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Neutral Models

Neutral Models. Landscape Ecology. Questions/Comments . Purpose of models? . Testing hypotheses Simplest hypothesis: Null – no effect Why are null hypotheses difficult for ecological studies? Complex systems Some effect, but from what?

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Neutral Models

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  1. Neutral Models Landscape Ecology

  2. Questions/Comments

  3. Purpose of models? • Testing hypotheses • Simplest hypothesis: • Null – no effect • Why are null hypotheses difficult for ecological studies? • Complex systems • Some effect, but from what? • Rejecting the Null, often doesn’t tell us anything.

  4. Caswell & Hubbell • Other Neutral Models • Built from Community Ecology • Hubbel – • Unified Neutral Theory of Biodiversity and Biogeography

  5. Neutral T of B and B • Theory about community organization • Constructed on Island Biogeography Theory

  6. IBG

  7. Neutral T of B and B • “the theoretical consequences of assuming that ecological communities are structured entirely by ecological drift, random migration, and random speciation.” • What is neutral in this theory? • treats organisms in the community as essentially identical in their per capita probabilities of giving birth, dying, migrating, and speciating.

  8. Null model for a landscape • A random map…. • Random in what way? • Pattern? • Process? • Number of patches? • Probability of patches?

  9. Why use neutral landscape models? • Determine the extent to which natural properties deviate from a theoretical spatial distribution • To predict how the processes (such as animal movement, seed dispersal, nutrient flow, genes, fire) are affected by landscape patterns.

  10. Simple Random maps. • Habitat vs. non-habitat • 0 or 1 • Determine proportion of landscape to be filled with habitat. • p • Randomly choose cells until reach p.

  11. What attributes exist for our map? • Number of patches • Size of patches • Amount of edge • Largest patch • Ability to percolate

  12. What is percolation? • A cluster is said to percolate when a the cluster connects from one edge of the map to the other. • What rules? • 4 edge rule • 8 edge rule • How do these rules change when percolation happens?

  13. Critical thresholds • Studying percolation gives evidence for important or critical thresholds. • Can an organism move through the system? • Or a nutrient? • Or a fire?

  14. Why would anyone want to use random methods to represent actual landscapes? • Same reason we test a null hypothesis! • Neutral landscape models don’t represent real landscape, but a standard to compare.

  15. Beyond just a value of p with random placement, what else can be done with a neutral landscape model? • Can clusters, patches develop from simple random processes?

  16. General Insights • Simply the amount of habitat changes patterns. • Thresholds exist at small changes of p.

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