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Cave-Dwelling Invertebrates Enjoy Exotic Cuisine

The views expressed are those of the author and are not necessarily those of Scientific American.


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Jenolan Caves, Australia. CC licensed image courtesy of flickr user avlxyz.

While the deep-sea may be the final frontier for marine biologists, caves remain one of the most elusive frontiers on (or rather, under) the land. Some caves extend dozens of miles below the ground in endless, sinuous networks all but cut off from the grassy hills and tree-lined horizons above. It’s not an easy environment to access and many explorers have perished attempting to map these subterranean labyrinths. Yet, for the last couple decades in particular, investigations keep finding astonishing communities of invertebrates inhabiting caves and existing nowhere else.

Nestled in Australia’s stunning Blue Mountain range is the 350 million year old Jenolan Caves Karst Conservation Reserve. Here, the base of the invertebrate community consists of decaying leaf litter. Historically, eucalyptus trees, which are native to the area, contributed most to the leaf litter pool. Over the years, introduced trees – like European sycamore brought in to stabilize steep, rocky slopes and North American Monterey pine planted for the timber industry – have naturalized around the cave opening.

To understand what effects the differences in leaf little composition have on cave communities, Hills and colleagues measured the rate of leaf litter decay and invertebrate diversity among the 3 leaf litter pools in “twilight” areas (i.e. nearer to cave openings) and “deep” areas (i.e. where the cave is always dark). The most rapidly decayed leaves were of the introduced Sycamore, which suggests their leaves release more carbon and nutrients into the cave ecosystem. Additionally, there was no difference in leaf decay rate between “twilight” and “deep” leaf litter, so it appears it doesn’t matter how close the litter is to above-ground features like light, rain and wind.

~Interlude~

Before discussing the rest of study, stand up and stretch, take a deep breathe and now sit back in your chair and relax… Close your eyes and envision a cave. Its dark, moist, there is only one small opening. All that remains of it is a singularity of daylight. It gets smaller as you glide further away. For thousands of years this cave has been fed organic matter from whatever vegetation surrounds it. Trees like the eucalyptus abound among the limestone hills, shedding off their leaves which casually drift on a wisp of wind towards the opening of the cave. Each day, over millennia, leaves are settling onto the stoop of Jenolan’s house and many are welcomed in by the forces of wind and gravity. Insects and arachnids feast on the leaves while the fungi and bacteria work their juices to break each product of sunlight into ever smaller pieces.

~End Interlude~

It is not difficult to imagine how generations of organisms become better adapted to consuming the food that is available to it if that food source doesn’t change over time. More efficient break-down of the plant matter, resistance to bitter tannins, adjusting body size and metabolism to food availability  - all strategies to adapt to food supply. Fast forward to our current century and globalism has rapidly introduced new organisms to every corner of the planet at an unprecedented pace. Are the invertebrate communities more diverse and abundant on the native vegetation that it may be better adapted towards?

The abundance and species diversity are much greater for the introduced european Sycamore than either the pine or Eucalyptus (see graphs below), especially in the “twilight” zone. This may be because sycamore leaves have higher specific leaf area (SLA) than both pine and eucalyptus. A low SLA is associated with long-lived leaves containing many structural and defensive compounds. These trees, like pines and eucalyptus, invest heavily to guard against plant-eaters whereas the broad-leafed sycamore does not invest as heavily against herbivory, so leaves break down more quickly. This faster release of nutrient may be part of the reason sycamore leaves have a more abundant and diverse community.

So what would happen if sycamore were to completely supplant eucalyptus? As a higher nutrient leaf that more quickly releases carbon, it should be better for the spineless society down under, right? One problem is that Sycamore is a deciduous tree. This means nutrient pulses to the caves occur seasonally. Since it’s leaves break down so quickly, it is a short lived pulse compared to the structurally-strengthened Pine and Eucalyptus, both of which keeps their leaves year-round. Since caves are below-ground they tend to be protected from seasonal variability and are relatively stable environments in terms of climate. Cave invertebrate communities need a more constant or stable supply of leaf litter to be sustained. As the authors propose,

“The short-term influx of energy provided by sycamore litter could be detrimental to subterranean invertebrate diversity in the long term. We would expect to see invertebrate species predisposed to utilizing sycamore derived energy dominating subterranean invertebrate communities and perhaps out-competing other invertebrate species, thereby reducing invertebrate diversity.”

Paradoxically, in an environment shaped by constancy over time and space, nutrient pulses may reduce diversity over time. If this is only a seasonal phenomenon, complete species replacement would be unlikely given that established invertebrates may utilize multiple plant sources over the year and persist over the long-term. There will always be a staple of of the community that are more generalist and can inhabit the caves under strenuous circumstances. But, in sort of a temporal intermediate disturbance hypothesis sort of way, one can envision the rapid change disrupting the system as a whole and having cascading effects throughout the cave ecosystem. Regardless, the moral remains: indifference towards species natural adaptions results in far-reaching and unforeseeable shifts to their ecology and evolution.

ResearchBlogging.orgHILLS, N., HOSE, G., CANTLAY, A., & MURRAY, B. (2008). Cave invertebrate assemblages differ between native and exotic leaf litter Austral Ecology, 33 (3), 271-277 DOI: 10.1111/j.1442-9993.2007.01814.x

Kevin Zelnio About the Author: Kevin has a M.Sc. degree in biology from Penn State, a B.Sc. in Evolution and Ecology from University of California, Davis, and has worked at as a researcher at several major marine science institutions. His broad academic research interests have encompassed population genetics, biodiversity, community ecology, food webs and systematics of invertebrates at deep-sea chemosynthetic environments and elsewhere. Kevin has described several new species of anemones and shrimp. He is now a freelance writer, independent scientist and science communications consultant living near the Baltic coast of Sweden in a small, idyllic village.

Kevin is also the assistant editor and webmaster for Deep Sea News, where he contributes articles on marine science. His award-winning writing has been appeared in Seed Magazine, The Open Lab: Best Writing on Science Blogs (2007, 2009, 2010), Discovery Channel, ScienceBlogs, and Environmental Law Review among others. He spends most of his time enjoying the company of his wife and two kids, hiking, supporting local breweries, raising awareness for open access, playing guitar and songwriting. You can read up more about Kevin and listen to his music at his homepage, where you can also view his CV and Résumé, and follow him twitter and Google +.

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The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 3 Comments

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  1. 1. Lou Jost 7:06 am 12/29/2011

    Interesting result, thanks for featuring it. I have not read the original. Did they use any frequency-sensitive diversity measures, or only richness? Richness is not necessarily the most informative measure, A more complete (and possibly very different) picture would be revealed by looking at diversity measures that include species relative abundances, as the authors suggest in the quote you gave near the end.

    For example, invasive plants have become dominant in some Galapagos habitats. Plant richness increased, because we now have new species on the islands. The native species hang on, but become rare and marginal. This is not a good thing, even though “diversity” has increased. The same applies to starlings in the eastern US–the bird communities that contain starlings are more diverse than the original community, according to species richness (because they have starlings in addition to native birds), but in many habitats the starlings are super-dominant and some native birds (particularly hole-nesters) have been driven nearly to extinction. An abundance-sensitive diversity measure would detect the decrease in equitability in the ecosystem, and hence a decrease in diversity when species abundances are taken into account.

    The best way to report these kinds of studies is by drawing a “diversity profile” using Hill numbers (= “true diversities”); this graph starts with species richness on the left and puts increasing emphasis on species frequencies as it goes to the right.

    In this cave system, it could be that non-specialized detritivores can eat sycamore leaves but not eucalyptus, and these non-specialists might swamp the specialists and reduce their numbers. It is necessary to see the complete picture before concluding anything from this study. I hope the authors have such an analysis. If not, their paper is incomplete and may mislead. Their quote in your post suggests they are aware of this. Did they do something about it?

    Lou

    Link to this
  2. 2. Kevin Zelnio in reply to Kevin Zelnio 10:48 am 12/29/2011

    Thanks for your comment!

    No, they didn’t use a frequency-dependent diversity measure. Not sure why considering they used PRIMER which calculated Shannon, Pielou and Fisher’s alpha easily enough. Only richness was used. They also used nMDS with presence/absence data instead of proportion of individuals. The nMDS plot didn’t show any strong patterns (1-2 sites grouped together but not by leaf composition or zone). After re-reading, I suspect they didn’t analyze their data more quantitatively because they could only ID their species down to morphospecies. So this may be a conservative approach in view, yet, as you pointed out, certainly masks their patterns of community assemblage.

    From the writing, I gather they were limited in their analysis, but I wish they confronted this head-on in the writing. I think it is OK to publish results conservatively, but authors need to address why they analyze their data the way they do. This should have been picked up by the editors and reviewers. Perhaps it was addressed to the reviewers but readers are unaware of goes on behind closed doors.

    I still think the data holds, but their interpretations are limited. There is more individuals and more species on non-native leaves during their study period (from February to May, 2006). But I agree that any community assemblage interpretation is mostly meaningless.

    Link to this
  3. 3. Lou Jost 8:48 pm 12/29/2011

    Thanks for your answer!

    Link to this

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