Niches and competition
Today the niche is a central concept that describes the adaptation of the species to the environment. One aspect is the environmental conditions (temperature, nutrient, pH, etc.) under which the species can live, which is abundantly exemplified when Linnaeus describes the processes or preservation (conservatio) of plants and animals. As an example, he notes at what altitude different plants can live in the alpine region in the Dalarna province, and draws parallels to the Alps and the Pyrenees.
Another aspect of the niche is how the species feeds (seed eaters, predators, decomposers, etc.). Here, Linnaeus describes how almost every plant is food for an insect, and many insects live only on specific plants, and exemplifies with the silkworm that cannot survive without the mulberry tree.
Today's ecologists discuss how species with overlapping niches can compete with each other for resources, while niche separation (specialization) reduces competition, or in the words of Linnaeus: From this it follows that one species does not seize the other's food, because if this were to happen, it would endanger both life and health. Although Linnaeus describes nature as the war of all against all, he emphasizes conditions that limit this war: Each plant thus keeps within its own growing place so that it does not oust the other.
This leads Linnaeus to a reasoning about species richness: If, for example, there are in Sweden thirteen hundred plants that all grow in their designated place, it is still rare to find more than fifty or a hundred within one and the same locality. Even these species can be spared from causing each other damage by having different flowering times, what we would today call phenological niche separation.
Herbivory, predation, and other trophic interactions
Today, we describe the various functions of organisms in the ecosystem as trophic levels. Green plants are primary producers that form the basis for all other life through their photosynthesis. The trophic level of an animals depends on whether it is herbivores or carnivores (predator), and then there are microorganisms as decomposers.
Linnaeus concludes that the animals are primarily created for the sake of the plants and not the plants for the sake of the animals. The conclusion seems a little strange when Linnaeus simultaneously describes how the common nettle, which is eaten by almost all livestock, is forced to feed fifty species of insects. The reasoning is based on the fact that the animals' task is to maintain the balance among the plants, and without the insects the stinging nettle would suppress many other plants.
In what can be considered the world's first ecological experiment, Linnaeus, with the help of a number of students, studied domestic animals and recorded which plants they ate and which they ignored. Based on 2,314 observations, he was able to summarize how many plant species were eaten and grazed by cattle, goats, sheep, horses and pigs, and notes that in 868 cases the plant was left untouched.
"Carnivorous" plants are a kind of reverse plant-animal interaction, which in Sweden is represented by several species of sundews, butterworts, and bladderworts. They catch insects and have enzymes that allow them to break down the insects and assimilate nitrogen and phosphorus. In a letter to Nicolaas Laurens Burman in Amsterdam in 1768, Linnaeus describes with a drawing how the North American species Venus flytrap (Dionaea muscipula) catches insects by quickly folding the leaves together, but that the plant had any return from the catch eluded Linnaeus.
It is also a task for the animals to help the plants with their dispersal, and for this they receive a reward in fruit that they eat; a mutualism in today's terminology. The Eurasian jay collects nuts in caches sometimes forgets their hiding place and they germinate into a bountiful brood, and the careless eating of squirrels and crossbills helps spruce and pine seeds to end up in damp moss.
Considering Linnaeus' vast knowledge of flowers and bees, it is surprising that he did not come to the realization of how another extremely important mutualism, namely insect pollination, works. In accordance with the opinion of the time, Linnaeus believed for a long time that the insects damaged the plant by taking nectar (which was thought to be nutrition for the fruit) and pollen. Later, he admittedly believed that bees can contribute to pollination, but only by moving around the flower so that the pollen is spread from stamen to pistil (Eriksson 1983).
In nature's governance, the grasses are of course farmers and the mosses poor crofters. But there are also soldiers: plants such as thistles and barberry have been armed to protect themselves against the animals. Without knowledge about chlorophyll and photosynthesis, Linnaeus described how, for example, dodders and certain orchids parasitize green plants, or as it is expressed in Linnaeus' political analogy, they are under the guardianship of foster parents.
In his travels to Dalarna and Lapland, another example of parasitism is given when Linnaeus describes how ox warble fly and reindeer warble fly torment cattle and reindeer when they lay their eggs in the fur.
All bird watchers know how the Arctic skua (Stercorarius parasiticus) steals food from seagulls, but Linnaeus has more knowledge to offer about the skua: in autumn, when the fish hide in the depths, it is Mergus merganser, that keeps Lari with the supply, since this bird can dive even deeper into the sea (Mergus merganser is goosander, and ’Lari’ here refers to skuas). Linnaeus gave the skua the species epithet parasiticus.
Although the concept of succession (change in species composition over time) was hardly used in the 18th century, it was clear to Linnaeus that vegetation is not static. An example is changes in vegetation caused by land uplift along the Baltic coast (although this was at the time referred to "water subsidence"). He finds deposits of sea shells far inland, and from the inhabitants along the northernmost part Bothnian shore, he hears how much the sea sinks in ten years, and calculates that it will be (with some overestimation) 4 feet and 5 inches (about 135 cm) for every century. Linnaeus describes how plants invade bare rocks rising out of the sea and also explains the mechanisms: Crustaceous lichens are the only ones that can grow on bare rocks, and they provide a thin soil as substrate for leafy lichens. Next come mosses, and it is only when these rot away that the mulch is sufficient for herbs and trees to establish. For Linnaeus, the lichens were assigned an important task in the economy of nature, even though we consider them useless. Nowadays, we explain how species replace each other during a succession by differing in their ability to colonize and compete in a certain environment.