Landscape Modification and Extinction of Species
There is a growing body of literature including some from Dr. Ruchira Somaweera’s team that lizards adapt to changed environments in ways scientists didn’t previously think of.
Speaking to Counterpoint, renowned scientist, Dr. Somaweera, says it is very common to claim that all change is bad and habitat change will destroy everything. But, he says scientists are convinced otherwise.
Dr. Somaweera, who is currently based in Australia, is a Research Scientist at the Ecosystem Change Ecology (ECE) team of CSIRO and an Adjunct Research Fellow and University of Western Australia, working on the impact of invasive species on freshwater crocodiles in tropical northern Australia. He is also a National Geographic Explorer and co-manages a specialized, science-based wildlife expedition venture–Aaranya Wildlife Odysseys.
The scientist points out that as of August 2019, there were 11,050 species of reptiles in the world. They inhabit all continents except Antarctica.
Landscape modification is a key driver of global species extinction. Thus, understanding how species react to changes is essential for effective conservation management in modified landscapes.
In a brief interview with Counterpoint, on the work he and his research team has conducted, Dr. Somaweera, said they had examined the impact of selected land use patterns on the critically endangered Ceratophora tennentii in the Knuckles mountain range of Sri Lanka, where lizards occupy patches of both natural undisturbed forests and modified plantations – evidently, those with a forest canopy. His research team tested three potential explanations for non-random habitat selection: availability of suitable micro habitat pockets, availability of prey and direct threats from humans. The micro habitat pockets occupied by the lizards were characterised by shade, humidity and the density of perches.
Most lizards were found in mixed cardamom forests followed by natural forests and cardamom plantations, but none were observed in the pine plantations. Food availability showed similar patterns among habitats.
Direct mortality due to human activity did not influence the distribution of this species. Dr. Somaweera says: “Our work indicates that habitat modifications that retain the structural complexity of the vegetation would still permit the existence of the species in densities equal to or greater than that of undisturbed forest patches.”
The study adds to a growing body of literature that signifies the importance of disturbed habitats (intermediate disturbance hypothesis) in protecting threatened species of fauna. It is highly unlikely that some disturbed habitats will ever be returned to their former pristine state in timeframes that are important for species’ conservation. Hence, Dr. Somaweera thinks, attention should be paid to developing suitable approaches to the management and conservation of species across disturbed habitats.
Rapid alteration of natural habitats, driven by the ever-increasing demand for agricultural land, adds a significant component of habitat variability to tropical environments, according to the researchers led by Dr. Somaweera. Human population-driven habitat alteration is a trend particularly evident in developing countries, where more humans are dependent on the environment for basic survival and where land management practices are relatively poor. The team has observed that the loss, degradation and fragmentation of natural habitats through these anthropogenic activities have caused most of the contemporary species extinctions and population declines. Dr. Somaweera said he was of the firm belief that a sound understanding of the determinants of the distribution of organisms through space and time was essential for conservation management of modified landscapes. “Furthermore, given the low availability of resources for conservation, improved understanding of the processes of threat also enables wildlife managers to prioritise expenditure more effectively.”
Dr. Somaweera warns that Sri Lanka has been known for its high endemism as well as the alarming rate at which its natural habitats are being degraded and that steps must be taken to arrest this trend. Among the most biologically diverse but also highly disturbed habitats within the island are the montane forests that once occupied almost the entire area above 1500 m elevation, he notes.
In Sri Lanka, there are more than 220 identified reptiles. More will be identified soon by dedicated researchers, according to senior herpetologists here.
The loss of habitat to less than the critical amount necessary for the survival of the species is by far the most serious threat to all Sri Lankan reptiles, including the lizards. This can be attributed to deforestation, urbanization, agriculture, habitat alteration, forest fires, habitat fragmentation and large-sale development programmes, Dr. Somaweera says.
Early settlements and rice cultivation in the Anuradhapura and Polonnaruwa periods (approx. from 300 B.C. to A.D. 1300) were probably the first to cause a drastic impact on the forests in the region (Abeywickrama, 1956).
Sri Lanka has lost nearly half its forest cover within just half a century, to a current forest cover of 23.5% (IUCN & MOENR, 2007). The loss of forest cover over the past five decades has averaged 30,000 ha. The rate of depletion of forests and wildlife habitats in Sri Lanka is considered one of the highest in South Asia.
Some lizards have adapted their lifestyle, following deforestation, habitat fragmentation and illegal encroachment of wild habitats Dr. Somaweera says. Driven away from their natural habitats these species have established themselves in cultivated lands and home gardens. In the case of agamids, geckos and skinks the situation is clearly visible. However, within these secondary habitats the lizards have become more vulnerable to natural or domestic predators and native opportunistic fauna including domestic cats, poultry, house crows (Corvus splendens), large-billed crows (Corvusmacrorhynchos) and common coucals (Centropus sinensis). Terrestrial and subfossorial species such as Otocryptisspp., Eutropis spp. and Lankascincusspp are more prone to predation by domestic animals.
The application of chemicals (weedicides, fertilizers, insecticides etc.) in agriculture and public health is increasing in the country, the research team has concluded. “This decreases the insect, earthworm and amphibian populations, which are important food sources for most lizards. In certain areas around Naula and Dambulla in the intermediate zone, following the spraying of malathion for malaria mosquitos in the 1970’s and ‘80’s, house geckos have completely disappeared. Other than for effects from chemicals, terrestrial and fossorial lizards associated with cultivated lands are destroyed during field preparation activities. Additionally, as a result of habitat fragmentation by roads, lizards are commonly killed when crossing roads, either during foraging or migrations.”
Dr. Somaweera is of the view that during a gecko study done in the southeastern dry zone of the country, a considerable variation in the species subjected to roadkill was observed. Hemidactylus triedrus lankae accounted for the highest number of gecko roadkill in forested areas, whilst H. frenatusac counted for the highest number within anthropogenic habitats. The high incidence of H. triedrus lankae roadkill is due to its terrestrial behaviour, which is not as common in the other Hemidactyluss species of Sri Lanka. However, it is also true that some agamids (especially Calotes versicolor and Calotes calotes and Otocryptis spp.) benefit from creation of open areas beside roads as feeding grounds and basking sites and certain skinks prefer disturbed, loose soil patches along roads as egg laying sites.
Another negative impact of habitat fragmentation is the isolation of populations. This is particularly evident in the gecko Callodactylodes illingworthrum where the boulder outcrops they inhabit are separated from in-hostile matrices such as chena cultivated lands and extensive grasslands (resulted from deforestation) which are unsuitable for the species, according to Dr. Somaweera. “Human consumption of lizards in Sri Lanka is a minor threat and is largely restricted to the land monitor (Varanus bengalensis). However, some studies showed that the land monitor is one of the most preferred bushmeat among the people in Northern Sri Lanka, especially by pregnant women and hunting has made the species rare in the North. “
The study has found that an average of 66 monitor lizards was hunted monthly in the study areas. Other prevailing threats include the killing of lizards due to myths and poor knowledge (e.g. skinks); to extract oils (e.g. monitor lizards) and due to being considered a pest in agriculture by feeding on tender shoots (agamids). In Sri Lanka, most reptiles have an image problem due to which most people fear and loathe them. To many people, they are worth more dead than alive.
Forest die-back is a silent threat, especially in the tropical moist montane forests of the Knuckles and Horton Plains and researchers think this could be due to the rain and cloud-water acidification. Dr. Somaweera says, noting that no detailed studies have yet been conducted in Sri Lanka to investigate the potential impacts of global climatic change on inland species, including the lizards. However, several observable distribution shifts have taken place in the recent past as certain lizards which were previously rare in the hills (e.g. Varanus bengalensis) are now more or less frequently seen in the wet hills.
Pictures Courtesy Dr. Ruchira Somaweera