My long search for rules on how fish communities are put together

This blog post is provided by Andrew L. Rypel and tells the #StoryBehindthePaper for the paper ‘Ecosystem size filters life-history strategies to shape community assembly in lakes’, which was recently published in Journal of Animal Ecology. In this study, he fuses theories from island biogeography and life-history studies to understand how fish assemblages filter along a lake size gradient.

As a child, I had the privilege of growing up exploring nature and the ecology of northern Wisconsin lakes. I was utterly fascinated by how each natural lake differed in unique ways – almost as if the lakes had personalities. Our family owned a minnow trap, and I had a keen interest in fish and fishing. As I got older and more independent, I started deploying the trap in various lakes, carefully observing the types and diversity of species. There were clear patterns. Big predator fishes like muskellunge (Esox masquinongy) and walleye (Sander vitreus) did not occur in small lakes – just the big lakes. And the smallest lakes, like the tiny bog lake I knew best, had just a few tolerant species. In between the small lakes and the big lakes, other species would pop in-and-out depending on the lake’s size and ‘personality’. The landscape was clearly dynamic, but there seemed to be rules, and they were governing the types of species that could occur in each lake and how the community was stitched together.

Figure 1: Image of one of the Wisconsin lakes, I grew up exploring – Banach Lake, Burnett County, WI. Photo by the author.

Later, as a graduate student, I became familiar and highly interested in the life-history model developed by Kirk Winemiller and Ken Rose (Winemiller & Rose 1992). In their model, fishes could be classified into one of three kinds of life-history strategies, providing considerable nuance and realism versus the classic “r vs k” framework. The model also worked extremely well for fishes, which express hyper diversity in life-history traits. Winemiller and Rose seemed to have figured a way to boil trait diversity down. Fish can be either ‘periodic’, ‘equilibrium’ or ‘opportunistic’ strategists. Equilibrium species become sexually mature later in life, females have low-to-moderate numbers of eggs, and babies have high survival. Periodic species also become sexually mature later in life, but females produce very high numbers of eggs – sometimes in the millions! The babies of these species often have very low survival rates. Opportunistic species become sexually mature very fast; they are small-bodied, and have good survivorship as babies. These life-history strategies seem to also map onto their habitats. For example, opportunistic species seem capable of rapid population increases, meaning they are quite good at colonizing new habitats, including those that might be harsh, like the small bog lakes I knew as a child.

Fast forward, and during my 30s, I was working as a research scientist for the Wisconsin DNR. I also maintained a research fellow position at the Center for Limnology at UW-Madison. It was here that I came into close contact with the work of John Magnuson, Bill Tonn and others (Tonn & Magnuson 1982; Magnuson et al. 1998). I was also fortunate to become engaged with the North Temperate Lakes – Long-Term Ecological Research Project (NTL-LTER). Magnuson, Tonn and others had long recognized some of the same patterns in lake fish assemblages that I did as a child. Could their ideas be married with those of Winemiller and Rose to develop new rules for how assemblages filter along the lake size gradient?

In my new study in Journal of Animal Ecology, I demonstrate how these concepts can be fused. I leverage data collected as part of NTL-LTER, and also the original Tonn and Magnuson data. In total, fish assemblage data were available for 40 Wisconsin lakes. For each species, I used the methods of Winemiller and Rose to classify each species into one of the three life-history strategies. I also provide a measure for their relative strength as a member to each strategy. Then, I looked at how species-area relationships differ among the strategies. Small lakes have low species diversity, and are typically dominated by opportunistic strategists, like central mudminnow (Umbra limi). As lakes increase in size, species richness increases. However, diversity of periodic and equilibrium species increases at a faster rate. Thus, large lakes are increasingly dominated by these kinds of species. This finding clearly shows how ecosystem size is filtering life-history strategies of species, which in turn, drives community assembly.

Figure 2: Smallmouth bass (Micropterus dolomieu). An equilibrium species that occurs in medium- to larger-sized lakes, especially those that are cool and clear. Photo by the author.

It appears there are several aspects of lake size involved in the filtering. Firstly, life-history niche space increases with ecosystem size. Thus, as there are more kinds of habitats available, more kinds of fish arrive to fill the niches. Second, when lakes reach a large enough size, they appear to generally transition from being small low connectance lakes isolated in the upper part of the landscape, to large lakes that connect to one another via stream and river networks. These ecosystems begin to favor large-bodied periodic species that simply can’t survive in small isolated lakes, and in fact often spawn and live parts of their lives in the rivers too. This includes species like walleye, lake sturgeon (Acipenser fulvescens), and freshwater drum (Aplodinotus grunniens). Finally, I show using limnology data from the NTL-LTER, that small lakes are simply less stable ecosystems. Attributes of water quality are more volatile in small lakes, and pioneering opportunistic species, with their rapid generation times and fast sexual maturation schedules, appear to be the only species that can survive.

 Figure 3: Author recently visiting a north Wisconsin lake. Photo by the author.

This was a fun and important project for me, and I did it largely because I love these ideas and ecosystems. It connected me back to some of the observations about lakes I made as a child. However, it also represents a marker for how I’ve come to see how ecology and community assembly works. It seems that there are rules, and while they can be bent, it is hard to break them. The rules may apply to other taxonomic groups on other islands, and this idea would be neat to explore in other landscapes/contexts – I predict they do hold. Some of these findings are important for managers to come to grips with. For example, stocking periodic species (e.g., walleye and muskellunge) into small lakes may be expensive and futile. The work also shows how good ideas can live in corners of ecology that rarely interact. Island biogeography and life-history theory have been around for a while, but have mostly been pursued separately. Bringing ideas together in this way is equal parts interesting and fun.

Read the paper

Read the full paper here: Rypel, A. L. (2023). Ecosystem size filters life-history strategies to shape community assembly in lakes. Journal of Animal Ecology, 00, 1–15.

Literature Cited

Magnuson, J.J., Tonn, W.M., Banerjee, A., Toivonen, J., Sanchez, O., & Rask, M. (1998). Isolation vs. extinction in the assembly of fishes in small northern lakes. Ecology, 79, 2941-2956.

Tonn, W.M., & Magnuson, J.J. (1982). Patterns in the species composition and richness of fish assemblages in northern Wisconsin lakes. Ecology, 63, 1149-1166.

Winemiller, K.O., & Rose, K.A. (1992). Patterns of life-history diversification in North American fishes: implications for population regulation. Canadian Journal of Fisheries and Aquatic Sciences, 49, 2196-2218.

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