“When” – Not Just “Where” – Key Question for Species Abundance

Cayelan Carey pulls a sample of plankton out of Lake Mendota. Photo: A. Hinterthuer
Cayelan Carey pulls a sample of plankton out of Lake Mendota. Photo: A. Hinterthuer

When it comes to studies of species abundance, scientists have spent a lot of time looking into where, exactly, particular plants or animals thrive or survive in a particular ecosystem. But, harnessing the relatively recent rise of long-term datasets, a new study published online in the journal PLOS ONE found that, for some species, it matters much more “when” you look.
The study was conducted by Cayelan Carey and Gretchen Hansen (both former post doctoral researchers at the Center for Limnology). Hansen is now a fisheries research scientist at the Wisconsin Department of Natural Resources. Carey is an assistant professor of biological sciences at Virginia Tech.
We asked Carey a few questions about the study and what it was like exploring fish and phytoplankton on the – ahem – space/time continuum.
What was the central question that led to this study?
Carey: If you think back to what ecology is, it’s the study of the distributions of organisms and their interactions with the environment. So a big question is why are some species rare and others common. People have tried to answer this by, for example, going out into a forest and surveying many many plots at one time and then writing about what they found.
What we’re working on now, thanks to long-term datasets like the Long-Term Ecological Research program, is looking at what species are present at those locations over time.
We wanted to see if the spatial patterns hold if you’re looking over time. So rather than the space question of “why are some species found at every single site and others are rare,” we asked “why are some species always present and others rare?”
What has the prevailing thought been based on the more spatial studies of species abundance in ecology? 
Fish population dynamics tend to follow a pattern - common species are abundant, rare species aren't.
Fish population dynamics tend to follow a pattern – common species are abundant, rare species aren’t.

Carey: The dominant paradigm going into this paper is that there are two distinct groups of species – “core” species which are always present and really abundant, and then “occasional” species which are very very rare, and maybe only detected once in any given site.
A key part of that is not just commonness versus rarity, but that the common species are usually really abundant and the rare ones are usually found in low population densities.
For your study, you looked at 17 years of data on a northern Wisconsin lake and what the population abundances were for various fish and phytoplankton species. What did that long-term dataset reveal?
Carey: Well, I should make the caveat that this was only looking at a single lake, but, for temporal part, all of the fishes did exactly everything you would expect to based on ecological theory – the really common fish were really abundant and the rare ones were not abundant at all.
The phytoplankton were, I think, the cool story. They showed some of the trend but, sometimes, the most rare species were also the most abundant and vice versa. So the really rare [species of phytoplankton] were usually existing at really low numbers but, once in 17 years, they would take off. So the most common species were not always the most abundant and the most rare were sometimes the most abundant.
A mid-October algae (or phytoplankton) bloom shows up on the Lake Mendota shoreline. Photo: Sam Oliver
A mid-October algae (or phytoplankton) bloom shows up on the Lake Mendota shoreline. Photo: Sam Oliver

So….now what? Will this study prompt further research?
Carey:  I think one of the cool things that needs to be done following up is to look at which species were driving this pattern in the phytoplankton . A lot of the cyanobacteria (also called bluegreen algae) were both rare and abundant, as well as common and not abundant.  The cyanobacteria are the ones that would show up only once but be super, super abundant that open time. Because cyanobacteria are such a public health concern we need to understand the conditions that allow them to bloom.
Anything else you found notable about the study? 
Carey:  I thought [the collaboration with Hansen] was really cool. In general, fish people and phytoplankton people don’t really talk – they’re two really distinct subdisciplines in limnology – fish people only hang out with fish people and phytoplankton people don’t hang out with fish people! So the long-term dataset let us ask questions not just across time, but also across disciplines.
And, also, I want to make sure LTER keeps going with these datasets. Very few people share their phytoplankton data. It’s kind of like gold, hundreds of thousands of dollars [and countless hours] go into counting phytoplankton. It’s not like fish where you can catch it and immediately ID it in your net!

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