Population ecology

Reproduction and dissemination

Regarding propagation (generatio), Linnaeus gives detailed descriptions of the sexual reproduction of plants and animals, but he also discusses vegetative reproduction in plants (which was, of course, established knowledge among gardeners). We say that a genetic individual (genet) originates from a seed, but can be divided into genetically identical individuals (ramets) through cloning. In a strikingly modern reasoning, Linnaeus says: This does not in the least overturn the unshakable truth that all plants are propagated by seeds, since all these parts of the plant are sprung from a seed. Strictly speaking, new plants never arise without seeds. Cloning produces new ramets, but it is still only one genet.

It is not enough to form many seeds - they must also be able to disperse, and Linnaeus uses all his knowledge of the biology of the dispersal of plants when he wants to show how the plants from a single place have been able to spread all over the earth. Linnaeus builds general reasoning, but bases them on well-chosen examples:

Wind dispersal

The North American Erigeron canadensis (Canadian fleabane) was cultivated in France, and had within a century spread throughout France, Italy, Sicily, Belgium and Germany, thanks to its wind-dispersed seeds. Given Linnaeus modest interest in mosses, it is a little surprising that he writes about their ’seeds’ (i.e. spores) which can hardly be seen with the naked eye: They mix with the air like atoms and spread everywhere... Therefore the same mosses grow in North America and Europe.

Ballistic seeds

Linnaeus mentions that some plants shoot out their seeds, with Ruellia (a member of the Acanthus family) as an example. This is a topical issue as Mexican petunia, Ruellia simplex, a species from Central America that is now spreading invasively in the USA (comparable to Himalayan or Indian balsam, Impatiens glandulifera, that is invasive in the same way in Sweden), and sterile varieties are now being cultivated to prevent further dispersal.

Animal dispersal

In addition to seeds with hooks, Linnaeus lists numerous plants whose seeds survive in the stomach of animals and are then dispersed, not only efficiently, but also to suitable germination environments: currants and other berries, fruit trees, juniper and mistletoe.

Water dispersal

The importance of water dispersal is often underestimated because, unlike other methods of dispersal, it is not obvious from their morphology if they are adapted to water dispersal. However, Linnaeus realizes that water dispersal can be important when he finds alpine plants along the rivers in the north, as far as 300 km from the alpine region. Another plant is Veronica longifolia var. maritima (a seashore variety of garden or longleaf speedwell) which Linnaeus founds at the northernmost coast of the Baltic Sea. He had previously only seen it at the coast north of Stockholm, but he speculates that further stopovers may have been necessary on the long journey for this plant from Germany.

Seed bank

Linnaeus is completely clear that seeds can retain their viability for up to 40-50 years in the ground. Today we see this as an adaptation to unpredictable conditions, or as Linnaeus puts it: The Creator has set it up that way, so that they will not be destroyed by age if they happen to be in a dry or unsuitable place, but rest in the hope that the conditions will change to the better.

Population growth

Linnaeus anticipates theories of exponential population growth when he calculates that an annual plant that produces only two seeds per year will, after twenty years, have given rise to 91,296 individuals. How he arrives at this number is unclear, and the reasoning is flawed since a population of an annual plant with two seeds would not grow at all! But he gets away with it by adding that there is no plant so poor in seeds, for all bear more seeds than two, and with such widely different examples as the oak and trees that propagate by root suckers, he states that it is evident that a single plant left untouched by the animals could reach and cover the whole earth with its offspring.