I think every graduate student could produce a list of things they wish they would have known going in. For me, “I Wish I Knew Then What I Know Now” could be a recurring feature on this blog. One of the hardest things for me to shake is a propensity to think about stream ecology in multiple dimensions. With the shift from emphasizing fish stocking to habitat management in traditional fisheries management, the idea that fish chose “homes” reflecting specific environmental tolerances was hammered into my brain. Slowly but surely I am learning that environment is only part of the picture.
One of the first exercises in my quantitative ecology class last semester was to partition variation in species communities among environmental and spatial factors. I immediately wondered what are these eggheads talking about? Theoretical ecology is interesting and all, but I am into applied fish conservation. My species could go extinct while I sit here worrying about frivolous things such as how spatial factors affect Clinch Dace distribution. So I responded how most do when confronted by inconvenient and uncomfortable new knowledge: I ignored it. I went back to my simple, comfortable niche-based world view. A world of conductivity, canopy cover, substrate, and watershed land use that alone should dictate where a Clinch Dace decides to live. Countless other published studies have done the same thing, why couldn’t I?
This strategy was somewhat successful for me until a few months ago. One of my committee members, Emmanuel Frimpong, happens to be a particularly brilliant landscape ecologist. During a committee meeting he burst my bubble of ignorance. So with the help of Dr. Frimpong and his former student Brandon Peoples, I began to confront the elephant in the room- space. I calculated the distance a fish would need to swim to travel between all my sites and began to think about how spatial correlation among the habitat measurements at my sites could be removed.
Metacommunity ecology uses four major paradigms to explain how space effects species distribution and community structure. These are: species sorting, mass effects, neutral theory and patch dynamics. I will focus on the first two paradigms in this post.
Source: Falke and Fausch 2010.
Species-sorting theory has historically been the bread and butter of ecological thinking. Species find the habitats that best suit them and generally stay in there. It is essentially the niche theory that is in some form recognizable to even a lay audience.
So then what are heck mass effects? Mass effects refer to source-sink population dynamics where individual fish are highly mobile. They are all about how fish moving among patches of habitat in the stream frequently; persisting in the good habitats called source patches at all times and occasionally blinking in and out of the less optimal habitats called sinks.
Although I’ve struggled mightily at times implementing space into my statistical analysis, so far my results suggest that perhaps a species-sorting model where environmental factors prevail in explaining species distribution is more dominant in Clinch Dace. Conceptually this makes sense. Clinch Dace are likely fairly sedentary; dependent on headwater pool habitats and nearby gravelly runs for feeding, reproduction, and most of the other important events in their short lives. However space and dispersal could definitely play a role when we start thinking about impassible road crossings, genetic connectivity among populations, and stream drying in the late summer/early fall.
A nice clean, well-shaded pool might fulfill most of the requirements for a Clinch Dace
There are certainly circumstances where the mass effects or a space dominated model might be a better fit. On the Arikaree river on the arid plains of Colorado researchers found that pure spatial mass effects explained 31.4% of the variation in fish community structure while pure environmental effects only explained 6.5%. This make sense, the Arikaree river’s hydrology is seasonally variable. Fish must move between habitats as they become available throughout the year. Deep pools that teem with fish in the spring may be dry by the fall. In other words, while environment may influence which habitats are best (sources) and which are worse (sinks) you may not be able to distinguish between the two with your sampling data. The species dispersal abilities can overwhelm the effects of microhabitat variation. Managers could construct the perfect pool for the imperiled brassy minnow in the Arikaree but if the right spatial configuration of habitat patches did not surround it, it wouldn’t do a lick of good.
Arikaree River. Source: Yubanet.com
All ecological research should tell a story which is a simplified but ultimately useful version of the focal ecosystem. While thinking beyond habitat may muddy the waters on the clean narrative that has prevailed for decades, native fish need biologists who can think in multiple dimensions. Yes, habitat is one, but there are also human, genetic, and of course spatial dimensions to consider as well. Without understanding how space and movement influences fish distribution, we are ill prepared to incorporate spatial effects in our graduate research and even less in our future careers managing of native fish communities. Multidimensional thinking will ensure that the stories we tell our peers and the public are nuanced and a step closer to reality.
Sources and Other Links:
Another cool blog post on meta-community ecology: http://evol-eco.blogspot.com/2011/06/metacommunity-data-and-theory-tortoise.html.
Book chapter I was reading while I wrote this: Falke, J. A. K. D. Fausch. 2010 From metapopulations to metacommunties: linking theory with empirical observations of the spatial population dynamics of stream fishes. In: Gido, K. B., and D. A. Jackson, editors. 2010. Community ecology of stream fishes: concepts, approaches and techniques. American Fisheries Society, Symposium 73, Bethesda, Maryland.