Habitat Fragmentation Habitat Fragmentation in the Temperate Zone in the Temperate Zone

D.S. Wilcove, C.H. McLellan D.S. Wilcove, C.H. McLellan and A.P. Dobsonand A.P. Dobson

Reviewed by Jeff Bowes and Lauren BealReviewed by Jeff Bowes and Lauren Beal

Originally published in M.E. Soule (ed), Originally published in M.E. Soule (ed), Conservation Biology: The Conservation Biology: The Science of Scarcity and DiversityScience of Scarcity and Diversity, (Sunderland, Mass.: Sinauer, , (Sunderland, Mass.: Sinauer,

1986).1986).

Purpose Purpose

What are the effects of fragmentation on What are the effects of fragmentation on species originally present in the intact species originally present in the intact habitat? habitat?

How does fragmentation lead to the loss of How does fragmentation lead to the loss of species? species?

What guidelines should be in place for the What guidelines should be in place for the selection and management in nature selection and management in nature reserves (of already fragmented reserves (of already fragmented landscape?)landscape?)

So what is Fragmentation? So what is Fragmentation?

A large expanse of A large expanse of habitat is transformed habitat is transformed into a number of into a number of smaller patches, with a smaller patches, with a smaller total area smaller total area

Can be isolated habitat Can be isolated habitat islands or samples islands or samples from original habitat from original habitat

Dispersal: migration Dispersal: migration

Study Specifics Study Specifics

Goal: long-term preservation of species Goal: long-term preservation of species whose existence is jeopardized by habitat whose existence is jeopardized by habitat destructiondestruction

Focus: vertebrate communities (especially Focus: vertebrate communities (especially birds) and forested habitats birds) and forested habitats

Suggestions based on computer model results Suggestions based on computer model results

How Can Fragmentation Lead to How Can Fragmentation Lead to Extinction? Extinction?

Reduction in total habitat area Reduction in total habitat area Affects populations sizeAffects populations size

Redistribution of the remaining area into Redistribution of the remaining area into separate fragmentsseparate fragments Affects dispersal and immigration rates Affects dispersal and immigration rates

Why Focus on Temperate Zone Why Focus on Temperate Zone Fragmentation? Fragmentation?

Temperate communities are more resistant Temperate communities are more resistant to effects of habitat fragmentation than to effects of habitat fragmentation than tropical communities tropical communities

However, most temperate damage was However, most temperate damage was done before humans started paying done before humans started paying attentionattention Great BritainGreat Britain Eastern United States Eastern United States

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A Model of FragmentationA Model of Fragmentation

Island biogeography framework Island biogeography framework Equilibrium between immigration and extinctionEquilibrium between immigration and extinction Computer model simulates the effects of Computer model simulates the effects of

fragmentation on two species fragmentation on two species Resistant pool (good dispersal ability, less prone to local Resistant pool (good dispersal ability, less prone to local

extinction) extinction) Susceptible pool (poor dispersal ability, more prone to Susceptible pool (poor dispersal ability, more prone to

local extinction) local extinction) Based on heath land in Dorset, England Based on heath land in Dorset, England Habitat is reduced from 5 large tracts to more than Habitat is reduced from 5 large tracts to more than

450 fragments, which total 5% of original area 450 fragments, which total 5% of original area

Computer Models Computer Models

Formula is based on probability that a given Formula is based on probability that a given species occurs as a breeding population in a species occurs as a breeding population in a fragment fragment

Probability increases with fragment area and Probability increases with fragment area and decreases in distance from another source decreases in distance from another source

Incidence Functions Incidence Functions

Incidence Functions Incidence Functions

And the model says … And the model says …

When a large habitat amount remains, few species are lost When a large habitat amount remains, few species are lost from either poolfrom either pool

Fragmentation reaches a critical level and species begin to Fragmentation reaches a critical level and species begin to die outdie out

Susceptible pool loses species earlier, loses more total Susceptible pool loses species earlier, loses more total species species

When resistant pool loses species, it loses them quicklyWhen resistant pool loses species, it loses them quickly More species stay at equilibrium if the remaining habitat is More species stay at equilibrium if the remaining habitat is

in one large patch instead of small fragments in one large patch instead of small fragments

Original Species v. Original HabitatOriginal Species v. Original Habitat

Too Simple? Too Simple?

Species carrying capacity is assumed to be Species carrying capacity is assumed to be directly proportional to fragment size directly proportional to fragment size

Assumes island rather than a fragment Assumes island rather than a fragment affected by surrounding habitat affected by surrounding habitat

Does not include habitat heterogeneity Does not include habitat heterogeneity Does not include population dynamics Does not include population dynamics

YET … still shows that fragmentation must be YET … still shows that fragmentation must be minimized or a rapid loss of species will occur minimized or a rapid loss of species will occur

Mechanisms of Extinction Mechanisms of Extinction

Home range size Home range size Loss of habitat Loss of habitat

heterogeneity heterogeneity Effects of habitats Effects of habitats

surrounding the surrounding the fragments fragments

Edge effects Edge effects Secondary extinctions Secondary extinctions

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So … What Should

We Do?

Four Areas of Guidelines for Temperate Zone Reserves

- Size

- Proximity to other reserves

- Shape

- Management

#1- Size Does Matter #1- Size Does Matter

How much available habitat must be set How much available habitat must be set aside as reserves, and in what distribution of aside as reserves, and in what distribution of sizes? sizes? As much as possible! As much as possible! Priority goes to largest remaining fragments Priority goes to largest remaining fragments Size matters for low density species, or species Size matters for low density species, or species

that need large areas that need large areas Large fragments might be sources of Large fragments might be sources of

immigrants for populations in neighboring small immigrants for populations in neighboring small fragments fragments

#2 - Distance Matters, Too #2 - Distance Matters, Too

Should reserves be clustered together in close Should reserves be clustered together in close proximity to each other, or spread out? proximity to each other, or spread out? Difficult to cluster large national reserves and parks Difficult to cluster large national reserves and parks

together together On local level, there is great benefit in reserves close to On local level, there is great benefit in reserves close to

each other each other Larger reserves can serve as sources of colonists for small Larger reserves can serve as sources of colonists for small

reserves reserves Species can pass through the habitats in the surrounding areas Species can pass through the habitats in the surrounding areas

#3 - Reserve Shape #3 - Reserve Shape

Reserves should be as circular in shape as Reserves should be as circular in shape as possiblepossible Minimizes dispersal distances in reserveMinimizes dispersal distances in reserve Minimizes forest edgesMinimizes forest edges

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#4 - Management#4 - Management

Reserves need active managementReserves need active management

• Prevents ecological imbalance created by fragmentation Prevents ecological imbalance created by fragmentation or human activityor human activity

What is active management?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 battle is not over once the land has been saved.the land has been saved.Indeed, it has just begun.”Indeed, it has just begun.”

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