Chapter 4 Fragmentation assessment

Transcription

1 Chapter 4 Fragmentation assessment 4.1. Introduction Fragmentation has become a central issue in landscape ecology and conservation (Forman and Godron, 1986; Skole and Tucker, 1993; Pimm and Raven, 2000; Cotler and Ortega-Larrocea, 2006). The breaking up of large land area into smaller patches is known to influence many ecological patterns and processes. It could negatively affect 76

2 all taxonomic groups including birds and mammals (Andrén, 1994), reptiles (Gibbons et al., 2000), amphibians (Stuart et al., 2004), invertebrates (Didham et al., 1996) and plants (Hobbs and Yates, 2003). Fragmentation may be due to natural factors such as fires, earthquakes, flood, hurricanes and drought, or by human induced operations such as logging, clearing for agriculture and plantations, road building and mining activities. Fragmentation leads into a condition of archipelago where the islands of forested habitats are surrounded by a sea of another type of environment, which is inhospitable for biota living in the islands. Fragmented forests have a relatively large ratio of perimeter to area, and the resultant forest edges may be different from interior forests in many ways. Fragmentation and the resulting increase in forest edge lead to changes from microclimate to species composition at various distances from the edge. Fragmentation lowers species number and alters community composition as a result of both, reduction in forest area and a change in forest shape. It has been reported that small fragmented patches experience lesser density of population and increase in the risk of extinction (Farina, 1998). Fragments up to 100 and 400 ha in size have been shown to exhibit major changes in forest dynamics, based on studies in the Amazon (Laurance et al., 1998) and Malaysian tropical rain forests (Forman and Godron, 1986). The persistence of population is lower in fragmented habitats than large habitats (Diamond, 1976; Tilman, 1994) and is susceptible to demographic extinction pressure (Shaffer, 1981) and environmental stresses (Simberloff and Abele, 1976). Habitat isolation can negatively affect day-to-day movements of a given species (e.g. between 77

3 nesting and foraging resources; Luck and Daily, 2003). Metapopulations, i.e. set[s] of local populations which interact via individuals moving between local populations sometimes develop as a result of habitat isolation (Hanski and Gilpin, 1991). The extent to which landscape modification results in habitat isolation depends on the interaction between a given species dispersal behaviour, mode and scale of movement, what constitutes suitable habitat for it, and how a given landscape has been modified. In addition, large-scale movements of species such as seasonal migration or range shifts in response to climate change may also be affected by habitat isolation (Soulé et al., 2004). Other notable effects include increases in wildfire susceptibility (Alencar et al., 2004; Cochrane and Laurance, 2002) increased rate of tree mortality and canopy-gap formation near forest edges (Laurance et al., 1997, 1998, 2001), affecting plant phenology (Adler and Kiepinski, 2000; Ackerly et al., 1990, Nason and Hamrick, 1997), changes in plant and animal species composition (Barlow et al., 2006; Cushman, 2006) and easier access to interior forest, leading to increased hunting and resource extraction or conversion to agroscape (Peres, 2001). Quantifying the pattern of fragmentation is essential to establish criteria for relating a particular patch pattern to its causes (Levin, 1992). Presently, a number of landscape indices and concomitant software s are available, e.g., FRAGSTATS - McGarigal and Cushman, (2002); Patch analyst - Rempel and Carr, (2003); BIOCAP - IIRS (Roy et al., 2002) and new measures are also being proposed (Bogaert et al., 2002a, b). The simplest way to summarize the pattern of fragmentation is through a frequency distribution of patch sizes. Pattern maps are useful because they quantify biologically relevant information that is not evident from a land cover map (Riitters et al., 2002). 78

4 However, this does not provide any information concerning other consequences of fragmentation, such as edge effect or isolation (Groom and Schumaker, 1993). Moreover, the spatial distribution of patch sizes is sometimes hard to measure and interpret in a complex landscape (Baskent and Jordan, 1995). Fragmentation of ecological units have been well documented at landscape level using patch number, size, shape, abundance and forest matrix characteristics (Forman and Godron, 1986; Skole and Tucker, 1993; Roy and Tomar, 2000; Cushman et al., 2006; McGarigal et al., 2009; Haire and McGarigal, 2010). These patch characteristics indicate the spatial organization of vegetation types, biotic disturbance, availability of nutrients, energy flow and habitat suitability. Today, remote sensing is being considered as an excellent tool for the analysis and effective monitoring of forest fragmentation. Several studies have used remote sensing to map patterns of forest fragmentation and to analyze the rates of forest-cover change in the tropics and elsewhere (Vogelmann, 1995; Riitters et al., 2002; Wickham et al., 2000, 2007). The present study is intends to analyze the landscape level fragmentation of the study area in general and forest categories in particular Methodology The characteristics of data products and the various processing stages are detailed in Chapter 3 (section 3.2.). The following matrices were selected for the fragmentation analysis. A software program, FRAGSTATS, developed by McGarigal and Cushman (2002) was used for the computation. The analysis was done at two levels; i.e., landscape level (for the entire landscape) and class level (for each forest type). 79

5 1. Number of Patches (NP) It means the total number of patches in each class or the landscape. 2. Patch Density (PD) - PD equals the number of patches in the class or landscape, divided by total landscape area (m 2 ), multiplied by 10,000 and 100 (to convert to 100 hectares). N PD = (10,000)(100) (4.1) A N = Total number of patches in the particular class or landscape. A = Total landscape area (m 2 ). 3. Area Weighted Mean Patch Area (AM) and Standard Deviation (SD) - AM equals the sum, across all patches in the landscape, of the corresponding patch metric value multiplied by the proportional abundance of the patch [i.e., patch area (m 2 ) divided by the sum of patch areas] (Equation 4.2). SD (standard deviation) equals the square root of the sum of the squared deviations of each patch metric value from the mean metric value computed for all patches in the landscape, divided by the total number of patches; that is, the root mean squared error (deviation from the mean) in the corresponding patch metric (Equation 4.3). AM m n a ij = Xij (4.2) = = m n i 1 j 1 aij = = i 1 j 1 80

6 SD = m n i= 1 j= 1 X ij N m n i= 1 j= 1 N X ij (4.3) 4. Total Edge TE equals the sum of the lengths (m) of all edge segments in a class or the landscape. It is given in meters. 5. Edge Density - ED equals the sum of the lengths (m) of all edge segments in the landscape, divided by the total landscape area (m 2 ), multiplied by 10,000 (to convert to hectares). It is represented in meters per hectare. E ED = (10,000) (4.4) A E = Total length (m) of edge in landscape. A = Total landscape area (m 2 ). 6. Euclidian Nearest Neighbourhood distance - equals the distance (m) to the nearest neighbouring patch of the same type, based on shortest edge-to-edge distance. It is represented in meters. approaches zero as the distance to the nearest neighbour decreases. It is perhaps the simplest measure of patch context and has been used extensively to quantify patch isolation. Here, nearest neighbour distance is defined using simple Euclidean geometry as the shortest straight-line distance between the focal patch and its nearest neighbour of the same class. 81

7 7. Splitting Index SPLIT equals the total landscape area (m 2 ) squared divided by the sum of patch area (m 2 ) squared, summed across all patches of the corresponding patch type. SPLIT 2 A = n 2 a ij j= (4.5) a ij = area (m 2 ) of patch ij. A = total landscape area (m 2 ). Split is based on the cumulative patch area distribution and is interpreted as the number of patches with a constant patch size when the corresponding patch type is subdivided into S patches, where S is the value of the splitting index. SPLIT = 1 when the landscape consists of single patch. SPLIT increases as the focal patch type is increasingly reduced in area and subdivided into smaller patches. The upper limit of SPLIT is constrained by the ratio of landscape area to cell size and is achieved when the corresponding patch type consists of a single one pixel patch Results Fragmentation at landscape level Fragmentation analysis at landscape level showed that number of patches, patch density, total edge, edge density, Euclidian nearest neighbourhood distance and split index increased during , whereas area weighted mean decreased, during the 82

8 same period. Number of patches in 1973 was 19,645, which increased to 26,951 in 1992 and 30,601 in 2001, and 32,586 in Patch density which was 10.37/ha in 1973 reached 14.24/ha in 1992, 16.17/ha in 2001 and 17.46/ha in Edge density was raised from 59.91m/ha in 1973 to m/ha in 2005 while split index increased from 6.25 to during the same time period. Area weighted mean patch area which was 3.03 ha in 1973, reduced to 1.18 ha in Table 4.1 shows the details of the landscape level analysis for the period of 1973, 1992, 2001 and Table 4.1. Change in landscape level fragmentation matrices from a period of for Attapady Landscape, Western Ghats. NP PD TE ED SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index Fragmentation at Class (vegetation type) level Evergreen forest The analysis showed that the number of patches, patch density, total edge, edge density and Area weighted mean patch size reduced considerably during a period of 32 years. Number patches that were 1530 in 1973 dropped to 1429 in 1992, 1318 in 83

9 2001 and 1285 in Contrary to that, Euclidian Nearest Neighbourhood Distance and Split index showed an increasing pattern during the same period. Split index observed in 1973 was 1728, which increased to 2328 in 1992, 2493 in 2001 and 2509 in The detailed results are given in table 4.2. The overall result indicate that the pristine evergreen forest have undergone changes both at the levels of small scale encroachments and large scale clearing of forest. Table 4.2. Change in evergreen forest fragmentation matrices from for Attapady Landscape, Western Ghats. NP PD TE ED SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index Semievergreen forest Fragmentation analysis of Semievergreen forests showed that number of patches, patch density, total edge, edge density, Euclidian nearest neighbourhood distance and Split Index increased during the period of Number of patches was 2141 (in 1973), 2895 (in 1992), 3247 (in 2001) and 3470 (in 2005) while Patch density per hectare was 1.14 (in 1973), 1.54 (in 1992), 1.62 (in 2001) and 1.69 (in 2005). Area weighted mean patch size showed a decreasing trend during this period. It was 0.18ha 84

10 in 1973, 0.16ha in 1992, 0.14ha in 2001 and 0.13ha in Details are provided in Table 4.3. Table 4.3. Change in semievergreen forest fragmentation matrices from for Attapady Landscape, Western Ghats. NP PD TE ED _ AM _ SD _ AM SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index Moist Deciduous forest Results of fragmentation analysis for moist deciduous forests showed that number of patches, patch density, total edge, edge density, Euclidian nearest neighbourhood distance and Split Index increased during the period of Number of patches which were found to be 2407 in 1973 increased to 4345 by the year of 2005, while the average Euclidian distance increased from 132m to 156m during the period of 1973 to The mean patch area decreased from ha to ha during the same time period. Detailed results are given in Table

11 Table 4.4. Change in moist deciduous forest fragmentation matrices from for Attapady Landscape, Western Ghats. NP PD TE ED SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index Dry Deciduous forest In dry deciduous forests, number of patches, patch density, total edge, edge density, Euclidian nearest neighbourhood distance and Split Index showed an increasing trend from 1973 to Total edge of dry deciduous forest which was 3172km in 1973 increased to 5201km in Similarly, Edge density also increased from 16.74m/ha to 27.48m/ha from 1973 to Results are given in table 4.5. In general, the pace of degradation was higher during the period of in comparison with

12 Table 4.5. Change in dry deciduous forest fragmentation matrices from for Attapady Landscape, Western Ghats. NP PD TE ED SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index Thorny Scrub forest Number of patches, patch density, total edge, edge density, Euclidian nearest neighbourhood distance and Split Index have also showed a degradation tendency similar to other forest types. Number of patches increased from 3141 in 1973 to 4580 in In contrast to other forest types, the mean patch area slightly increased from 0.049ha to 0.085ha during the same time period. This may be due to the spreading of the thorny scrub vegetation in the abandoned and barren areas of the hills. Detailed results are given in Table

13 Table 4.6. Change in thorny scrub forest fragmentation matrices from for Attapady Landscape, Western Ghats. NP PD TE ED SPLIT NP number of patches, PD Patch Density (ha), TE Total Edge (km), ED Edge Density (m/ha), _MD Area weighted mean (ha), Area Standard deviation (ha), - Euclidian Nearest Neighbourhood Distance Area Weighted Mean (m), - Euclidian Nearest Neighbourhood Distance_ Standard deviation (m), SPLIT Split Index 4.4. Discussion Selection of the landscape fragmentation matrices The selection of fragmentation indices was purposive i. e., each index indicates one aspect of fragmentation. The number of patches of a particular ecosystem might indicate that it suffers a higher rate of disturbance (e.g. encroachment and deforestation). Nevertheless, information on the number of patches alone does not have any interpretive value because it has no information about area, distribution or shape of the fragments (McGarigal and Marks, 1995), for this reason this index was calculated together with other matrices that could together be more interpretable. For eg., Patch density (PD) increases with a greater number of patches and serves as an indication of the extent to which a landscape is fragmented (Gillanders et al., 2008). Another example is the area-weighted mean patch area which indicates the mean 88

14 patch area. Progressive reduction in the size of ecosystem fragments is a key component of ecosystem fragmentation, thus a landscape with a smaller mean patch size for the target ecosystem than another landscape might be considered more fragmented (McGarigal and Marks, 1995). Also number of patches and mean patch area are often used complementary since high number of patch and low mean patch size values reinforces an interpretation of fragmented landscape conditions (Matsushita et al., 2006). Edge density (ED) is calculated as the length of all borders between different classes in a reference area divided by the total area of the reference unit and is a measure of the complexity of the shapes of patches and an indicator of the spatial heterogeneity of a landscape (Gillanders et al., 2008). In a similar way, the higher the Euclidian Nearest Neighbourhood Distance, i e., distance between two patches of similar type denotes a higher rate of patch isolation. This is a measure of fragmentation of an ecosystem type since the distance from a patch to another might be increasing due to human disturbances to that ecosystem type (e.g. deforestation, land use change, and so on.). Split Index is an overall measurement of fragmentation at different time scales. Thus, altogether these parameters could explain three aspects of fragmentation 1) number of patches, 2) edge effects and 3) patch isolation. Landscape fragmentation is commonly characterized using pattern indices such as number of patches (NP), mean patch size (MPS), the distance between patches, and measures of edge habitat (Langford et al., 2006). 89

15 Fragmentation at landscape level Landscape-level indices are measures of all patch types or classes over the full extent of the data (McGarigal and Cushman, 2002). It is evident from the Fragstat analysis that the forests of the Attapady landscape have been fragmented considerably during For example, the number of patches and patch density which are the simple and direct measures of fragmentation increased during this period. Number of patches were 19,645 in 1973 which raised into 32,586 in In similar way, patch density which was 10.37/ha in 1973 increased to 17.46/ha in This clearly indicates that individual patches of Attapady landscape have undergone fragmentation resulting in sub-patches. Menon and Bawa, (1997) observed a four fold increase in number of patches and 83% reduction in average patch size for the entire Western Ghats during the period of They have also noted that 40% of the original natural vegetation in the Western Ghats was converted into open/cultivated lands, coffee and tea plantations, and hydro-electric reservoirs during the same time period. The studies in the temperate forests of Rize, Turkey indicated that the number of patches and patch density increased (610 to 2,085 and 0.61 to 2.09/ha respectively) and mean patch size decreased (163.6 to 47.9/ha) during the period of 1984 to 2007 (Günlü et al., 2009). Increase in number of patches and patch density, and decrease in mean patch size demonstrated that the forest landscape has gone into a more fragmented structure, negatively affecting biodiversity and the resilience of the ecosystem (Günlü et al., 2009; Crk et al., 2009). It has been reported that small fragmented patches experience lesser density of population and increased risk of extinction (Farina, 1998). The increased value for Euclidian Nearest Neighbourhood distance shows that the distance between two homogenous patches also increased 90

16 during the period which is a measure of patch isolation. Landscape connectivity, a measure of spatially contiguous landscape matrix, has been shown to exert strong influences on ecological processes, such as the movement and dispersal of organisms, the use of resources by animals, gene flow, and the spread of disturbance (Pearson, 1993). Splitting index also increased during the period which proves the focal patch type is increasingly reduced in area and subdivided into smaller patches. Numerous studies point out that land use and forest cover changes are attributed to the human population growth and urbanization processes (Wakeel et al., 2005). Changes in forest landscape patterns are generally known to be affected by socio-economic factors and are being increasingly identified as critical factors influencing environmental change (Abdullah and Nakagoshi, 2006; Nagendra et al., 2004). Many landscape ecological studies at the temperate and Neotropical regions have shown that socio-economic factors (Zhao et al., 2003), forestry expansion (Nagashima et al., 2002), urbanization and agricultural use patterns (Doygun and Alphan, 2006) could make the landscape more fragmented Fragmentation at forest type levels Evergreen forests have undergone changes both at the extent (deforestation) as well as homogeneity (fragmentation). The reduction in patch and edge densities per hectare and mean patch area were probably due to the clearing of forest, while, the increase in Euclidian neighbourhood distance indicated that homogeneous patches are becoming more fragmented. A similar result was observed for Kalakkad Mundanthurai Tiger Reserve (KMTR) where number of evergreen patches decreased during the period of (Giriraj et al., 2010). All other forest types such as semi-evergreen, moist 91

17 deciduous, dry deciduous and thorny scrub showed a more or less similar pattern i.e., increase in number of patches, patch density, total edge, edge density, distance between homogeneous patches and split index, and decrease in mean patch size. The results indicated that none of the forest types are away from the influx of human population and associated pressure. The encroachment and overdependence might have led the pristine forests into a degraded system as is evident from the landscape matrices. The absence of specific studies in the Attapady landscape, in particular and in the Western Ghats, in general makes it difficult to compare the present results with other observations. However, the results were in agreement with the global scenario that tropical forests are highly fragmented due to intensive human activities (Murphy and Lugo, 1986; Gerhardt and Hytteborn, 1992; Miles et al., 2006) Chapter Summary Fragmentation analysis was carried out at two levels; i.e., landscape level (for the entire landscape) and class level (for each forest type). The variables selected for the fragmentation analysis were (1) number of patches, (2) patch density, (3) total edge, (4) edge density, (5) area weighted mean patch size and its standard deviation, (6) euclidian nearest neighbourhood distance and its standard deviation and (7) split index. Fragstat analysis proved that the forests of the Attapady landscape have fragmented considerably during the period of During this period the number of patches and patch density were increased which points out that individual patches of Attapady landscape were fragmented into sub-patches. The detailed analysis of fragmentation at class level revealed that evergreen forests have undergone changes both at the extent (deforestation) as well as homogeneity 92

18 (fragmentation). While, the increase in Euclidian neighbourhood distance indicated that homogeneous patches of evergreen forest are becoming more fragmented. All other forest types such as semi-evergreen, moist deciduous, dry deciduous and thorny scrub showed a more or less similar pattern i.e., increase in number of patches, patch density, total edge, edge density, distance between homogeneous patches and split index, and decrease in mean patch size. The results indicated that none of the forest types are away from the influx of human population and associated pressure. The result was in agreement with the studies in the temperate and the Neotropics that the prevalent human activities and its associated impacts could exerts pressure on the contiguity of the forest. Species level assessment of such human influenced landscape is a requisite for the effective conservation and eco-restoration activities, and therefore the following chapters are planned to address the response of species and communities to disturbance gradients. 93