The Wageningen branch of the KNNV has documented the wealth of insect species in the small arboretum. And it is a tremendous wealth, they tell us. The researchers have therefore written an open letter to appeal for the whole park to be preserved.
Besides its significance as a living collection and as cultural-historical heritage, the small arboretum at the Dreijen clearly has special value as a biodiversity hotspot as well. By quality of life, Wageningen UR understands 'living, working and spending leisure time in a balanced green environment with a wide variety of plants and animals'. So what does Wageningen UR mean by this slogan? More green space in and near the city, with more plants and animals? Not in our own back yard, obviously...
The insect working group of the Wageningen area branch of the KNNV has been inventorying the botanical garden at the Dreijen since 2005. We have used many different techniques to catch the insects: ground traps for ground-dwelling insects, window traps and tent-like malaise traps for aerial insects, and various types of bait traps. Light was used at night, and the insects were sucked, dragged or knocked into nets. This extensive sampling produced a very large number of insects and other arthropods, some of which have since been identified by various taxonomy specialists. In particular, a number of beetles, bugs, flies and wasps.
Once our research is finished, the idea is to publish a book about the wealth of insects and other arthropod species in this garden. Recent developments around the ownership and management of the arboretum have lent a new urgency to the inventory. Yet it is impossible to process such a massive amount of data in a short period. So we want to give a brief overview of the current status here. The initial data show that the garden is extraordinarily rich in arthropods, and that many rare species occur.
The table below gives an overview of the number of families within this group that have been collected, plus the number of species and individuals. A very large number of species have been found in the garden, of various classes and families. To give one example, nowhere else in the country have so many species of bug been found on a small area as in the small arboretum. Once all the data have been processed, the total number of species of arthropod will certainly exceed 1000. So the arboretum can fairly be described as a biodiversity hotspot.
Number of families
Number of species
Number of specimens
It has been established that not only is there a very high biodiversity in the arboretum, but also several rare species. We shall just take a few examples. A specimen of the long-horn beetle Tetrops starkii was found. Up to that point, this species was only known from a single specimen collected in Limburg in 1967. The second example was a crawling beetle, Perigona nigriceps, one specimen of which was caught. This is a very rare species which was collected in the past in South Limburg, but of which the only specimens to be spotted since 1966 have been in the Wageningen area. A final example is the digger wasp Pemphredon montana, which was caught in a malaise trap. This species, which has a boreal-alpine distribution, is extremely rare in the Netherlands, where it has only been spotted twice before.
One of the reasons this garden is so species-rich is the presence of very varied habitants close together - various ponds, a drought garden, old trees, both deciduous and coniferous, shrubs, hedges and herbs, walls, a rockery, piles of leaves, dry and sunny places and damp and shady corners... The age of the garden plays a role too. Both the great biodiversity within a range of groups, and the presence of rare species indicate the unique character of this garden.
To maintain or even extend this diversity will require that the insect- and fauna-friendly management of the garden is continued, if not intensified. Certainly the innumerable micro-residents of the garden should be maintained and well managed. In their larval stage, many of the beetles inventoried live off dead branches or twigs of old trees. Plans to cut down a large number of trees to improve the view from building known as the 'banana' suggest that the good management needed cannot be expected from Het Depot art foundation.
Part of the garden has recently been fenced in. There are building plans for the part that includes the ponds, the system garden, the herb garden and the location of the former rose collection, which together constitute one third of the original area of the arboretum. This area contributes to the total diversity of microhabitats in the garden, and thus to the species diversity. For example, the ground beetle Bembidion octomaculatum was found on the banks of the large pond. This species is considered extremely rare and is hardly found outside of South Limburg in the Netherlands. In the rosebed, Helophorus porculus has been collected. This is a rare water beetle, which had rarely been sighted before 1950. Building on the pond side of the gardens will therefore seriously affect the diversity of insects and other invertebrates.
We agree with the 50 Wageningen professors that the Dreijen arboretum deserves to be preserved as cultural-historical heritage. Moreover, our inventory has clearly shown that the garden in its current form is of outstanding natural value. The garden should therefore be kept at its current size, and should be managed in keeping with its value. In the Veluwepost of 5 November 2009. Louis van Kessel suggests donating the two arboreta which are currently under the management of Wageningen UR to the Gelders Landschap foundation. An excellent proposal: this would be a way for Wageningen UR to fulfil its social responsibilities...
Theodoor Heijerman, on behalf of the insect working group of the KNNV, Wageningen branch.