Context-Dependent Species Interactions

For a few weeks, I’ve been writing about branchiobdellids and crayfish, and about how the relationship between the symbiont and host can vary with ecological context (e.g., crayfish size, how fast the crayfish can be colonized by bacteria in a given environment, branchiobdellid density).  I intended to change topics this week, but a really cool Ecology Letters paper just came out about the context-dependent nature of species interactions, so I MUST tell you guys about it.

Let’s start with the worst part of the paper, first:  they didn’t include parasites!  😛  The interaction types they considered were predation, competition, and mutualism.  However, as I showed a few weeks ago, mutualists can act like parasites, so I’m deeming this paper worthy of the Parasite Ecology blog.

When we say that “interactions are context-dependent,” we mean that either the strength or the sign of the interaction changes as conditions change.  For instance, if parasites kill their hosts when the hosts are in low resource conditions but the same parasites have no noticeable affects on hosts when the hosts are in high resource conditions, the strength of the interaction changes with resource conditions.  A “sign change” happens if the interaction goes from positive (mutualism) to neutral (commensalism) or negative (parasitism), or vice versa.

So, what kind of “conditions” are we talking about?  In their meta-analysis, Chamberlain et al. (2014) considered abiotic conditions (e.g., pH, sunlight), space (e.g., United States vs. Europe), time (this year vs. next year), and the presence of third party species.  The last one may seem like a non sequitur, but here’s an example: do you remember that post I did about the crabs that protect urchins from parasitic snails?  We might expect the crabs to act like mutualists of urchins (=positive interaction) in the presence of parasitic snails but as commensalists of urchins (=neutral interaction) in the absence of parasitic snails.  That would be a sign change.  Ok, finally, Chamberlain et al. (2014) didn’t look at one condition that I find particularly interesting – the abundances of the species.  Like, remember how branchiobdellids are mutualistic at intermediate abundances but parasitic at high abundances?  Chamberlain et al. (2014) actually couldn’t look at species abundance in their meta-analysis because there are too few examples in the literature!  NEED. MORE. DATA.

The meta-analysis includes all kinds of stuff about the variation in the strengths of interactions, and I recommend that you click through and check out their open access paper to find out more.  Here, I’m just going to tell you about their results related to sign changes for predation, competition, and mutualism.  Just eye-balling their Figure 1, out of 70 predation experiments, the sign of the interaction changed with conditions in ~45% of the experiments.  Isn’t that crazy?!  45% of the time, people found that predators go from having negative effects on prey to having no effects on prey – or to even having positive effects on prey! – under varying ecological conditions.  An even higher proportion of competition experiments found sign changes, and a still greater proportion of mutualism experiments found size changes (~60%).

So, yeah.  This was mind blowing.  One more piece of evidence that the universal answer to ALL ecological questions is: IT DEPENDS.

Reference:

Chamberlain, S. A, J. L. Bronstein, and J. A. Rudgers. 2014. How context dependent are species interactions? Ecology Letters.