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Vreemdelingen in de natuur
Vreemdelingen in natuur en landschap: inleiding
Laura I. Kooistra
Vreemdelingen in de Nederlandse flora? De tijd zal het leren
[Aliens in the Dutch flora? Time will tell]
Paleobotanical research (on pollen, seeds, fruits and wood) indicates that the timescale determines whether the designation ‘alien’ is applicable to Dutch plants. Factors influencing the flora are climatic changes, the landscape, the flora itself, the fauna and man. The results of archaeological research on seeds arid fruits are collected and stored in the national archaeobotanical database RADAR. When the records are classified by age, it is notable that especially plants of wet meadows spread from the Iron Age (800 BC) onwards. After this period deforestation became stronger, and the resulting open landscape presumably improved migration opportunities for herbaceous plants, including meadow plants. Apparently, man promoted the natural migration of meadow plants. Many pioneers, particularly arable weeds, were introduced by man, be it inadvertently. The introduction of arable weeds started more than seven thousand years ago, when the first peasants settled in the Netherlands. They brought grain with them for cultivation, carrying with it the seeds of arable weeds. These examples from paleobotany demonstrate that concepts such as ‘alien’ or ‘indigenous’ are difficult to define, since the flora changed over time due to a number of factors, one of them being man.
Bert Maes en Otto Brinkkemper
Autochtone bomen en struiken. Een historisch-ecologische benadering
[Indigenous trees and shrubs, a historicalecological approach]
This contribution gives a survey of the research on indigenous trees and shrubs in the Netherlands and Flanders. Maes has developed a method torecognise and describe authochthonous trees and shrubs in the field. Besides useful criteria pertaining to the individual tree or shrub and its habitat, historical and archaeobotanical information turn out to be very significant. Brinkkemper is a specialist on both these latter subjects. The article focuses on the history and importance of indigenous gene material and its protection and preservation. The authors urge that the existing sources of indigenous genes be preserved and that indigenous plant material be used in the afforestation of nature protection areas and their surroundings.
Barbara C. van Dam
Vreemde eiken in het bos
[Non-native oaks in the forest]
Oaks survived the last lce Age in Southern Europe. In the Netherlands the first oaks established themselves about nine thousand years ago. Their history can be traced from the current geographical distribution of descendency lines of Pedunculate and Sessile Oaks. Native oaks in the Netherlands originate from oaks that grew in Spain and Italy during the last lce Age. Descendants of the Balkan line also occur in the Netherlands, but they were imported by man, so in principle they are aliens in the Dutch forests. DNA-research (on genetic patterns) shows that the diversity within forests is very wide and that no distinction can be made between natural, indigenous forests and planted forests. Remarkably, trees that belong to various descendency lines, do not differ in a number of morphological
characteristics derived from forestry. The conclusion is that descendency lines exchange genetic material, as a result of which the correlation between descendency lines (determining the genotype) and morphological characteristics (the phenotype) disappears. Individual oaks can grow very old and survive under a variety of circumstances, because the oak adapts rapidly to changes in habitat and frequently reproduces a great number of very diverse descendants. The spectre of diversity of genetic material of oaks could be preserved by maintaining a number of small forests, which can exchange genetic material and rejuvenate themselves regularly.
Stinzenplanten en inburgering van vreemdelingen
[‘Stinsen plants’: native plants and established aliens]
‘Stinsen plants’ are alien plants that were introduced after 1780 and never spread beyond special environments created by man, such as landscape parks and gardens near castles (some in Friesland are called ‘stins’, hence their name), country estates, old farmyards and parish gardens. This article describes the origin, distribution and naturalisation of this particular group of plants. Taking the plants as a starring point, the author provides a general introductory survey of the terminology regarding alien species as used by scientists and policy makers.
Petra J.E.M van Dam
De rol van de warande, Geschiedenis van de inburgering van het konijn
[The role of the warren. History of the naturalisation of the common rabbit]
The rabbit was introduced in the Netherlands at the end of the Middle Ages. Initially it enjoyed a high status and both its fur and meat were reserved for the nobility only. Later it became a consumer article for the middle classes. The rabbit spread through warrens, from whence it became feral and developed into a plague for agriculture. The success of the rabbit may be explained in terms of biological characteristics as well as human influence. The foundation of warrens played an important role. Here the rabbit was protected against predators and fed, which was essential for its survival during harsh winters. In the warrens rabbits were selected for size for the purpose of commercial hunting, which probably changed the genetic pattern of the rabbit. This may explain why rabbits in Northern Europe are bigger than in Southern Europe, where they can survive without human interference due to the mild climate. Essential for the success of the feral rabbit was the fact that man changed the landscape over time through the expansion of agriculture, which created a habitat better suited to the rabbit.
Vreemde vogels gedragen zich voorspelbaar
[Predictable behaviour of exotic birds]
Many bird species occur outside their natural area of distribution through intentional or unintentional human interference. Successful exotic birds increase in numbers and their dispersal becomes wider. Among native species, too, increase in numbers may go hand in hand with wider dispersal. For both groups of species, the colonisation of unoccupied territories evolves at a certain rate that is connected with characteristics of the species, such as reproduction, survival and dispersal. Since a few years mathematical models are available with which the rate of dispersal can be predicted. This contribution compares the rate of dispersal as observed in the field with the rate as predicted by one of the models. When the three mentioned parameters are measured at the front of colonisation, there is a striking similarity between the observed and the predicted rates: exotic birds behave predictably.