Homogenization of freshwater lakes

What happened to fish communities in Ontario lakes over the last few decades? We have been relentlessly looking for answers to this question! Indeed, we did our best to determine whether global change has affected those communities over the last 40 years. It turns out that fish communities in Ontario lakes are pretty different from what they were, but climate change does not explain those changes! But, but… how can we say that? Well, that’s the topic of this post.

We had two very good surveys from the Ministry of Natural Resources and Forestry: one that was carried out back in the 1970’s and the other one in the 2000’s. These included presence-absence data for tens of fish species for >500 freshwater lakes throughout Ontario.

Kevin Cazelles - Homogenization of freshwater lakes - Ocean Fact

Fig. 1 Maps of the lakes

Copyrights © Kevin Cazelles

To put it simply

After selecting a meaningful set of species, we compared different aspects of the biodiversity. Overall, we found a surge in local diversity (a.k.a. α-diversity) and a drop in regional diversity (a.k.a. β-diversity). You may wonder what means, exactly. We can understand these changes using a bakery analogy (a good one, at least for French people). Let’s assume that in your home town, years ago, there were 3 bakeries:

  1. from the first, you were able to buy cholatines, croissants, rhubarb pies and carrot cakes;
  2. from the second one, only croissants (but the best in town!);
  3. in the last you could buy chocolatine and lemon pie

but nowadays, the choices at these same three bakeries are a bit different:

  1. cholatines, croissants, carrot cakes;
  2. cholatines, croissants, carrot cakes;
  3. cholatines, croissants.

That’s one example of homogenization because the pastries offered by these bakeries are now more similar than they were previously! Now, swap bakeries for lakes and pastries for fish species! Our research found that fish communities are more similar now than they were historically.

To understand this homogenization, we searched for plausible explanations. We used climate data, lakes characteristics (e.g. lake size) and species characteristics (e.g., preferred temperature) to build statistical models to determine the most plausible causes. It turns out that climate data and lake characteristics failed to explain the observed changes in fish communities; however, we found a clear difference in between gamefish species (the top 5 species in Northern Eastern Ontario) and the baitfish species: the former group of species thrives in Ontario but not the latter group! This explains pretty well the compositional shift in fish communities and led us to conclude that the homogenization trend is due to gamefish movements ; but it’s important to bear in mind that climate change may be making lakes more suitable for some gamefish species to occupy.

Kevin Cazelles - Homogenization of freshwater lakes - Graph - Ocean Fact

Fig. 2 Homogenization of the lakes: recent times shifted towards more gamefish species in Ontario lakes.

Copyrights © Kevin Cazelles

Freshwater lakes of Ontario are now dominated by gamefish species and the true nature of the movement of these species: is is natural ( colonization, human-driven or a bit of both? Anyway, the compositional shift we described will likely affect the ecological dynamics within lakes over the long-term. This is also a question that certainly requires further investigations.

To conclude this post, I (Kevin Cazelles) would like to add a few remarks. The first concerns the homogenization trend. As a scientist, I am trying to focus on facts and being as less judgmental as possible but it is hard not to worry about the loss of regional biodiversity. Going back to the analogy used above, homogenization may be welcome news for carrot cake lovers but a very sad one for people that used to enjoy rhubarb pies!

The second remark is about the importance of collaboration. Even though I am relatively comfortable with community analysis, I must say that my expertise in fish communities was minimal (though it is slightly better now). So, I would like to stress how crucial it was for me to benefit from other researchers expertise, from people that actually know their fish and the data. Without them, this paper would not have been possible. I think in modern science it is important to know your strengths (and always keep building up your skill set and extending your knowledge), but it is even more important to be aware of what you don’t actually know and to identify the people that can help you as their input may bring your paper to the next level!

 This was originally posted at the McCann Lab.

The full story is now available on Global Change Biology website and we also released the analysis pipeline for this study. Please, do not hesitate to reach out to Kevin Cazelles if you cannot access the paper or have further questions.

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Kevin Cazelles
Kevin CazellesPostdoc - Guest at Ocean Fact
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