Amur honeysuckle (Lonicera maackii) and autumn olive (Elaeagnus umbellata) are two prominent invaders of rural and suburban situations in the lower Midwest. Honeysuckle colonizes a wide range of light conditions, including open grassland, forest edge, and even deeply shaded forest interior habitats. Autumn olive is restricted to grassland and forest edge habitats. Both species produce large quantities of red berries in late summer, which disappear over several months. In this study we monitored fruit disappearance rates at 6 sites in southwestern Illinois from mid-September to the end of December. We tested the following hypotheses: (1) Fruit disappearance rates for autumn olive are typically higher than for honeysuckle (honeysuckle is of poor nutritional quality); (2) Fruit disappears more quickly from forest edge honeysuckle than interior shrubs owing to its greater apparency in the edge; and (3) Fruits positioned high on shrubs disappear more quickly than fruits lower in the canopy, again because of greater apparency.
Results/Conclusions
Fruit disappearance rates were significantly higher in autumn olive than in honeysuckle. It required about 23d for half of autumn olive fruits to be taken, as compared to 45d for honeysuckle. After 58d 90% of autumn olive fruits were gone, as compared to 85d for 90% of honeysuckle. Both edge and interior honeysuckle required the about same period of time for complete removal (ca. 100d), but removal rates for interior shrubs were about 85% of those for edge shrubs. For honeysuckle, fruits in the lower branches of edge shrubs disappeared more rapidly than fruits in any other situation. Fruits in the lower branches of autumn olive also disappeared more quickly than those in upper branches. Presuming honeysuckle and autumn olive share the same pool of frugivores, it is clear that autumn olive is preferred. Whether this has any significance to its invasive capacity depends on the behavior of the dispersers and the habitats they visit after eating. For honeysuckle, edge dwelling birds represent a pool of dispersers that is not only more likely to take fruit, but to redeposit it in the forest edge, a more favorable habitat for honeysuckle. The preference for fruit on lower branches perhaps reflects the desire for concealment on the part of bird dispersers. Alternatively, these fruits may be taken by small climbing rodents. Very few fruit are found beneath shrubs, indicating nearly complete removal by animals.
