Causes and Consequences of Spatial Heterogeneity

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Transcript Causes and Consequences of Spatial Heterogeneity

Causes and Consequences of Spatial Heterogeneity
Ecolog(ists) use(s) the concept of a landscape in two
ways. The first, which considers a landscape as a specific
area based on human scales, is intuitive: Landscapes are
ecological systems that exist at the scale of kilometers and
comprise recognizable elements such as forest patches,
fields, and hedgerows, human settlements, and natural
ecosystems. The second use of landscape is as an
abstraction representing spatial heterogeneity at any scale.
(Pickett and Cadenasso 1995, Science)
In this guise, the landscape is an ecological
criterion for a spatial approach to any
ecological system.
From Webster’s Dictionary (10th edition), 2001
Heterogeneity: The quality or state of being heterogeneous.
Heterogeneous: consisting of dissimilar or diverse ingredients or
constituents.
Relative term (i.e., similar?)
Measured by ecological characteristics
Emphasis on changes across the space
Depending on interests, objectives
Function of time. Again, “ephemeral” is a relative concept.
Mosaic: a surface decoration made by inlaying pieces of variously
colored (attentions here!) materials to form pictures or patterns.
Coherent structure or relationship
Examples of
mosaics and
heterogeneity
Examples of
mosaics and
heterogeneity
Causes Spatial Heterogeneity
Natural disturbances: fires, tornados, hurricanes, flooding,
volcanoes, outbreaks of diseases and insects, …
Human disturbances: harvesting, urbanization, conversion
(to agricultural lands, fragmentation, wars, …
Climatic Differences: continental vs marine climate, global
climate change, …
Landform: elevation, slope, aspect, …
Differentiations in ecosystem dynamics:
Other unpredictable events: introductions of invasive
species, migration of keystone species, …
Consequences of Spatial Heterogeneity (ecological)
Pros
Cons
Choices of habitats
Loss of key habitat
Encouraging metapopulation
Loss of large, continuous patch
High species richness
Increases in AEI
Effects on disturbances
Invasive species
Effects on landscape processes
Effects on disturbances
Effects on landscape processes
Quantifying Spatial Heterogeneity:
Evenness, Contagion, Fractal dimension, Patchiness (Li & Reynolds 1995)
Evenness:
 n 2
ln  pi 
 i 1 
E   100 
ln n
Pi = the probability that a random chosen pixel belong to type i
N = the total number of patch types
E, ranges between 0 and 1, responds to the number of patch types and their proportions
in a landscape. Higher values indicate a more homogeneous landscape.
Quantifying Spatial Heterogeneity:
Evenness, Contagion, Fractal dimension, Patchiness (Li & Reynolds 1995)
   P  ln P 
s
1
Contagion:
C
s
i 1 j 1
ij
ij
2  ln s
Pij = the probability that two randomly chosen adjacent pixels belong to cover type I and j;
S= number of cover types
C ranges from 0 to 1, with a high value indicating a landscape with a generally clumped patterns of
cover across the landscape and a low value indicating a landscape with a dispersed pattern of cover
types.
Quantifying Spatial Heterogeneity:
Evenness, Contagion, Fractal dimension, Patchiness (Li & Reynolds 1995)
Fractal Dimension:
A fractal will look the same whatever the level of resolution used
to observe the object, i.e. a shape made of parts similar to the
whole in some way. (Mandelrot 1985)
ln( N )
D
ln(r )
Where N is number of steps used to measure a
pattern unit length, and r is the scale ratio.
What is the fractal
of (a), (b), ©, and
(d)?
Quantifying Spatial Heterogeneity:
Evenness, Contagion, Fractal dimension, Patchiness (Li & Reynolds 1995)
  E
Patchiness:
P  100 
ij
 Dij

Nb
N= the total number of patches
Eij = the number of edges between patch type i and j
Dij = the dissimilarity value between patch type i and j
Nb = the total number of edges of pixels (i.e., each pixel has 4 edges)
P, ranges between 0 and 1, measures the contrast of neighboring patch types in a
landscape mosaic and may indirectly reflect the spatial arrangement. Higher P values
indicate more diverse land mosaic.
Handout: Exercise during the class