Last week was a bit of a stroll down memory lane, thinking about the ecological concepts that really resonated with me. I am going to travel a bit further down memory lane this week before bringing it to the present in the next couple of weeks.
Populations and the models we use to describe them are rather simple, at heart, but the devil is in the details. Nothing can happen to an individual that affects a population other than their birth, their death, or movements in and out of that population (immigration and emigration). This BIDE approach to modeling is the basis for all populations but we can get much more complex from this starting point. This post is about how births, deaths, immigrants, and emigrants vary across the riverscape - which is where it gets more complex but also more interesting.
A rather unforgettable graduate student experience was seeing where Brook Trout (Salvelinus fontinalis) would spawn in Appalachian watersheds. One of our undergraduate technicians who later became a Masters student in the lab was looking at spawning habitat in the upper Shavers Fork (WV) watershed and it was mind-blowing where Brook Trout would spawn. Upper Shavers is a different world from Wisconsin. Appalachian headwaters are steep, often cascading streams that have cut to bedrock. And in Appalachia, bedrock can be very important to buffer the acidic precipitation that they receive but not all bedrock is created equal. Because of the deep stream cutting, different bedrock layers were intercepted by the streams rather quickly. Many of these layers were shales, sandstones, and other non-buffering rocks but sometimes, dolomite and limestone are bisected and these provided both springs and acid buffering - the lifeblood of these watersheds. These tiny little springs, less than a gallon a minute in some cases, would be where trout would spawn. Sometimes, the tiny tributaries they formed would go underground, yet Brook Trout would find a way to move into the little pockets of flowing water above where the stream had gone underground. These were prime spawning grounds in the watershed as much of the rest of the watershed did not have the water quality to sustain the eggs through incubation. This experience really showed that nature will find a way and that really small areas can have an importance that greatly outweighs the amount of space they take up on the landscape.
Sources and Sinks
Pulliam's 1998 paper, "Sources, Sinks, and Population Regulation" was another of those seminal papers - it has been cited over 6,000 times - that really started a new line of inquiry by researchers. Pulliam starts by telling us that we don't pay enough attention to basic spatial population dynamics and goes on to introduce sinks - habitats where populations would not persist without sources. Simply put, in source habitats, births (B) are greater than deaths (D) so "excess individuals" emigrate from sources to sinks. Sinks are places where deaths are greater than births and they are dependent upon immigrants from source habitats to be sustainable. Sources can survive without sinks but sinks can not persist without immigration from sources.
In the West Virginia Brook Trout example above, these buffered spring tributaries are sources and parts or the whole of the rest of the system are sinks. These little - and I mean LITTLE - tributaries are responsible for a large number of Brook Trout in the watershed. Trout spawned in other places but water most everywhere else in the watershed had pH's that were at best marginal for egg survival. And I know your mind is churning by now and you are thinking that if only we could get rid of sink habitats or turn them into sources. But therein lies the rub...
Coaster Brook Trout provide a story of how sources and sinks often work together. "Coasters" spawn in Lake Superior tributaries but leave for the lake to access more and higher quality food. Living on aquatic macroinvertebrates in small streams is a tough way to make a living and larger forage fishes allow for a greater scope for growth. Move out to Lake Superior and you can get fat, in a hurry. In the animal world, getting fat is a good thing. It means you can produce many more eggs or sperm which is what life is all about if you are a Brook Trout. The source stream is obviously important but Lake Superior, a classic sink in this case, is also important in allowing individuals to grow large and produce lots of offspring. The story is much the same as other anadromous fishes except that the Coaster life history seems to quite plastic. There is no genetic basis for Coasters, or at least not one we have found, but we think that streams reach carrying capacity and some individuals make their way downstream to the lake. These "spill over" fish become Coasters.
Brook Trout in the Shavers Fork watershed share a similar story, albeit at a much smaller scale. Brook Trout move into larger streams with more abundant food and space where they can grow larger to be able to produce more eggs and sperm. It is a simple bioenergetics story - the mainstem for parts of the year is at a near thermal optimum for Brook Trout and it has a more abundant and profitable forage so fishes grow faster. Brook Trout that access this higher quality food resource grow larger and produce more offspring, thus they pass on more of their genes than do their competitors.
Complementary and Supplementary Habitats
As science does, our thinking about how different patches are tied together by dispersal evolved over time. As researchers put out new ideas, themselves and others test those theories and see if they can apply them to new places, refine them, and improve upon these ideas. Spatially explicit models - basically models where location matters - sprung from Pulliam's and others' sources and sinks ideas. Another Isaac Schlosser paper, this one from a few years later than the Stream Fish Ecology: A Landscape Perspective I highlighted in last week's post, that I really got into was Critical Landscape Attributes that Influence Fish Population Dynamics in Headwater Streams, a mouthful to be sure. And at the same time, a paper that was not about fish but a rather beastly 27 page population dynamics paper by Wu and Louchs (1995), From Balance of Nature to Hierarchical Patch Dynamics: a Paradigm Shift in Ecology, was no light read but an interesting and influential one.
These papers were far reaching but each tied together heterogeneity. They each described how landscapes and riverscapes are comprised of different patches with different population dynamics for different species (sources and sinks) within those patches, and how dispersal ties these patches together. And this is where things start getting really complex but for this post, just two more ideas - complementary and supplementary habitats.
Complementary habitats are habitats that are required for an aspect of a fish's life history whereas supplementary habitats are not required but may provide some additional benefit. For example, Brook Trout in Lake Superior tributaries can live their entire lives in the tributaries in which they were born without entering the Great Lakes and "becoming" Coasters. Lake Superior is a supplementary habitat; it it not required but it provides a significant foraging habitat not provided by the natal stream. Complementary habitats would be the ocean and natal streams for anadromous salmon with a life history that requires freshwater natal streams and time in the ocean before returning to freshwater to spawn.
I will not tell much of the story here and instead save it for next week, in a bit less dense post about how we use the ideas of sources and sinks, complementary and supplementary habitats, and the dynamic landscape model to manage fishes. A good example of this comes from a paper out of my old lab from Shavers Fork which uses stable isotopes to link Brook Trout diets to location - a spatially explicit way to sample what a fish has consumed over its life. Brook Trout are quite mobile in the system but larger Brook Trout were almost always found to have had "mainstem signature". The authors ponder if the loss of large Brook Trout in many watersheds are tied to a number of degradations - dispersal barriers, mainstem warming, harvest and elevated mortality risk of mobile fish, and presence of exotic trout in the mainstem river.
Next week, I plan to spend a bit more time in West Virginia before writing about how the same approach is being used Trout Unlimited and others in Wisconsin to restore and reconnect rivers.
Related Posts:
Literature Cited
Huntsman, B. M., Petty, J. T., Sharma, S., and Merrian, E. R. (2016). More than a corridor: use of a main stem stream as supplemental foraging habitat by a brook trout metapopulation. Oecologia182, 463–473. https://doi.org/10.1007/s00442-016-3676-4
Pulliam, H. R. (1988). Sources, sinks, and population regulation. The American Naturalist, 132(5), 652-661.
Schlosser, I. J. (1995). Critical landscape attributes that influence fish population dynamics in headwater streams. Hydrobiologia 303, 71–84. https://doi.org/10.1007/BF00034045