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.