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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.
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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
Habitat Fragmentation • 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
Habitat Fragmentation • Another key component is the ‘landscape matrix’ that surrounds the fragments • E.g. consider moving from center to edge
Habitat Fragmentation • 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
Habitat 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
Fragmentation & Heterogeneity • However, at some scale, everything becomes a mosaic (GSMNP)
Fragmentation & Heterogeneity • What are some factors that could lead to the observed distribution?
Fragmentation & Heterogeneity • 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)
Fragmentation & Heterogeneity • 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
Fragmentation & Heterogeneity • 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
Fragmentation & Heterogeneity • 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
Fragmentation & Heterogeneity • 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
Fragmentation & Heterogeneity • 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
Fragmentation & Heterogeneity • But numbers are not consistent through time and space
Habitat Fragmentation • So is patchiness good? • Does it promote diversity? • Doesn’t heterogeneity promote diversity? • If this is the case, isn’t more better?
Habitat Fragmentation • 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)
Habitat Fragmentation • 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
Habitat Fragmentation • 2) a naturally patchy landscape has rich internal structure (lots of gaps, logs, snags, vertical layers) whereas fragmented landscapes are more simplified
Habitat Fragmentation • 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
Habitat Fragmentation • 4) certain features of fragmented landscapes, such as roads and various human activities, pose specific threats to population viability
Habitat Fragmentation • 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)
The Fragmentation Process • 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)
The Fragmentation Process • 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
The Fragmentation Process • Many landscape metrics are derived from theory
The Fragmentation Process • However, given the various dispersal abilities, response to landscapes, habitat preferences, theory definitely needs to ‘ground-truthed’
