Social insects saturate most of the terrestrial environment. In ways that become fully apparent only when we bring our line of sight down to a millimeter of the ground surface, they lay heavily on the rest of the fauna and flora, constraining their evolution.
That fact has struck home to me countless times during my life as a biologist. Recently it came again as I walked through the mixed coniferous and hardwood forests on Finland’s Tvarminne Archipelago. My guides were Kari Vepsalainen and Riitta Savolainen of the University of Helsinki, whose research has meticulously detailed the distribution of ants in the archipelago and the histories of individual colonies belonging to dominant species. We were in a cold climate, less than 800 kilometers from the arctic circle, close to the northern limit of ant distribution. Although it was mid-May, the leaves of most of the deciduous trees were still only partly out. The sky was overcast, a light rain fell, and the temperature at midday was an unpleasant (for me) 12°C. Yet ants were everywhere. Within a few hours, as we walked along trails, climbed huge moss-covered boulders, and pulled open tussocks in bogs, we counted nine species of Formica and an additional eight species belonging to other genera, altogether about one-third the known fauna of Finland. Mound-building Formicas dominated the ground surface. The nests of several species, especially F. aquilonia and F. polyctena, were a meter or more high and contained hundreds of thousands of workers. Ants seethed over the mound surfaces. Columns traveled several tens of meters between adjacent mounds belonging to the same colony. Other columns moved up the trunks of nearby pine trees, where the ants attended groups of aphids and collected their sugary excrement. Swarms of solitary foragers deployed from the columns in search of prey. Some could be seen returning with geometrid caterpillars and other insects. We encountered a group of F. polyctena workers digging into the edge of a low mound of Lasius flavus. They had already killed several of the smaller ants and were transporting them homeward for food. As we scanned the soil surface, peered under rocks, and broke apart small rotting tree branches, we were hard put to find more than a few square meters anywhere free of ants. In southern Finland they are in fact the premier predators, scavengers, and turners of soil. Exact censuses remain to be made, but it seems likely that ants make up 10% or more of the entire animal biomass of the Tvarminne Archipelago.
Two months earlier, in the company of Bert Hölldobler of the University of Wurzburg, F. R. Germany (then at Harvard University, USA), I had walked and crawled on all fours over the floor of tropical forest at La Selva, Costa Rica. The ant fauna was radically different and much more diverse than in Finland. The dominant genus was Pheidole, as it is in most tropical localities. Within a 1.5 km2 area along the Rio Sarapiqui, my students and I have collected 34 species of Pheidole, of which 16 are new to science. The total ant fauna in the sample area probably exceeds 150 species. That is a conservative estimate, because Neotropical forests have some of the richest faunas in the world. Manfred Verhaagh (personal communication) collected about 350 species belonging to 71 genera at the Rio Pachitea, Peru. That is the world record at the time of this writing. I identified 43 species belonging to 26 genera from a single leguminous tree at the Tambopata Reserve, Peru (Wilson, 1987a). From my experience in ground collecting in many Neotropical localities, I am sure that an equal number of still different species could have been found on the ground within a radius of a few tens of meters around the base of the tree. In other words, the fauna of the Tambopata Reserve is probably equivalent to that at the Rio Pachitea.
The abundance of ants at Neotropical localities, as opposed to species diversity, is comparable to that on the Tvarminne archipelago, and they occupy a great many more specialized niches as well. In addition to a large arboreal fauna, lacking in Finland, leaf-cutter ants raise fungi on newly harvested vegetation, Acanthognathus snare tiny collembolans with their long traplike mandibles, Prionopelta species hunt campodeid diplurans deep within decaying logs, and so on in seemingly endless detail. Roughly one out of five pieces of rotting wood contains a colony of ants, and others harbor colonies of termites. Ants absolutely dominate in the canopies of the tropical forests. In samples collected by Terry Erwin by insecticidal fogging in Peru, they make up about 70% of all of the insects (personal communication). In Brazilian Terra Firme forest near Manaus, Fittkau and Klinge (1973) found that ants and termites together compose a little less than 30% of the entire animal biomass. These organisms, along with the highly social stingless bees and polybiine wasps, make up an astonishing 80% of the entire insect biomass.
While few quantitative biomass measurements have been made elsewhere, my own strong impression is that social insects dominate the environment to a comparable degree in the great majority of land environments around the world. Very conservatively, they compose more than half the insect biomass. It is clear that social life has been enormously successful in the evolution of insects. When reef organisms and human beings are added, social life is ecologically preeminent among animals in general. This disproportion seems even greater when it is considered that only 13,000 species of highly social insects are known, out of the 750,000 species of the described insect fauna of the world.
In short, 2% of the known insect species of the world compose more than half the insect biomass. It is my impression that in another, still unquantified sense these organisms, and particularly the ants and termites, also occupy center stage in the terrestrial environment. They have pushed out solitary insects from the generally most favorable nest sites. The solitary forms occupy the more distant twigs, the very moist or dry or excessively crumbling pieces of wood, the surface of leaves - in short, the more remote and transient resting places. They are also typically either very small, or fast moving, or cleverly camouflaged, or heavily armored. At the risk of oversimplification, the picture 1 see is the following: social insects are at the ecological center, solitary insects at the periphery.
This then is the circumstance with which the social insects challenge our ingenuity: their attainment of a highly organized mode of colonial existence was rewarded by ecological dominance, leaving what must have been a deep imprint upon the evolution of the remainder of terrestrial life.
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The most advanced social insects are referred to as eusocial, an evolutionary grade combining three traits: some form of care of the young, an overlap of two or more generations in the same nest site or bivouac, and the existence of a reproductive caste and a nonreproductive or “worker” caste. The eusocial grade has been attained by 4 principal groups of insects: the ants (order Hymenoptera, family Formicidae. 8800 described species), the eusocial bees (order Hymenoptera, about 10 independent evolutionary lines within the families Apidae and Halictidae. perhaps 1000 described species overall), the eusocial wasps (order Hymenoptera. mostly in the family Vespidae and a few in the family Sphecidae. 800 described species), and the termites (order Isoptera, 2200 described species). The presence of a worker caste is by far the most important feature, because it enhances division of labor and a more complex society overall. The most familiar social insects, those with the striking social adaptations such as honeybees, mound-building termites, and army ants, all have strongly differentiated worker castes.
Evolutionary grades below the eusocial state abound in the insects. They are lumped together in the category “presocial,” in which one or two but not all three of the aforementioned eusocial traits are displayed. One of the most frequently remarked forms of presocial behavior is subsocial behavior, which simply means that the parents care for their own nymphs or larvae. For example, the females of many true bugs (order Hemiptera) remain with their young to protect them from predators and sometimes even to guide them from one feeding site to another. Some scolytid bark beetles not only guard their young but feed them fungi in specially constructed nursery chambers. In neither case, however. do the offspring later function as nonreproductive workers. Hence neither hemipterans nor scolytid beetles, remarkable as they are, qualify as social insects.
Ants, bees, and wasps, being members of the order Hymenoptera, have a life cycle marked by complete metamorphosis. To use the appropriate adjective, they are holometabolous. As illustrated in Figure 3. the individual passes through four major developmental stages radically different from one another: egg, larva, pupa, and adult.
The significance of this tortuous sequence is the difference it allows between the larva and the adult. The larva is a feeding machine, specialized for consumption and growth. It typically travels less, remaining sequestered in a nest site or other protected microenvironment. The adult, in contrast, is specialized for reproduction and in many cases dispersal as well. It often feeds on different food from that of the larva or no food at all, living on energy stores built up during the larval phase. Finally, the pupa is simply a quiescent stage during which tissues are reorganized from the larval to the adult form. The effect of complete metamorphosis on social evolution is profound. The larva can do little work and must be nurtured. Its dependence on the adults is increased by its limited mobility, since even if it were capable of independent feeding it could not travel to distant food sources. Consequently a large part of adult worker life is devoted to larval care, during which individuals search for food to give to the larvae, then feed, clean, and protect them.
~~Success And Dominance In Ecosystems: The Case Of The Social Insects -by- Edward O. Wilson
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