Habitat Fragmentation

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

"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."

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

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

Habitat Fragmentation

Habitat fragmentation is an anthropogenic disturbance

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

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

with two components:

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

Habitat Fragmentation

Habitat fragmentation is an anthropogenic disturbance

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

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)

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

Natural disturbance

regime

500 yr 1000 yr

Green = Eastern hemlockPurple = American beech

Red = Red mapleYellow = Yellow birch

Nature is Inherently “Patchy” & Dynamic

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

500 yr 1000 yr

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

Natural disturbance

regime

Anthropogenic clearcut

Nature is Inherently “Patchy” & Dynamic

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

Nature is Inherently “Patchy” & Dynamic

Nature is inherently “patchy,” but anthropogenic disturbanceoften results in landscapes different from (and potentiallyless hospitable than) those resulting from natural causes

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

Log10 (Area)

Log 1

0 (N

o. s

peci

es)

Patch (Fragment) Size & Isolation

Data for Bismark Archipelago birds from Diamond (1972) PNAS

Patch (Fragment) Size & Isolation

E. O. Wilson(b. 1929)

Robert MacArthur(1930-1972)

Patch (Fragment) Size & Isolation

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

and to restore ecological / evolutionary processes

Island Biogeography Theory emphasizes dynamism & patchiness of natural processes

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

Island Biogeography Theory

Concerns the dynamics of immigration from a mainland source pool and extinction on islands or patches

surrounded by inhospitable matrix

Immigration rate (e.g., new species

per yr)

Number of species (S)

Island Biogeography Theory

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

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

Number of species (S)

Island Biogeography Theory

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

Immigration rate (e.g., new species

per yr)

Number of species (S)

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

Turn-over rate (T)

Equilibrium S

Island Biogeography Theory

Immigration rate (e.g., new species

per yr)

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

Near island

Far islandTNear

SNear

Number of species (S)

SFar

TFar

Island Biogeography Theory

Why does the probability of immigration for each species vary with island isolation?

Immigration rate (e.g., new species

per yr)

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

Number of species (S)

Small island

Large islandTSmall

SLargeSSmall

TLarge

Island Biogeography Theory

Why does the probability of extinction for each species vary with island size?

Immigration rate (e.g., new species

per yr)

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

Number of species (S)

Small island

Large island

Near island

Far island

SNear,Small

SFar,Large

SNear,LargeSFar,Small

Island Biogeography Theory

Ecological Assembly Rules

From from Wikipedia

Jared Diamond(b. 1937)

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

Single Large or Several Small (SLOSS) Debate

Nested Subsets

From from Wikipedia

Relaxation – loss of species that occurs after fragmentation event

A B C D E

A B

A B C

Single Large or Several Small (SLOSS) Debate

Jared Diamond(b. 1937)

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)

Images from Wikipedia

Arctic tern

Ground nutDesert pup fish

Heliconius erato

Cougar

Coyote

Species Especially Vulnerable to Fragmentation

Wide-ranging

Poor dispersal abilities

Specialized requirements

Low fecundity

Vulnerable to human exploitation or persecution

Photo from www.env.duke.edu

John Terborgh(b. 1936)

Perturbation that propagates downward through two or more

trophic levels, resulting in alternating positive and negative

impacts on successive levels

Lago Guri Islands, Venezuela

Not just relaxation, but devastating ecological meltdown owing to top-down trophic cascades

Photos from Wikipedia

– +

– +

++

– +

Tree seedlings Tree seedlings

Top-Down Trophic Cascades

Photos from www.mongabay.com

Thomas Lovejoy Bill Laurance

Recipients of the 2009 BBVA Foundation Frontiers of Knowledge Award in Ecology & Conservation Biology

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

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

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

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

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

NASA false-color remotely sensed image of BDFFP

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

Figure from Laurance et al. (2006) PNAS

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

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

Map from www.enviro-map.com

Corridors can help connect fragments

E.g., United Nations Educational, Scientific &

Cultural Organization (UNESCO) World

Heritage Sites in the Wet Tropics of Queensland,

Australia

Corridors

a

Image from www.rewilding.org

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

& how to achieve them

Image from www.rewilding.org

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

& how to achieve them