Warming alters herbivore control of plant life history New paper in Ecology

Insect herbivores play an important role in plant populations. The can determine population size by consuming and killing plants. They determine plant fitness, either by consuming seeds, reducing photosynthesis rates by consuming leaves, or by forcing plants to invest limited resources into defensive chemistry rather than seed production.

Warming, caused by climate change, affects both plants and herbivores. Rising temperatures increase plant growth rates, affect the production of plant defensive chemicals, and can also affect plant seed production. On top of that, warming often increases herbivore metabolic rates, causing herbivores to increase their consumption of plant material. Such increased consumption rates may strengthen the effect of herbivores on plants, but the direct effects of warming on plants (i.e. higher growth rates), could offset the increased herbivore damage. My co-authors and I just published a new paper in the journal Ecology, titled “Responses of plant phenology, growth, defense, and reproduction to interactive effects of warming and insect herbivory“, that examines whether rising temperatures might disrupt the ability of insects to influence plant life history.

What we found surprised us. At normal temperatures, plants were able to compensate for herbivory.  Sure, plants exposed to insects were shorter and produced fewer fruits, but each fruit was packed with more seeds. As a result, total seed biomass was unaffected by insects (although seeds were smaller).

Warming changed all that.  Plants were shorter and they produced more fruits. However, fruits were smaller, and packed with many small seeds. It was quite astonishing, however, because herbivores no longer had any effect. Despite high leaf damage, plant life history was unaffected by the presence of herbivores.

Why would this happen, why would warming negate any influence of insects on plants? We don’t know for sure, but we suspect that warming reduced soil moisture (a common effect of rising temperatures). Low soil water content appears to have induced a stress response, causing stunted growth in the plants and altering their life history strategy. Interestingly, the stress response to low soil moisture appears to have superseded any effect of herbivores.

Herbivores play an incredibly important role in plant ecology; they drive plant population dynamics and evolution. Our results suggest that climate warming might drastically alter the role of insects in plant communities.

Undergraduate Research Paper – Phosphorus and Grasshoppers

I’d like to congratulate my REU from last summer, Maddi Rode, whose paper “Prospective evidence for independent nitrogen and phosphorus limitation of grasshopper (Chorthippus curtipennis) growth in a tallgrass prairie” was just published in PLOS One

 Nitrogen, a critical component of amino acids and proteins, has long been considered the primary limiting nutrient of terrestrial insects. Other nutrients have generally received much less attentions. However, phosphorus is a crucial component of larval growth, given the tight coupling between phosphorus-rich RNA and growth rates. Indeed, the strength of phosphorus limitation in terrestrial insects is often just as strong as nitrogen limitation. However, few studies have enriched plants with both nitrogen and phosphorus (separately and together) to determine the relative strengths of nutrient limitation.

Maddi spent the summer at Konza Prairie Biological Station doing just that. She enriched plots of Andropogon gerardii, or big bluestem, with nitrogen, phosphorus, or a combination of the two. We then tracked the growth of the marsh meadow grasshopper, Chorthippus curtipennis, under all conditions.

Chorthippus curtipennis. http://bugguide.net/node/view/699668

She found that nitrogen enrichment led to higher grasshopper growth rates. Surprisingly (or unsurprisingly to us), phosphorus enrichment stimulated grasshopper growth by nearly the exact same amount as nitrogen enrichment. 

This work adds to the building body of literature that grasshoppers, and indeed most terrestrial insects, are limited by a suite of nutrients beyond simply phosphorus. What this means for herbivore feeding behavior and climate change remains to be seen…

New Paper on Mutualisms in Ecology Letters

Good news, everyone! My friend and collaborator Andy Shantz and I, along with our PhD. advisor Deron Burkepile, just published a new paper in Ecology Letters regarding the effects of nutrient enrichment (i.e. fertilizer, phosphorus runoff, sewage outfalls) on nutrient-sharing mutualisms.

Nutrient-sharing mutualisms occur when two species cooperate to exchange necessary nutrients between them. The most common example is plants and mycorrhizal fungi. These fungi live on or in the roots of plants and are very good at scavenging rare nutrients, such as nitrogen or phosphorus, from the soils. However, they are not very good at obtaining sugars, carbohydrates, and other carbon-based nutrients. Plants have the opposite problem. They fix sunlight and carbon dioxide into sugars and carbohydrates, but often cannot take up enough nitrogen from the soils. Mycorrhizae and plants share nutrients, with fungi receiving sugars in exchange for nitrogen, to the benefit of both parties. Corals and algae are another common example, wherein corals provide nitrogen to their photosynthetic algae partners in exchange for sugars.

Beneath the snow and dirt lies a vast network of roots and fungi that support this Ponderosa pine forest
Beneath the snow and dirt lies a vast network of roots and fungi that support this Ponderosa pine forest

The stability of these mutualisms hinges on the relative rarity of important nutrients, in this case nitrogen and phosphorus. There is some evidence that the plant, or more generally the photosynthesizing partner, regulates the amount of nutrients in the trade. Theory predicts that if limiting nutrients, like nitrogen, suddenly become non-limiting, then the mutualism should fall apart. After all, why should the plant continue to trade away valuable sugars when it no longer needs the nitrogen from the fungi?

Andy and I tested this prediction using a meta-analysis, which is a quantitative synthesis of published results. Sure enough, we found that nutrient-sharing mutualisms subject to nutrient enrichment showed signs of collapse. That is, when soils or water were fertilized with nitrogen or phosphorus, the heterotrophic partner (corals, fungi) suffered lower performance because the phototrophic partner (plants, algae) ceased trading sugars for nutrients.

What does this mean? Well, nutrient-sharing symbioses form the foundation of most ecosystems: most plants harbor mutualistic fungi, corals cannot exist with their algal symbionts, and lichens exist as a mutualism between fungi and algae. If these mutualisms weaken, then the foundation of most communities also weakens, although just how much remains to be seen.

Atala Season!

Over the past few weeks, Blue Atala caterpillars have been out in force. A single coontie plant in the park by my house could have 15 – 20 caterpillars. A week ago, the caterpillars all formed their chrysalids, so the coontie plants look like Christmas trees, evergreen shrubs with little red ornaments.

Atala chrysalids hanging from a coontie plant.
Atala chrysalids hanging from a coontie plant.

I can’t wait for the next week or so, when the park will be swarming with Atala butterflies.

Effects of Herbivory on Ecology of Treefall Gaps

Nate Lemoine, FIU PhD candidate and Smithsonian researcher, sprays treefall gaps within the Smithsonian Environmental Research Center with herbicide. Photos by D. Doublet
Nate Lemoine, FIU PhD candidate and Smithsonian researcher, sprays treefall gaps within the Smithsonian Environmental Research Center with herbicide. Photos by D. Doublet

Naturally-occurring treefall gaps are an important part of forest ecology, playing a prominent role in the regeneration of both pioneer and non-pioneer tree species. Nate Lemoine is setting out to understand how insect herbivory plays a role in the growth and health of plants at treefall gaps. By caging small plots within gaps, he is deterring deer and other animals from eating the plants. He is also using herbicide to deter insects from some plots to compare to controlled plots where only water is sprayed.

By Dejeanne Doublet

Caterpillar Food 101

By Dejeanne Doublet, intern Conducting research with insects means that you must take on the roles and duties of a caretaker.

This summer we worked with the caterpillar species Spodoptera exigua (beet armyworms) and their close relative Spodoptera frugiperda (fall armyworms). Both species were shipped to us in sheets of eggs containing roughly 1,000 caterpillars. Most caterpillars start out their lives as eggs on leaves. They usually don’t get to pick and choose their food at the beginning of their lives, and are usually forced to eat from the plant or tree where they hatched.

Some caterpillars will enter a wandering stage once they’re big enough. They may wander about from plant to plant, picking and choosing what they like best or they may stay put at one type of plant for the rest of their life.

  • The first step involves mixing the agar into boiling water until it is completely dissolved. Then, the temperature should to reduced to 85 degrees before adding in the remaining ingredients.

When you’re a caterpillar shipped to a lab with 999 other caterpillars, though, you don’t necessarily get to have an all-you-can-buffet of cuisines. Our lab doesn’t quite function like a Subway and you can’t always have it your way. However, we do make sure you’re eating all your nutrients and vitamins with a homemade concoction of Lepidoptera food.

The recipe consists of more than a dozen ingredients that are simply mixed into boiling water. Click here for the full recipe. Once the ingredients are mixed together, they form a thick substance that cools down to form a tapioca pudding-like concoction.

We poured the finished food goo into 8 oz. plastic containers, allowed it to cool, then placed small pieces of the sheets containing caterpillars eggs on top of the food. We then placed the containers in a 30 ºC chamber under lights that mimic a 16-hour day and 8-hour night cycle. The caterpillars usually begin to hatch a day from when they arrive as eggs. Within a few days, they’re growing and eating and growing and eating.

Spodoptera exigua at about a week old
Spodoptera exigua at about 10 days old. Photos by D. Doublet