To make a meadow it takes a clover and a bee: the entomophilous flora of N.W. Europe and its insects

An analysis of the anthophilous fauna of N.W. Europe is presented, stressing the role plants play for insects. The study is based on some 29,000 relations between about 2,600 insect species and 1,300 plant species (569 genera). The data are derived from our database (“CrypTra”) of biotic relations between Cryptobiota and Tracheophyta, that is based on published sources. It is suggested that a ratio of 2 to 5 anthophilous insect species per entomophilous plant species is the rule in N.W. Europe, where other types of zoophily are virtually absent. A small minority of the plant species/genera play a disproportionally important role as hosts to flower visitors; many of these so-called cornucopian taxa belong to the commonest entomophilous plants in the region, and occur also in moderately disturbed habitats. There is a significant positive correlation between the commonness of a plant species and the fraction this plant represents of the trophic resources exploited by an insect species. There is, on the other hand, a significant negative correlation between the number of insect species visiting a given plant species, and the number of plant species visited by a given insect species. These two elements together demonstrate that the anthophilous fauna and the entomophilous flora of N.W. Europe as a whole form a loose system, not predominantly characterised by specialisation. In accordance with this, factor analysis suggests that there is no ground to recognise more than three visitor types, viz., the allotropous, hemitropous, and eutropous visitors as defined by Loew. A minority of the plant taxa – essentially the cornucopian ones – can with some difficulty be associated with these three types of visitors, and a very few narrowly specialised plant taxa can be associated with more specific visitor groups. However, the large majority of plants cannot be fitted in any typology. These results have practical implications for the nature management of the anthophilous fauna, in that the important role of the cornucopian floral element is underlined. The fact that the majority of the cornucopian species are perennial, or even woody, places constraints to agricultural practices intended to foster beneficial anthophilous insects.