Interactions between meat ants (Iridomyrmex spadius) and bellyache bush (Jatropha gossypiifolia)

Abstract

Understanding the dispersal of weed species is important for the development of effective control strategies. In this study, a series of experiments was conducted to clarify the role that meat ants (Iridomyrmex spadius) play in dispersing bellyache bush (Jatropha gossypiifolia), an exotic shrub currently invading the rangelands of northern Australia. The nutrient composition of food [lipids (fatty acids), fat and soluble carbohydrates] provided by bellyache bush seed components [caruncle, exotegmen and seed (without caruncle and exotegmen)] was identified. Seed components were rich in lipids, particularly palmitic, oleic, stearic, linoleic and eicosenic acids. Oleic and palmitic were most abundant in the caruncle (30% each), linoleic in the seed (61%) and palmitic in the exotegmen (36%). Over all seed components, fat concentration was relatively high (6.3%) compared with soluble carbohydrates (2.3%). The impact of feeding was then determined by comparing germination and viability of intact, non-carunculate (caruncle manually removed) and ant-discarded bellyache bush seeds. Feeding by meat ants significantly increased seed germinability, whilst having no adverse affects on viability. The quantity of seeds dispersed and the seasonal pattern of dispersal was recorded by collecting seed from the middens of randomly selected meat ant nests on a monthly basis. On average, 12 330 ± 603 seeds were retrieved from the middens of individual meat ant nests over 12 months, with highest numbers recorded between February and June (>1200 seeds/ant nest). The effect of this dispersal was determined through comparisons of plant densities within core infestations of bellyache bush, meat ant nest middens and pastures located directly adjacent to core infestations and that were being invaded primarily through localised ballistic dispersal. The density of bellyache bush plants growing from the seed reserves within middens averaged 79 plants/m2, just 18% less than that within core infestations. Seedling survival (1 year) and growth within core infestation and meat ant sites was also quantified. The middens of meat ant nests provided an environment conducive to higher seedling survival and faster growth rates than occurred within core infestations. Mutualistic interaction between bellyache bush and meat ants is likely to build local ‘infestation pressure’ that may be conducive to range extension in years of exceptionally wet seasons. Management of seed dispersal by meat ants may reduce that risk.

Understanding invasion history: genetic structure and diversity of two globally invasive plants and implications for their management

“…Seeds are produced in a capsule, each containing three to four orange‐brown seeds. Capsules dehisce explosively with most seed falling within a few metres of a parent plant (Bebawi & Campbell, 2004). Long distance dispersal (hundreds of metres) may be mediated by meat ants (Bebawi & Campbell, 2004).…”

Invasive ants in Australia: documented and potential ecological consequences

“…eating the eliaosome but not dispersing the seed. In the case of several weed species in Australia that have ant-dispersed seeds (Piggin 1982;Brown & Brooks 2003;Bebawi & Campbell 2004), cheaters would be preferable to an ant assemblage that facilitates seed germination.…”

Seed Dispersal Distances by Ants Increase in Response to Anthropogenic Disturbances in Australian Roadside Environments

“…Whitney (2002) analyzed the density of discarded seeds from Iridomyrmex nests (sometimes up to 25 m), and recorded a high seed viability ratio (40%). Furthermore, middens of Iridomyrmex nests were reported to provide environmental conditions suitable for seedling growth, and facilitate range expansion of plant species in northern Australia (Bebawi and Campbell, 2004). Other species can also discard seeds from their nests (Beaumont et al, 2013), sometimes with the elaiosomes remaining intact on discarded seeds.…”