The Importance of the Work of Ants in the Environment
Seed dispersal is central to plant reproduction, recruitment, population genetics, and ecology because it determines the movement of plant genes in space and, in many cases, in time. Seed dispersal links the usually sessile individuals to ecological processes operating at larger geographic scales such as long-distance dispersal, colonization, range expansion, isolation, and speciation.
Even subtle changes in traits that influence seed dispersal can therefore have far-reaching consequences on fitness and a multitude of other biological traits, and thus, dispersal mode is one of the most fundamental life-history traits in plants. Seed dispersal modes are usually classified into five broad types (‘syndromes’): unaided (passive, ballistic, etc.), by wind (anemochory), by water (hydrochory), by vertebrates externally (e.g. on fur: exozoochory) or internally (passing through the gut: endozoochory), and by ants (myrmecochory). Although it is recognized that these modes differ dramatically in their benefits (and costs) to plants, the number of origins and global significance of different modes of seed dispersal remain poorly understood.
Seed dispersal mutualisms are an ideal context in which to understand the factors driving trait convergence because of their multiple origins, simple morphological bases, diffuse suites of animal partners, and ecological importance. Ant-mediated dispersal or myrmecochory is one of the more enigmatic animal-plant mutualisms. To date, estimates of the number of origins of elaiosomes have been based purely on the number of families in which the trait occurs. Nonetheless, the number of plant families in which myrmecochory occurs indicates that it may have evolved frequently and that the selection pressures acting on the mutualism are or were strong. When myrmecochory is successful for the plant, ants take the entire seed (or, more generally, diaspore) back to the nest, where they remove the elaiosome and discard the seed in a midden or outside the nest. In any given site, multiple species may disperse seeds, but one or a few ant species tend to be numerically dominant in terms of the number of seeds removed.
The dispersal of seeds or fruits by ants is called myrmecochory and represents a very common phenomenon: seed dispersal by ants has evolved more than 100 times and can be found in four subfamilies of ants (out of seven) and more than 11 000 species from at least 77 families of plants.
A major concern in evolutionary science is the evolution of mutualisms between free-living organisms (as opposed to parasites) involving many unspecialized partners that exert incongruent selection forces on each other. Because the outcome of such mutualisms may be difficult to predict, the role of reciprocal selection in their evolution is sometimes underestimated. However, if the interacting species are structured geographically, coevolution can act at the population level on a reduced number of partners, allowing local coadaptations.
Such “diffuse” mutualisms are well exemplified by seed-ant dispersal interactions (myrmecochory) in which seeds bearing a lipid-rich appendage (the elaiosome) can be transported by large guilds of nongranivorous ants. More than 3,000 myrmecochores are known around the world, mainly in temperate and Mediterranean regions where ant communities can change rapidly along latitudinal gradients. As a consequence, across their range, myrmecochory are likely to interact with a variety of dispersers which have varying impacts on their reproductive success.
Many ants collect myrmecochorous seeds to ingest the elaiosome, an attached lipid-rich food body. Potential benefits to the seed include the colonization of new patches, escape from predators and/or related plants, and directed dispersal to safe, nutrient-rich microsites such as ant nests.
Therefore, the ants collect elaiosomes and treat the seeds as waste, which is cut off the valuable elaiosome before it is carried into the nest and used as a food source. Foundresses that fed elaiosomes had survival rates similar to foundresses fed on a standard diet, whereas brood production was significantly reduced. Thus, elaiosomes appear to supplement rather than completely replace the diet of ants. The plants gain transport because the seeds are carried by the ants and often reach nest moulds and sites nearby, which are characterized by soil that is rich in nutrients. Although dispersal distances that can be reached using ants as vectors appear limited, myrmecochory has an important function in structuring ecosystems and in evolution: myrmecochory appears a key evolutionary innovation that has considerably driven the evolution of diversity in angiosperms.
Sevigne and Detrain classify seed dispersal by ants into three stages: the collection of seeds and movement to a nest or central location (stage I), the removal of elaiosomes within the nest (stage II), and the abandonment of seeds either inside the nest or rejected outwards from the nest (stage III).
Seed dispersal by ants is considered to be a “diffuse” mutualism, meaning that multiple species of ants disperse the seeds of multiple plant species, though it is frequently skewed toward one particularly important ant species. This type of mutualism is termed an “unevenly diffuse” mutualism and the disproportionately important partner is known as the keystone mutualist.
The benefits of dispersal of seeds by ants to modern plants appear to vary among ecosystems. Attempts to understand the evolution of myrmecochory have focused on two research strategies. Studies have either compared the features of ecosystems in which myrmecochory is rare with another in which it is common or have followed the fate of seeds dispersed and not dispersed by ants to infer the current selective pressures on ant-dispersed seeds.
To date, most research has tended to highlight the role of side effects. Ant nests are thought to be preferred sites for seed germination or storage because of the reduced risk of mortality due to fire in Mediterranean ecosystems (Auld, 1986), predation (Heithaus, 1981), or nutrient limitation (Westoby et al., 1991a) relative to areas outside the nest. Alternatively, dispersal by ants may actually offer benefits in terms of dispersal distance. Several authors (Gomez and Espadaler, 1998; Whitney, 2002) have suggested that for the relatively small plants that tend to be dispersed by ants, a distance of even a meter may represent several plant widths and so still yield substantial fitness benefits. Although average dispersal distances are typically on the order of just a few meters, longer-distance dispersal events can be achieved, either by ants with large foraging areas.
The plant adaptation eliciting myrmecochory is the elaiosome, a fleshy growth that develops on the outside of a ripe seed or fruit. Elaiosomes contain specialized fatty acids, sterols, and other nutrients. Foraging ants transport fruit or seeds bearing elaiosomes to their nests, where they eat the elaiosomes or feed them to developing larvae. The remaining intact fruit or seeds, minus elaiosomes, are either left in the nest or deposited outside the nest in middens or in scattered locations around the nest.
Seeds dispersed by ants are usually moved short distances, typically less than 2 m, though seeds may occasionally be moved more than 5 m. Dispersal distances tend to be related to ant size, with dispersal distance increasing with increasing body length. Despite short distances of transport, this mode of dispersal provides seeds with several distinct advantages.
