Impacts of invasive spotted deer (Axis axis) herbivory on mangrove vegetation in South Andaman Island, India

Abstract

The invasion of spotted deer (Axis axis) in the Andaman Islands, India, is a serious concern for the native flora and fauna of this insular ecosystem. We assessed how invasive spotted deer herbivory is affecting the regeneration and vegetation composition in the mangrove forest of the South Andaman Islands by comparing sites with and without herbivory pressure. Vegetation data was collected across 18 sites (ten sites with herbivory in the Mahatma Gandhi Marine National Park, and eight sites without herbivory (control) in South Andaman). We used circular plots (7m radius) along linear transects for data collection (125 plots each for herbivory site and control site). The results highlighted a strong influence of herbivory on mangrove vegetation and the impacts varied across the three mangrove zones (landward, ecotone, and seaward). Control sites consistently exhibited higher species richness and abundance across all categories (tree, sapling, and seedling) except for seedling abundance. Herbivory disturbance varied significantly along the three zones, (p < 0.05) with the landward zone being the most affected, followed by ecotone to seaward zones. Invasive spotted deer herbivory altered the vegetation structure and recruitment, especially in landward mangroves, leading to shifts in mangrove distribution and the emergence of the least palatable Ceriops tagal as the dominant species. Management and policy-level interventions are an immediate requirement for the removal of invasive spotted deer in the Andaman Islands for conserving mangroves and marine natural resources unique to the island ecosystem.Invasive species significantly threaten local biodiversity, disrupt ecological functions, and outcompete native species (Mooney & Hobbs, 2000;Courchamp et al., 2003;Beltran et al., 2014). This negative interplay could lead to cascading impacts such as habitat loss, soil erosion, nutrient loss, reduced recruitment opportunity, and degradation of the whole ecosystem (Simberloff, 2011;Beltran et al., 2014;Russell et al., 2017). Particularly, the introduction of invasive herbivores can lead to changes in vegetation composition and a decline in species richness in the long term (Coomes et al., 2003;Webster et al., 2005). For instance, the invasive ungulate (Sus scrofa Linnaeus, 1758) introduced in the Great Smoky Mountains National Park, USA, altered the native vegetation through predation, habitat damage, competition, and disease transmission (Webster et al., 2005;McClure et al., 2018). The impact of invasive herbivory species on the island ecosystem is especially detrimental as most of the island species have evolved without intense competition due to herbivory and predation (