Photo: Goats eating noxious weeds, central Texas. © 2018 Delena Norris-Tull
The Role of Herbivory in Biological Invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Schierenbeck, Mack, and Sharitz, 1994, examined the effects of herbivory on growth and biomass allocation in native and introduced species of Lonicera. They carried out field experiments in an abandoned field, to compare responses to herbivory between the invasive Lonicera japonica (Japanese honeysuckle, a woody vine from Asia) and the North American native Lonicera sempervirens (coral honeysuckle, a woody vine). The two species share some common traits. Both grow in the southeastern US, can grow rapidly through vegetative propagation, tend to bear leaves throughout the year, and are found in similar ecological conditions and soils. Japanese honeysuckle was introduced into the US as forage for wildlife and cattle. The study sites were agricultural fields (corn and cotton) that had been abandoned over 40 years prior, and that had become forests of mixed hardwoods and pines, with both species of Lonicera growing in the forests. The experiments involved three treatments on both species: no herbivory, insect herbivory, and mammal and insect herbivory. They carried out the randomized treatments in 180 plots. Lonicera cuttings were collected from the field sites and propagated in greenhouses, and then planted in the plots. Some plots were given unlimited access to herbivory of insects and mammals. The treatment plots with no herbivory and with insect herbivory were enclosed in cages of PVC pipe and hardware cloth. The no-herbivory plots were treated with the insecticide Isotox, or with water in the control plots. To determine whether Isotox would affect the growth of either Lonicera species, cuttings were grown and treated in a greenhouse. The Isotox was found to have no affect on Lonicera growth in the greenhouse experiments.
One plant of each Lonicera species was harvested above and below ground, from each plot, on seven harvest dates from July 1990 to August 1991. Leaf areas were measured. After oven-drying, biomass of roots, leaves, stems and branches were measured. “Relative growth rate, net assimilation rate, leaf area ratio, specific leaf area, and root: shoot ratio were calculated at each harvest.” Herbivory was visually assessed by the percent of leaf area missing, per plant.
Results of insect herbivory treatment: Schierenbeck, et al, 1994, found that insect herbivores were mainly indigenous insects, hawk moths and wax moths. Lonicera sempervirens had more damage than did Lonicera japonica for seven of the nine harvest dates, but difference in damage was significant only for the final harvest date.
Results of insect and mammal (unlimited) herbivory treatment: Schierenbeck, et al, 1994, found that differences in the amount of damage, with L. sempervirens receiving greater damage, were more pronounced with the addition of mammal herbivory.
Results of no-herbivory treatment: While they were unable to prevent all herbivory, very minimal herbivory (less than or equal to 5%) occurred on either Lonicera species.
Growth comparisons, no-herbivory treatment: for L. japonica, Schierenbeck, et al, 1994, found no significant differences occurred in net assimilation rate or leaf area ratio, at any harvest date. And specific leaf area was significantly higher in four harvest dates. L. sempervirens experienced no differences in net assimilation rate for any harvest dates. Specific leaf area was highest for the January harvest date. Leaf area ratios for L. sempervirens were significantly higher than in the insect + mammal herbivory treatment, for the final four harvest dates. L. sempervirens had relative growth rates about equal to or sometimes greater than L. japonica, at eight harvest dates.
Total leaf area comparisons, no-herbivory and insect herbivory treatments: L. sempervirens had total leaf areas about equal to or greater than L. japonica at eight harvest dates, with L. sempervirens having significantly higher total leaf areas in four harvest dates, including the August date. In the insect + mammal herbivory treatment, L. japonica exhibited significantly higher total leaf area by September.
Root: shoot ratio: L. japonica showed no between-treatment differences at any harvest. L. sempervirens had an increased ratio with herbivory, which was significantly higher with unlimited herbivory in the final three harvests, when compared to the other two treatments.
Biomass allocation: Schierenbeck, et al, 1994, found that L. sempervirens and L. japonica showed opposite trends in biomass allocation. With no-herbivory, L. japonica had little increase in total biomass. L. japonica had its greatest increase in biomass in the insect+mammal herbivory treatment. With unlimited herbivory, L. japonica also had a higher allocation to secondary leaves and stems than L. sempervirens, after one year. L. sempervirens showed the greatest biomass increase with no-herbivory, and the least increase with unlimited herbivory.
Schierenbeck, et al, 1994, found that “the ability to display compensatory regrowth complicates any assessment. A compensatory response… adds a new dimension to the long-held view of the link between invasive ability and herbivory… Without herbivory, the native honeysuckle… expresses a comparative advantage in vegetative growth… Nevertheless, the biomass allocation patterns… indicate that [the invasive] L. japonica can compensate for leaf area lost to herbivory, whereas [the native] L. sempervirens shows progressively less of an increase in total biomass with increased herbivory… With unlimited herbivory, L. japonica allocates more carbon than L. sempervirens to secondary leaves and stems; these differences… between the species become more pronounced as the plants grow… This continually larger allocation to leaves could result in an exponential increase in growth.” Due to slight differences in the timing of peak leaf expansion, “L. japonica has two advantages,...more time to construct leaves and as much as two months of growth before insect herbivory begins.” And a slight “difference in leaf retention may give L. japonica an advantage… in resource-limiting environments.”
Schierenbeck, et al., 1994, concluded their report with the following, “Much of the difficulty in explaining the causes of plant invasions may stem from seeking a single explanation when invasions more likely stem from multiple traits interacting with multiple features of the new range.”
Next Section:
Evolutionary Reduced Competitive Ability
For summaries of other research on the success of invasive species, click on any of these links:
The Role of Herbivory in Biological Invasions
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
July 2020.
Schierenbeck, Mack, and Sharitz, 1994, examined the effects of herbivory on growth and biomass allocation in native and introduced species of Lonicera. They carried out field experiments in an abandoned field, to compare responses to herbivory between the invasive Lonicera japonica (Japanese honeysuckle, a woody vine from Asia) and the North American native Lonicera sempervirens (coral honeysuckle, a woody vine). The two species share some common traits. Both grow in the southeastern US, can grow rapidly through vegetative propagation, tend to bear leaves throughout the year, and are found in similar ecological conditions and soils. Japanese honeysuckle was introduced into the US as forage for wildlife and cattle. The study sites were agricultural fields (corn and cotton) that had been abandoned over 40 years prior, and that had become forests of mixed hardwoods and pines, with both species of Lonicera growing in the forests. The experiments involved three treatments on both species: no herbivory, insect herbivory, and mammal and insect herbivory. They carried out the randomized treatments in 180 plots. Lonicera cuttings were collected from the field sites and propagated in greenhouses, and then planted in the plots. Some plots were given unlimited access to herbivory of insects and mammals. The treatment plots with no herbivory and with insect herbivory were enclosed in cages of PVC pipe and hardware cloth. The no-herbivory plots were treated with the insecticide Isotox, or with water in the control plots. To determine whether Isotox would affect the growth of either Lonicera species, cuttings were grown and treated in a greenhouse. The Isotox was found to have no affect on Lonicera growth in the greenhouse experiments.
One plant of each Lonicera species was harvested above and below ground, from each plot, on seven harvest dates from July 1990 to August 1991. Leaf areas were measured. After oven-drying, biomass of roots, leaves, stems and branches were measured. “Relative growth rate, net assimilation rate, leaf area ratio, specific leaf area, and root: shoot ratio were calculated at each harvest.” Herbivory was visually assessed by the percent of leaf area missing, per plant.
Results of insect herbivory treatment: Schierenbeck, et al, 1994, found that insect herbivores were mainly indigenous insects, hawk moths and wax moths. Lonicera sempervirens had more damage than did Lonicera japonica for seven of the nine harvest dates, but difference in damage was significant only for the final harvest date.
Results of insect and mammal (unlimited) herbivory treatment: Schierenbeck, et al, 1994, found that differences in the amount of damage, with L. sempervirens receiving greater damage, were more pronounced with the addition of mammal herbivory.
Results of no-herbivory treatment: While they were unable to prevent all herbivory, very minimal herbivory (less than or equal to 5%) occurred on either Lonicera species.
Growth comparisons, no-herbivory treatment: for L. japonica, Schierenbeck, et al, 1994, found no significant differences occurred in net assimilation rate or leaf area ratio, at any harvest date. And specific leaf area was significantly higher in four harvest dates. L. sempervirens experienced no differences in net assimilation rate for any harvest dates. Specific leaf area was highest for the January harvest date. Leaf area ratios for L. sempervirens were significantly higher than in the insect + mammal herbivory treatment, for the final four harvest dates. L. sempervirens had relative growth rates about equal to or sometimes greater than L. japonica, at eight harvest dates.
Total leaf area comparisons, no-herbivory and insect herbivory treatments: L. sempervirens had total leaf areas about equal to or greater than L. japonica at eight harvest dates, with L. sempervirens having significantly higher total leaf areas in four harvest dates, including the August date. In the insect + mammal herbivory treatment, L. japonica exhibited significantly higher total leaf area by September.
Root: shoot ratio: L. japonica showed no between-treatment differences at any harvest. L. sempervirens had an increased ratio with herbivory, which was significantly higher with unlimited herbivory in the final three harvests, when compared to the other two treatments.
Biomass allocation: Schierenbeck, et al, 1994, found that L. sempervirens and L. japonica showed opposite trends in biomass allocation. With no-herbivory, L. japonica had little increase in total biomass. L. japonica had its greatest increase in biomass in the insect+mammal herbivory treatment. With unlimited herbivory, L. japonica also had a higher allocation to secondary leaves and stems than L. sempervirens, after one year. L. sempervirens showed the greatest biomass increase with no-herbivory, and the least increase with unlimited herbivory.
Schierenbeck, et al, 1994, found that “the ability to display compensatory regrowth complicates any assessment. A compensatory response… adds a new dimension to the long-held view of the link between invasive ability and herbivory… Without herbivory, the native honeysuckle… expresses a comparative advantage in vegetative growth… Nevertheless, the biomass allocation patterns… indicate that [the invasive] L. japonica can compensate for leaf area lost to herbivory, whereas [the native] L. sempervirens shows progressively less of an increase in total biomass with increased herbivory… With unlimited herbivory, L. japonica allocates more carbon than L. sempervirens to secondary leaves and stems; these differences… between the species become more pronounced as the plants grow… This continually larger allocation to leaves could result in an exponential increase in growth.” Due to slight differences in the timing of peak leaf expansion, “L. japonica has two advantages,...more time to construct leaves and as much as two months of growth before insect herbivory begins.” And a slight “difference in leaf retention may give L. japonica an advantage… in resource-limiting environments.”
Schierenbeck, et al., 1994, concluded their report with the following, “Much of the difficulty in explaining the causes of plant invasions may stem from seeking a single explanation when invasions more likely stem from multiple traits interacting with multiple features of the new range.”
Next Section:
Evolutionary Reduced Competitive Ability
For summaries of other research on the success of invasive species, click on any of these links: