Seed dispersal by ants : behavioural and chemical ecology of a mutualism

Gammans, Nicola R (2005) Seed dispersal by ants : behavioural and chemical ecology of a mutualism. University of Southampton, Doctoral Thesis.

Abstract

I investigated the interaction between seeds of gorse (Ulex minor and U. europaeus) and the ant Myrmica ruginodis, with the aim of discovering whether this interaction is a mutualism. Firstly I investigated whether elaiosome chemicals attract M. ruginodis to disperse the diaspore, and if so, whether they also attract a granivorous ant species Tetramorium caespitum. Our results showed M. ruginodis are attracted from a distance to the diaspore by a lipid cue, whereas T. caespitum do not respond to any cues. This implies a mutualistic ant would find the seeds before a granivore, having great benefits for the Ulex seeds. To investigate the ant benefits within the mutualism, I measured the effects of elaiosome consumption on M. ruginodis colony fitness. Results showed larvae fed with elaiosomes increased in number by 100% compared to a control, and the weight of larvae increased by 50%. Chemical analysis of the elaiosomes showed the presence of nutrients known to be essential to the ant’s diet, which can only be obtained from plant matter.

I then investigated the possible benefits to the plant. I found seeds, which had had their elaiosome removed by the ants, germinated faster than controls. I also tested ant nest soil compared to control soil and found the former contained higher levels of available nitrogen, phosphorus and potassium. This is of great importance for the plant as heathland soil is very nutrient poor. Also seed dispersal was considerably enhanced by ants, in particular for U. minor. Finally I found that ants take a significant number of seeds over a 24 hour period, reducing the number available to a seed predator the wood mouse, Apodemus sylvaticus.

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