Not only are there plants that are protected from herbivory by ants , there are plants with a mutualistic relationship to ants for their seed dispersal. Readers who live in the eastern deciduous forest may know some of the plants: Erythronium americanum, Sanguinaria canadensis, Asarum canadense, Dicentra canadensis, Viola species, and Claytonia virginica. Many of these species are shade-tolerant, and their leaves persist in mature woodlands throughout the growing season. Several members of the rich woods flora are herbaceous plants called spring ephemerals. Their whole life cycle – from emergence to dormancy – is completed in the short span of time between the first weather for active pollinators to full leaf-out and canopy closure.
Within the sugar maple/white ash/basswood mesic forest association there are habitats with the favorable bedrock (limestone predominates), aspect (SE to NE), landform (slightly concave slope), soil (neutral to high pH and evenly moist), and land use history (no cultivation) . Because the growing conditions are so good here, plants thrive, resulting in more species richness than most other habitats. And in these rich woods 30 to 40 percent of the species will be myrmecochores – having seeds dispersed by ants. (Matlack 1994). The ant species doing this work are the foraging type: Aphaenogaster, Myrmica, Formica, and Lasius (Handel et al 1981).
The ants are interested in the tissue rich in lipids (eliaosome) which is attached to the outside of the seed. They harvest the seeds and carry them to the ant nest.
After eating the eliaosome, they discard the intact seed into a refuse pile within the nest – a perfect environment that protects the seed from predation and fosters germination. No surprise that researchers have found the highest density of ephemerals near tree bases or decaying logs where ants prefer their nests (Thompson 1980).
Needless to say, seeds dispersed by ants do not move very far in any one year. Using historic records to determine whether a woodland was previously cultivated, grazed, or logged, Professor Henry Art and his biology students at Williams College have found that colonies of various myrmecochores expand at a rate of no more than 1-3 meters per year. Their research and my own has found that refuges within formerly cleared lands were important seed sources…places that were too steep to have been plowed, niches at the base of stone walls where grazers or mowers could not reach, strips of woodlands separated from pastures or hayfields by wetlands too soggy to cross. (Parnall 1998). Small colonies of ephemerals survived in these refuges long enough for an abandoned field to be repopulated. In the case of land that has been plowed, however, the seed bank has been destroyed and the soil profile disturbed, so recovery takes several decades.
Dzwonko and Loster (1989) found that older woods had more ant-dispersed herbaceous species than young woods; that numerous patches of woodlands within agricultural uses had more richness than isolated patches; and that where there is more species richness, there is higher probability of finding rare species – meaning that the rare species they found were only in old or ancient woods.
South African myrmecochores:
The South African fynbos is an area of world-class floral richness within the Cape Floral Kingdom. Many species exist only in this area of the world, and some of these are only in very small populations. Researchers in this fire-adapted landscape reported myrmecochores in almost every plant genus in the Proteaceae family, resulting in the highest frequency of myrmecochores in the world. Seeds from these plants, stashed underground in ant nests, would have a better chance of surviving the natural ground fires. Of the shrub species they studied, they found that winged, fringed, hairy, and nut seeds were dispersed fairly far from the parent plant which could establish well-separated new populations after fire. The ant-dispersed seed, however, would not travel so far and would serve to fill in some of the gaps. Pierce and Cowling (1991) noticed that sometimes ants in the dune fynbos community do indeed eat the seeds and not just the eliaosome. They suggest that this unreliability for restoring particular plant species from the seed bank after fire actually contributes to species richness, amounting to species taking turns with successful regeneration,resulting in no particular species able to dominate to the detriment of others.
Preservation and restoration implications:
As designers, foresters, and landowners in areas where ant-dispersed plant species, and especially spring ephemerals, can be found, there are implications for planning changes on the land. Since evidence of ephemerals disappears from the flora by mid-June, such a woodland might look very sparse and ‘uninteresting’ in the ground and shrub layer in other parts of the year. We would do well to pay attention to the tree species, bedrock, and soil type, which will predict for us whether there might be a trove of beautiful woodland ground flora worth preserving. Or have the patience to wait until spring to see what emerges. Where new home building is the project in rich woods, good site planning that ensures a small footprint of disturbance is essential. These plants are better preserved in situ, because replenishment from nursery stock is unrealistic (both unavailable and expensive).
Selective logging (not clearcuts) is not necessarily harmful to these populations, but the cutting must take place when the ground is frozen. For the educators among us, any displays showing pollinator gardens should contain more than the typical meadow wildflower species. The spring ephemerals belong there too, since some are obligate to rare butterflies and many are essential to the earliest bumblebees and flies. And any landowner with rich woods would be well advised to leave fallen logs in place to decompose and favor ant habitat, encouraging ants to plant a forest garden unequalled in beauty.
Bond, William. South African National Biodiversity Institute, Kirstenbosch, Cape Town. protea.worldonline.co.za/watch.htm
Dzwonko, Z. and S. Loster. 1989. Distribution of vascular plant species in small woodlands on the Western Carpathian foothills. Oikos 56: 77-86.
Handel, S.N., S.B. Fisch, and G.E. Schatz. 1981. Ants disperse a majority of herbs in a mesic forest community in New York State. Bulletin of the Torrey Botanical Club 108 (4): 430-437.
Matlack, Glenn R. 1994. Plant species migration in a mixed-history forest landscape in Eastern North America. Ecology 75 (5): 1491-1502.
Parnall, Ruth. 1998. Vegetation and land use history of nine mesophytic forest stands in western Franklin County, MA. Masters thesis, Dept. of Botany, CT College.
Pierce, S.M. and R.M. Cowling. 1991. Dynamics of soil-stored seed banks of six shrubs in fire-prone dune fynbos. Journal of Ecology 79: 731-747.
Thompson, J.H. 1980. Treefalls and colonization patterns of temperate forest herbs. American Midland Naturalist 104: 176-184.
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