Transcript Habitat Fragmentation in the Temperate Zone

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
http://ofiroren.tripod.com/test/brown-bear.jpg
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

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
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 …
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
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?


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
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
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Home range size
Loss of habitat
heterogeneity
Effects of habitats
surrounding the
fragments
Edge effects
Secondary extinctions
http://www.hlasek.com/foto/dendrocopos_major_2415.jpg
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
http://www.zionnational-park.com/images/jpegs/dixie-national-forest-9.jpg
#4 - Management

Reserves need active management
• Prevents ecological imbalance created by fragmentation
or human activity

What is active management?
• Controlled treatment of vegetation
• Elimination of foreign species
• Controlling 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.