Replication: step 1 in PhD research

Here are a few statements that won’t surprise anyone who knows me. I think replication has the potential to be really useful. I think we don’t do nearly enough of it and I think our understanding of the world suffers from this rarity. In this post I try to make the case for the utility of replication based on an anecdote from my own scientific past.

A couple of years ago Shinichi Nakagawa and I wrote a short opinion piece about replication in ecology and evolutionary biology. We talked about why we think replication is important and how we can interpret results from different sorts of replications, and we also discussed a few ideas for how replication might become more common. One of those ideas was for supervisors to expect graduate students to replicate part of the previously published work that inspired their project. When Shinichi and I were writing that piece, I didn’t take the time to investigate the extent to which this already happens, or even to think of examples of it happening.

Then out of the blue the other day, it occurred to me that I’d seen this happen up-close with one of my own findings. First some background. Bear with me and I’ll try to be brief. When I was a naïve master’s student (with a hands-off adviser who had at least one foot in retirement), I decided to test Tom Martin’s ideas about nest predators shaping bird species co-existence, but in a new study system: the shrub nesting bird community at Konza Prairie in Kansas (by the way, this anecdote is NOT about my choice to do a conceptual replication for my MSc work). Anyway, I was gathering all the data myself, trying to find as many nests as I could from multiple species, monitoring those nests to determine predation outcomes, and measuring vegetation around each nest. I bit off more than I could chew, but I wanted to be done in one field season. I was in a hurry for some reason – not a recipe for sufficient statistical power. Instead, it was a recipe for an ambiguous test of the hypothesis since I didn’t find many nests for most bird species. I did, however, find a decent number of nests of one species: Bell’s vireo. Among the more than 60 vireo nests I found, I noticed something striking – brood parasitic cowbirds laid eggs in many of them, and if a cowbird egg hatched in a vireo nest, all vireo chicks were outcompeted and died. What was really interesting is that vireos abandoned many parasitized nests before cowbird eggs hatched and these vireos appeared to re-nest up to seven times in a season. I first thought this was evidence of an adaptation in Bell’s vireos to avoid parasitism by cowbirds via re-nesting (that’s another story), but I ended up publishing a paper that pointed out that the number of vireo eggs in the nest (rather than the number of cowbird eggs) was the best predictor of vireo nest abandonment. Thus it seemed like a response to egg loss (cowbirds remove host eggs) by Bell’s vireos might explain their nest abandonment and therefore how they could persist despite high brood parasitism. Now on to the heart of the story.

Several years later, after doing a PhD elsewhere, I found myself back in Kansas. A new K-State PhD student (Karl Kosciuch – who was one of Brett Sandercock’s first students) arrived and was excited about the Bell’s vireo –cowbird results I had reported. Looking back on it, this is a textbook case of how exploratory work and replication should go together. I found a result I wasn’t looking for. Someone else came along and thought it was interesting and wanted to build on it but decided to replicate it first. Karl did several things for his PhD, but one of them was simply to replicate my observational data set with an even bigger sample. He found the same pattern, thus dramatically strengthening our understanding of this system, and strongly justifying follow-up experiments. I actually joined Karl for one of these experiments, and it was very satisfying behavioral ecology. It turned out that it really is loss of their own eggs that induce Bell’s vireos to abandon and that cowbird eggs do not induce nest abandonment on their own.

This study had a happy ending for all involved, but what if Karl’s replication of my correlative study had failed to support my result? Well, for one it hopefully would have saved Karl the trouble of pursuing an experiment based on a pattern that wasn’t robust. Such an experiment would presumably have failed to produce a compelling result, and then would have left Karl wondering why. Were the experimental manipulations flawed? Was his sample size too small? Was there some unknown environmental moderator variable? Further, although the population of Bell’s vireo we studied is not endangered, the sub-species in Southern California is and one of the primary threats to that endangered population has been cowbird parasitism. My result had been discussed as evidence that Bell’s vireo populations might be able to evolve nest abandonment as an adaptive response to cowbird parasitism. If no replication had been conducted and only an unconvincing experiment had been produced, this flawed hypothesis might have persisted with harmful outcomes to management practices of Bell’s vireo in California.

I think there’s a clear take-home message here. Students benefit from replicating previously published studies that serve as the basis for their thesis research. Of course it’s not just students who can benefit here – anyone who replicates foundational work will reduce their risk of building on an unreliable foundation. And what’s more we all benefit when we can better distinguish reliable and repeatable results from those which are not repeatable.

I’m curious to hear about other replications of previously published results that were conducted as part of the process of building on those previously published results.