Wednesday, August 21, 2013

Fighting foreigners with foreigners: aphid vs. vine

Apparently the NYC Department of Parks and Recreation is releasing thousands of weevils originally from Asia to fight mile-a-minute, an invasive vine also native to Asia, which has taken over parts of the city.
I want your MAM(my)! 

When I first read this, it immediately triggered alarm bells to go off in my head. I don't know if it is the combination of quasi-hippie environmentalist and old-school naturalist professors I had during my undergraduate, which insisted on letting things be, or my mother's insistence that two wrongs don't make a right, but I thought this would be yet another human manipulation destined for failure. Think cane toads, native to central America and introduced to sugar cane fields in the Caribbean and Australia to keep down pests, but now spreading well beyond its range and killing off many of its would-be predators with its poisonous skin. Or, the story (for which I can find absolutely no evidence for now) that rats were introduced to an island, then snakes were introduced to eliminate the rats, and the plan backfired and the snakes have taken over the island, killing much of the native wildlife (it almost sounds like the story of Guam and brown tree snakes, but isn't).

However, I am glad to learn that we have learned from our mistakes, and when we say that extensive research was done to evaluate both safety and efficacy of the aphids in targeting mile-a-minute, hopefully we mean it (see references here). As with most pest invasion studies, potential control mechanisms were identified by looking for the herbivores which keep the plant in check in its native environment. Researchers identified Rhinoncomimus latipes aphids (they really need a good common name - can we nickname them munch-a-minutes?) as good potential targets for further research, and their breeding began in controlled environments in the US.

The potential heroes of the story...

Of course, organisms don't always behave the same in a new environment as they did in their original environment (this is why things may be minor members of a diverse community in their native habitat, and invade in another), so the next step was to evaluate whether they still feed primarily (or ideally uniquely) on mile-a-minute. Researchers at the University of Delaware dusted these aphids red, and placed them at the base of non-target plants, or yellow, and placed them at the base of mile-a-minute, and followed them through time to see where they ended up. The researchers found that aphids which originally started on mile-a-minute did not stray onto non-host plants, and those on non-host plants found their way onto mile-a-minute more and more as time went on. This is all good.

However, there are still other questions which remain unanswered. For example, do the aphids have natural predators? If mile-a-minute declines, or is in low density in some places, will the aphids switch to other food sources (like us and previously disregarded fish)? Will they reproduce with native mites and make new, super cabbage-eating mites which will upset community gardeners? And, as far as I can tell, although we know it munches on the weed, we don't know whether the mite occurs in sufficient density to have a noticeable impact on mile-a-minute population.

While some of these worries may seem a little far-fetched, all have previously emerged as problems. That said, we cannot be frozen by fear; we live in a dynamic world we change for better or worse.  We don't have enough knowledge to predict the future, so we should try and balance gathering enough information to make an informed decision, and making a decision in a timely manner. After all, researchers need results for grant applications or to appease investors, and some are willing to release their experimental creatures into the wild without full ecological impact assessments in order to undercut scientists with possibly more ethical methods.

Sunday, August 11, 2013

The chemistry of decomposition - what is really going on down there?

I recently visited Jerry Melillo's warming plots at the Harvard Forest, and I don't know why, but I was amazed by how much the leaf litter had decomposed over the past few months. This got me thinking about the structure of carbon of the remaining litter. The historic view has been that labile sugars go first, then cellulose, then lignin, but what about the waxes and other compounds which don't fit into these categories? To a certain extent, our understanding of litter decomposition has been limited by our ability to detect and distinguish these other compounds, but tools like NMR are changing that.

The husband-wife duo of Nishanth Tharayil and Vidya Suseela at Clemson have once again teamed up with Baoshan Xing at UMass to study the fine chemistry of litter. In one of their previous papers, the authors noted that reducing precipitation increased the relative abundance of tannins in red maple litter; in this paper they looked at how the chemistry of Japanese knotweed litter, that horrible invasive which is actually quite delectable when young, changes through time in litter subject to different warming and precipitation treatments at the Boston Area Climate Experiment.

In this experiment, the authors made "old" and "new" litter bags by harvesting Japanese knotweed which had either been decomposing upright following senescence the previous year, or just-senesced stems. They were placed at the edges of the high (~+3C) and ambient temperature plots, under drought (50% of precipitation removed year-round), ambient, or wet (an extra 50% of rain applied during the growing season) treatment, and harvested at four time points over a period of three years. By pulling peaks from various methods I don't understand (DRIFT Spectroscopy, and 13C cross-polarization magic angle spinning NMR spectroscopy), they confirmed that decomposition of recalcitrant litter is generally more temperature sensitive than more labile stuff. Again, as previously noted, decomposition is greatest when supplemental precipitation is applied in conjunction with warming. But perhaps the most interesting point was that while the effect of climate treatment on overall decomposition rate did not differ between new and old litter, specific (recalcitrant) compounds did decompose more fully in the older litter, which the authors cite as evidence that initial litter chemistry does matter.

As my PI pointed out, there are a few problems with the way in which this experiment was designed. First, the authors used litter from a plant not found in the warming experiment, collected at a site a hundred miles away. The authors state that this litter was chosen because all litter is clonal, and therefore should have been initially identical, but wouldn't litter taken from one of the trees at or near the experimental site not be adequate? Perhaps the problem would be that litter allowed to decompose in-situ for a year would start with clearly different microbial communities than the litter which had just-fallen from a tree. That said, we know that there is often microbial succession during decomposition, and therefore the litter going in old probably had a different microbial community associated with it than the "new" litter, whether or not it was harvested from immediately adjacent stems from a clonal population. In this instance, differences in decomposition with litter starting chemistry could be due to the presence of a microbe at the litter source site initiating decomposition of compounds that microbial populations are not as well-suited to break down. For instance, there could be fungi at the source litter site which are much less abundant at the experimental warming site because it is a well-trodden former agricultural field with relatively low organic matter content.

My PI also pointed out that using litter which starts from the same plant but is in different stages of decay is not a particularly biologically informative way to answer a question. She said that using various kinds of leaf litters which naturally differ in their starting chemistry, as occurs in ecosystems today and potentially exacerbated by climate-induced shifts in species composition and litter chemistry, would make the results of the experiment more useful in ecosystem carbon models. I believe the litter bags for that experiment are decomposing in-situ as I type.

As I alluded to above, my main beef with the paper was, of course, that they didn't talk about whether the microbial community differed between warming treatments. I am interested in knowing whether the differences in decomposition are due to purely physical effects, or whether changes in the microbial community also played a role. I would also like to know if succession of microbes on the different litter ages and in the different plots followed the same pattern, just being accelerated in some instances, or whether the communities were completely different.

This is a kind of chronic problem with ecologists; they generally ignore microbes (or suppose what is happening without validating the assumption). Almost every paper I read, I hope with all my heart that they have soil cared for nicely and kept in an ultra-low freezer somewhere. But anytime I inquire, the answer is no. If they want to really understand what happened in their system, it is their loss.

But please, if you are doing anything involving soil, or are doing a study in which you expect soil or litter-degrading microbes to be adhered to your object of interest, please flash freeze that soil/litter at collection and place in a -80C freezer. If you don't have access to one, contact me before you collect your samples and I will send a self-addressed cooler with dry ice, and I will fill my PI's freezer with random samples as long as I can without her noticing.