Habitat Fragmentation in the Temperate Zone D.S. Wilcove, C.H. McLellan and A.P. Dobson Reviewed by Jeff Bowes and Lauren Beal Originally published in M.E. Soule (ed), Conservation Biology: The Science of Scarcity and Diversity, (Sunderland, Mass.: Sinauer, 1986).

Purpose What are the effects of fragmentation on species originally present in the intact habitat? How does fragmentation lead to the loss of species? What guidelines should be in place for the selection and management in nature reserves (of already fragmented landscape?)

So what is Fragmentation?  A large expanse of habitat is transformed into a number of smaller patches, with a smaller total area  Can be isolated habitat islands or samples from original habitat  Dispersal: migration

Study Specifics Goal: long-term preservation of species whose existence is jeopardized by habitat destruction Focus: vertebrate communities (especially birds) and forested habitats Suggestions based on computer model results

How Can Fragmentation Lead to Extinction?  Reduction in total habitat area  Affects populations size  Redistribution of the remaining area into separate fragments  Affects dispersal and immigration rates

Why Focus on Temperate Zone Fragmentation?  Temperate communities are more resistant to effects of habitat fragmentation than tropical communities  However, most temperate damage was done before humans started paying attention  Great Britain  Eastern United States

A Model of Fragmentation  Island biogeography framework  Equilibrium between immigration and extinction  Computer model simulates the effects of fragmentation on two species  Resistant pool (good dispersal ability, less prone to local extinction)  Susceptible pool (poor dispersal ability, more prone to local extinction)  Based on heath land in Dorset, England  Habitat is reduced from 5 large tracts to more than 450 fragments, which total 5% of original area

Computer Models  Formula is based on probability that a given species occurs as a breeding population in a fragment  Probability increases with fragment area and decreases in distance from another source  Incidence Functions

Incidence Functions

And the model says …  When a large habitat amount remains, few species are lost from either pool  Fragmentation reaches a critical level and species begin to die out  Susceptible pool loses species earlier, loses more total species  When resistant pool loses species, it loses them quickly  More species stay at equilibrium if the remaining habitat is in one large patch instead of small fragments

Original Species v. Original Habitat

Too Simple?  Species carrying capacity is assumed to be directly proportional to fragment size  Assumes island rather than a fragment affected by surrounding habitat  Does not include habitat heterogeneity  Does not include population dynamics  YET … still shows that fragmentation must be minimized or a rapid loss of species will occur

Mechanisms of Extinction  Home range size  Loss of habitat heterogeneity  Effects of habitats surrounding the fragments  Edge effects  Secondary extinctions

So … What Should We Do? Four Areas of Guidelines for Temperate Zone Reserves - Size - Proximity to other reserves - Shape - Management

#1- Size Does Matter  How much available habitat must be set aside as reserves, and in what distribution of sizes?  As much as possible!  Priority goes to largest remaining fragments  Size matters for low density species, or species that need large areas  Large fragments might be sources of immigrants for populations in neighboring small fragments

#2 - Distance Matters, Too  Should reserves be clustered together in close proximity to each other, or spread out?  Difficult to cluster large national reserves and parks together  On local level, there is great benefit in reserves close to each other  Larger reserves can serve as sources of colonists for small reserves  Species can pass through the habitats in the surrounding areas

#3 - Reserve Shape  Reserves should be as circular in shape as possible  Minimizes dispersal distances in reserve  Minimizes forest edges

#4 - Management  Reserves need active management Prevents ecological imbalance created by fragmentation or human activityPrevents ecological imbalance created by fragmentation or human activity  What is active management? Controlled treatment of vegetationControlled treatment of vegetation Elimination of foreign speciesElimination of foreign species Controlling populations of “nuisance” animalsControlling populations of “nuisance” animals

“…the battle is not over once the land has been saved. Indeed, it has just begun.” Thank you for listening to our presentation.