5
Discussion Of Modern
And Historical
Migrations

In the abstract model presented in the Introduction, geographic patterning of seed plant species is the product of interplay between opposing forces tending to expand or restrict the distribution. The main centrifugal force is usually seed dispersal; the main centripetal force is environmental selection. When dispersal and environment are sufficiently stable, fluctuating within normal limits, these forces eventually equilibrate and the species borders become static. Migration, whether advance or retreat, occurs when equilibrium is upset by changes in dispersal or environment or both. The case histories that have been discussed thus far show a variety of migration patterns resulting from such changes.

Changes In Dispersal

Changes in dispersal alone, with no change in environmental selection, can result in both local and long-range colonization. An example of local colonization is the arrival on Baltic islets of new plant species after a change in the foraging grounds of gulls nesting on the islets. An example of long-range colonization is the introduction from overseas of ruderal plants with wool imported to France. New colonies produced by such long-range dispersal are commonly ephemeral and nonreproductive. However, there are also a great

many cases of successful establishment of widely disjunct populations following long-range dispersal. Most of these are weeds of artificially disturbed habitats or wild pioneers of seashores, streambanks, and other naturally disturbed habitats, but there are also cases of introduced exotic plants invading undisturbed native vegetation, both on remote islands and on continents.

As would be expected, recent establishment of new, widely disjunct colonies is mainly due to human introduction, the major new agency of long-range dispersal. Only occasionally are new colonies attributed to natural long-range dispersal, for example, the arrival of an orchid, Spiranthes romanzoffianum , in the British Isles in the nineteenth century by wind-borne seed from North America. For two reasons, however, formation of disjunct colonies by natural long-range dispersal may be going on without being recognized. First, the mode of dispersal may be unrecognized. For example, if plant species were to arrive in Hawaii by ocean, wind, or bird dispersal in the present day, as they undoubtedly did many times prehistorically, they would inevitably be assumed to have been artificially introduced. Second, the recency of the colonization may be unrecognized. For example, Madroño: A West American Journal of Botany regularly has a section reporting new collections that extend the known ranges of native plant species. Some of these involve disjunctions of only a few tens of kilometers, but many report populations several hundred kilometers outside the formerly known ranges. These are normally assumed to have been simply newly discovered rather than newly established.

None of the case histories showed retreats of species due to decreased dispersal. Conceivably, extinction of animal species could have caused contraction of seed shadows of plant species, followed eventually by dying out of marginal colonies outside the reproductive core of the plant species. Perhaps this occurred with oaks and other trees that provided mast for the North American passenger pigeon or with aquatic plants with seeds carried in the fur of the European beaver, but no examples are known. A more extreme case, in which the whole population of a plant species may have ceased to reproduce after extinction of an animal, involves two species endemic to Mauritius. The dodo, a giant, flightless relative of the pigeon, became extinct about 1680, soon after the island was colonized by the Dutch. Temple (1977) suggested that a tree species, Calvaria major , was totally dependent on the dodo for reproduction. Calveria was originally common on the island and was cut for lumber in the early colonial period but is now nearly extinct. Only 13 trees survive, all believed to be over 300 years old. Temple fed some of the thick-shelled Calvaria seeds to domestic turkeys as stand-ins for dodos; the seeds germinated after passing through a turkey gizzard.

Changes In Environment

In a great many cases, especially but not exclusively local migrations, advances and retreats are evidently due to changes in the environment, while any dispersal changes appear irrelevant. A change in a single factor in the plant environment, such as introduction of a new herbivore, a new disease, or a new fire regime, commonly causes retreats of some species and advances of others. Such linked migrations are not necessarily on the same scale; the catastrophic retreat of elms in North America was accompanied only by micromigration of the species that replaced them. Sunflowers marched across the continent along roadways that had pushed previous plant occupants back only a few feet.

Disjunctions in species ranges can result from environmental change both by colonization of isolated new habitats and by fragmentation of formerly continuous distributions. Examples of the former include new islands emerging within the reach of sea-dispersed beach plants and artificial marshes formed under the flyways of migratory waterfowl. Many plant distributions have been fragmented by clearing and grazing. Dramatic recent changes in plant environments, like changes in dispersal, have been largely but not exclusively due to human activities.

Changes In Both Dispersal And Environment

Some cases of plant migration involve interconnected changes in dispersal and environment. For example, herds of sheep driven into the Great Basin of western North America evidently carried in their wool seeds of Eurasian grasses that invaded ranges overgrazed by the sheep. On Krakatau, the establishment of figs and other trees dispersed by birds and fruit bats evidently attracted more of those animals to the island and started a positive feedback between forest development and animal introduction of more tree species.