ANT-MEDIATED SEED DISPERSAL

Dispersal is a fundamental aspect of nature. Organisms disperse for the simple reason that life, on average, is better the further away you get from your parents (sorry mom). The appliances of dispersal are evolution's means of achieving slightly longer dispersal distances or, in many cases, dispersal to slightly better sites (sites where it is easier to have more children, grandchildren and so on). We are familiar with the burrs that fasten to us as we walk and hitch a ride, the awns of grasses to catch the wind or the fruits that sweeten our breakfasts but which were intended as rewards for the vertebrates that might carry them. But there are other, stranger ways of getting around. Some mites ride in the nostrils of hummingbirds to get from place to place. Other mites ride beetles. Bird lice ride from bird to bird clinging to hypoboscid flies. And then, as strange as any of these, we find that many thousands of plant species of more than a hundred different plant lineages are carried from the foot of their parents to (slightly) more distant locations in the mouths of ants. A major focus of my lab group is to understand the relationship between ants and seeds.

Consequences of Seed Dispersal by Ants

We began our work on ants and seeds to understand whether short-distance dispersal of seeds by ants has contributed to the high diversity of the Kwongan heaths in Australia, Fynbos in South Africa and understory plants of the Eastern U.S. Causally, in this project founded by the Australian Research Council, we seek to understand how dispersal mode affects dispersal distance, how dispersal distance affects speciation rate and geographic range size frequency distributions, and how speciation rate and range size distributions relate to continental patterns of species richness. In Southwest Australia perhaps a third of all plant species are dispersed by ants. Ants carry the seed through the heathland landscape (pictured at right) and deposit the seed in a garbage pile in or near their nest (photo to the left). The seed is carried by a fleshy appendage called the elaiosome which is later removed and fed to the larvae. In addition we seek to understand the selective pressures that have favored elaiosomes in particularly ecological regions, such as the heathlands of southwestern Australia.

One of our projects in Australia attempts to link aspects of local dispersal of seeds by ants to large-scale patterns of diversity. Aaron Gove (Curtin University), Jonathan Majer (Curtin University), Tim Barroclough (Imperial College) and I are studying whether the evolution of ant-mediated dispersal leads to smaller geographic ranges and faster speciation rates. Because ant-mediated dispersal has evolved more than a hundred times it has allowed us to compare the net speciation rates of ant-dispersed plants and plants dispersed by other means. In addition, we can compare the conditions under which the evolution of ant-mediated dispersal is favored and the extent to which ant community diversity, abundance and composition structure plant communities.

Ecology and Evoluton of Seed Dispersal by Ants

Recently, we've discovered that seed dispersal by ants in Australia is disproportionately due to ants of one genus, Rhytidoponera (Gove et al. 2008, Oecologia). Neil Mccoy, a masters student in my lab, is following up on this work to understand the dynamics of the relationship betweenRhytidoponera metallica and Rhytidoponera violacea and seeds and then also the main drivers of the occupancy and abundance of Rhytidoponerapopulations.

Judith Canner, a PhD student in the Biomath department at NCSU is working understand when plants should specialize on particular ant partners and when it is more advantageous to attract a diverse group of ant species. Judith's work will include both a strong modeling component and field experiments with local ants and ant-dispersed plants in North Carolina. Judith's recent work has focused on understanding what happens to seeds after they are carried into ant nests, a neglected stage in myrmecochory.

Benoit Guenard, a PhD student in my lab, is collaborating withJudith Canner,and Neil McCoy to understand the consequences of the invasion of Piedmont forests of North Carolina by the nonnative ant Pachycondyla chinensis (the Asian Needle Ant) for dispersal of native myrmecochorous plant species.

Seed Dispersal by Ants and Local Gene Flow

Finally, in a project with Byron Lamont (Curtin University), Neal Enright (University of Melbourne), Sieggy Krauss (King's Park Botanic Garden) Tianhua He (King's Park Botanic Garden) and Ben Miller (Curtin University/University of Melbourne), we are using AFLP and Microsatellite markers to characterize differences in patterns of maternal and paternal gene flow among ant and wind dispersed plant species restricted to island-like dune crests in the Eneabba sandplains of SW Australia. Ultimately we hope to compare the patterns of gene flow of ant and wind dispersed species and to estimate the frequency of long-distance seed dispersal events. Toward this end, we are also now developing models of seed dispersal of ant-dispersed seeds that consider the relative importance of dispersal of those seeds by their primary vectors, ants, versus occasional longer distance dispersal events by emus, which in separate work we have found to occassionally consume many "ant dispersed" seed species.

The story of Myrmecochory (Jaws of Life, Natural History 2005)

"I displaced a rock in Tennessee. Underneath, huddled at one edge of the exposed dirt, was a colony of ants. The slender ants moved slowly in the cool spring morning, and I had a long look at them before they vanished down their hole. The queen was fat and glamorous. Around her were tiny, silver eggs, chubby larvae, and pupae folded like mummies inside translucent cases. At the edge of the colony, surrounded by a small pile of ant garbage— heads, legs, and other shiny but unidentifiable parts—were two seeds. How had the seeds found their way into the ants’ tunnels? With patience and the fortitude to sit still when an ant clambers over you, you might easily learn the answer for yourself. The first thing to do is to turn over the next rock, or poke into a nearby log. Rocks and logs are windows into the secret life of the soil; the views are fleeting but, at least to children and biologists, mesmerizing. Over many seasons of rock-turning... (PDF)