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? ◦ Rejecting the Null, often doesn’t tell us anything.

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

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

IBG

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.

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

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.

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.

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

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?

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

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.

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?

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