Unlocking the Secrets of Niches and How They Apply to Stream Fishes
- Jason G. Freund
- Jul 13
- 7 min read
Niches are about the most complex, yet seemingly simple idea that there is in ecology. There are two different ways we started to recognize niches - the Grinnellian and Eltonian niche. These two niche concepts were instrumental in today's view of what an ecological niche is.
The Grinnellian niche is named for Joseph Grinnell, a scientist and museum director in the early 1900's. A meticulous note taker, today many field biologists and those in training, use "the Grinnell Method" of keeping records of their field observations. The Grinnellian niche recognizes the habitat in which a species lives and its adaptations to live in that habitat. For example, Brook Trout (Salvelinus fontinalis), our native stream trout's niche is as an inhabitant of coldwater lakes and streams ranging from small streams to the Great Lakes. This niche is broad in the sense that they are widely distributed and not limited to a single type of environment, but it is narrow in that they have a narrow range of temperatures in which they exist.
Charles Elton, for which the Eltonian niche earns its name, was an Oxford University trained zoologist that was born a couple of decades after Grinnell. Both were born of some means - a common theme for scientists of that era. The Eltonian Niche is often distilled to "a species role in the community" or something similar. It recognizes the interactions among species and how each taxon plays a role in the community which they are part of. This is largely a trophic niche - they are a top carnivore, for example, and as such they move nutrients from lower-level consumers to higher level consumers and play a role in limiting the populations of their prey species. Using Brook Trout as an example again; they have a wide trophic niche, being predators of both primary consumers (aquatic macroinvertebrates) and larger, higher-level consumers like small fishes and rodents. In the streams in which they live, they are often near - but not at the top - of the food web.
We can apply these concepts to maybe the most famous of taxa, Darwin's finches, which are not actually finches but are tanagers (FWIW, the Scarlet Tanager is NOT a member the tanager family but a member of the cardinal family). Darwin's finches are the classic example of adaptive radiation, the idea that driven by competition and open niches, relatively rapid evolution leads to differentiation in what was once a single species. Probably the best example of adaptive radiation are the cichlids of the African rift lakes where individual lakes like Malawi and Victoria each have hundreds of unique cichlid species.
Darwin's finches have diversified to take advantage of the different niches available on the Galápagos Islands. There are 18 main islands that differ in elevation, vegetation, and size and thus number of available niches. These islands, one of the most isolated archipelagos in the world, were created by volcanos up to 90 million years ago and, in fact, some islands are still growing today. Low elevation areas are largely desert and shrublands, however higher elevation areas are cooler and wetter and are home to forests and grasslands. This means that the Galápagos Islands are rich in diversity due to the ability of species to be geographically isolated not only from the mainland but among the islands of the archipelago as well. And they can also be isolated by preferring different habitats on the larger, more diverse islands.
Darwin's finches are often characterized by their beak size and shape, a trait that should make us think about the Eltonian niche as the beak is a window into what a bird consumes. Their names - the Common Cactus Finch, Medium Ground Finch, and Mangrove Finch - give us insights into what habitats they inhabit and the niche they fill.
Fundamental and Realized Niches
In the lack of competition, a species has a larger niche - its fundamental niche. However, when other species are present, species may only occupy part of their niche - their realized niche. As we add species, we have to recognize that species compete and there is potential niche overlap, that is, species share similar niches. Competition has a negative effect on both species and thus, they should evolve to minimize competition. This leads us to the idea of niche partitioning or how evolution drives species to different niches - both in the Grinnellian and Eltonian senses.
This led to the conceptualization of the Competitive Exclusion Principle or Gause's Law or Principle which states that no two species can share the same niche and coexist. In fact, Grinnell was truly the first to recognize this idea. Georgy Gause, a Russian scientist, experimentally tested the concept with paramecia, small eukaryotic freshwater ciliates. He found that, eventually, one species drove the other to extinction when the environment was held constant. However, as he allowed for environmental variability, the two species would often coexist. This was a major revelation.
Modern Niche Concepts
As with most scientific ideas, they "evolve" as we make observations and conduct experiments to test these concepts. As we moved towards the middle of the last century; ecology became more mathematical, and our understanding and ability to describe niches changed. G. Evelyn Hutchinson and his graduate students Robert MacArthur, Howard Odum, Larry Slobodkin, and others as well as colleagues like Richard Levins were instrumental in this "evolution" of the field of ecology from a descriptive science to a more quantitative and experimental field. New niche models - the Hutchinsonian Niche - embraced a multivariate niche rather than the more simple niches proposed by Elton and Grinnell. This view came about, in part, by the Paradox of the Plankton which asked, why are there so many species of plankton when they have such similar ecological roles?
As with most ecological concepts, we quickly figure out that the answers are rarely simple and are location-dependent at scales which we have a difficult time understanding. Today, our niche concept(s) are quite complex but still probably do not truly capture the variability that occurs in nature. We often use Lotka-Volterra equations, basically a population model that accounts for competition between two species. Even these equations only allow us to understand the effects of two species on one another when we know that there are often many more than one other species affecting our species of interest.
Niches and Fishes
The reason I started this post was thinking about our research project this summer on the distribution of two sculpin species in the Kickapoo River watershed. To not rehash much of what I wrote about the post on sculpins previously, I will be brief. There are two species of sculpin in Wisconsin streams - mottled (Cottus bairdii) and slimy (C. congnatus) sculpin in addition to a couple of species that live in the Great Lakes (as do mottled and slimy sculpin). These two species look nearly identical - in fact we tell them apart by small differences in a couple of fins. Both are benthic insectivores - as best we know, they have a near perfect trophic overap. That caveat is an important one as we do not know that much about if/how their food preferences differ. And both live in coldwater streams (and lakes), although we know that Slimy Sculpin have a lower thermal preference. After publishing the sculpin post, a Great Lakes researcher reached out and shared in an email how they find the two species on the same underwater rock humps but Mottled Sculpin are higher in the water column where the water is a little warmer and Slimy Sculpin are deeper where the water is colder. They seem to have differentiated themselves based on water temperature.
This brings me back to our idea of niches and what, other than small thermal differences, separate these two sculpin and other sculpin species. Obviously, some of it is geography - they only overlap in relatively narrow parts of their ranges. One of the questions we are most interested in is, will we find them in sympatry - that is, both species co-ocurring together? There is some limited evidence of the two species hybridizing. Maybe even the two species have a difficult time recognizing their own species? If they do occur in the same stream, what conditions lead to this overlap?
To take a step back, we know that few fishes are able to survive, much less thrive, in cold streams. I wrote about this idea in a post on indices of biotic integrity (IBI) and how coldwater streams with the highest IBI scores have nothing but trout and sculpin present. Temperature is a resource and the "lack" of it, leads to fewer species. This is linked to a number of bioenergetic constraints, such as enzyme function. And because of these thermal limitations, relatively few niches are present in coldwater streams. Aside from the odd North American River Otter (Lontra canadensis), mink, Great Blue Heron, or Bald Eagle; trout are at the top of most trout stream food webs. They have a wide trophic niche - ranging from zooplankton when young to macroinvertebrates and eventually fishes and anything else they can get in their mouths when older and larger.
As we move into warmer streams, the diversity of niches, and thus fishes, increases. Again, we can view this both in the Grinnellian and Eltonian sense. There are more thermal niches available - in fact, we often talk about streams in terms of their thermal regimes - coldwater, coolwater, warmwater and then the transitions between these three categories. As species diversity increases - because thermal constraints are reduced, because larger habitats have more niches, and there are a greater number of energy sources as well as a greater amount of available energy - the number of niches increases. For example, warmwater streams have a greater diversity of predators, in part, because there is a greater diversity of prey. We now have predator that specialize in eating different prey. Muskellunge can't "make a go of it" eating shiners weighing a few grams each and Smallmouth Bass can't choke down eight inch and larger suckers.
As I have been adding recently - below is an AI generated annotated bibliography about the two sculpin species.
From my own experience. Thermal niches. A stream I fish has a fair amount of spring water, and also snow melt and rain runoff. In multi-year droughts, when the stream is mostly spring water, the brook trout predominate in the upper water and the brown trout move downstream where the water is a bit warmer. When the rain returns, especially after thunderstorms on hot summer days, the browns move up into what had been brookie water the weeks before. The small headwaters are almost exclusively the haunt of brook trout, the downstream water holds only browns, and there is a range of several miles where the overlap occurs, brookies and browns both; brookies rule when the water is cold, the…