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Seed – Ant Interactions Experiments

Funding
Thiess Pty. Ltd and The Faculty of Science & IT Summer Scholarship (Matthew Hamilton).

Background
When direct seeding native plant seed onto a substrate for rehabilitation, it can be partially or even totally removed by ants. It was previously thought that once in the nest, ants removed the aril (growths on seeds relished by ants and birds) from seeds and discarded the rest of the seed outside the nest, leading to seed dispersal. If that were the case, ants would not collect seed that had their arils removed. Also, it was not known if the practice of hydrating some seed before sowing would affect removal rates.

Aim
To compare seed removal rates of three plant species from Petri dishes in a spoil rehabilitation area and in a remnant forest reference site. Also to determine whether seed hydration and aril removal influenced predation rates.

Summary of Results
Total seed removal for three species (Pultanaea retusa, Hardenbergia violacea, Acacia parvipinnula ) was studied in the forest and on the spoil rehabilitation area. 80% of seeds on the rehabilitation area were removed irrespective of species within 2 hours (below). In the forest there were still many seeds in the dishes at the conclusion of the study (5 hours).

In a second experiment, seed was placed in Petrie dishes: dry, hydrated, or with aril removed, in the forest and on the spoil rehabilitation area.

Results for Pultenaea retusa are shown below. On the spoil, irrespective of treatment, all seeds were removed. In the forest, seeds were removed more slowly, with those that had their arils removed slowest of all.

The reasons for these differences are mainly the ant species assemblages. On the rehabilitation area large generalist ants were present in large colonies and had no difficulty carrying away even the seeds without arils (ants will preferentially grab the seeds by the aril to carry them). In the forest, many more ant species were present but most of them were smaller and colonies were less dense than on the rehabilitation area. Some ants from different species did not forage in the same areas, leading to some plates being untouched if the dominant species was not interested.

No evidence of seed being discarded out of the nest was observed, indicating that while some dispersal of seed may occur, when seed consumers collect the seed it is generally eaten.

Conclusions
Ants can have a significant impact on the success of a rehabilitation plan through direct seeding. Methods can be developed to avoid this. For example, by choosing a time of year to seed when germination is possible but foraging activities of ants are lower or focused upon other food sources. Alternatively, ant repellents can be trialled as seed coatings to deter predation. Ant assemblages in both areas and ant seed consumption activities need further study.