Habitat Fragmentation
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Habitat Fragmentation

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Habitat fragmentation

Habitat Fragmentation

"Let's start indoors. Let's start by imagining a fine Persian carpet and a hunting knife. The carpet is twelve feet by eighteen, say. That gives us 216 square feet of continuous woven material. Is the knife razor sharp? If not, we hone it. We set about cutting the carpet into thirty-six equal pieces, total them up--and find that, lo, there's still nearly 216 square feet of recognizably carpet like stuff. But what does it amount to? Have we got thirty-six nice Persian throw rugs? No. All we're left with is three dozen ragged fragments, each one worthless and commencing to come apart."

Quote from David Quammen’s (1996) Song of the Dodo; Image from www.floridahabitat.org


Habitat fragmentation

Habitat Fragmentation

Habitat fragmentation is an anthropogenic disturbance

Disturbance – a discrete event that removes biomass (and thereby can create heterogeneity or “patchiness”)

Photo of a fragmented Valdivian forest in Chile from: www.tncfire.org


Habitat fragmentation

Habitat Fragmentation

Habitat fragmentation is an anthropogenic disturbance

with two components:

(1) A reduction in area of the focal habitat type

(2) A change in habitat configuration; remaining patches are smaller and more isolated than in the original configuration

Photo of a fragmented Valdivian forest in Chile from: www.tncfire.org


Habitat fragmentation

Nature is Inherently “Patchy” & Dynamic

“Water, earth, and fire are Louisiana’s three special ingredients…

The lowlands flood. The uplands burn… if you live in Louisiana,

there are only two possibilities: either your land will eventually flood,

or it will eventually burn. Most of our native plants and animals are

therefore dependent on either flooding or fire or, in some cases, both.”

Paul Keddy

(b. 1953)

Photo of Paul Keddy from www.drpaulkeddy.com; quote from Keddy’s (2008, pg. 14) Water, Earth, Fire


Habitat fragmentation

Nature is Inherently “Patchy” & Dynamic

Space-time Mosaic (Watt 1947); Shifting Mosaic (Bormann & Likens 1979); Patch Dynamics; Crazy Quilt (H. S. Horn)

Natural disturbance regime

Green = Eastern hemlock

Purple = American beech

Red = Red maple

Yellow = Yellow birch

500 yr

1000 yr

Images from Deutschman et al. (1997); www.sciencemag.org


Habitat fragmentation

Nature is Inherently “Patchy” & Dynamic

Nature is inherently “patchy,” but anthropogenic disturbanceoften results in landscapes different from (and potentially

less hospitable than) those resulting from natural causes

Natural disturbance regime

Anthropogenic clearcut

500 yr

1000 yr

Images from Deutschman et al. (1997); www.sciencemag.org


Habitat fragmentation

Nature is Inherently “Patchy” & Dynamic

Nature is inherently “patchy,” but anthropogenic disturbanceoften results in landscapes different from (and potentially

less hospitable than) those resulting from natural causes

Fragmentation reduces the extent and connectivity of habitats

Fragmented landscapes typically have simplified internal structure of patches and matrices

Fragmented landscapes typically have more contrast between

adjacent patches (including patch-matrix juxtaposition)

Features of fragmented landscapes (e.g., roads and dams)

pose special threats to population viability


Habitat fragmentation

Patch (Fragment) Size & Isolation

Log10 (No. species)

Log10 (Area)

Data for Galapagos plants from van der Werff (1983) Vegetatio


Habitat fragmentation

Patch (Fragment) Size & Isolation

Data for Bismark Archipelago birds from Diamond (1972) PNAS


Habitat fragmentation

Patch (Fragment) Size & Isolation

Island Biogeography Theory emphasizes

dynamism & patchiness of natural processes

Conservation Biologists (and managers) must understand natural processes, to make sense of anthropogenic disturbances

and to restore ecological / evolutionary processes

Robert MacArthur(1930-1972)

E. O. Wilson(b. 1929)


Habitat fragmentation

Island Biogeography Theory

Concerns the dynamics of immigration from a mainland

source pool and extinction on islands or patches

surrounded by inhospitable matrix

Map on left from www.mapsofworld.com; map on right from www.peloncillo.org


Habitat fragmentation

Island Biogeography Theory

Why does the immigration rate decline as a function of S?

Immigration rate

(e.g., new species

per yr)

Number of species (S)


Habitat fragmentation

Island Biogeography Theory

Why does the extinction rate increase as a function of S?

Extinction rate (e.g., number of species per yr)

Number of species (S)


Habitat fragmentation

Island Biogeography Theory

Immigration rate

(e.g., new species

per yr)

Turn-over rate (T)

Extinction rate (e.g., number of species per yr)

Equilibrium

S

Number of species (S)


Habitat fragmentation

Island Biogeography Theory

Why does the probability of immigration for each

species vary with island isolation?

Near island

Immigration rate

(e.g., new species

per yr)

Far island

TNear

TFar

Extinction rate (e.g., number of species per yr)

SFar

SNear

Number of species (S)


Habitat fragmentation

Island Biogeography Theory

Why does the probability of extinction for each

species vary with island size?

Small island

Immigration rate

(e.g., new species

per yr)

Large island

TSmall

TLarge

Extinction rate (e.g., number of species per yr)

SSmall

SLarge

Number of species (S)


Habitat fragmentation

Island Biogeography Theory

Near island

Small island

Immigration rate

(e.g., new species

per yr)

Far island

Large island

Extinction rate (e.g., number of species per yr)

SFar,Small

SNear,Large

SNear,SmallSFar,Large

Number of species (S)


Habitat fragmentation

Single Large or Several Small (SLOSS) Debate

Ecological Assembly Rules

E.g., Sometimes we find nested subsets in which larger areas contain the same subset of species as smaller areas, plus additional area-sensitive species

Jared Diamond(b. 1937)

From from Wikipedia


Habitat fragmentation

Single Large or Several Small (SLOSS) Debate

Nested Subsets

A B

A B C D E

A B C

Jared Diamond(b. 1937)

Relaxation – loss of species that occurs after fragmentation event

If fragments contain nested subsets of species, then a single large reserve is better than several small ones of the same total area (SLOSS debate)

From from Wikipedia


Habitat fragmentation

Species Especially Vulnerable to Fragmentation

Wide-ranging

Poor dispersal abilities

Specialized requirements

Low fecundity

Vulnerable to human exploitation or persecution

Cougar

Arctic tern

Desert pup fish

Coyote

Ground nut

Heliconius erato

Images from Wikipedia


Habitat fragmentation

Lago Guri Islands, Venezuela

Not just relaxation, but devastating ecological meltdown

owing to top-down trophic cascades

Perturbation that propagates downward through two or more trophic levels, resulting in alternating positive and negative impacts on successive levels

John Terborgh(b. 1936)

Photo from www.env.duke.edu


Habitat fragmentation

Top-Down Trophic Cascades

+

+

+

+

+

Tree seedlings

Tree seedlings

Photos from Wikipedia


Habitat fragmentation

Biological Dynamics of Forest Fragments Project (BDFFP), Amazonas, Brazil

Thomas Lovejoy

Bill Laurance

Recipients of the 2009 BBVA Foundation Frontiers of Knowledge

Award in Ecology & Conservation Biology

Photos from www.mongabay.com


Habitat fragmentation

Biological Dynamics of Forest Fragments Project (BDFFP), Amazonas, Brazil

Photo of a forest fragment, surrounded by newly created cattle pasture in Brazil


Habitat fragmentation

Biological Dynamics of Forest Fragments Project (BDFFP), Amazonas, Brazil

NASA false-color remotely sensed image of the confluence of Río Negro & Río Solimões (Amazon)


Habitat fragmentation

Biological Dynamics of Forest Fragments Project (BDFFP), Amazonas, Brazil

NASA false-color remotely sensed image of BDFFP


Habitat fragmentation

Biological Dynamics of Forest Fragments Project (BDFFP), Amazonas, Brazil

Edge effects – negative effects of a habitat edge on interior conditions

Some species can only inhabit the interior or core, and some are specifically attracted to the edge

Figure from Laurance et al. (2006) PNAS


Habitat fragmentation

Corridors

Corridors can help connect fragments

a

E.g., United Nations Educational, Scientific & Cultural Organization (UNESCO) World Heritage Sites in the Wet Tropics of Queensland, Australia

Map from www.enviro-map.com


Habitat fragmentation

Conservation Biologists (and managers) must understand natural processes, to determine conservation targets

& how to achieve them

Image from www.rewilding.org


Habitat fragmentation

Conservation Biologists (and managers) must understand natural processes, to determine conservation targets

& how to achieve them

Image from www.rewilding.org


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