Trophic cascades, detrital dynamics, and invasive crayfish

Welcome to my summary of Day 1 of the NABS/ASLO joint meeting, almost live and definitely on location!  I’ve decided I’m going to start each NABS meeting posts with a short list of things I learned during the previous day.  Here goes!

Things I learned yesterday:

— There are a whole lot of invasive fish species in the world and they can cause massive problems in stream systems.  The round goby is a big problem in the Great Lakes region and the armored catfish dominates some rivers in southern Mexico (and is feared by the locals who think the fish is poisonous).
— Dams and other barriers to fish can cause nitrogen shortages in streams above the barriers.
— Global warming could cause a downward shift in body size in a huge variety of organisms because body size generally goes down as temperature goes up.

My favorite talk at the meeting yesterday was the one titled above.  For my entomological or scientific readers out there, this title likely makes sense.  For the rest of you – don’t worry if it doesn’t!  We scientists use a lot of big words that normal, er, I mean, non-scientific, people wouldn’t ever have a reason to know.  Let’s go over the concepts in the title before I get into the content of the talk.

I’m going to start with detrital dynamics.  Detritus is a fancy word for biological debris or debris made up of organic matter.  In aquatic systems, detritus is typically made up of plants parts (such as leaves that have fallen into the water), dead algae, and the occasional dead animal.  Detritus is an important food source for many aquatic insects, so a lot of aquatic scientists will put mesh bags full of leaves into streams and see what happens to them as part of their studies.  This is what detrital dynamics means: how detritus in a stream or lake is processed and broken down by the organisms living in the water.

Invasive means exactly what you think it might mean.  An invasive species is an organism that has invaded another environment, a habitat in which it has not lived before, either naturally or via human activities.  Most of the time, invasive species are problematic.  For example, yesterday I learned that round gobies were probably transferred from their native habitat to the Great Lakes in the ballast water of tansoceanic ships that entered the Great Lakes.  The species does well in the Great Lakes and is causing problems for native fish.  It is currently expanding its range into the streams that flow into the lakes.

Trophic cascades are a little more complicated.  Most people know about food chains or food webs.  Let’s quickly review the concept.  At the bottom, you have producers, things like plants and bacteria, that convert solar energy or other forms of non-organic energy into body mass.  Then you have the herbivores.  Herbivores eat the producers, converting the energy they get from digesting the producer into their own body mass.  Then you have the predators, the carnivores.  These organisms eat the herbivores to develop their body mass.  The producers, herbivores, and predators form what are known as trophic levels, groups of organisms that have similar feeding requirements and styles.  The suffix “troph” refers to food, so any time you see this term used in biological terms you know you’re supposed to consider food or feeding modes.

In the past, biologists used to think that the relationships between these organisms was a simple chain or a simple web: a predator eats an herbivore that ate a producer.  They didn’t think that a predator had much of an impact on other predators or the herbivores it didn’t eat, and early biologists definitely didn’t expect that the predator would have any impact on the producers.  We now know that this is a considerably oversimplified way to think about the relationship of organisms in nature and that the relationships among and between trophic levels can be incredibly complex.  As an example, consider a bird species that eats grasshoppers.  The birds love grasshoppers and eat as many as they can.  Because the birds are eating the grasshoppers, they are decreasing the number of grasshoppers in the population.  This in turn allows the grasses and other plants the grasshopper consumes to grow better because there are fewer grasshoppers eating them.  So, the bird has an indirect affect on the grasses.  And this is what is known as a trophic cascade: one organism impacts an organism in the next lower trophic level, which in turn impacts the next lowest trophic level, etc.  Everything is kept in a sort of balance, at least until something comes along to change that balance.  And that brings me to the subject of the talk I liked most yesterday: the signal crayfish.

The signal crayfish. Photo taken from Wikipedia.

The signal crayfish (Pacifastacus leniusculus) is an American crayfish that has become an invasive species in certain parts of the world.  They’ve been purposefully introduced into streams in Europe on more than one occasion and they’ve been transported beyond their natural range within the United States as well.  They’re omnivores and they eat almost everything they can get their mouthparts on, including several things that other organisms have a hard time eating.  As a result, they can cause major problems if they become established in an area outside of their native range.

Dr. Jonathan Moore of the University of California Santa Cruz studied the species in a stream in northern California, Scott Creek, where the crayfish is an invasive species.  He suggested that biologists still don’t have any means of accurately predicting what will happen if a species is added or removed from a stream because we don’t understand how everything ties together in a system.  He stated that trophic cascades are really more like trophic tangles, masses of interactions that we can’t disentangle from one another.  Some of his work focuses on trophic cascades and what happens when a species is eliminated or introduced into a system.

The study he presented was one in which he looked at the effect of the presence of signal crayfish on the populations of aquatic insects, algae production, and leaf litter processing in Scott Creek.  To do this, he did two different studies, one short term experiment and one long term observational study.  For the short term experiment, he compared the aquatic insects, algae levels, and leaf breakdown in small, isolated pools with various numbers of crayfish in them.  In the long term experiment, he looked at the same factors in pools that had never had crayfish in them compared to pools that had crayfish.

Dr. Moore found that in the short term, signal crayfish decreased the number of aquatic insects in the pools significantly.  This in turn allowed an increase in the amount of algae to occur, likely because the insects were not as abundant and were not able to eat as much of the algae in the pool and the crayfish don’t eat as much of the algae as the insects do.  He also found that leaf breakdown increased with an increase in crayfish abundance in the pools.  The crayfish were eating the leaves, so the more crayfish, the faster the leaves were broken down and consumed.

In the long term, Dr. Moore found similar results.  The number of aquatic insects was higher in pools without crayfish compared to pools containing crayfish.  The algae levels were higher in pools with crayfish than in those without.  He didn’t see the expected result with the leaf litter though: pools with and without crayfish had nearly identical leaf breakdown rates.  He attributed this to the fact that pools with low or no crayfish in them had more insects, particularly caddisflies, that broke the leaves down in the place of the crayfish.  So, the leaf litter being broken down or consumed at similar rates regardless of which organism was doing the work.

The take home message with which Dr. Moore ended his talk was that the impact of an organism on its environment is a combination of direct and indirect effects.  He also suggested that these direct and indirect effects can operate on distinctive timescales as evident from differences the leaf breakdown rates in the short and long term studies.  He then emphasized the importance of these types of studies so that we better understand the relationship between organisms and might someday be able to predict what will happen if a species is added or subtracted from a system with more precision.

And with that, I’m off for another day of talks!  Tonight I am also doing my presentation, a poster on my work on the respiratory behaviors of the giant water bug.  I will not talk about my own work tomorrow and focus instead on another talk I find interesting, but I’ll get back to my own research eventually.  Hope you’ll check back again tomorrow!

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Posts in this series:
Day 0 – Introduction to the Series
Day 1 – Invasive Crayfish
Day 2 – Giant Water Bug Dispersal
Day 3 – Dragonfly Captive Rearing
Day 4 – Integrating Service-Learning Programs into College Courses
Day 5 – Impact of a Small Preserve on Stream Health

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