BY Ruwan Laknath Jayakody
The monitoring and managing of temporal changes in habitats, particularly in elephant dominated terrain, should be considered a top priority as it has been found that the grassland habitat of the Udawalawe National Park has significantly decreased over the 2005-2019 period, mainly owing to the invasion by scrub, which would ultimately affect the fodder availability for wild herbivores, especially elephants in the park.
This finding and recommendation was made in a research article on “Changes in habitat coverage from 2005 to 2019 in the Udawalawe National Park” which was authored by W.P.T.A. Perera, P.H.K.L.A. Prematilaka, M.H.A. Haseena, A.H.L.C.M. Athapaththu and M.R. Wijesinghe (all attached to the University of Colombo’s Zoology and Environmental Sciences Department with the latter also being attached to the same university’s Science Faculty’s EcoServ Research Lab) and published in the Ceylon Journal of Science 50 (4) in December 2021.
Habitat heterogeneity, on a landscape scale, is a key determinant of biological diversity, per “Animal species diversity driven by habitat heterogeneity/diversity: The importance of keystone structure” by J. Tews, U. Brose, V. Grimm, K. Tielborger, M.C. Wichmann, M. Schwager and F. Jeltsch and “Functional landscape heterogeneity and animal biodiversity in agricultural landscapes” by L. Fahrig, L. Brotons, F. Burel, T. Crist, R. Fuller, C. Sirami, G. Siriwardena and J.L. Martin.
According to Perera et al., changes that occur in the extents and distribution of habitats (driven mainly by human-induced disturbance factors such as logging, reforestation, agricultural expansion, establishing human settlements, and urban development while certain natural changes also occur over time as a result of ecological succession) in the landscape would be expected to lead to alterations in the diversity of its biotic communities.
Protected areas are, N. Dudley noted in “Guidelines for applying protected area management categories”, primarily set aside for conserving biological diversity, where Perera et al. explained that in such areas with a variety of different habitats, proper management would require that the mosaic of habitats is maintained in a manner that will offer the protected species the best chances of survival and in such areas primarily aimed at conserving wildlife, effective management should ensure that the habitats within them would be capable of meeting the food and shelter requirements of the faunal species they sustain, per S. Schindler, H.V. Wehrden, K. Poirazidis, T. Wrbka and V. Kati’s “Multi-scale performance of landscape metrics as indicators of species richness of plants, insects and vertebrates” and J. Nori, R. Torres, J. Lescano, J. Cordier, M. Periago and D. Baldo’s “Protected areas and spatial conservation priorities for endemic vertebrates of the Gran Chaco, one of the most threatened eco regions of the world”.
This might however, J. Briggs, A. Knapp, J. Blair, J. Heisler, G. Hoch, M. Lett and J. Mccarron’s “An ecosystem in transition: Causes and consequences of the conversion of mesic (a type of habitat with a moderate or well balanced supply of moisture) grassland to shrubland” prove to be a challenge in landscapes that are exposed to continual change brought about by natural and anthropogenic factors.
The Udawalawe National Park is a wildlife reserve with an extensive network of protected areas spread across for conserving the rich biodiversity heritage. According to the Wildlife Conservation Department, the key objective of establishing it was to provide a safe refuge for the herds of elephants whose habitat had been affected by the construction of the Udawalawe Reservoir and associated development activities. Importantly, the area also functions as a corridor for the movement of elephants between the surrounding areas, and therefore, the park is a vital link in extending the elephants habitat range. It is located in the dry zone.
C.H. Holmes mentioned in “The broad pattern of climate and vegetational distribution in Sri Lanka in the study of tropical vegetation” that the typical natural climax vegetation characteristic of the dry zone is described as dry mixed evergreen forest.
The Asian elephant (Elephas maximus) is a flagship species of the park. The elephants move in and out of the park, and the total population in the park was recorded as being between 804 and 1,160 individuals per S. de Silva, A.D. Ranjeewa and D. Weerakoon’s “Demography of Asian elephants at the Udawalawe National Park”. This comprises about a fifth of the total population of elephants in Sri Lanka, according to the department. This wildlife park is one of the most visited protected areas with its primary attraction being the presence of large herds of elephants, according to the Sri Lanka Tourism Development Authority. Therefore, Perera et al. pointed out that a decline in the elephant population, besides posing a threat to the survival of this species, would also reduce the park’s tourism potential.
The park falls within the two administrative Districts of Ratnapura and Monaragala. It covers an area of 308 square kilometres (km²) and encompasses two reservoirs for storing water for irrigation; the Udawalawe Reservoir (accounting for 34.05 km² at full capacity) and the much smaller Mau Ara Reservoir (E. Ranaweerage, A.D. Ranjeewa and K. Sugimoto’s “Tourism-induced disturbance of wildlife in protected areas: A case study of free ranging elephants in Sri Lanka”).
The dry season extends from June to September. The mean annual rainfall is 1,500 millimetres (mm), with two rainy spells – an inter monsoonal spell between March and May and the North East monsoon from October to January. The mean annual temperature is 32° Celsius and the relative humidity, per the department, ranges from 61% to 94% in the daytime.
The area carries a mix of vegetation types that include (i) dry mixed evergreen forest, (ii) scrub, (iii) grassland, (iv) riverine forest, and (v) forest plantations, the last two being very small in extent, per the department, and has been influenced by both natural and anthropogenic factors. The grasslands in the park, dominated by Panicum maximum (Guinea grass) and the scrub are mainly the outcome of past anthropogenic activities including the clearance of large areas of old growth forest for shifting cultivation (chena), per the department. Recent studies (M.D.B. Alahakoon, E.M.A.B. Pushpakumara, G. Ellepola and K.B. Ranawana’s “Food and feeding patterns of Asian elephants in the Udawalawe National Park” and C. Sampson, P. Leimgruber, D. Tonkyn, J. Pastorini, H.K. Janaka, E. Sotherden and P. Fernando’s “Effects of illegal grazing and invasive Lantana camara [Common lantana – a flowering plant] on Asian elephant habitat use”) have indicated threats to the grasslands owing to both the spread of invasive exotic plant species and illegal livestock grazing.
In this regard, Perera et al. carried out field observations throughout the park in September 2019 and identified three major habitat types for the study, namely, (i) forest (including old growth natural forest i.e. closed canopy dry mixed evergreen forest; disturbed and secondary dry mixed evergreen forest; riverine forest, and forest plantation), (ii) scrub (low, bushy, thorny vegetation, also including small trees), and (iii) grassland (areas dominated by grass, with or without scattered trees). A total of 10 points, well spread out within the park, were geo referenced for each habitat type using a Global Positioning System (GPS) – a satellite based radio navigation system. Remote sensing and Geographic Information System (GIS) tools and techniques which can be used to assess changes in the habitat cover were used to analyse temporal changes in the distribution and extent of the selected habitat types over a 14-year period, from 2005 to 2019. The boundary of the park which was obtained from satellite images from the United States Geological Survey were used for generating habitat cover maps of the three selected habitats – forest, scrub and grassland – in order to determine temporal trends. Each of the three habitats: forest, scrub, and grassland, were considered for assessing accuracy with a total of 50 reference points being used for each.
In terms of the temporal changes in the extents of the selected habitats, the study considered three broad habitat types for assessing temporal changes – forest, scrub, and grassland, and the analyses of the satellite images show significant changes that have occurred in the distribution and extents of these three major habitat types. Perera et al. elaborated that being changes on a landscape scale, this is remarkable considering the relatively short space of time (14 years) over which they have occurred. The forest cover had increased by 29 km², from 13% to 22% between 2005 and 2019. This means that the forest cover had increased by as much as 68% of its original extent within the space of 14 years. Scrub habitat was the dominant vegetation type in 2005 and it increased further by about 12 km², from approximately 47% to 51%. Most importantly, the grass cover had decreased considerably – it had reduced by about 51 km², from 27% to 11%. The satellite images show that the forest had spread into areas which were in scrub, and that the scrub had spread into the areas that were in grass. The net effect was a large reduction in the extent of the grassland.
The high level of accuracy (over 80%) attained in the habitat assessment using multi-temporal satellite images combined with ground truthing in the present study, indicates that this is a reliable method for determining changes in the coverage of broadly defined, major habitat types in protected areas.
The study shows that the extents of the three habitat types – forest, scrub, and grassland have changed substantially over a relatively short period of time i.e. over the 14 years, from 2005 to 2019. Whereas both forest and scrub have increased – forest by 29.3 km² (a 67.9% increase), scrub by 11.9 km² (a 7.9% increase) – grassland has shown a marked decrease of 51.2 km² (a 59% decrease). The forest area had increased replacing scrub and scrub had spread into the grasslands, both owing to the natural process of plant succession, aided in the case of scrub invading the grassland, by the aggressive spread of the exotic invasive species, Common lantana. Interestingly, the increasing trends in the extents of the forest and scrub habitats show marginal reversals between 2015 and 2019 (forest) and between 2010 and 2015 (scrub). This could be attributed to the difficulty in classifying transitional stages between the two habitats, scrub and forest. Acts of illegal clearing and the burning of forest habitats for shifting cultivation or for the cultivation of Cannabis sativa, which were among the recorded offences in the park during this period (U.L.N.S. Perera, P.A.A. Jayawardana, D.D. Dharmaratne and M.R. Wijesinghe’s “An analysis of the offences at the Udawalawe National Park”), may have also contributed to the decrease in the forest area observed between 2015 and 2019.
Regarding the grass cover, grasses come into full flush mainly during the rainy season. This factor was recognised in the present study and the four sets of satellite images used were those taken towards the end of the period during which the dry zone experiences wet weather caused by monsoonal and inter-monsoonal rains.
The present study recorded an increase in the extent of scrub which is one of the typical vegetation types in the dry zone. Scrub habitats often occur in juxtaposition with grasslands as seen in the satellite images and as previously reported by Alahakoon et al. In the park, the scrub habitats consist of native species and exotic invasive species. The aggressive spread of alien invasive species would have contributed to the increase in the extent of the scrub habitat. Two species, Common lantana and Chromolaena odorata (a tropical and subtropical species of flowering shrub in the sunflower family) have been identified as problematic species which are spreading rapidly in the scrub and encroaching onto the grasslands, per the department. It is documented by S.T. Fernando, N. Kodippili, C. Suraweera and B. Kumari in the “Identification of the distribution of Common lantana (exotic invasive species) and its impacts on the Udawalawe National Park” that Common lantana has invaded an area of around 2,650 hectares (ha) in this park. Elsewhere, habitat loss from such invasive species has become an important conservation focus for habitat management for the Asian elephant, according to G. Wilson, A.A. Desai, A.S. Dalice and L.L. Wayne’s “The influence of the invasive weed Common lantana on elephant habitat use in the Mudumalai Tiger Reserve, Southern India”.
Grasslands have been recognised as one of the most important wildlife habitats in the park. It has arisen owing to the practice of shifting cultivation in this area until around 1972. In Udawalawe, it is reported that following the designation of the national park in 1972, people living inside its boundaries were relocated and the land, abandoned after raising food crops for a few seasons, reverted to grassland, per the department. H. Knicker’s “How does fire affect the nature and stability of soil organic nitrogen and carbon? A review” stated that the recovery of soil nutrients, especially nitrogen and phosphorus, is really slow in a burnt land and may take as much as 35 years for full recovery. During this time, ecological succession is known to take place replacing grass species with other herbaceous vegetation, followed by scrub species. The spread of invasive species from adjoining scrub habitats would accelerate this process.
The under brushing of scrub vegetation and the removal of Common lantana and other invasive species carried out on an ad hoc basis in the park, per Fernando et al., might have helped to control the spread of scrub vegetation, albeit only marginally, considering the scale and frequency of the operation. In successional habitats, such as the scrub and grasslands, temporal changes occur naturally, but might be affected by anthropogenic factors leading to the spread of invasive species. For instance, a study conducted on the sides of jeep tracks within the park (G.K. Hettiarachchi, M.S.L.R.P. Marasinghe, S. Pilapitiya and M.R. Wijesinghe’s “Would quantitative assessments of adverse impacts of tourism signal the need for improved management? A case study from Sri Lanka”) reports that the density of C. odorata is significantly greater on the sides of intensively used trails in comparison with less used trails, suggesting that human disturbance might have facilitated the spread of this species. Domestic cattle being released into the park also lead to the spread of invasive plants by dispersing seeds (W.O. Odadi’s “African wild ungulates compete with or facilitate cattle depending on season”, J. Chuong, J. Huxley, E. Spotswood, L. Nichols, P. Mariotte and K. Suding’s “Cattle as dispersal vectors of invasive and introduced plants in a California annual grassland” and Sampson et al.). They can also alter the vegetation structure and diversity within an ecosystem, possibly posing an additional threat to the elephants (T.T. Schulz and W.C. Leininger’s “Differences in riparian [relating to wetlands adjacent to rivers and streams] vegetation structure between grazed areas and enclosures” and L. Porensky, R. Mcgee and D. Pellatz’s “Long-term grazing removal increased invasion and reduced native plant abundance and diversity in a sagebrush grassland”).
Concerning the implications such habitat changes have on the elephants and the wider faunal community, among the fauna that would be affected considerably by the habitat changes in the park, is the elephant. The elephant population in Sri Lanka is on the decline and hence the protection of this species is a high conservation priority. It is well documented (P. Fernando and P. Leimgruber’s “Asian elephants and seasonally dry forests” and Alahakoon et al.) that the most preferred habitat of the elephant in Sri Lanka is grassland. According to P. Fernando’s “Managing elephants in Sri Lanka: Where we are and where we need to be”, the elephant feeds on grasses which show dense growth, are easy to gather, and have few secondary compounds. In reservoirs, as the water level drops in the dry season, parts of the reservoir bed get exposed, allowing seasonal grasses to sprout and the elephants are known to, per A.D. Ranjeewa, J. Pastorini, K. Isler, D. Weerakoon, H. Kottage and P. Fernando’s “Decreasing reservoir water levels improve habitat quality for Asian elephants”, gather on the exposed reservoir bed to feed on this grass. In the park, the grass cover in the exposed tank beds would provide a seasonal addition to the elephants food resource, compensating for the loss of habitat owing to the construction of the reservoirs. Moreover, this resource becomes available at a time when the resources in the high grassland areas get reduced owing to the drought conditions.
In the park where elephants are the prime attraction, the considerable reduction and continuing decline in grassland would be a matter of grave concern. Adding to the decline in the grass cover, the grasslands are prone to invasion by exotic species that are not consumed by the elephants (Alahakoon et al.). Apart from grass, elephants are also known to feed on some woody species – e.g. bark of the Bauhinia racemosa [bidi leaf tree which is a medicinal species of flowering shrub], a common native species in scrub habitats (Alahakoon et al.), but high forest and riverine forest are seldom used for feeding (Alahakoon et al., and Sampson et al.). Elephants have also to compete with other herbivores that feed on grass, and the limited extent of grassland and its continuing decline, will seriously impact on all these faunal species. Given this condition, the overgrazing by domestic cattle within the park, which is illegal, aggravates the situation. One of the indirect but important consequences of the dwindling fodder resources within a protected area is the movement of elephants to border villages and to cultivations around human habitations in search of food, which aggravates the human-elephant conflict (Fernando).
The mosaic landscape of the park, while being an important habitat of elephants, also provides refuge to many other vertebrates, some endemic and/or nationally threatened. Of the habitats in the park, grassland recorded the highest species richness for birds and mammals and relatively high species richness for reptiles, according to the department, indicating that the observed decline in grassland cover would be detrimental to many other species besides the elephant. This adds to the importance of recognising habitat management as an important issue for maintaining the integrity of this protected area.
Some of the locally applicable management practices that would be useful for maintaining good quality grassland and increasing its spread are the periodic, controlled burning of selected areas of scrub, and under brushing and removing invasive plants in other areas with fire being shown to be important in the park for reducing the spread of invasive plants and promoting the lush growth of grass (C. Sampson’s “Conserving the land of the giants: Critical Sri Lankan protected areas”). What is important is that these actions, at a suitable scale, should be incorporated into management plans and be carried out systematically.
On a wider perspective, the many development activities carried out in Sri Lanka over the past few decades (the restoration of irrigation reservoirs, the expansion of agriculture, and the establishing of human settlements) have greatly reduced the extents of habitats for the elephant and other species of wildlife. This has been compensated for, to a certain extent, by the establishment of a network of protected areas. Perera et al. showed the need to continually monitor the food resources available to the wildlife in the protected areas, in the present case, to the elephant, and to carry out management practices to conserve and further increase the required resources.