habitat fragmentation in the temperate zone d.s. wilcove, c.h. mclellan and a.p. dobson reviewed by...
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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).
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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?)
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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
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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
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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
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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
http://ofiroren.tripod.com/test/brown-bear.jpg
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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
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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
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Incidence Functions Incidence Functions
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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
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Original Species v. Original HabitatOriginal Species v. Original Habitat
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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
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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
http://www.hlasek.com/foto/dendrocopos_major_2415.jpg
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So … What Should
We Do?
Four Areas of Guidelines for Temperate Zone Reserves
- Size
- Proximity to other reserves
- Shape
- Management
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#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
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#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
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#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
http://www.zionnational-park.com/images/jpegs/dixie-national-forest-9.jpg
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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
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“…“…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.”
Thank you for listening to our presentation.