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University of Michigan Biological Station

The University of Michigan Biological Station (UMBS) was founded in 1909.

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Herbivory as a limit to maximum prey capture

Organismal
nutrient responses
herbivore
carnivorous plant
Project Abstract: 
Coping with low-nutrient environments has led to the repeated evolution of plant carnivory. Given the repeated evolution of carnivory as well as the facultative nature of this otherwise costly trait, why are carnivorous plants not more speciose in wet, sunny, nutrient-poor sites? Recent evidence suggests herbivores may play an important role in limiting the success of plants with specialized nutrient acquisition strategies (e.g. nitrogen-fixing bacterial associates), as herbivores are drawn to more nutrient-rich plant tissue. To test this hypothesis in carnivorous plants, I will conduct a factorial herbivore exclusion and prey addition experiment on Sarracenia purpurea, the purple pitcher plant. Specifically, I will examine whether 1) plant growth rate is maximized at intermediate levels of prey intake, and 2) if this pattern is caused by preferential consumption by herbivores of plants with high nutrient intake. If my hypotheses are supported, I expect to see 1) growth is maximized at intermediate prey additions, with herbivory increasing with increasing prey intake 2) in the absence of herbivory, growth will not decrease at high prey concentrations. This work examines a broad pattern in herbivory at the within-species scale and expands the hypothesis to a very different nutrient acquisition strategy.
Investigators: 
Sylvie Martin-Eberhardt
Associates: 
webermg
Status of Research Project: 
Active
Years Active: 
2024 to 2025
Methods: 
The proposed experiment uses 200 adult S. purpurea plants growing at Mud Lake Bog (open area of bog accessed from the puncheon trail). At the beginning of the growing season in late May, half of the plants will be covered in a fine mesh bag (tule) to exclude herbivores, and half will have a sham treatment (bags with holes). The treatment is designed so that light and water can pass through the bag, but insects will be excluded. This herbivory exclusion treatment will be crossed with a prey addition treatment; each plant will be randomly assigned a prey addition from 0 to 200 mg of insect biomass added to each adult pitcher. By evacuating the pitcher before the start of the experiment and obstructing the pitcher opening with cotton after the treatment has been added, I can isolate the effect of nutrient intake while allowing herbivores access to pitcher leaves. The prey addition treatment will be repeated at monthly intervals throughout the growing season so as not to starve the plants of prey. After a the growth period (mid to late August), I will remove the treatment equipment (bags, cotton) and record RGR (relative growth rate, calculated as the total increase in leaf number and rosette size over the growth period) and flowering attempts. If more than 30 plants from each treatment flower, I would like to collect seed in the fall (October, November) from experimental plants to quantify the total seed set. Also, I would like to randomly harvest one pitcher per plant for foliar nitrogen analysis (C:N) and caterpillar choice bioassays using the generalist S. exigua. If my hypothesis is supported, caterpillars will preferentially feed on leaf disks from plants with a higher prey intake, which in turn will have a greater foliar nitrogen concentration. Finally, because S. purpurea is known to store nitrogen from year to year, I would like to quantify plant size and leaf number at the beginning of the following growing season (May 2025). This would require durable plant ID markers to be left in the bog during the 2024-2025 winter, to be retrieved in May 2025 upon collecting the second season new growth data.
Funding agency: 
Botanical Society of America