What's the Neighborhood Like? The Importance of Connectivity in Stream Networks (Part 1)
There are certain ideas that just "click" with us. As a Master's and eventually a Ph.D. student, Isaac Schlosser was something of a hero to me; an inspirational researcher that was responsible for a lot of how I thought about fisheries ecology and management. He, his students, and his colleagues wrote a number of influential papers on two ideas that really resonated with me; the role of movement in the life history of fishes and the importance of terrestrial habitats on fishes and their habitats. His dynamic landscape model was an idea that shaped how I looked at streams and stream fishes.
Image is from a tweet by Eric Larson, University of Illinois. Schlosser's dynamic landscape model described how fishes move to access food, refuge, and spawning areas. For some fishes such as sculpin, these three critical areas may be within a small area. For other species, like anadromous Salmonids, these locations may be hundreds or more miles apart. They spawn in freshwater streams far from the ocean environments where they have lived from one to several years and have put on the majority of their weight. A similar idea in the Great Lakes are "coasters", Brook Trout (Salvelinus fontinalis) that grow much larger by accessing forage-rich waters of the Great Lakes rather than continuing to live in their natal streams.
His paper, Stream Fish Ecology: a Landscape Perspective from 1991, was a revelation for me. It put into perspective how and why fishes moved, or did not move. It set the stage for many researchers to better understand and test the relationships among fishes, instream habitat, and terrestrial environments.
Like all ideas we encounter, we experience it within our knowledge and past experiences. As a new graduate student at West Virginia University, I had previously been a research technician in an agronomy lab at the University of Wisconsin, where among other things we examined how rotational grazing affected streams and birds. I came into a project that with an advisor that sort of disappeared overnight, a story for over a campfire some evening... So I fell into a Master's project exploring the movement of Largemouth Bass (Micropterus salmoides) on the Belleville Pool of the Ohio River with Kyle Hartman. My Ph.D. research with Todd Petty looked at the effects of acid mine drainage and habitat fragmentation on stream fish communities. Each of these projects explored how fishes interacted with their riverscape.
Why Streams are Unique
While it seems rather obvious, streams are linear landscapes - riverscapes, if you will. It is part of what makes rivers so unique. Part of why, where oceans make up almost all of the liquid water on Earth, freshwaters are home to nearly half of all the fishes species. The reason is distance and isolation which is much less likely to occur in oceans. For example, there are but six species of bonefishes (family Albulidae) and two species of tarpon (family Megalopidae) - one in the Atlantic and one in the Pacific. In freshwaters, there are over 200 species of daters (family Percidae), over 1,250 minnows (family Cyprindae), between 2,000 and 3,000 species of cichlids (family Cichlidae), and at least 220 species of trout, salmon, whitefishes, and other salmonids (family salmonidae).
There are some great examples of adaptive radiation in lakes. Rift lakes in Africa which each contain several hundred unique cichlid species are maybe the single most impressive example of adaptive radiation. However, streams by their linear nature and changing course are more easily isolated. In West Virginia, Kanawha Falls separated the New River, above the falls, from the Kanawha River, below the falls. Eight endemic fish species live in the New River, above Kanawha Falls - by comparison, there are no endemic fishes in Wisconsin.
From Walsh and Meador 1998 - the number of described native freshwater fish species by state. The trends are rather clear - the Southeastern US is the center for fish diversity for North America. Like the answer to so many biological questions, the answer has everything to do with evolution. In this case, time and isolation worked together to produce the diversity we see today. The Tennessee River drainage and a number of drainages to the Gulf of Mexico (Mobile River, in particular) was refuge from the effects of glaciers. Further north, like here in Wisconsin, the fish species we have are those that retreated north with the retreating glaciers.
At a very long time frame, the Southeastern United States served as a glacial refugia. At shorter time frames, fishes seek refuge from floods, high temperatures, predators, and other things that can cause mortality or the expenditure of a great amount of energy. Life is an energy balance equation - more energy out than is going in can only persist for so long before it results in death. Refugia can be seasonal - like fishes moving to overwintering habitats where they may not be able to gain much energy but they also are not expending much energy. More often than we once knew, winter refuge is often downstream of trout streams, in warmwater rivers and lakes. Meyer et al. (2011) found trout from Beaver Creek overwintered in the Peshtigo River, at that point, no longer a trout stream. And around the Driftless, most every year there are accounts of trout being caught by ice fishermen in the Mississippi River.
During the summer, refugia from warm water temperatures are important, sometimes for a few hours, other times, for weeks at a time. This is a time where unscrupulous anglers can literally make a killing. I know a number of places where trout seek springs in an otherwise warm river. While you may hook them in 67*F water (19.5*C), they are quickly pulled into 72*F (22*C) or warmer water. Musky face similar challenges, albeit at warmer water temperatures.
The riverscape is a fish's world - the variety and spatial arrangement of different patches on the landscape. The river continuum concept (blog post on the RCC) describes a physical, and thus, a biological predictability to rivers. Within this predictable upstream to downstream continuum, there is heterogeneity or "patchiness" as ecologists like to talk about. I can see my homewaters traveling through a heavily grazed pasture where the water may warm a bit more quickly and be a patch where some trout move to forage in the spring. And as that stream crosses the fence, it enters pasture that has not been grazed in a few years due to floods ripping out the fences. Trout may move into this reach in the summer when terrestrial insects abound and become their main food source.
The two most significant movements tend to be around the spawn and to access overwintering habitats. But for some fishes, they have no reason to make these movements. During my Masters research, I tracked Largemouth Bass (Micropterus salmoides) in the Ohio River - a really big stream. Bass used area with little current for much of the year - embayments, off-channel habitats, and some of the tributaries. Some fishes that used the embayments had no reason leave these flooded tributaries whereas other bass moved 20 or more miles to find a place to overwinter or spawn. Other fishes, like sculpin (family Cottidae) in streams may spend their entire lives in an area no larger than your kitchen table - never knowing, or caring that a world exists outside of those 10 square feet. Their riverscape is much more intimate.
Effects of Fragmentation
The effects of fragmentation are different for different fishes but in general, the trends are quite distinct. We can talk about fragmentation in two different ways - natural and anthropogenic. As I wrote above, natural barriers to dispersal like waterfalls or oceans are responsible for high endemism - the number of species in the New River above Kanawha Falls or in the Tennessee and Mobile river watersheds of the Southeastern US. Time + isolation = Evolution.
At much shorter time frames, humans have built dams, poisoned rivers, built terrible bridges and culverts, and other manmade barriers to fish passage. The result of these has largely been the reduction of fish species, nearly the exact opposite effect of natural barriers. Studies from across the decades have shown the same thing, there are fewer native species above dams. But to confuse the whole issue a bit more, fisheries managers have used barriers to isolate native species from the non-native species that are otherwise kicking their butts. And that is the problem with this post and why it became part 1 of who knows how many posts on the topic of the role of dispersal and the riverscape.
Our understanding of the role dispersal in a complex environment continues to evolve and requires a lot more than one post so as not to totally overwhelm. My plan will evolve as well, I am sure, but the next post is about restoring the riverscape and efforts to improve dispersal of stream fishes.