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?

antacacialongtermfitness

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.

Elepunts

Are elephants afraid of mice? Well, maybe. There isn’t much evidence, unless you like sample sizes of 1 individual. However, elephants are definitely afraid of ants, and that is a much more interesting ecological story.

Last week, I posted an introduction to the symbiosis between ants and plants. One of the services provided by ants is protection from herbivores. Those herbivores may be insects, like caterpillars and grasshoppers, but they may also be megafauna, like elephants.

Elephants love munching on Acacia trees, but some Acacia species are protected by ants. With the ants removed, elephants will gladly eat species that typically have ants. But when the ants are present, the elephants avoid defended trees (Goheen and Palmer 2010). This decision to avoid getting viciously stung by hordes of ants may have far-reaching consequences in savanna ecosystems: tree community composition is affected, because defended tree species are more likely to survive in areas with many elephants (Goheen and Palmer 2010).  Tiny symbionts can play big roles in ecological communities!

antsandelephant

References:

Goheen JR, Palmer TM (2010) Defensive plant-ants stabilize megaherbivore-driven landscape change in an African savanna. Curr Biol 20:1768–72.

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!

Guard Crab Functional Diversity

Continuing my theme of Odes to Awesome systems, I want to tell you guys about one of my favorite animals: guard crabs.  These crabs live on corals and deter starfish predators from attacking the corals by pinching the starfish.  The guard crab cute factor is out of this world:

Photo credit: Bryan Mayes

I’ve been fascinated by guard crabs ever since I read the really thorough studies of Peter Glynn from the 1970’s and 1980’s.  He did all kinds of work showing that guard crabs protect corals from predation by Ancanthaster starfish.  Not all coral species have guard crabs, and Peter Glynn showed that populations of unprotected coral species are decimated during Ancanthaster outbreaks, while protected species suffer few losses.  Furthermore, protected corals sometimes indirectly defend unprotected corals, because starfish won’t cross barriers of protected corals.

Previously, it seemed like some guard crab species were a lot better at protecting corals than others.  This is true in other defensive symbiont systems, too (e.g., ants that protect Acacia trees).  But I just read a really cool study by McKeon and Moore (2014) that shows that the slacker symbionts may not be as lazy as we thought!  Specifically, small guard crabs aren’t very good at protecting corals from starfish predation.  However, small guard crabs are good at protecting corals from smaller corallivores, like predatory snails.  Furthermore, big crabs are no good at protecting against those smaller corallivores.  (This is a cool parallel to the paper I talked about last week, where hydroids protect hermit crabs from limpets but not blue crabs.)

So, corals need functionally diverse guard crab communities – communities with large and small guard crabs – to be protected from multiple corallivore species.  Awwwwesome!

Crabbies

Reference:     

McKeon CS, Moore JM (2014) Species and size diversity in protective services offered by coral guard-crabs. PeerJ 2:e574.

50 Shades of Symbionts

When we “sell” our science to journals and policy makers and even the general public, we often pitch our work in broad, abstract strokes (“trait-mediated indirect effects of…”) and/or in a highly applied context (“acid runoff tolerance of two functionally important species”).  I’m not saying that’s wrong.  But I think that most scientists – and most non-scientists – fall in love with ecology because the systems (the actual plants/animals/etc.) are cool, and then we have to gloss over the insanely awesome systems that we study in order to talk about the general applicability of our results.  Well, no more brushing cool systems under the rug!  Parasite Ecology is taking action by doing a few weeks of Odes to Awesome Systems.

The best way to prove that you have an awesome study system is to graphically illustrate the unquestionable adorableness of your study species.  EXHIBIT A – Hermit Crabs with Pink Afros:

Photo from here.

BUR-100721 Gewone heremietkreeft, Pagurus bernhardus met Ruwe zeerasp, Hydractinia echinata

Photo from here.

Did you know that hermit crabs have over 500 symbiont species?  More than 100 of those symbionts are obligate symbionts, meaning that they are only found on/in/with hermit crabs.  I learned that while perusing a heartwarming tale entitled, “The Not So Lonely Lives of Hermit Crabs: Studies on Hermit Crab Symbionts.

One of those obligate symbionts is the pink afro (also called “snail fur”) in the photos above.  Those colonial hydroids (genus Hydractinia) are found exclusively on gastropod shells, and especially shells that are occupied by hermit crabs.  Unsurprisingly, scientists who go out and find hermit crabs with pink afros just have to ask this question:  do the hydroids affect their hermit crab hosts?

As I’ve blogged about before (here and here), some symbionts protect their hosts from natural enemies.  Buckley and Ebersole (1994) wondered if the hydroids could protect hermit crabs from being eaten by blue crabs.  They found that blue crabs were just as likely to attack hermit crabs with or without hydroids, so the hydroids didn’t have any effect on predator preference.  However, blue crabs were much more successful when attacking hermit crabs with hydroids.  Having hydroids actually made hermit crabs more susceptible to predation!

BUT… gastropod shell strength wasn’t associated with the presence of hydroids.  So what was it about hydroids that made it easier for blue crabs to successfully attack hermit crabs?  Well, a second, parasitic symbiont – shell-boring Polydoran worms – decreased shell strength, and those worms were more likely to be present if the shells had hydroids.  So, one symbiont mediated the occurrence of a second symbiont, which in turn mediated blue crab predation success.  Nuts!

Hermits

This could be the part of the story where we conclude that hydroids decrease hermit crab fitness.  But remember how most symbioses are context-dependent, where the strength and even the sign of the interaction depends on environmental and ecological conditions?  Well, it turns out that hydroids protect hermit crabs from a different enemy: ectoparasitic slipper limpets.  Therefore, Buckley and Ebersole (1994) suggest that the relationship between hydroids and hermit crabs changes throughout the year, depending on whether blue crabs and/or limpets are abundant.  That really emphasizes the importance of studying symbioses across broad time scales and under varying ecological and environmental conditions.

So, there you have it.  You can’t figure out hermit crab ecology without thinking about hermit crab symbionts.   Pink afros are more than just fashion statements.

Reference:

Buckley WJ, Ebersole JP (1994) Symbiotic organisms increase the vulnerability of a hermit crab to predation. J Exp Mar Bio Ecol 182:49–64.

Resistance vs. Tolerance to Parasites

In disease ecology and parasitology, we often talk about a host’s ability to resist or tolerate parasites.  What’s the difference?  Resistance is a measure of a host’s ability to reduce parasite establishment.  For instance, imagine that two hosts are each exposed to 10 parasites.  In the first host, 8 of those parasites manage to evade the host’s immune system and successfully establish, and in the second host, only 2 of the parasites successfully establish.  The second host is more resistant to infection.  Tolerance is a measure of a host’s ability to “deal with” a given parasite load.  Now imagine that two hosts each have 5 parasites.  Those parasites hardly affect the first host’s ability to survive or reproduce, but the same number of parasites causes a huge reduction in the second host’s ability to survive and reproduce.  The first host is more tolerant.  (A really great figure from Raberg et al. (2007) sums this up.)

What determines a host’s ability to resist or tolerate parasites?  Good question!  This is a hot topic for research.  Body condition (i.e., overall health) likely has something to do with resistance and tolerance.  And then there is that ever-present explainer of all the things: genetics (Raberg et al. 2007).  But today, I want to talk about something else.  Do paternal effects influence resistance and tolerance?

In a recent, awesome, open access study, Kaufmann et al. (2014) exposed three-spined stickleback “sires” (fathers/dads/sperm-makers) to nematodes.  Then they used sperm from either these exposed sires or unexposed sires to fertilize strickleback eggs.  Here’s what they found: when the sires were exposed to parasites, the eggs were less likely to develop and the juveniles were less likely to survive.   But if they took surviving offspring from both exposed and unexposed sires, and then exposed some of those offspring to nematodes, the offspring from exposed sires had higher tolerance to parasites.  Specifically, parasites had a big effect on the body condition of offspring from unexposed sires, but no effect on offspring from exposed sires.  Neat!  Surprisingly, parental effects didn’t influence offspring resistance to parasites.  Unsurprisingly, genetics also played a role in both resistance and tolerance.

References:

Raberg, L., D. Sim, and A.F. Read. 2007. Disentangling Genetic Variation for Resistance and Tolerance to Infectious Diseases in Animals. Science 318(5851): 812-814.

Kaufmann, J., T.L. Lenz, M. Milinski, and C. Eizaguirre.  2014. Experimental parasite infection reveals costs and benefits of paternal effects. Ecology Letters. (Open access.)