Introduced more than a century ago, Prosopis juliflora, an invasive alien species (IAS), has colonised India against all odds of geographic variability. The vilayati keekar (Prosopis juliflora) is the most dominant species in India’s arid and semi-arid tracts, especially in the states of Gujarat, Rajasthan and Haryana. Aravalli, the oldest mountain range in India, cuts diagonally across these states, running from the southwest to the northeast. In a regime where biodiversity is closely adapted to the arid environs, Prosopis juliflora occupies extensive patches, threatening to oust the endemic flora. Native plant diversity is relied upon not only for cultural and recreational reasons but also because they act as a store of carbon, enable sustainable water management and house diverse biological species.
Prosopis juliflora or Vilayati Keekar
Prosopis juliflora is one of the most extensively found species in the Aravalli, especially in its northern and central trails in the states of Haryana and Rajasthan. However, knowledge on controlling and eradicating IAS is still limited, which has resulted in numerous problems including the extinction of endemic and threatened species (Sandilyan and van’t Klooster, 2016). This is likely to create ecological problems with severe socio-economic implications in days to come (Reshi and Khuroo, 2012) unless there is an attempt to control it through a biocontrol agent.
So far, IAS related issues in India have not received the attention that they deserve; scientists’ and policy makers’ knowledge on control and eradication of IAS is limited (Adhikari et al., 2015). For instance, several control methods have been applied to eradicate and control IAS plants such as Prosopis juliflora, Lantana camara, Parthenium hysterophorus, Eichhornia crassipes and Mimosa diplotricha, which have proved futile so far. In fact there is a history of wrong testing in India. It is, therefore, imperative to find alternative ways for better management and eradication of IAS in India.
A recent (2016) study by the University of Adelaide, Australia has revealed that a native parasitic vine, ‘Devil’s twine’ (Cassytha pubescens) has effectively controlled invasive plants such as European gorse (Ulex europaeus) and Scotch broom (Cytisus scoparius). The role of native Devil’s twine in controlling European gorse is a major breakthrough in IAS management in Australia. Interestingly, in some parts of southern India, infestations of C. reflexa on Prosopis juliflora and L. camera were noticed. All three plants are alien to India. During field observations, it was noticed that C. reflexa gradually covered the entire canopy of Prosopis juliflora (Fig. 1a). After two months of infestation, the leaf and twigs of the canopy developed blight and canker while the growth was also stunted as compared to the non-infected Prosopis juliflora species nearby (Fig. 1b & c). As the infected vilayati keekar died, C. reflexa on the canopy died too—but its offshoots extended its territory and infected other plants consequently (Fig. 2).
Earlier studies had pointed out that C. reflexa had developed a highly efficient absorption system that allowed the species to suck resources such as water and amino acids from the host. Moreover, the parasitic plant can grow about 7 cm/day and can cover 3 sq m in a growing season. Also, it can easily extend its range rapidly with a dense vegetative mat of intertwined stems (Lanini and Kogan, 2005). It was also reported that C. reflexa transmits viral diseases to host plants in addition to affecting the seed germination process of the host (Mishra, 2009). These characteristics make it highly suitable as a control measure for Prosopis juliflora. Known for high seed production, efficient dispersal mechanisms, fast growing ability and resistance to browsing of cattle, the vilayati keekar is extremely difficult to eliminate. It is worth mentioning here that the leaf litters of Prosopis juliflora lead to heavy seed mortality among the native species such as babool (Acacia nilotica), Indian mustard (Brassica juncea), windmill grass typically found in arid areas (Chloris dolichostachya), Indian rosewood (Dalbergia sissoo) and khejri, scientifically known as Prosopis cineraria (Kauret al., 2012).
Studies have revealed that Prosopis juliflora is one of the worst IAS species in India, having the potential to successfully survive in hostile climatic conditions and establish themselves in several vital habitats including forests, wetlands, bunds of crop fields and wastelands (Jayshankar, 2009). It becomes imperative at this stage to investigate the role of C. reflexa as an effective biocontroller.
However, control of IAS is not an isolated event. It requires a long-term effort, aided by constant monitoring and investigation (Bibhuti et al., 2011). Preliminary observations clearly established the biocontrol potential of C. reflexa. However, in order to confirm and quantify the biocontrol efficiency of C. reflexa on Prosopis juliflora, field trials in different climatic zones and habitats are needed. Universities and colleges need to proactively award such projects to post graduate and other students engaged in research who can evaluate the efficacy and viability of the biocontrol agent. It needs to be understood that the success of a biocontrol agent is very difficult to foresee; quite often they do not operate in the predicted manner. But if they do, they become the most effective agents to control invasive populations. Before the introduction of a biocontrol agent into a new habitat, international guidelines need to be adopted which entail the use of precautionary measures. The Convention on Biological Diversity, recommended the following for a safe biocontrol practice (COP 13, 2016).
The potential for direct and indirect non-target impacts on the ecosystems, habitats, native species and related human health and safety in the area where the biological control agents are planned to be released.
The potential for indirect non-target impacts on the ecosystems, ecosystem functions and services, human health and safety and social, economic and cultural values in the areas where the biological control agents are planned to be released and in areas in which they might spread.
The potential influence of climate and its current and future variability and other sources of environmental variation on establishments, spread and impact of the biological control agent.
The risks to ecosystem functions and services, social, economic and cultural issues, including the values and priorities of indigenous peoples and local communities.
Apart from the aforementioned, occurrence and distribution of threatened, endemic and keystone species in that habitat is also essential.
It emerges from the preceding discussion that there is a growing need amongst the research community to focus on IAS, as these are major drivers of global environmental change. It has also been revealed that climatic variability in India helps in accelerating biological invasion (Adhikari et al., 2015). Despite this, there is little attention to the study of IAS.
Limited studies on the infestation of C. reflexa on Prosopis juliflora suggest that the former exhibits an efficient mechanism that can help control the negative impact on native plant diversity, subject of course to further field trials and validation. If proved successful, C. reflexa can be promoted as an effective biocontrol agent to stop proliferation of invasive species in the Aravalli as also the rest of
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