Landscape Ecology - Lecture Notes

Landscape Fragmentation and Connectivity

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• What is "landscape fragmentation"?
  
• Related term: habitat fragmentation
  
• Habitat loss vs. fragmentation per se

   

   

• The degree (or a measure) of spatial connectedness among landscape elements (patches, corridors and matrix; Forman 1995).

(1) Structural connectivity: A measure of how connected or spatially continuous landscape elements are.

(2) Functional or behavioral connectivity: Refers to how connected an area is for an ecological process, such as dispersal of plants and animals, and energy and nutrient flows.

Note: Landscape connectivity is dependent upon both the scale of observation and ecological processes under consideration. For a given landscape, its connectivity may vary radically with respect to different processes (e.g., beetle movement, bird flying, wind-driven seed dispersal, fire spread).





(Kool, J.T., A. Moilanen, and E.A. Treml. 2013. Population connectivity: recent advances and new perspectives. Landscape Ecology 28:165-185.)

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Percolation theory

- Percolation theory was developed to describe physical properties of gels, polymers, and glassy materials, and forms the basis for studies of the flow of liquids through material aggregates.

- A simple random process is assumed in percolation models.

- A striking feature in percolation is that the threshold phenomenon occurs at the critical density or critical probability (Pc). For example, in a 2-dimensional square lattice, Pc = 0.5928 (if 4-neighbor rule is used) or Pc = 0.407 (if 8-neighbor rule is used).

- Percolation models suggest that there may be thresholds in connectivity which influence flows of energy, materials and organisms.

- Real landscapes may have a Pc lower than the theoretical value derived from percolation theory.

- Empirical studies indicate that landscape connectivity is a function not only of the interconnectedness among landscape elements but of how organisms perceive and move through complex mosaics (Wiens et al. 1996).

     

Taubert, F., R. Fischer, J. Groeneveld, S. Lehmann, M. S. Muller, E. Rodig, T. Wiegand, and A. Huth. 2018. Global patterns of tropical forest fragmentation. Nature 554:519-522+.
    

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Neutral Models

(1) Definitions:

- Neutral models: Models that generate expected patterns in the absence of specific ecological process (Caswell 1976).

- Neutral landscape models: "Models lacking effects due to topography, contagion, disturbance history, and related ecological processes" (Gardner et al. 1987).


(2) Purposes of using neutral models in landscape ecology
 

- To derive a baseline against which relationships between landscape pattern and processes can be rigorously tested to establish significant departures from the expected patterns.


(3) Percolation theory as a neutral model in landscape ecology
 

- Percolation theory has been used, as a neutral model, by landscape ecologists to study spatial patterning and its influences on ecological processes (e.g., fire spread, animal movement).

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Examples of Critical Phenomena in Ecology

(1) Epidemic processes: The spread of disease is known to depend on critical levels of infection rate and density of susceptibles and vectors. Reducing these factors below their critical levels is a standard containment strategy.

(2) Fire disturbance: The spread of a fire across a landscape depends on the density and/or spatial connectivity of fuel being above a critical level.

(3) Population outbreaks: For example, studies of the spread of Crown of Thorns starfish outbreaks on the Great Barrier Reef suggest that reef to reef spread depends on inter-reef connectivity being above a critical threshold.

(4) Species invasion: The invasion of some new species in a region depends on a critical density of sites available for colonization. For example, pollen studies of post-glacial plant migrants in eastern Canada show that many taxa were suppressed by lack of growing sites.

(5) Biodiversity: Changes in biological diversity in a landscape may show threshold phenomenon in response to a gradient of disturbance (frequency and intensity) such as fire.

Roles of corridors

• Corridors may increase or decrease landscape connectivity, depending on their structural and functional characteristics and the organisms or processes under consideration.

• For characteristics of corridors, click here.

• The role of corridors in conservation biology is controversial.  While they are important for enhancing landscape connectivity and habitat integrity and thus favor the persistence of certain species, there are also negative consequences (see discussion earlier on connectivity).  Use of corridors in conservation programs must be considered separately based on the specifics of the systems themselves.

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Debate on connectivity and experimental evidence

• Fletcher, R. J., R. K. Didham, C. Banks-Leite, J. Barlow, R. M. Ewers, J. Rosindell, R. D. Holt, A. Gonzalez, R. Pardini, E. I. Damschen, F. P. L. Melo, L. Ries, J. A. Prevedello, T. Tscharntke, W. F. Laurance, T. Lovejoy, and N. M. Haddad. 2018. Is habitat fragmentation good for biodiversity? Biological Conservation 226:9-15.

• Fahrig, L., V. Arroyo-Rodriguez, J. R. Bennett, V. Boucher-Lalonde, E. Cazetta, D. J. Currie, F. Eigenbrod, A. T. Ford, S. P. Harrison, J. A. G. Jaeger, N. Koper, A. E. Martin, J. L. Martin, J. P. Metzger, P. Morrison, J. R. Rhodes, D. A. Saunders, D. Simberloff, A. C. Smith, L. Tischendorf, M. Vellend, and J. I. Watling. 2019. Is habitat fragmentation bad for biodiversity? Biological Conservation 230:179-186.

• Empirical and experimental evidence

• Haddad, N. M., L. A. Brudvig, J. Clobert, K. F. Davies, A. Gonzalez, R. D. Holt, T. E. Lovejoy, J. O. Sexton, M. P. Austin, C. D. Collins, W. M. Cook, E. I. Damschen, R. M. Ewers, B. L. Foster, C. N. Jenkins, A. J. King, W. F. Laurance, D. J. Levey, C. R. Margules, B. A. Melbourne, A. O. Nicholls, J. L. Orrock, D.-X. Song, and J. R. Townshend. 2015. Habitat fragmentation and its lasting impact on Earth’s ecosystems. Science Advances 1:e1500052.

• Connecting natural habitats could help plant diversity endure | Science News   [PDF]
  
• Scientists connected fragments of pine savanna and new species keep showing up
  

Then what?

• Miller-Rushing, A. J. et al. 2019. How does habitat fragmentation affect biodiversity? A controversial question at the core of conservation biology. Biological Conservation 232 (2019) 271–273.

• Valente, J. J., D. G. Gannon, J. Hightower, H. Kim, K. G. Leimberger, R. Macedo, J. S. Rousseau, M. J. Weldy, R. A. Zitomer, L. Fahrig, R. J. Fletcher, J. Wu, and M. G. Betts. 2023. Toward conciliation in the habitat fragmentation and biodiversity debate. Landscape Ecology:10.1007/s10980-10023-01708-10989.

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