Habitat Fragmentation - Introduction to Conservation Biology - Lecture Slides, Slides of Biology

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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 Docsity.com 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 Docsity.com 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) Docsity.com Fragmentation & Heterogeneity • A simplistic view of fragmentation is larger patches being broken into smaller ones Docsity.com Fragmentation & Heterogeneity • However, at some scale, everything becomes a mosaic (GSMNP) Docsity.com 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 Docsity.com 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 Docsity.com 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 Docsity.com Fragmentation & Heterogeneity • But numbers are not consistent through time and space Docsity.com Habitat Fragmentation • So is patchiness good? • Does it promote diversity? • Doesn’t heterogeneity promote diversity? • If this is the case, isn’t more better? Docsity.com 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) Docsity.com 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 Docsity.com Habitat Fragmentation • 4) certain features of fragmented landscapes, such as roads and various human activities, pose specific threats to population viability Docsity.com 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 Docsity.com 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) Docsity.com 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 Docsity.com The Fragmentation Process • Many landscape metrics are derived from theory Docsity.com Habitat Fragmentation & Edge Effects • 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 Docsity.com Habitat Fragmentation & Edge Effects • 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 Docsity.com Habitat Fragmentation & Edge Effects • Following grazing, invasive grasses filled the spaces between natives Docsity.com Biological Consequences of Fragmentation • 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) Docsity.com Biological Consequences of Fragmentation • 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) Docsity.com Biological Consequences of Fragmentation • Ovenbirds are positively correlated with forest area whereas WOTH are equally likely in most (10ha>) Docsity.com Biological Consequences of Fragmentation • 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? Docsity.com Biological Consequences of Fragmentation • Oceanic and land-bridge islands over time; which is which? Docsity.com Biological Consequences of Fragmentation • 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 Docsity.com Biological Consequences of Fragmentation • Also important to remember that most species utilize multiple habitats at various stages of development or during their life-cycle Docsity.com Biological Consequences of Fragmentation • Location A is preferable to B Docsity.com Biological Consequences of Fragmentation • 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 Docsity.com