Culture of crops or livestock

13.3.1 Human agriculture

At least in terms of geographic extent, some of the most dramatic mutualisms are those of human agriculture. The numbers of individual plants of wheat, barley, oats, corn and rice, and the areas these crops occupy, vastly exceed what would have been present if they had not been brought into cultivation. The increase in human population since the time of hunter-gatherers is some measure of the reciprocal advantage to Homo sapiens. Even without doing the experiment, we can easily imagine the effect the extinction of humans would have on the world population of rice plants or the effect of the extinction of rice plants on the population of humans. Similar comments apply to the domestication of cattle, sheep and other mammals.

Similar 'farming' mutualisms have developed in termite and especially ant societies, where the farmers may protect individuals they exploit from competitors and predators and may even move or tend them.

13.3.2 Farming of insects by ants

Ants farm many species of aphids (homopterans) in return for sugar-rich secretions of honeydew. The 'flocks' of aphids benefit through suffering lower mortality rates caused by predators, showing increased feeding and excretion rates, and forming larger colonies. But it would be wrong, as ever, to imagine that this is a cosy relationship with nothing but benefits on both sides: the aphids are being manipulated - is there a price that they pay to be entered on the other side of the balance sheet (Stadler & Dixon, 1998)? This question has been addressed for colonies of the aphid Tuberculatus quercicola attended by the red wood ant Formica yessensis on the island of Hokkaido, northern Japan (Yao et al., 2000). As expected, in the presence of predators, aphid colonies survived significantly longer when attended by ants than when ants were excluded by smearing ant repellent at the base of the oak trees on which the aphids lived (Figure 13.5a). However, there were also costs for the aphids: in an environment from which predators were excluded, and the effects of ant attendance on aphids could thus be viewed in isolation, ant-attended aphids grew less well and were less fecund than those where ants as well as predators were excluded (Figure 13.5b).

Another classic farming mutualism is that between ants and many species of lycaenid butterfly. In a number of cases, young lycaenid caterpillars feed on their preferred food plants usually until their third or fourth instar, when they expose themselves to foraging ant workers that pick them up and carry them back to their nests - the ants 'adopt' them. There, the ants 'milk' a sugary secretion from a specialized gland of the caterpillars, and in return protect them from predators and parasitoids throughout the remainder of their larval and pupal lives. On the other hand, in other lycaenid-ant interactions the evolutionary balance is rather different. The caterpillars produce chemical signals mimicking chemicals produced by the ants, inducing the ants to carry them back to their nests and allowing them to remain there. Within the nests, the caterpillars may either act as social parasites ('cuckoos', see Section 12.2.3), being fed by the ants (e.g. the large-blue butterfly Maculinea rebeli, which feeds on the crossleaved gentian, Gentiana cruciata, and whose caterpillars mimic the larvae of the ant Myrmica schenkii), or they may simply prey upon the ants (e.g. another large-blue, M. arion, which feeds on wild thyme, Thymus serpyllum) (Elmes et al., 2002).

13.3.3 Farming of fungi by beetles and ants

Much plant tissue, including wood, is unavailable as a direct source of food to most animals because they lack the enzymes that can digest cellulose and lignins (see Sections 3.7.2 and 11.3.1). However, many fungi possess these enzymes, and an farmed aphids: do they pay a price?

ants and blue butterflies

1.0

0.8

te

rat

0.6

"¡0

>

■>

0.4

Cfl

0.2

0.0

Ant attended

-

-

S. \

^*|Ant excluded

,1,1,1,1,

, i, i,i, i, i, i, i,T, i,

0 2 4 6 8 10 14 18 22 26 30 Days after the start of experiments

0 2 4 6 8 10 14 18 22 26 30 Days after the start of experiments

CO CD erl0.44

0.42

12 Season

12 Season

Figure 13.5 (a) Ant-excluded colonies of the aphid Tuberculatus quercicola were more likely to become extinct than those attended by ants (X2 = 15.9, P < 0.0001). (b) But in the absence of predators, ant-excluded colonies perform better than those attended by ants. Shown are the averages for aphid body size (hind femur length; F = 6.75, P = 0.013) and numbers of embryos (F = 7.25, P = 0.010), ± SE, for two seasons (July 23 to August 11, 1998 and August 12 to August 31, 1998) in a predator-free environment. •, ant-excluded treatment; •, ant-attended treatment. (After Yao et al., 2000.)

animal that can eat such fungi gains indirect access to an energy-rich food. Some very specialized mutualisms have developed between animal and fungal decomposers. Beetles in the group Scolytidae tunnel deep into the wood of dead and dying trees, and fungi that are specific for particular species of beetle grow in these burrows and are continually grazed by the beetle larvae. These 'ambrosia' beetles may carry inocula of the fungus in their digestive tract, and some species bear specialized brushes of hairs on their heads that carry the spores. The fungi serve as food for the beetle and in turn depend on it for dispersal to new tunnels.

Fungus-farming ants are found only in the New World, and the 210 described species appear to have evolved from a common ancestor: that is, the trait has appeared just once in evolution. The more 'primitive' species typically use dead vegetative debris as well as insect feces and corpses to manure their gardens; the genera Trachymyrmex and Sericomyrmex typically use dead vegetable matter; whereas species of the two most derived (evolutionarily 'advanced') genera, Acromyrmex and Atta, are 'leaf-cutters' using mostly fresh leaves and flowers (Currie, 2001). Leaf-cutting ants are the most remarkable of the fungus-farming ants. They excavate 2-3-liter cavities in the soil, and in these a basidiomycete fungus is cultured on leaves that are cut from neighboring vegetation (Figure 13.6). The ant colony may depend absolutely on the fungus for the nutrition of their larvae. Workers lick the fungus colonies and remove specialized swollen hyphae, which are aggregated into bite-sized 'staphylae'. These are fed to the larvae and this 'pruning' of the fungus may stimulate further fungal growth. The fungus gains from the association: it is both fed and dispersed by leaf-cutting ants and has never been found outside their nests. The reproductive female ant carries her last meal as a culture when she leaves one colony to found another.

Most phytophagous insects have very narrow diets - indeed, the vast majority of insect herbivores are strict monophages (see Section 9.5). The leaf-cutting ants are remarkable amongst insect herbivores in their polyphagy. Ants from a nest of Atta cephalotes harvest from 50 to 77% of the plant species in their neighborhood; and leaf-cutting ants generally may harvest 17% of total leaf production in tropical rainforest and be the ecologically dominant herbivores in the community. It is their polyphagy that gives them this remarkable status. In contrast to the A. cephalotes adults though, the larvae appear to be extreme dietary specialists, being restricted to nutritive bodies (gongylidia) produced by the fungus Attamyces bromatificus, which the adults cultivate and which decompose the leaf fragments (Cherrett et al., 1989).

Moreover, just as human farmers may be plagued by weeds, so fungus-farming ants have to contend with other species of fungi that may devastate their crop. Fungal pathogens of the genus Escovopsis are specialized (never found other than in fungus gardens) and virulent: in one experiment, nine of 16 colonies of the leaf-cutter Atta colombica that were treated with heavy doses of Escovopsis spores lost their garden within 3 weeks of treatment (Currie, 2001). But the ants have another mutualistic association to help them: a filamentous actinomycete bacterium associated with the surface of the ants is dispersed to new gardens by virgin queens on their nuptial flight, and the ants may even produce chemicals that promote the actinomycete's growth. For its part, the acti-nomycete produces an antibiotic with specialized and potent inhibitory effects against Escovopsis. It even appears to protect the ants themselves from pathogens and to promote the growth of the farmed fungi (Currie, 2001). Escovopsis therefore has ranged leaf-cutting ants:

remarkably polyphagous ants, farmed fungi and actinomycetes: a three-way mutualism

Figure 13.6 (a) Partially excavated nest of the leaf-cutting ant Atta vollenweideri in the Chaco of Paraguay. The above-ground spoil heap excavated by the ants extended at least 1 m below the bottom of the excavation. (b) Queen of A. cephalotes (with an attendant worker on her abdomen) on a young fungus garden in the laboratory, showing the cell-like structure of the garden with its small leaf fragments and binding fungal hyphae. (Courtesy of J. M. Cherrett.)

Figure 13.6 (a) Partially excavated nest of the leaf-cutting ant Atta vollenweideri in the Chaco of Paraguay. The above-ground spoil heap excavated by the ants extended at least 1 m below the bottom of the excavation. (b) Queen of A. cephalotes (with an attendant worker on her abdomen) on a young fungus garden in the laboratory, showing the cell-like structure of the garden with its small leaf fragments and binding fungal hyphae. (Courtesy of J. M. Cherrett.)

against it not just two two-species mutualisms but a three-species mutualism amongst ants, farmed fungi and actinomycetes.

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment