Changing Shades of Green

by DJ and USW

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Picture taken in 2011 shows scattered trees and grass species in between

If your garden is left unattended it grows wild after several seasons. Eventually, there will be a mini-forest of a few species carefully chosen by a natural process. This is an ecological succession. The same takes place in the wild. Once a habitat is disturbed but then left undisturbed by humans, it goes through a series of structural changes in the vegetation with time.

Uda Walawe National Park was declared in 1972 following the establishment of Uda Walawe reservoir for generation of hydro-electricity. The park acquired areas of human settlements, timber plantations, mature forests and scrublands. It subsequently played an important role as a catchment of the reservoir and a wildlife refuge. A substantial area of newly declared park was consisted of abandoned slash-and-burn (chena) cultivations.

Traditionally in Sri Lanka, cultivated and then harvested chena plots are abandoned for another 5-10 years before burning it again. During the period chena goes through ecological changes starting from grasses and herbs to young trees of less lignified tissues. If disturbed again, the chena will never grow into a mature forest but the structural changes already in the ecosystem over time support an array of species diversity. It is identified that many herbivores are benefited by early stages of these changes and particularly elephants for a great extent [1].

Until late 2000s, Uda Walawe’s abandoned chena cultivations had biomass of grasses (predominantly Panicum maximus or Guinea grass, an introduced grass species of invasive nature but in this case encouraged as herbivore fodder) and many herbs which contained less cellular deposits like lignin. Elephants enjoyed grazing in these plots specially after monsoon showers. Presence of year-round grasses and water in the park favoured the elephant populations and in 2011 we estimated about 1000 elephants used the park seasonally [2].

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Panicum maximum (foreground) used to nourish elephants of Uda Walawe

 

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Elephants in their version of heaven in 2008

However, the primary vegetation Uda Walawe was making a comeback through ecological succession and the grasslands gradually started changing into young forests with species like Bauhinia racemosa (Bidi leaf tree), Cassia fistula (Golden shower tree), Ziziphus mauritiana (Chinese apple) and Flueggea leucopyrus (Bushweed). Concurrently, Lantana camara (wild-sage) took upper hand in many parts of the national park, including changing grasslands. It is known to be an invasive species in South Asia and is propagated through seed dispersal by perching birds.

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Lantana camara shows the widest spread in Uda Walawe replacing elephant food species (2016)

 

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Eupatorium, a bush not readily consumed by elephants, is widely spread in the park

Young forest species don’t meet elephants’ food demand and they are not as nutritive as the grass and herb species of primary succession. Further, elephants have to compete with the domestic cattle inside the park for remaining grazing grounds. Subsequently in mid 2010s wild elephants of Uda Walawe started exhibiting poor body conditions. Our studies continue to monitor demographic changes of the population along with the changing green shades of the park.

There is a school of thoughts that fire management can be a solution for non-elephant-friendly vegetation changes in Uda Walawe. Annual fires regenerate grass and tender herb species. That is a suggestion to interfere with the natural ecological succession in the park and to set it at square one! It could be a strategy to sustain herbivores for another few years, but conservation doesn’t mean protecting only a few chosen species – the total ecosystem deserves attention as a whole. The questions for Uda Walawe are practical ones – what’s the right balance, and what risks do we incur in altering them?

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Elephants with lower body condition as seen in 2016

References

[1] Pastorini, Jennifer, et al. “A preliminary study on the impact of changing shifting cultivation practices on dry season forage for Asian elephants in Sri Lanka.” Tropical Conservation Science 6.6 (2013): 770-780.

[2]de Silva, Shermin, Ashoka DG Ranjeewa, and Devaka Weerakoon. “Demography of Asian elephants (Elephas maximus) at Uda Walawe National Park, Sri Lanka based on identified individuals.” Biological Conservation 144.5 (2011): 1742-1752.

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