Habitat Fragmentation Sometimes habitat is not lost nor converted, it is simply fragmented...

Habitat Fragmentation

• Sometimes habitat is not lost nor converted, it is simply fragmented

• Fragmentation has 2 components:– 1) a reduction in the area covered by a

habitat type– 2) a change in habitat configuration,

with the remaining habitat apportioned into smaller and more isolated patches


• In reality, very rarely is the landscape ‘reconfigured’ with the same amount of habitat as previously

• However, there can be varying degrees to how many ‘fragments’ are generated in the process

• Furthermore, there is a great deal of variation in the pattern of those fragments


• Another key component is the ‘landscape matrix’ that surrounds the fragments

• E.g. consider moving from center to edge


• Over the past several decades scientists have sought to better understand the processes associated with fragmentation, predict which kinds of species are sensitive to fragmentation and suggest measures to reduce or mitigate the effects of fragmentation


• However, with a phenomenon as complex as fragmentation, empirical generalizations that apply to all systems in all areas are hard (impossible?) to come by

• We will compare ecological processes in naturally heterogeneous landscapes and fragmented landscapes (e.g. sp-area, island effects, landscape matrix)

Fragmentation & Heterogeneity

• A simplistic view of fragmentation is larger patches being broken into smaller ones


• However, at some scale, everything becomes a mosaic (GSMNP)


• What are some factors that could lead to the observed distribution?


• Natural disturbances create additional heterogeneity beyond that generated by the physical environment

• The grain of a landscape is often determined by the spatial scale of disturbance

• A pattern may be coarse- (e.g. fire) or fine-grained (e.g. canopy gaps)


• At any given time, there are a blend of grains across the landscape

• Furthermore, disturbances are patchy across time (e.g. a fire season, hurricane season)

• This pattern is called ‘space-time mosaic’

• Remember, an ‘intermediate’ amount of disturbance can increase diversity


• Every landscape is patchy, some are just more than others

• As a consequence, habitat quality (and sometime suitability) varies across space and can result in spatially discrete populations

• A system of local population interacting (linked through occasional dispersal) is termed a metapopulation


• Because of the fragmented nature of these populations and frequently their relative small size, local extinction events are relatively common

• Furthermore, the persistence of the local metapopulation is tied to the ability (or inability) of individuals to move between populations


• There is a great deal of variability in species ability to disperse– E.g. the samango monkey in S Af has

group dynamics that make dispersal into and across fragments highly unlikely (hence cannot act like a metapopulation) and are consequently restricted to large, intact blocks of suitable habitat

– E.g. flightless insects, ant-dispersed herbs are other poor dispersers


• Dispersal is more likely to maintain metapopulations in naturally patchy landscapes than in formerly continuous habitat…why?

• The metapopulation model also suggests that habitat patches currently unoccupied may be critical to survival because they represent possible sites of recolonization

• Establishing populations on vacant patches may help downward population spirals












• But numbers are not consistent through time and space














• So is patchiness good? • Does it promote diversity?• Doesn’t heterogeneity promote

diversity?• If this is the case, isn’t more better?


• The differences between naturally patchy and fragmented landscapes are only beginning to be understood

• In the marine realm, we understand very little

• Let’s consider distinctions between fragmented systems and naturally patchy systems (and make some predictions)


• 1) fragmentation has resulted in a reduction of the extent and connectivity of habitats and species may or may not adjust to this change in habitat availability and configuration


• 2) a naturally patchy landscape has rich internal structure (lots of gaps, logs, snags, vertical layers) whereas fragmented landscapes are more simplified


• 3) largely because of the previous point, a natural landscape often has less contrast (less pronounced structural differences) between adjacent patches than does a fragmented landscape, and therefore potentially less-intense edge effects


• 4) certain features of fragmented landscapes, such as roads and various human activities, pose specific threats to population viability


• In other words, there are differences between naturally patchy landscapes and fragmented ones

• Furthermore, the greater the differences, the greater the threat to species persistence in them

• However, there are unpredictable thresholds and specific outcomes are somewhat tenuous

The Fragmentation Process

• In terrestrial systems, fragmentation typically begins with a gap formation or intrusion at the edge

• Impact may be minimal for a while• As the gaps increase, the matrix

becomes ‘residual habitat’ and something else (e.g. agriculture, open space, road)



• Fragmentation can be quantified as landscapes differ in their spatial pattern, size, shape, isolation to another patch, and complexity of their edge (read Box 7.1)


• Many landscape metrics are derived from theory, which makes their practical value more difficult to ascertain

• E.g. percolation theory predicts that at 59.28% cover, the probability of finding a corridor across the landscape decreases abruptly


• Many landscape metrics are derived from theory


• However, given the various dispersal abilities, response to landscapes, habitat preferences, theory definitely needs to ‘ground-truthed’

Habitat Fragmentation& Edge Effects

• Fragmentation and edge effects in landscapes that are still predominately forested fail to find any strong impacts

• However, some behaviors can obscure eventual patterns– E.g. birds and site fidelity

• The time lag between habitat loss and eventual loss species loss defines a region’s extinction debt


• The extinction debt for African forest primates is estimated to be over 30% in many forest fragments

• It can be relatively easy to determine the effects of fragmentation (i.e. species lost) without observing the process generating it

• Consequently, it continues to merit study and discussion


• It is important to remember that the process can occur at many different spatial and temporal scales and in any kind of habitat

• E.g. biogeographic scale (Bering Strait); this level is unlikely to occur naturally

• E.g. Atlantic Rainforest; many studies currently in fragments

• E.g. finer scales; internal fragmentation of pristine natural areas


• Following grazing, invasive grasses filled the spaces between natives

Biological Consequences of Fragmentation

• The effects of fragmentation range from the obvious losses to the more subtle and indirect

• Some effects can be ‘repaired’– Forests of E coast of US

• However, most are not being restored or regenerated

• Freshwater systems are highly fragmented (remember the number?)


• Initial Exclusion• An obvious effect of fragmentation is

the loss of species in the portion of the matrix that was destroyed

• Many species have relatively narrow distributions and can easily be excluded (remember Centinela Ridge)


• Crowding Effect• When the habitat in the matrix is

destroyed, those individuals in the destroyed portion seek refuge in the remaining fragments

• Frequently see a spike in abundance, but inevitably falls

• Where is persists, can have negative impacts (i.e. density-dependent effects)


• Insularization and Area Effects• Small reserves are unlikely to harbor

a number of species with large area requirements (e.g. cougar 400km2)

• Other species avoid tracts of suitable habitat and more than enough room for a territory (e.g. area-sensitive)


• Ovenbirds are positively correlated with forest area whereas WOTH are equally likely in most (10ha>)


• Oceanic vs. Land-bridge islands• Land-bridge islands were typically

connected to mainland during Pleistocene glaciation events (where sea level dropped 100m or more)

• Those peninsulas probably contained same diversity as mainland habitat of equal area


• However, following isolation, these islands began to lose species (relaxation)

• Do you think species were random?• However, species richness is typically

higher on land-bridge islands than oceanic islands of similar size. Why?

• Some want to use this analogy for mainland ‘islands’ that have become isolated


• Appropriate paradigm for terrestrial ‘islands’ that have become isolated?

• However, evidence is relatively weak• E.g. in National Parks in the west, the

number of extinctions > colonizations and extinction rates are inversely correlated with park area

• In Canadian parks, this paradigm does not hold. Why?


• Oceanic and land-bridge islands over time; which is which?


• So is it better to have many smaller populations or a single larger one? (SLOSS)

• Another potential problem is the replacement of native species

• E.g. in a 400-ha woodland in Boston, native declined by 0.36%/yr over last century and exotics increasing 0.18%/yr


• Isolation• Isolation of habitats and population is

an effect of fragmentation as consequential as reduction in habitat and population size

• If no single patch is large enough, perhaps a cluster of suitable habitat can suffice


• Location A is preferable to B


• Also important to remember that most species utilize multiple habitats at various stages of development or during their life-cycle


• Location A is preferable to B


• What constitutes a barrier to movement is highly species-specific

• Is a fencerow of brush a fence or corridor?

• How various habitat are perceived is poorly known for most species

• In most cases, human generated features are barriers


• The long-term consequences of dispersal barriers on population dynamics and genetic structure are becoming better known

• Many studies have shown that measures of genetic diversity are generally associated with population size and isolation from other populations

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Habitat Fragmentation Sometimes habitat is not lost nor converted, it is simply fragmented...

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