Landscape Ecology - Lecture Notes

Landscape Structure: Pattern Formation and Patch Dynamics

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• Landscape pattern includes both landscape composition (diversity and relative abundance of landscape elements) and landscape configuration (shape and spatial arrangement of landscape elements). 
  
• Landscape ecological studies often focuses on the problems of spatial pattern: the formation, dynamics, and consequences of landscape pattern.

• Spatial pattern affects ecological processes (population dynamics, species persistence, biodiversity, ecophysiological and ecosystem processes, etc.);
• Pattern and process are interrelated, so ecological processes may be inferred or better understood by studying spatial pattern, although pattern-process relationship may be very complex (e.g., highly nonlinear or not apparent for various reasons).  It is important to understand, however, the relationship between pattern and process is often scale-dependent.
  
• Because structure is usually easier to study than function, characteristics of pattern may be used as an indicator of process for applied purposes (e.g., ecological monitoring and assessment), assuming that a reliable relationship between the two can be established.

• Causes or agents of spatial pattern may be categorized into three groups:
 
• Abiotic (physical), biotic, and anthropogenic

• Physical and anthropogenic agents operate over all scales, whereas biological factors are usually responsible for patterns at finer scales.
• The abiotic factors (e.g., climate, topography, geomorphology) presents a physical template in which biological and anthropogenic processes often interact to generate spatial pattern.
• Temperature and precipitation determine the spatial pattern of biomes, within which ecosystem types are modified by elevation and other topographical features. Within local ecosystems, biological processes, such as predation, competition, parasitism, and allelopathy, as well as abiotic factors at local scales, affect the formation of spatial pattern.
• Disturbances, either abiotic or anthropogenic, are a major agent of pattern across a wide range of scales. The unique characteristics of anthropogenic disturbances must be explicitly recognized.

  
 

• Scale dependence and scale multiplicity of landscape pattern
• Landscape pattern is scale-dependent, meaning that landscape pattern changes with scale in space and time. 
  
• Causes and mechanisms of pattern formation are also scale-dependent.
• Scale multiplicity refers to the fact that there are multiple characteristic scales for the different patterns and processes in a landscape.
  

  

• Summary of Causes and Mechanisms of Patchiness (Wu and Loucks 1995)

This figure represents a conceptual framework of patchiness and its causes and mechanisms
in ecological systems. Causes and mechanisms can operate at a variety of spatio-temporal
and organizational scales, forming different hierarchies. Disturbances may be natural (e.g.
fires, windthrows, storms) or human-induced (e.g. introduction of exotic species, harvesting
of biomass), which can in turn be divided into finer categories according to, for example,
spatial and temporal scales. Biological interactions include competition, predation, selective
herbivory, parasitism, commensalism, and allelopathy. (From Wu & Loucks 1995)

• Major anthropogenic drivers for landscape change

• Agriculture
- Farming practices, grazing, introduction of exotic species, etc.


• Urbanization
- Rapid urbanization worldwide with the continuous human population "explosion"


• Habitat fragmentation
- For purposes of extracting natural resources, transportation and communication, constructions, farming, etc.


• Deforestation
- For harvesting purposes, building roads, constructions, farming, etc.
 
• Desertification
- Accompanied by soil erosion which is often induced by overgrazing, overharvesting, vegetation fragmentation
 
• Pollution
- Air pollution, water contamination, and solid wastes
- Point pollution vs non-point pollution
 


• A gradient of landscape modification by humans (Forman and Godron 1986)
  

(2) Managed landscapes - Native species & ecosystems managed & harvested

(3) Cultivated landscapes - Cultivation, villages, and patches of natural ecosystems

(4) Suburban landscapes - Residential & commercial areas, cropland, managed vegetation, and natural areas

(5) Urban landscapes - Dense buildings, roads, residential areas, and commercial centers, high population density

   

1. What Is the Patch-Corridor-Matrix model?

• Richard Forman and Michael Godron (1981, 1986) developed the Patch-Corridor-Matrix conceptual model to describe the pattern and process in landscapes.
• The Patch-Corridor-Matrix model provides a useful spatial language for the study of pattern and process.

2. Patches, Corridors and Matrix

(1) Forman & Godron (1986) and Forman (1995): A relatively homogeneous nonlinear area that differs from its surroundings

(2) A spatial unit differing from its surroundings in nature or appearance (Wiens, 1976)

(3) A bounded, connected discontinuity in a homogeneous reference background (Levin and Paine, 1974)

(4) A region of the environment where the abundance of something, organisms or resource, is high (Roughgarden, 1977)

(5) A relatively discrete spatial pattern which may vary in size, internal homogeneity, and discreteness (White and Pickett, 1985)

- Patches can be characterized by their size, shape, content, duration, structural complexity, and boundary characteristics.

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(1) Remnant patches: Caused by widespread disturbance surrounding a relatively small area

(2) Disturbance patches: Resulting from alteration or disturbance of a small area; the inverse of remnant patches

(3) Environmental resource patches: Caused by spatial patchiness in environmental resources (e.g., soil types, topography-related factors)

(4) Regenerated patches: Formed on a previously disturbed site, resembling remnant patches in appearance

(5) Introduced patches: Created by people introducing organisms to an area, e.g., plantations, golf courses, habitations

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B. Corridors:
 
• Strips that differ from their surroundings on both sides

Corridor types based on causative mechanisms

(1) Disturbance corridors
(2) Remnant corridors
(3) Environmental resource corridors
(4) Regenerated corridors
(5) Introduced corridors

Structural attributes of corridors

(1) Internal structure
• Width characteristics (environmental gradient from side to side or from interior to edge)
• Presence of a distinctive internal entity (e.g., in a stream, river, road, patch, ditch, wall) or an interior environment (e.g., in wide habitat corridors)
• Biotic communities (e.g., species composition, diversity, and vertical structure)
  

(2) External structure

• Relationship to surroundings
• Curvilinearity and width
• Connectivity and gaps
  

- Width and the presence of an internal entity or interior environment are key spatial variables controlling corridor function.

Major Corridor Functions

1. Habitat (e.g., riparian ecosystems, vegetated corridors)
2. Conduit (when objects move along it)
3. Filter/Barrier (e.g., windbreaks, roads)
4. Source (an area or reservoir that gives off objects)
5. Sink (an area or reservoir that absorbs objects)




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C. Network and Matrix

Network

• Corridors of a single type interconnect to form a network, which are composed of nodes and linkages (corridors) that are usually surrounded by a matrix .

Nodes and Linkages

• Linkages are corridors that connect nodes , and thus nodes are usually located at intersections .

Network Attributes

- Network density: the amount or abundance of corridors in an area.
- Network connectivity: the degree at which all nodes are connected.
- Network circuitry: the degree to which loops or circuits are present in the net.

Network Functions

1. A habitat for organisms
2. A conduit for organisms and materials moving along corridors
3. A barrier against flows in the enclosed matrix or patches

Major Attributes and Identifying Criteria for a Matrix

1. Largest relative area in the landscape
2. Highest connectivity
3. Most control over dynamics of the landscape

1. Patch size and number

Theory of Island Biogeography

 
dS/dt = I(D) - E(A)

 
[Wu, J., and J. L. Vankat. 1995. Island biogeography: Theory and applications. Pages 371-379 in W. A. Nierenberg, editor. Encyclopedia of Environmental Biology. Academic Press, San Diego.

[A short video explaining the theory of island biogeography
 


[Whittaker, R. J., J. M. Fernández-Palacios, T. J. Matthews, M. K. Borregaard, and K. A. Triantis. 2017. Island biogeography: Taking the long view of nature’s laboratories. Science 357:eaam8326.]

E. O. Wilson on the theory of island biogeography

Interior and edge environments

Edge species: species that live primarily near the perimeter of a landscape element. Interior species: species that live primarily in the interior of a landscape element.

• - Effects of patch size on edge/interior species richiness and species density (packing).
• - Most species in small patches are generalists (edge species), whereas probably most in large patches are specialists (interior species).

 

[From Wu, J. andJ. L. Vankat. 1991. An area-based model of species richness dynamics of forest islands. Ecological Modelling 58:249-271.]

Edge Effects

• The phenomenon that the outer band of a patch has a significantly different environment and thus different ecological characteristics (e.g., species composition and abundance, ecosystem productivity) from the patch interior.

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Problems and pitfalls of applying Island Biogeographic Theory in fragmented terrestrial landscapes
• Internal habitat diversity
• Disturbance
• Patch dynamics
• More important role for edge effect and the interior-to-edge ratio
• Interactions of patches with matrix and corridors
• Multiple sources for species pool


A general relationship between species richness and patch characteristics (Forman 1995)


Species richness = f [habitat diversity(+), disturbance(+/-), patch size(+), age(+/-), matrix heterogeneity(+), isolation(-)]




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Effects on Ecosystem Processes
 
Productivity and biomass, e.g.,
• - Through effects of micrometeorological factors; inter/edge effects, susceptibility to external disturbances; etc.


Erosion and mineral nutrients, e.g.,

• - Clear cut size affects erosion, soil organic matter, and nutrient runoff.
• - Where wind or water flow accelerates along edges, erosion typically increases.


Water and aquatic systems, e.g.,

• - A large forest can completely cover an aquifer and protect its water quality.
• - A large forest also can enclose a complete stream network.

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The debate of SLOSS (Single large or several small)

Ecological Values of Large Patches and Small Patches (Forman 1995)

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Large Patches:
• 1. Water quality protection for aquifer and lake.
• 2. Connectivity of a low-order stream network.
• 3. Habitat to sustain populations of patch interior species.
• 4. Core habitat and escape cover for large-home-range vertebrates.
• 5. Source of species dispersing through the matrix
• 6. Microhabitat proximities for multihabitat species.
• 7. Near-natural disturbance regimes.
• 8. Buffer against extinction during environmental change.


Small Patches:

• 1. Habitat and stepping stones for species dispersal, and for recolonization after local extinction of interior species.
• 2. High species densities and high population sizes of edge species.
• 3. Matrix heterogeneity that decreases fetch (run) and erosion, and provides escape cover from predators.
• 4. Habitat for small-patch-restricted species.
• 5. Protect scattered small habitats and rare species.


The bottom line:

• Large patches, large benefits, and small patches, small supplemental benefits.

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2. Patch Shape

Common patch shapes

- Natural patch shapes: usually curvilinear or irregular
- Human-created patch shapes: usually geometric

Major patch shape attributes

1. Patch elongation (L:W ratio)
2. Patch convolution (# of major lobes present)
3. Perimeter-to-area ratio

Effects of Patch Shape

- Form-function principles

(i) Compact forms are effective in conserving energy and resources.
• The rounded or compact form with minimal appendages (with a minimal perimeter-to-area ratio) is characteristic of systems where it is important to conserve resources like energy, materials or organisms;


(ii) Convoluted forms are effective in enhancing interactions with the surroundings.

• A convoluted shape with a high perimeter-to-area ratio is characteristic of systems with considerable interchanges of energy, materials, or organisms with the surroundings.


(iii) Network or labyrinthine forms tend to have a conduit system for transport.

• A road network, stream system, or telephone system transmits objects or information from one portion of the network to a different portion.

- Relationship between patch shape and interior/edge ratio

• Patch shape ---> perimeter-to-area ratio ---> interior-to-edge ratio

-Ecological effects

• - Effects on movements and flows of energy, materials and biological entities (e.g., important in the dispersal and foraging of organisms
• - Only sparse research has been done.

VI. PATCH DYNAMICS

What Is Patch Dynamics?
 
(1) The temporal change in spatial pattern and variability resulting from within-patch turnover and interpatch interactions, i.e., change in patchiness in time;

(2) The field of study of the spatial pattern , formation , evolution , and decay of patches, as well as mechanisms and consequences of patchiness;

(3) An emerging ecological perspective that emphasizes spatiotemporal heterogeneity, non-equilibrium properties, and hierarchical nature of ecological systems. The patch dynamics paradigm focuses on the coupling of pattern and process at different scales, offering a promising approach to the bridging of the conceptual and methodological gaps between terrestrial and marine ecology and between micro- and macro-ecology.


Patch dynamics provides a unifying framework to understand the formation and dynamics of spatial pattern across different systems and scales. Patch dynamics provides an essential conceptual framework for modern landscape ecology.

The Five tenets of HPDP:


Ecological systems as nested, discontinuous hierarchies of patch mosaics
 
• - Ecological systems are hierarchical mosaic systems of patches that differ in size, shape, and successional stage, with discrete levels at particular scales. In contrast with the traditional paradigm in which the individual organism, population, community and ecosystem are the basic ecological units, the new paradigm takes a natural spatial unit, the patch, as a fundamental structural and functional unit.
  

Dynamics of ecological systems as composite patch dynamics


• - In hierarchically structured ecological systems, the phase change of individual patches at the local scale, and temporal change in patch mosaics at the broader scale give rise to temporal dynamics. Therefore, the dynamics of ecological systems are composed of the dynamics and interactions of constituent patches on different scales, which is an emergent property in that it is not simply the sum of the individual patch dynamics. For example:
• - Forest dynamics = composite gap dyn (gap formtn, local succn + interactions with other gaps and the background)
• - Regional population dynamics = composite patch dyn (local + interactions)
• - Landscape dynamics = composite dyn of indiv. ecosystems (indiv. ecosystem dyn + interactions)

The pattern-process-scale perspective
 
• - The interrelationship between pattern and process is central to HDPD. Processes modify existing pattern and create new pattern; on the other hand, pattern can either facilitate or constrains processes. Therefore, one should study ecological processes in their context and search for pattern based on underlying processes. The pattern-process perspective is evident in studies of metapopulation dynamics.
• - Pattern changes with scale. Pattern and scale, be it spatial, temporal or functional, are inseparable in theory and practice. The choice of scale affects the particular pattern likely to be observed. Thus, the choice of scale can behave like a blinder, a filter, or a magnifier, depending on the appropriateness of the observation window size.
• - For a particular pattern, there usually exists a domain of scales in which the pattern can be most effectively studied.
• - Because of the scale-dependence of the pattern being measured, results of pattern analysis must be presented with explicit specification of the scale at which the data have been obtained. Moreover, conducting pattern analysis over multiple scales is now possible and is recommended.
  

The non-equilibrium perspective  

• - As opposed to the classical equilibrium viewpoint, a hierarchical patch dynamics paradigm permits emphasis on non-equilibrium and stochastic processes as elements of ecological stability.
• - Ecosystems exhibit non-equilibrium dynamics on small scales where stochasticities usually dominate, and on very large scales where climatological, geological, and evolutionary changes come into play.
  

Incorporation and metastability  

• - Two further, but interrelated components of the hierarchical patch dynamics paradigm are the incorporation principle, derived from hierarchy theory, and the concept of metastability.
• - Lower-level processes are inherently an integral part of higher-level structure and processes in a hierarchical system. Through incorporation, non-equilibrium patch processes at one level often translate to a quasi-equilibrium state at a higher level.
• - This homeorhetic, quasi-equilibrium state has been termed metastability (Naveh, 1987; ONeill et al., 1989). This illustrates the idea of order out of apparently random fluctuation.
• - For example, single tree-falls induce local gap dynamics that create non-equilibrium outcomes at the gap level; however, gaps are incorporated readily as an area-wide mean process at a larger forest stand level, leading to a shifting mosaic steady state. A large blowdown can not be incorporated in forest stand dynamics, but it can be at a landscape of larger scale. By the same token, wildfires may be incorporated by a larger regional landscape.

• Stabilizing mechanisms (Waterhouse and DeAngelis 1985)
  

• Quantifying landscape stability in terms of the relative interval and extent of disturbances (Turner et al. 1993).