Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

A retrospective analysis of pollen host plant use by stable and declining bumblebee species.

Abstract

Understanding population declines has been the objective of a wide range of ecological studies. When species have become rare such studies are complicated because particular behaviour or life history traits may be the cause but also the result of the decline of a species. We approached this problem by studying species' characteristics on specimens that were collected before the onset of their decline and preserved in Natural History Museums. In north-western Europe some bumblebee species have declined dramatically during the twentieth century whereas other, ecologically similar species have maintained stable national populations. A long-standing debate focuses on whether this is caused by declining species having stricter host plant preferences. We compared the composition of pollen loads of five bumblebee species with stable populations and five with declining populations using museum specimens collected before 1950 in Belgium, England and the Netherlands. Prior to 1950, the number of plant taxa in pollen loads of declining species was almost a third lower than that in stable species even though individuals of stable and declining species generally originated from the same areas. There were no systematic differences in the composition of pollen loads between stable and declining species but the plant taxa preferred by declining species before 1950 had experienced a stronger decline in the twentieth century than those preferred by stable species. In 2004 and 2005, we surveyed the areas where bumblebees had been caught in the past and compared the composition of past and\present pollen loads of the stable, but not of the by now locally extinct declining species. The number of collected pollen taxa was similar but the composition differed significantly between the two periods. Differences in composition reflected the major changes in land use in north-western Europe but also the spread of the invasive plant species Impatiens glandulifera. The main question now is why declining species were apparently not able to switch to less preferred food plants when stable species were. This study shows that natural history collections can play an important role in improving our understanding of the ecological mechanisms driving species population change.