Fishing for Ecological Opportunity in Ashby Lake

How have pumpkinseed fish in Ashby Lake, Ont., and elsewhere split into two groups with different characteristics and behaviours, and why does it matter?  A new study by Dr. Beren Robinson and recent MSc student Will Jarvis from the Department of Integrative Biology is answering these questions, and shedding light on an important evolutionary phenomenon in the process.

A species can split into distinct groups when it is exposed to what biologists call “ecological opportunities”, such as access to a new habitat or food source.   Over time, the population diverges into distinct groups that have adapted to different resources.

Jarvis and Robinson wanted to determine if ecological opportunities available to pumpkinseed fish in Ashby Lake had contributed to this process.

“There are very few studies that have directly examined the effect of ecological opportunities on adaptive divergence, it is usually just inferred,” explains Jarvis.   But Ashby Lake gave them a valuable opportunity to test their theory directly because they could see what the early stages of divergence in a species looks like in real time, rather than guess what happened in the distant past.

Ancestrally, pumpkinseed are a shallow water fish, specialized to feed on prey that dwell near the shoreline, like snails and insect larvae. But the shallow waters of Ashby Lake are not a very nutrient rich habitat and the deep open water of the lake offers a new habitat and resources in the form of zooplankton.

To find out if the open water habitat has led to pumpkinseed fish in Ashby Lake diverging into two groups, the researchers asked two specific questions.  First, do fish from open water and shoreline habitats have distinct characteristics, and second, how much do pumpkinseed fish move between the two habitats? These questions assess divergence and the idea of specialization in response to a new opportunity in the open waters, respectively.     

To answer these questions, Jarvis and Robinson designed a mark and recapture study where they caught over 1,000 pumpkinseed from the two habitats, tagged them and then put them back where they originally captured them. The next summer, they recaptured the tagged fish and compared physical characteristics, such as oral jaw size and body length.  They also looked at how many fish were found in the same habitat a year later versus the number of fish that moved between habitats.

They discovered that the shoreline pumpkinseed fish had larger oral jaws and smaller body sizes compared to their open water counterparts. They also found that pumpkinseed fish were over six times more likely to remain within either the open water or shoreline habitat rather than move between them – an important finding because it shows strong habitat “fidelity” over time that contributes to ecological specialization by individuals from a single habitat.

“We were surprised,” Jarvis said. “We expected to find more migration between habitats.” 

Such a finding would have indicated that the fish could be more “generalist in their ecologies”, meaning that they can use a wider variety of resources instead of being limited to specific habitats or food sources. 

The results support the hypothesis that the open water area of Ashby Lake represents an ecological opportunity in the form of additional habitat for the pumpkinseed fish. The physical differences between the open water and shoreline fish suggest that the fish are adapting and diversifying, because each group’s characteristics made them better suited to their particular habitat.

Because some of the fish have adapted to this new open water habitat, say the researchers, Ashby Lake can now support about twice as many pumpkinseed fish than if they only lived in the shoreline habitat.

Interestingly, Jarvis and Robinson also found evidence that there are still enough individuals moving between habitats to cause genes to ‘flow’ between the open water and shoreline populations. This means that so far, the pumpkinseed fish are diversifying without evolving into two separate species that no longer reproduce.

Jarvis says the findings are important because “they show how ecologically important diversity forms within species in an environmental context.”

The study also highlights that sometimes there is more diversity lurking in a population than may first appear. 

“When you start to look closely, the pumpkinseed fish are not homogenous, they’re more diverse than they look at first glance.”

This study was funded by the Natural Sciences and Engineering Research Council.  

Read the full study in the journal Evolutionary Ecology. 

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