Silybum, Notobasis and Onopordum blancheanum
A. In Bet Hakerem
The high prevalence of Silybum marianum near ant nests was seen also near my home. I started to watch this plant and the ants’ behavior in the area not far from my home in Bet Hakerem. At present most of the area is covered by buildings and highways, or planted with ornamental plants (planted and managed by the municipality’s gardeners with no remnant of the “wild” plants). My former student Dr. Rakefet Gabai started her M.Sc. thesis under my supervision (Fig. 8.3.1), dealing with the biology of Silybum marianum. A few of the issues were studied by Rakefet whereas others were investigated by me. Thus, I discovered that S. marianum has an oily-body (Figs. 8.3.2, 8.3.3) on each achene.
I took advantage of the plants and nests not far from my home and made a preliminary experiment at the afternoon hours, while in my role as a father, walking my 1 year old daughter in a stroller. I saw a nest far away from Silybum marianum plants. I collected some 250 achenes of that plant with their pappus (which assists seed dispersal by wind as a “parachute” in many other plants) and laid them on the ants’ path. The ants readily moved the achenes into their nest. I drew a line perpendicular to their path, and as each ant passed the line, the way it held the achene was registered in a table. 45 minutes later I could sum up that 64.2% of the achenes were held by their oily-body (Fig. 8.3.4 right), 24.1% were held in the opposite end of the achene, 6.5% were held by their lateral side and 5.2% by the pappus.
During the 2 months after that day I collected all the waste material deposited in the threshing zone by the ants of that specific nest. The S. marianum achenes were discovered one by one (Fig. 8.3.4 left). None of them had been opened and none had an oily-body (Fig. 8.3.5). I concluded from that that the worker ants on the path brought the achenes into the nest; other ants that work inside the nest dealt with them inside the nest and a third group (possibly) evacuated them to the threshing zone. Rakefet Gabai carried out a few experiments that complete our findings quantitatively. In one experiment she compared the germination ability of achenes with and without oily body.
The full germination (100% or close to it) in the two groups proves that the oily body functions as bait or a way to attract the ants. They bring the achenes to the nest, remove the oily body and lay it near the egg laid by the queen ant. The oily body contains the essential food needed by the developing young ant. Michal Motro who studied the biology of the harvesting ant in depth, clarified that the worker ants feed the young ones developing at that time of the year on oily food. This was in spring, when there are but small quantities of seeds in the field. The achenes of S. marianum are collected intensively because the oily body contains the food the ants search for in other seeds by opening and processing them. However, in S. marianum they do not have to work on the food reserves of the seed.
The S. marianum achene is too big for the ants’ jaws. Their smooth surface increases their protection and reduces the ability of ants to open them. Thus, the S. marianum “gains” transfer of its diaspores to the habitat best fitted to the development of offspring. The nest environment is rich in nutrients (the threshing zone in particular) and the S. marianum seedlings develop into plants that create shade over its competitors. Rakefet carried out experiments when she uprooted S. marianum seedlings from half the circle of the threshing zone. Comparing the two nest halves she found that other plants developed at a much higher biomass and species diversity compared to the area shaded by the S. marianum.
B. “Go to the ant…”
25 years have passed since my first experiment and when the seed collection season started I went to Bet Hakerem for a short visit, aiming to repeat the first collection experiment. Most of the natural areas with ant nests of the past have become covered with houses, asphalt roads, ornamental irrigated gardens, or piles of building debris or crushed rocks. Between “Ziv” school and the buildings of “Achuzat Bet Hakerem” there is a small area where mostly natural vegetation has developed. There, I found a single active nest of harvesting ants. As in the past, I found an active ants’ path: hundreds of ants were running along the path carrying single flowers of Avena barbata. For many years I taught my students the difference between Avena sterilis, which has 2-4 flowers in each diaspore, and Avena barbata where the diaspore is of only one flower. Here, finally, I received a lesson about the biology of the plant and of ants. One of the two flowers of each spikelet of the upper part of the inflorescence falls to the ground long before the vegetation and the soil have dried out.
When I came with my S. marianum achenes to that nest I thought the ants (having read my previous articles) would be very happy with the oily-body gift. I considered my deed akin to a natural occurrence, when diaspores of S. marianum arrive by wind dispersal. The Silybum achenes lay on the path for a long time and only one ant pulled an achene backwards toward the nest. All the other ants continued efficient transfer of Gramineae as if they had been ordered to continue collecting members of this family only. I estimate that 95% of the seeds transferred were of Avena barbata, 4% of Hordeum spontaneum and Lolium rigidum. The achene-gift that I brought to the nest was a small fragment of a percent.
Further on, I found in another place, a nest with low activity. A few ants walked around in desultory manner. If I could describe the first nest as a celebration of activity, the other nest looked desolate by the deficiency of seeds in their surroundings. A few moments after putting a few S. marianum achenes near the nest, they quickly organized themselves and transferred the gifts into the nest holes. They were holding the achenes by their oily body and in a short while they had inserted 5 achenes, photographed by my camera. In this way I was able to replace the old photos with new ones. I learnt more about the function of the single flower of Avena barbata. From now on I imagine that worker ants receive orders which they obey without a trace of any self initiative for collecting. Grass collection does not permit improvisations of collecting plants from other families. High school pupils and university students are invited to continue my voyage of “learning from the ants”.
When I started to search for literature sources for seed dispersal by ants (mirmecochory), I learnt that this phenomenon was dealt with in depth by researchers in S. Africa and Australia. These two areas developed in evolutionary time through the continued influence of wild fires in the natural vegetation. The conclusions I bring here from a few books and articles are: 1.”Plants dispersed by ants” (mirmecochorous) enter this group only if their diaspore has elaiosome (oily body|). 2. Seed ripening before the fire season and their collection into the nests protects seeds from being burnt, 3. Plants dispersed by ants are common among maquis species. Silybum marianum fulfils the first requirement by having an oily body. Rakefet Gabai proved that the presence or absence of the oily body on the achenes has no bearing on the germination ability. Checking a grassland area between Bet Hakerem and Givat Ram after a wild fire, revealed plenty of seedlings near the ant nests (Figs. 8.3.6, 8.3.7).
In an article we wrote on S. marianum we opened the way for plants that do not belong to maquis to be regarded as mirmecophytes. This statement opened a new direction in the understanding of the natural origin of plants that accompany human activity. The latter involves the formation of large areas of disturbed ground rich in nutrients. Many researchers and authors of floras around the Mediterranean Sea regarded S. marianum as a ruderal plant (confined to garbage heaps). Revealing the “mirmecochoric connection” of S. marianum explains where the plant passed through its adaptation or pre-adaptation to become one of the most successful ruderals.
Rakefet’s experiments show that the plant wins out over other plants by shading others while growing on fertile soil near the ant nests. It also grows abundantly below shade-giving trees and provides a shaded refuge in summer for cows and other animals. While resting there, their dung fertilizes the soil with nutrients. S. marianum conquers its competitors from roadsides and garbage heaps in the Mediterranean area and forms dominant patches (Fig. 8.3.9, 8.3.10). S. marianum plants developing near the nest were found to be 10 (!) times heavier than plants growing among the nests.