Causes and Consequences of Spatial Heterogeneity Ecolog(ists) use(s) the concept of a landscape in...

Causes and Consequences of Spatial HeterogeneityCauses 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.

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 characteristicsEmphasis on changes across the spaceDepending on interests, objectivesFunction 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.

From Webster’s Dictionary (10th edition), 2001

Coherent structure or relationship

Examples of mosaics and heterogeneity

Causes Spatial HeterogeneityCauses 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)Consequences of Spatial Heterogeneity (ecological)

Pros

Choices of habitats

Encouraging metapopulation

High species richness

Effects on disturbances

Effects on landscape processes

Cons

Loss of key habitat

Loss of large, continuous patch

Increases in AEI

Invasive species

Effects on disturbances

Effects on landscape processes

Quantifying Spatial Heterogeneity: Quantifying Spatial Heterogeneity: Evenness, Contagion, Fractal dimension, Patchiness (Li & Reynolds 1995)

Evenness:

E

p

n

ii

n

1 0 0

2

1

ln

ln

Pi = the probability that a random chosen pixel belong to type iN = 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.


Contagion: C

P P

s

ij ijj

s

i

s

1

211

ln

lnPij = 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.


Fractal Dimension:

DN

r

ln ( )

ln ( )Where N is number of steps used to measure a pattern unit length, and r is the scale ratio.

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)

What is the fractal of (a), (b), ©, and (d)?


Patchiness: P

E D

N

ij ij

b

1 0 0

N= the total number of patchesEij = the number of edges between patch type i and jDij = the dissimilarity value between patch type i and jNb = 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

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