Animals Make a Difference
Changing the distribution of predators and prey in an ecosystem can turn things upside down.
We all know that biodiversity is an important feature of our planet. Two papers published last week provide fascinating illustrations of why that is so.
Losing Big Fish Means More CO2 in the Atmosphere
Overfishing is depleting many fish populations, and because of fishers’ preferences for large predator fish and practices that favor removal of large predator fish, fish populations are trending toward a greater preponderance of small fish over larger ones and less fish at the top of the food web (an underreported phenom in my book). It might not be obvious, but such a trend could affect the amount of carbon dioxide in the atmosphere.
To understand why, we need to briefly review what is known as the biological pump. The ocean provides a major pathway for removing carbon dioxide from the atmosphere. The pathway begins when atmospheric carbon dioxide dissolves in the surface waters where it is taken up by phytoplankton during photosynthesis and converted to organic carbon. If that organic carbon is oxidized in the surface waters, it once again becomes dissolved carbon dioxide and can vaporize back to the atmosphere. No net removal.
But if the organic carbon sinks from the surface into deeper waters, the carbon is effectively isolated from the atmosphere and voila — you have atmospheric carbon dioxide removal.
The most direct way for that organic carbon to sink from the surface is for the phytoplankton to simply sink on their own. But the food web itself can assist that process. Fish that eat the phytoplankton can get eaten by even larger fish. These larger predator fish can then swim into deeper waters, where the organic carbon they ate in the surface waters is emitted as carbon dioxide via respiration. The net result is more carbon dioxide flows from atmospheric to deep waters.
I first came across this interesting notion that large predator fish could play an important role in the ocean carbon cycle in a 2006 paper published in the Journal of Marine Research by W.K. Dewar of Florida State University and co-authors (including my Duke colleague Doug Nowacek). It was also the subject of a more recent paper by Kakani Katija of the Woods Hole Oceanographic Institution published in The Journal of Experimental Biology.
Now Trisha Atwood of the University of British Columbia and co-authors have taken this idea one step further by asking how the fish-carbon pump would be affected by the fact that large predator fish are becoming less abundant. Presumably it would decrease the amount of carbon dioxide being removed from the atmosphere. But would the change be significant? To answer that question they examined the carbon cycle in ponds, streams and bromelaids with and without predator fish or insects. They reported their results in a paper published in the journal Nature Geosciences.
The authors found “substantially reduced carbon dioxide emissions [into the atmosphere] in the presence of predators in all systems, despite differences in predator type, hydrology, climatic region, ecological zone and level of in situ primary production.” The amount of reduction was startling: as much as 90 percent. The implication: if more predator fish means less carbon dioxide emissions from surface waters, then less fish means less net uptake of atmospheric carbon dioxide by surface waters and more greenhouse gases to warm the atmosphere.
What Kinds of Communities Are Best Suited to Survive Climate Change?
That was the question that Eric Post of Penn State University addressed in a paper published in the journal Proceedings of the Royal Society of London documenting “the results of a 10-year field experiment [in a low-Arctic plant community near Kangerlussuaq, Greenland] … which have focused on effects of ongoing warming on … stability within the plant community.”
To understand how biodiversity, climate change and herbivores affect the community, Post used about a dozen distinct paired plots exposed to either ambient or warmed temperatures and either grazed or ungrazed conditions during the growing season. The results: Most plots were destabilized by warming* and “[a]cross the entire plant community, stability increased with diversity…[and] stability and diversity were reduced by herbivore removal, warming and their interaction.”
Post speculates that diversity helps to stabilize communities subject to climate change because “species interactions … [either] stabilize interactions among competitors,” preventing one species from becoming overly dominant, or “lead to competitive exclusion of species for species” that would not otherwise have competitors.
As a species, human beings are already and will likely be increasingly challenged by climate change. We can best prepare ourselves by making sure our institutions and infrastructure are resilient. Of course one very important component of our infrastructure is the biosphere. As it appears, making that part of our infrastructure resilient means keeping it as diverse as we can. How do you think we are doing so far?
* There was some weak evidence that warming with grazing may have enhanced stability.