Bears Indirectly Affect Plant Fitness

If you haven’t seen it yet, there’s a really cool paper in Ecology Letters about the indirect effects of bears on ecological communities (Grinath et al. 2015). Did you know that bears will eat ants? Well, they will! Especially during periods when they are food limited. And when bears disturb ant nests, the ants stop tending the leaf and tree hoppers (=herbivorous insects) that they farm for honeydew. Without ants patrolling nearby, tree hoppers experience higher predation pressures from other arthropod predators, like lady beetles and spiders. And when the densities of herbivorous insects decline, plant fitness increases. So, by eating ants, bears can increase plant fitness! Nuts!

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I glossed over some of the details of this story, such as variation in the effects of bears across years. To get all of the details and to see some cool structural equation modeling, go check out the paper!

Reference:

Grinath, J.B., B.D. Inouye, and N. Underwood. 2015. Bears benefit plants via a cascade with both antagonistic and mutualistic interactions. Ecology Letters 18(2): 164-173.

The benefits of attracting many symbiont species

Textbook examples of mutualisms usually involve two interacting species; for instance, honey bees and clover. But of course, one host may have many mutualists (e.g., bees, butterflies) and one mutualist may have many hosts (e.g., clover, blueberries). In many cases, the benefits of having multiple possible host species are obvious. For instance, for ants that eat tasty elaiosomes and disperse plant seeds, it would be hard to get by while specializing on the seeds of only one plant species, because those seeds would only be available for part of the year. But how does the host benefit from having many symbiont species?

In ant-plant interactions, one plant species may have individual visitors from many ant species. For instance, the shrub Urera baccifera receives visitors from 22 facultative ant species, which visit to harvest food bodies and/or fruits (Dutra et al. 2006). Similarly, the yellow alder (Turnera ulmifolia) receives visitors from 24 facultative ant species, which visit to harvest seeds with elaisomes and/or to feed at extrafloral nectaries (Cuautle et al. 2005). While they’re on the plant, these ants can protect the plant from herbivores, like caterpillars. And finally, there may be multiple ant species visitors to a given plant species even when the associations are obligate: for instance, Acacia drepanolobium may be inhabitated by one of four ant species at any given time (Palmer et al. 2010).

Ok, but why so many species of ant visitors? Why shouldn’t the plant sanction any partners except the most beneficial species? This is still an active area of research, but at least part of the answer is that different ant species vary in the services that they provide to the plant. For instance, ant species may be more of less aggressive defenders, more or less likely to disperse seeds, more or less likely to farm scale insects, etc. (You might remember that this is true for guard crabs, too, where big crab species are good at protecting corals against starfish, while small crab species are good at protecting against vermetid snails.) Ant species also vary in how costly they are to harbor. For instance, one species may be a good defender, but it may also sterilize the host plant (e.g., Stanton 1999). So, a plant’s lifetime fitness may be determined not only by the guild of ant species that visits the plant during its life, but also on the timing and order in which those species visit the plant (Palmer et al. 2010). For instance, having sterilizing symbionts when you’re young can actually increase lifetime plant fitness. Isn’t that wild?

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References:

Cuautle M, Rico-Gray V, Diaz-Castelazo C (2005) Effects of ant behaviour and presence of extrafloral nectaries on seed dispersal of the Neotropical myrmecochore Turnera ulmifolia L. (Turneraceae). Biological journal of the Linnean Society 86: 67-77.

Dutra HP, Freitas VL, Oliveira PS (2006) Dual Ant Attraction in the Neotropical Shrub Urera baccifera (Urticaceae): The Role of Ant Visitation to Pearl Bodies and Fruits in Herbivore Deterrence and Leaf Longevity. Functional Ecology 20(2): 252-260.

Palmera TM, Doak DF, Stanton ML, Bronstein JL, Kiers T (2010) Synergy of multiple partners, including freeloaders,increases host fitness in a multispecies mutualism. PNAS 107(40): 17234–17239.

Stanton ML, Palmer TM, Young TP, Evans A, Turner ML (1999) Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant. Nature 401:578–581.

Myrmecophytic Plants and Their Ants

You can find ants and plants in almost every terrestrial habitat on the planet. Both groups can be incredibly abundant, so it isn’t surprising that ants and plants interact a lot. But some plants and ants have intimate symbiotic relationships that go far beyond the occasional interaction. Some of my favorite ecological stories involve these symbioses, and I’m going to post those stories in the coming weeks. But this week, I just want to introduce you to the system and let the insane photography skills of Alex Wild bring these organisms into your life.

Myrmecophytic plants: Who are they, and what do they provide their ant symbionts?

There are many genera of myrmecophytic plants, including flowers, shrubs, trees, and even ferns. These plants vary widely in the degree to which they invest in their ant symbionts. Below is a list of the structures that plants have evolved to provide their ants with resources, but not all ant-plants have all of these structures.

Domatia: Domatia are hollow structures that the ants can use for nests. Depending on the plant species, these domatia may be hollow stems or spines. For instance, check out this hollow thorn on an Acacia tree and the hollow base of this epiphytic plant. Both are homes for ants!

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"Pseudomyrmex peperi"

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Extrafloral nectaries: Many plants provide nectar rewards in their flowers in order to attract pollinators. Ant-plants may also have extrafloral nectaries – structures that provide nectar but that are not associated with flowers. On ant-plants with domatia and active ant colonies, these extrafloral nectaries can feed resident ants. On ant-plants without domatia, these nectaries can attract ant visitors. Here are two gorgeous ants feeding at an extrafloral nectary.

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Food Bodies: Ants can’t just survive on nectar; they need resources other than sugar, too. Some ant-plants have evolved to produce food bodies that contain proteins or lipids that ants can harvest for those vital nutrients. Here’s an ant harvesting one such food body.

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Plant-ants: Who are they, and what do they provide their plants?

There are also many genera of ants that have symbiotic relationships with their plants. These ants can be facultative or obligate symbionts (meaning that they are only found living on plants), depending on the species. In the coming weeks, I’ll talk a lot more about the services that ants do (and do not) provide to plants, but here are the main points:

Defense: Plant-ants are feisty plant protectors! They can bite and sting herbivores or even throw the herbivores off the plant. Plant-ants will also attack competing plants. For instance, they will bite encroaching vines or inject neighboring plants with formic acid. (Yeah. For real. Look up Devil’s Garden.) Here are some ants getting rid of a vine, and some different ants hauling away an intruder ant.

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Nutrients: When ants die and defecate, they fertilize their plants. (I bet you can imagine this one without a photo. Also, arboreal earthworms are a thing, and plants eat their poop, too. You’re welcome.)

Seed Dispersal: Some plants produce seeds with tasty exterior food bodies called elaiosomes. The ants collect these seeds and eat off the elaisomes, then put the seeds in their waste piles. During this process, the seeds are dispersed, and they’re also protected from predators while they’re in the ant nests.

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Aren’t plant-ants and ant-plants cool?! You should check out Alex Wild’s website for more awesome photos. Stay tuned for more on plant-ant ecology next week!