Ashoka Trust for Research in Ecology and the Environment, Royal Enclave Sriramapura, Jakkur PO 560064, Bengaluru, INDIA .
Invasive species have been recognized to be an important threat to biodiversity and ecosystem functioning. I examined the patterns of Lantana camara (hereafter, lantana) invasion and the effects of lantana invasion on native plant communities in a tropical dry forest in the Biligiri Rangaswamy Temple Wildlife Sanctuary (hereafter, BRT), Western Ghats, India. I then examined how patterns of lantana spread are related to factors such as disturbance (e.g., forest fires and historical habitat modification) and habitat structure. In order to link pattern with process, I investigated the role played by soil seed banks as a potential mechanism underlying lantana success in BRT. Lastly, I examined how local people perceive lantana invasions. I tabled results from both traditional ecological knowledge sources and scientific sources about the patterns and processes of lantana invasion and drew out implications for the future management of lantana-invaded landscapes.
Data on the distribution of lantana and native species in 1997 came from an existing study. In 1997, the entire 540 km2 study area was overlaid with a 2-km grid to yield 134 cells, and an 80 m x 5 m plot was established in the centre of each grid cell. I re-visited the same set of plots in 2008 to arrive at the change in the density and distribution of lantana and native species. Additionally, I explored the effects of lantana density on the native woody species community, particularly diversity, abundance, evenness, size class distribution, and relative dominance. Over 11 years there was a tremendous increase in lantana abundance and density. Lantana was present in only 41% of plots inventoried in 1997, but by 2008 lantana had spread to 81 % of all plots. Lantana invasion was accompanied by a reduction in native species density and diversity. Lantana invasion was also accompanied by a reduction in evenness in the native community. In addition there was evidence for drastic reductions in the regenerating size classes of trees, suggesting that tree population declines may occur in the future. Lantana was found to be the most dominant species in BRT.
The BRT landscape has experienced and is experiencing a variety of disturbances. Historical disturbance factors, such as selective- and clear-felling of trees, extraction of bamboo and grass resources, and slash-and-burn agriculture, could have created conditions favorable for lantana invasion. Contemporary disturbance factors, such as roads, and human-caused disturbance (e.g., collection of non-timber forest products or fuel-wood) may also play a role in driving lantana invasions. Lantana invasion could also be driven by an increase in fire frequency, since lantana resprouts in response to fire. Propagule pressure from sites already invaded by lantana could further enhance lantana invasion. Finally, lantana invasion would be influenced by the availability of suitable habitat. I modeled the rapid spread of lantana using explanatory variables such as historical habitat modification, current human disturbance, fire frequency, propagule pressure, and habitat suitability. Using an information-theoretic, model-selection approach, I focused the modeling exercise on the three distinct stages of biological invasion — arrival, establishment, and spread. Lantana arrival was best explained by propagule pressure, rather than disturbance factors or habitat suitability. Lantana establishment on the other hand, was limited by fire frequency. Lastly, I found that lantana spread was influenced largely by proximity to historical disturbance, such as old plantations.
Management efforts aimed at controlling the invasion of lantana and of other potentially problematic invasives are limited by the lack of information on the mechanisms that may enhance their success. The ecology of soil seed banks is one such mechanism. I explore the role played by soil seed banks in enhancing the success of two common invasives in BRT — lantana and Chromolaena odorata (hereafter, Chromolaena). Results from this study indicate that the soil seed bank is saturated with seeds of lantana and Chromolaena vis-à-vis native species. Lantana forms persistent seed banks, implying that for any kind of control, lantana seed output first has to be reduced. Reducing seed output could be achieved by reducing the density of adult lantana plants. Furthermore, repeated removals would have to be continued till lantana seed banks in soil are depleted. However, results from this study also indicate that lantana seed banks are negatively affected by fire. Repeated removals, combined with fire could be explored as a method to control lantana regeneration. Chromolaena, on the other hand, does not seem to be as pervasive as lantana.
Lastly, I explored how resident communities view lantana invasions. An indigenous community, the Soliga, have been residing in BRT for centuries. I used an open-ended interview schedule to solicit Soliga thoughts on lantana invasion in BRT. The Soliga cited three main reasons for lantana spread: its copious fruit output and wide dispersal, the decrease in fire frequency, and the historical extraction of grass and bamboo resources. According to Soliga views, the nature of the lantana-fire relationship depended on lantana abundance. At a low lantana density, the occurrence of early dry-season fires was seen as a way to control lantana from spreading. At a high lantana density, the occurrence of forest fires was seen as beneficial for lantana, since fires were more intense and negatively affected native species due to the additional fuel provided by lantana biomass. The hampering of natural regeneration of native species due to the thick growth of lantana, which, in turn, curtailed native species seedlings from accessing light was also cited as a further contribution to lantana success. The Soliga believed that lantana invasion has had a negative effect on forest composition and structure, and on their livelihoods.
Tabling scientific knowledge with traditional ecological knowledge has led to an improvement in our understanding of lantana invasions in BRT. Some aspects of lantana invasion, such as the role of propagule pressure and response of native tree communities were corroborated by both traditional and scientific sources. However, some aspects, such as the role of fire in lantana invasion were surprisingly contradictory. According to the Soliga, at low lantana densities fire may play a role in limiting lantana, a view contrary to scientific studies that hypothesize a positive relationship between lantana and fire. However, both traditional and scientific knowledge sources agree that fires in high lantana density areas, if they occur, could be destructive for native species. The patterns of lantana spread indicate the serious threat posed by lantana to native plant biodiversity and the structure of this tropical dry forest landscape. Although fire frequency was observed to limit lantana establishment, and also appeared to reduce the density of viable lantana seeds stored in soil, a precautionary approach should be adopted before utilizing fire as a management tool to control or eradicate lantana. Since fires are likely to burn more intensely due to the build-up of lantana biomass, reducing biomass via lantana removal may be required before fires could be used to control lantana.