Invasion biology and Ant-plant systems in Australia

Abstract

Introduction Australia is a continent known for its ecological idiosyncracies. It goes well beyond the abundance of furry animals with pouches and diversity of creatures that can kill you. Australia's geologically long period of geographic isolation has translated to remarkable floral and faunal endemicity; some 92 per cent of its vascular plants and over 80 per cent of its frogs, reptiles, and mammals are found nowhere else (Chapman, 2009). It spans 35 degrees in latitude, and although much of it is desert and xeric shrublands, it also comprises seven other ecoregions including Mediterranean forests and woodlands, temperate grasslands, and tropical and subtropical broadleaf forests (Environmental Resources Information Network, 2012) that also contribute to its plant and invertebrate diversity. It is the flattest and driest of the continents, and its old soils are notably nutrient poor (Orians & Milewski, 2007). What does any of this have to do with ant-plant interactions? These nutrient-poor soils have at least two consequences that affect ant-plant interactions. Low nutrient soils are more likely to have shrubby plants that depend on ants for seed dispersal (Westoby et al., 1991a; Berg, 1975). Australia is one of the three hotspots for myrmecochory (Lengyel et al., 2010) harbouring an estimated 32 per cent of myrmecochore genera (Warren & Giladi, 2014; Chapters 5 and 6). It is common for 30–50 per cent of a site’s flora to be ant-dispersed (Westoby et al., 1991b). Second, plants growing in nutrient-poor soils are limited in nitrogen and phosphorus and have an abundance of carbon. Plants can expend this excess carbon by producing lots of nectar (floral or extrafloral) and/or hosting of honeydew-producing insects (Orians & Milewski, 2007). The availability of carbohydrate-rich resources supports the extraordinarily high local ant productivity in Australia’s arid regions (Andersen, 2003) and plays an important role in shaping its rainforest ant assemblages (Blüthgen et al., 2004). The ant diversity and endemicity of Australia are also notable. In most regions of the world, higher ant species richness occurs in rainforest than in arid regions. The opposite is true in Australia (Andersen, 2003; AntWiki, 2010). Local richness of often greater than 100 species per hectare in the arid zone also far exceeds that in similarly arid climates (Andersen, 2007; Andersen, in review).