial relationships
with equal frequency or magnitude. that influence landscape connectivity
Catastrophic events, such as wildfire, and edge or interior forest habitats.
wind, landslides, snow avalanches, British Columbia’s forests have
flooding, and certain other weather- evolved under the influence of several
related phenomena, can be intense natural disturbance regimes, which
and act over large areas, resulting in have created the composition, size,
the death of entire populations and age, and distribution of specific forest
causing major changes to ecosystems. types, as well as the structural charac-
However, a relatively minor distur- teristics of forest stands.
bance, involving tree death or treefall
gaps, would affect only one or a few Seven Generalizations about the
individuals. Importance of Disturbance
Two categories of natural distur-
bances are: Disturbances are fundamentally
. Abiotic: from non-living agents, important in controlling landscape
such as wildfire, flood, landslides pattern and ecological function. Peter
and snow avalanches, and weather- White () listed seven generaliza-
related phenomena (e.g., wind, tions that help to explain natural
drought, frost/ice/snow). disturbances and their effects on
. Biotic: from living agents, such as ecosystems.
disease organisms, or grazing and
browsing by mammals or insects. . Disturbances occur on a variety of
The combined effects of abiotic spatial and temporal scales
and biotic natural disturbance agents Landscape mosaics reflect the temporal
determine natural disturbance re- and spatial distributions of distur-
gimes. These regimes are defined by bances. Disturbances can be of:
variables such as the area disturbed • small spatial scale (e.g., an individual
and the frequency and magnitude of tree dies or falls, creating a treefall
the disturbance (expressed as either gap), or
intensity or severity). • large spatial scale (e.g., fire may re-
Disturbance frequency, size, and turn a large forested area to an early
intensity vary among ecosystems, seral stage or wind may advance
helping to create landscapes with succession by releasing an
different attributes. Landscape at- understorey of shade-tolerant ad-
tributes include the relative vanced regeneration).
abundance of seral stages, or the Both small- and large-scale distur-
stages of ecosystem development that bances can occur, resulting in landscape
follow a major disturbance event. mosaics with patches of varying size,
Other attributes relate to specific species composition, and age structure.
Disturbances can affect an ecosys- than % in interior ponderosa pine
tem for: forests where conditions are different
• a relatively short time period (e.g., and fire is the prevalent disturbance
a tree falls; subsequent canopy agent.
closure occurs within a decade), or Small-scale wind events may create
• a relatively long time period (e.g., a disturbances of varying size in the
landslide or intense wildfire; landscape because of specific topo-
complete ecosystem recovery to graphic or vegetation conditions.
pre-disturbance conditions may Trees susceptible to blowdown include
take centuries). those that are:
• situated in rain-saturated soils;
. Disturbances affect many levels of • located where airflow may be fun-
biological organization nelled and thus accelerated (e.g., on
Most biological communities are re- a mountain ridge, at the head of a
covering from the last disturbance. valley, or next to clearcuts); and
The effects of disturbance are felt at • weakened by age, root disease, or
many levels of biological organiza- insect damage.
tion—from the individual to
ecosystem-wide. Natural disturbances . Disturbances overlay environmen-
can: tal gradients, both influencing and
• disrupt ecosystem and stand being influenced by those gradients
development, Underlying environmental gradients
• return areas to earlier stages of affect some natural disturbances. For
succession, and example, fires have the potential to
• change habitat mosaics. burn more intensely when moving
For example, severe fires may con- across dry terrain as opposed to
sume organic matter in soils, kill moister areas, where less fuel might
dominant tree species, change stream burn. Some disturbances, however,
chemistry, and shift the patterns of operate independently of physical
mammal movements, thus affecting gradients, as when severe windstorms
ecological, physiological, and behav- randomly destroy trees over wide
ioural processes and landscape areas. The landscape patterns that
patterns. result from this type of disturbance
are thus patchy and unrelated to the
. Disturbance regimes vary, both underlying environmental gradients.
regionally and within one landscape. Alternatively, some disturbances
Disturbances vary among specific reinforce changes in landscape com-
geographic areas and biogeoclimatic position and structure along physical
zones. Some regions or landscapes are gradients. Such events are important
subject to wind, landslides, and flood- mechanisms for energy flow and nu-
ing, while fire, insects, and disease trient cycling and for maintaining age,
affect others more. species, genetic, and structural
For example, damage during severe diversity.
wind events is strongly associated with
elevation and aspect, as well as 5. Disturbances interact
vegetation structure. Extremely large Various disturbance agents affect an
areas can be disturbed, especially already diverse physical and biological
along or near the west coast, where landscape to create and maintain eco-
large-scale storms with hurricane- system diversity. Some disturbance
force winds come ashore. Some % agents may promote or inhibit the
of individual tree mortality in coastal occurrence and effects of other distur-
Sitka spruce–western hemlock forests bance agents.
is wind-induced, compared to less For example, windthrow may affect
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