Mutualism vs. Symbiosis

Ecologists sometimes use the terms “mutualism” and “symbiosis” interchangeably, and I wish that they would not do so! I recently bought a copy of Judie Bronstein’s new book about mutualism, and the first two chapters are devoted to defining the terms mutualism and symbiosis, and distinguishing between the two. Here, I’ll outline the main points. For a longer treatment of this topic, check out the book!

Mutualism is an ecological interaction between at least two species (=partners) where both partners benefit from the relationship.

Symbiosis is an ecological interaction between at least two species (=partners) where there is persistent contact between the partners.

In symbioses, one partner is often smaller, lives on or in the larger partner, and has a shorter lifespan that the larger partner. The smaller partner is a symbiont, and the larger partner is a host.

Not all mutualisms are symbioses. Some mutualisms are non-persistent, like when a pollinator very briefly visits a flower and then never returns to that flower again. Additionally, not all symbiosis are mutualistic. For instance, parasites are also symbionts.

A few months ago, I posted this useful classification scheme for thinking about different types of ecological interactions. The two axes are the “relative duration of association” and “net effect of the relationship on the ‘host.’” Mutualisms are any relationships on the right-hand/positive side of the net effect of the relationship on the ‘host’ axis, regardless of where the relationship falls on the vertical axis. Symbioses are relationships at the ‘top’ of the relative duration of association axis – where much of the symbiont’s lifespan is spent in association with the same host – regardless of where the relationship falls on the horizontal axis.


Effects of Symbionts on Host Population Dynamics

Sometimes it can be really difficult to determine whether a symbiont is a mutualist, a commensal, or a parasite of its host. The context-dependent nature of these relationships is partially to blame for our inability to stick a label on any given symbiosis, because the net outcome of the relationship might vary every time we try to measure it! But even if we limit our focus to just one set of conditions, it can be difficult to say for sure what net impact the symbiont has on host fitness, because we may not be able to simultaneously quantify all of the ways that the symbiont might affect the host. For instance, we might find that the symbiont does not affect host survival, and then we might be tempted to call the symbiont a commensal. But if we didn’t measure host reproductive output as well as survival, how do we know that the symbiont didn’t affect that particular host vital rate? And to make things more confusing, how long-term must our measurements be? Do we need to record every detail from the second the host is born until the second it dies? And then do we need to do the same for each of the host’s offspring?

In this post, I’m not going to answer any of those questions. (GOTCHA!) Instead, let’s talk about a beautifully written and relevant paper about ants on cacti (Ford et al. 2015). If you need a refresher on the incredibly cool ecological relationships between ants and myrmecophytic plants (and/or you want to see some sweet photography by Alex Wild), check out this post. Otherwise, I’ll assume you’re on board with the terminology that I’m using.

The fishhook barrel cactus sports a bunch of extrafloral nectaries that are visited by ants. If insects visit the plant’s fruits or flowers when there are ants on guard, those insects might get attacked by the ants. If the ants are deterring herbivorous insects, they might be positively affecting host fitness. To determine whether that was the case, Ford et al. (2015) quantified the effects that ants had on three host vital rates: number of fruits, plant growth rate, and plant survival. Ants increased the number of fruits produced by the plants, but ants did not affect plant growth rates or survival.

If ants are increasing fruit output, that should have important implications for cactus population dynamics, right? Actually…maybe not! Ford et al. (2015) used some integral projection models to show that cactus population dynamics were really sensitive to cactus growth rates and survival probabilities, but cactus population dynamics weren’t affected by the number of fruits produced per plant – the one vital rate that ants affected.

So, Ford et al. (2015) used three years of very detailed survey data to show that ants don’t seem to be having any affect on cactus population dynamics. But what if they were to follow these populations for a longer period? What if ants don’t really affect the cactus population most years, but they have a big effect during the rare years where there are “catastrophes” (i.e., outbreaks of herbivorous insects) or “bonanzas” (i.e., relatively wet years)? Ford et al. (2015) used some simulations to show that ants could potentially have positive effects on host population dynamics in the long-tem, especially when there were high frequencies or intensities of certain catastrophes or bonanzas. Neat!



Ford, K.R., J.H. Ness, J.L. Bronstein, and W.F. Morris. 2015. The demographic consequences of mutualism: ants increase host‑plant fruit production but not population growth. Oecologia.

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.

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!


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.


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

Defended Hosts are Frassheads

Last week, I told you guys that parasitoid wasps respond to H. defensa, which is a bacterial endosymbiont that protects aphids from wasps.  Next week, I’m going to talk about how H. defensa affects aphid fitness.  But first, what is the magnitude of the protective effect?  Well, it varies with the strain of H. defensa and the aphid species and probably lots of other factors, too.  But in the example that I’m going to discuss next week, aphids without H. defensa have a 38% probability of becoming mummies if they get attacked by wasps, while aphids with H. defensa only have a 4% chance of becoming mummies if they get attacked by wasps (Vorburger et al. 2013).  So, H. defensa reduces the probability of mortality after attack by 89.5%!



Stay tuned to see what happens to Sal and Lisa.  Are they up Frass Creek without a paddle?


Vorburger, C., P. Ganesanandamoorthy, and M. Kwiatkowski. 2013. Comparing constitutive and induced costs of symbiont conferred resistance to parasitoids in aphids. Ecology and Evolution 3(3):706-13.


How do parasitoids respond to defended hosts?

Last week, I talked about the new Godzilla movie and how I thought that the MUTOs should have been parasitoids.  This week, let’s talk about some awesome, real life parasitoids: parasitoid wasps (Aphidius ervi).

Quickly, the life cycle works like this: the female wasp finds an aphid nymph, she stabs the aphid with her ovipositor, and then she typically lays one egg inside the aphid.  After one day, the egg hatches into a larval parasitoid, and the larva hangs out inside the aphid while eating the aphid’s innards.  After about one week of this, the aphid dies.  Actually, the aphid’s corpse becomes a “mummy,” and the larva pupates inside the mummy before eventually emerging as an adult parasitoid.  Mating happens, and then the female wasps go off to infect more aphids.

But here’s an interesting complication: some aphids are protected by bacterial symbionts (Hamiltonella defensa).  The degree to which aphids are protected varies with the strain of H. defensa, but the take-home message is that when a wasp lays an egg inside an aphid, the egg is much less likely to survive to adulthood if the aphid has H. defensa symbionts (Oliver et al. 2012).  However, if a wasp lays two eggs inside an aphid with H. defensa symbionts – which is not what wasps usually do – then one larva is more likely to survive than it would have been if it had been a single egg.  In other words, only one larva is going to make it out of there alive, even when two eggs are laid, but one of the larvae is better off than it would have been if it were a single egg.

You might be thinking, “Why, what an interesting tidbit.  Who cares?”  NATURAL SELECTION CARES.  Just kidding, natural selection isn’t sentient, but natural selection should favor any wasp strategies that increase wasp fitness.  And wasp fitness is higher when more wasp eggies turn into wasp larvae and then adult wasps.  And wasp larvae are less likely to die in aphids with H. defensa if two eggs are laid in the aphid, instead of the typical single egg.  See where I’m going with this?

Yes, wasps can differentiate between aphids with and without H. defensa.  And when aphids have H. defensa, wasps are much more likely to lay two eggs in those defended aphids than they are to lay two eggs in undefended aphids.  And that, my friends, is amazing.  While wasps still probably have reduced fitness when infecting defended aphids, the superinfecting tactic (=laying two eggs) likely compensates for some of the reduced fitness.



(Yes, sometimes aphids have conversations in my head, and I write them down. You’re welcome.)

Check out the open access paper to learn about the mechanism behind the success of superinfection:


Oliver, K.M., K. Noge, E.M. Huang, J.M. Campos, J.X. Becerra, and M.S. Hunter. 2012. Parasitic wasp responses to symbiont-based defense in aphids. BMC Biology 10:11.