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Plants get a helping hand from ants
Seed dispersal is a vital stage in the life history of all plants — not only ensuring the spread and survival of a species, but helping to shape the floral landscape. We usually think of seed dispersal as the movement of seeds by wind, water or vertebrates. Some plants have evolved an ingenious way of getting their seeds moved around using ants: myrmecochory.
Tasty rewards for ants
Plants exploit these industrious creatures by producing seeds which have an oil-rich appendage called an elaiosome. Chemicals within the elaiosome attract the ant, encouraging it to carry the seed back to the nest, where the elaiosome is removed and fed to larvae and the undamaged seed is discarded in or nearby the nest. This provides the ants with a food source and the plants with an effective dispersal mechanism, together with the added advantages of being hidden from predators, buried in a nutrient-rich ant nest, and protected from the intense heat of bush fires in some habitats.
Cape Floristic Region
Myrmecochory occurs globally in at least 11,000 plant species in 77 families. It is likely to be an important driver for the creation of new species in several regions of the world and so of considerable evolutionary importance. One of the best examples of this is in the South African Cape Floristic Region, home of the iconic Fynbos flora. Many native genera of this region rely solely on ants for their dispersal, like the pincushion proteas (Leucospermum).
An enemy underfoot
Unfortunately, this delicate ant-plant mutualism is at risk from the notorious, invasive Argentine ant (Linepithema humile). This species was brought to South Africa in the early 1900s during the Boer War in horse fodder from Argentina, and has since managed to establish in some areas of pristine Fynbos.
Argentine ants quickly displace keystone, seed-dispersing, native ants such as the pugnacious ant (Anoplolepis custodiens) and big-headed ant (Pheidole capensis). While this battle for supremacy may take place unnoticed by the average passer-by, over the long-term its effects may become clear, as the structure of the floral landscape begins to change due to the disruption.
With this threat in mind, part of my PhD research is to evaluate the seed-dispersal ‘palate’ of the two different communities: invaded vs natural. The aim is to understand the short-term effects that Argentine ants are having and consequently the long-term effects on plant species prevalence, as this may help identify those species that might be at significant risk of extinction and should be prioritised for collection by the Millennium Seed Bank Partnership.
The good, the bad and the downright frustrating
Our study focused on the Jonkershoek Nature Reserve, which has played host to the invasive Argentine ant since the early 1980s.
Step 1: Understanding ant community structure
To find out what was moving in and around our designated sites, we set up an elaborate series of pitfall and bait traps. As we suspected, there was a noticeable difference in both ant community structure and activity, with invaded communities suffering at least a threefold decrease in species abundance and richness.
Step 2: Gathering seeds for upcoming trials
Working with partners at the University of Cape Town and Kirstenbosch Botanical Gardens, seed from a number of species was collected across a range of plant genera, both native and invasive. Low seed set in many native plant species that rely on ant dispersal made this a frustratingly difficult task. This problem is not shared by the invasive counterparts, such as Port Jackson willow (Acacia saligna), which produces thousands of seeds, each with their own highly attractive elaiosome.
Step 3: Cafeteria experiments
Seeds in hand, we set out to test the preference and removal rate for these ant species in our invaded and native ant communities with a series of seed hubs. Each seed hub consisted of a card with a selection of seeds placed on top and covered with a wire mesh dome to discourage rodents from stealing the seeds. Each seed hub was monitored over several hours, both day and night. Preliminary results showed that the communities’ ‘palate’ varies considerably: larger Proteaceae and Restionaceae seeds were not removed in invaded sites.
Step 4: Nest excavations
While two different ant species are attracted to and remove the same type of seed, the fate of the seed might be considerably different. Some ant species quickly abandon the seed on the soil surface after removing the elaiosome, whereas others take it down into the safety of the nest.
To evaluate seed fate, a series of nests across a range of ant species were excavated using wax. This allowed us not only to identify what seeds are present within the nest, but also at what depths they are buried. Although, sadly, this method is destructive to the ant nest, it was necessary to be able to identify the fate of seeds belonging to a number of important plant species.
Interestingly, only nests of the pugnacious ant and small pugnacious ant (both species excluded from invaded zones) had nest entrances wide enough for a number of the important large-seeded plant species, such as the tree pincushion (Leucospermum conocaprodendron).
The story so far…
These experiments have shown that fewer seeds are removed in invaded sites. It is worrying that a number of large-seeded native plant species in invaded areas are not being dispersed at all. Even more concerning was that while removal of native seeds varied between invaded and non-invaded sites, there was no significant difference in removal of invasive seeds. This hints at a possible synergy between invasive ants and invasive plants, and the results paint a rather depressing picture for the seed fate of a number of native South African plant genera under Argentine ant rule.
On a more positive note, follow-up experiments scheduled at both the University of Bristol and Kew’s Millennium Seed Bank may help to further unravel this intricate relationship. The end game is to identify plant species at greatest risk and in potential need of in-situ and ex-situ conservation.