Forest Fragmentation

Question 1

There’s rapid conversion of tropical forest…

Answer 1

… to agriculture, often managed in intensive plantations

Question 2

But forest clearance to agriculture is rarely wholesale…

Answer 2

…and often fragments of forest are retained in the landscape from tiny patches to massive swathes that are now cut off from contiguous forest.

Question 3

Habitat fragmentation

Answer 3

Three stages to fragmentation process:

• Initial habitat loss
• Isolation of blocks of habitat
• Increased isolation due to further land-use change

Question 4

Deforestation is usually nonrandom

Answer 4

Some habitats cleared preferentially
Geography (topography, soils, climate)
Physical accessibility (e.g., roads, rivers)
Fragments are often a non-random subset of original habitats & microhabitats
Some species are absent or poorly represented in fragments from outset (“Sample effect”)

Question 5

How big is the problem?

Answer 5

Brazilian Amazon
- ~180,000 fragments 1-100 ha
- 25% forest within 1 km of edge
Brazilian Atlantic
- ~230,000 fragments 1-100 ha
- 91% forest within 1 km of edge

Question 6

Ecological Foundations to Fragmentation Research:Species-Area Relationships & Island Biogeography Theory

Answer 6

What do islands and mountain tops have in common?
They both demonstrate relationships between area of habitat and number of species that survive in that area
This fundamental field of ecology began when biologists started to see patterns in the number of species that occurred on islands and mountain tops and the relationship that appeared to occur between species richness and habitat area

Question 7

Species-area relationship

Answer 7

Power function: S = cA^z
The curve is characterized by an equation called the Arrhenius equation
S = species richness, A=Area, and c & z are constants (c = species richness factor and z = species accumulation factor)

Question 8

The Theory of Island Biogeography (MacArthur & Wilson 1967)

Answer 8

Oceanic islands were viewed as relatively static (i.e., slow change) in species composition
Colonization and extinction events were considered rare
M&W postulated that the number of species on islands is a dynamic equilibrium between the opposing forces of extinction and colonization

Question 9

Basic Tenets of Island Biogeography Theory

Answer 9

Local extinction rate is a function of island size
Bigger islands 
larger population sizes  
lower extinction rates  
more species
Colonization rate (arrival of new species) is a function of island isolation from the mainland
More isolated islands  
lower rates of colonization  
fewer species

Question 10

Can IBT be applied to biodiversity impacts of fragmentation in tropical forest fragments?

Answer 10

Area and isolation effects
Impact of area on species richness?
Species richness increases with area
There are also impacts on species composition, as smaller patches have different composition to larger forest patches

Question 11

Which species are lost?

Answer 11

Rapid loss of species with large area requirements:

• Predators
• Large-bodied species
• Elevational migrants

Plus forest-interior specialists

Question 12

Isolation effects

Answer 12

(1) Increasing distance from contiguous forest means that fragments are more isolated
(2) Habitat matrix impacts on connectivity

Question 13

What is the habitat matrix?

Answer 13

The habitat that surrounds forest fragments and ‘connects’ them to other fragments or contiguous forest
Depending on the matrix it often supports populations of some species found in forest
Fragments are connected for these species

Question 14

Disparity between forest and matrix

Answer 14

Inhospitable matrix for forest species
- Colonization
rates decline
- Useable habitat shrinks

Hospitable matrix reduce the effects of fragments size on some species
Also allows for dispersal movements and recolonisation of fragments where species have gone extinct

Question 15

Extinction debt over time

Answer 15

Major confounding issue in identifying impacts of fragmentation is effect of time
Immediately after fragmentation, many species remain, but…
Many species committed to extinction over time
Termed “Extinction debt” (Tilman et al. Nature 1994)
Future ecological cost of current habitat destruction

Question 16

Extinction debt in the Amazon

Answer 16

100 ha fragments lose half of their species in <15 yrs

Question 17

Reducing the extinction debt

Answer 17

Increase fragment size to reduce debt, but…

To reduce species loss 10-fold requires 1000-fold increase in fragment area (Ferraz et al. 2003 PNAS)

Question 18

Edge effects

Answer 18

A disturbance that penetrates into the fragment

Question 19

Abiotic and Biotic edge effects?

Answer 19

Wind, tree mortality, disturbance-adapted species invade, altered spp composition, increased temperature, reduced moisture.

Myriad changes:
Microclimate and wind
Vegetation structure (e.g., treefall gaps)
Floristic composition
Animal composition
Can extend very far into forest (to 400 m)
Critically, processes can effectively cause a reduction in fragment area for many species

Question 20

Functioning of ecosystems

Answer 20

Rainforest communities are underpinned by networks of interactions between species
Species play different functional roles in tropical forest systems:
Predators
Seed dispersers and pollinators
Trees  store carbon & structure

Question 21

An impact of fragmentation on ecological functions?

Answer 21

Large predator loss causes…

x10 increase in reptiles, x25-50 increase in monkeys, x100 increase in leaf cutter ants, x35 increase in rodents means reduced numbers of saplings

Reduced seed dispersal mutualism, fewer seeds dispersed and are dispersed less far.

Reduced wood density

Question 22

What does fragmentation tell us about protected area management?

Answer 22

Maintaining connectivity
IBT stresses the importance of isolation to colonization rates
Key recommendation for protected area management is to retain connectivity
Wider and better quality corridors

Question 23

A value of corridors?

Answer 23

Corridor is a linear habitat remnant, surrounded by the modified matrix, that facilitates wildlife movement between patches
Could retention or creation of corridors improve species survival?
E.g., Brazil requires land-owners to retain riparian (riverine) corridors of minimum width depending on size of river
Biodiversity value of Brazil’s corridors?

Question 24

Summary

Answer 24

Forest fragmentation major disturbance
Strong species-area and isolation effects
There is an extinction debt in fragments
Edge effects further degrade value of patches (especially small ones)
Functioning in fragments is reduced
Breakdown of interactions causing cascading impacts throughout communities
Creating corridors is important method