Cave Inverts Prefer Exotics

While the deep-sea may be the final frontier for marine biologists, caves are one of the least studied environments on land. Some caves can extend dozens of miles below the ground in sinuous networks, all but cut off from the grassy hills and tree-lined horizons above. Its not an easy environment to access and many explorers have perished attempting to map these subterranean labyrinths. Yet, recent investigations have found an astonishing community of invertebrates associated with caves, existing nowhere else. Many of these species are insects and spiders, adapted to the dark conditions, muggy conditions. Nearly every new cave expedition turns up species never before seen.

How do these animals exist down there? What is their source of food? It turns out that nestled in the Blue Mountains of Australia at the 350 million year old Jenolan Caves Karst Conservation Reserve, the base of the invertebrate community consists of decaying leaf litter. Eucalyptus trees, native to the area, historically contributed the most to the leaf litter pool. Over the years, introduced trees have naturalized around the cave opening. Sycamore from Europe was brought in to stabilize steep, rocky slopes and Radiata Pine from North America was provided for the timber industry. Hills and colleagues from the University of Technology Sydney compared leaf litter decomposition rates and invertebrate diversity between the 3 leaf litter pools in "twilight" areas (i.e. near cave openings) and "dark" areas deeper in the cave.

The introduced Sycamore leaves decomposed much faster than the radiata pine needles and native eucalyptus leaves. This suggests that Sycamore leaves release more carbon and nutrients into the cave ecosystem, potentially supporting a more abundant and diverse invertebrate community. Interestingly, there was no difference in leaf mass loss between twilight and dark leaf litter. Proximity to above-ground features like light, rain and wind appear not to affect leaf litter decomposition.

Before I discuss the trends for invertebrates there, I want you to sit back in your chair, take a deep breathe and relax. Close your eyes and envision a cave. Its dark, moist, there is only one opening. All that remains of it is a singularity of daylight. For hundreds or thousands of years this cave has been fed organic matter from the surrounding vegetation. Trees such as Eucalyptus abound in the limestone hills, shedding off their leaves which make their way by the cave opening eventually being blown in by a gust of wind. This occurs daily for millennia. Insects and arachnids feast on the leaves as well as the fungi and bacteria associated with the leaf matter. Over time one might suspect that the animals become adapted to the leaf litter type in some way.

Fast forward to the last 100 years and globalism has introduced new organisms to every corner of the planet at an unprecedented pace, including Sycamore and Radiata Pine to eastern Australia. Are the invertebrate communities more diverse and abundant on the native vegetation that is may have adapted to?

As you can see from the above figures, abundance and species richness are much greater for the introduced european Sycamore than either the pine or Eucalyptus, especially nearer the cave opening. The authors don't really nail the answer with their experiment, but narrow it down to attributes of Sycamore that favor colonization by invertebrates over Pine and Eucalyptus. For instance, Sycamore has a 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 invest heavily to guard against plant-eaters whereas the broad-leafed Sycamore does not invest against herbivory, so leaves break down quickly. This faster nutrient release 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? It is a higher nutrient leaf and releases carbon faster (i.e. breaks down faster), so 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 would occur on a seasonal basis. Since its leaves break down so quickly, this pulse would be short lived compared to the structurally-strengthened Pine and Eucalyptus, both of which keeps their leaves year-round. Being below-ground, caves are protected from the variability of the seasons and are relatively stable environments in terms of climate. The invertebrate community there needs a more constant or stable supply of leaf litter to be sustained. 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."

Interesting food for thought: nutrient pulses reduce diversity over time. On the other hand, if the authors are talking about partial species replacement, I would also expect to see increase in diversity pooled over the whole year. If this is only a seasonal phenomenon, complete species replacement would be unlikely given that established invertebrates that can utilize multiple plant sources over the year will persist over the long-term. It would certainly be an idea worth funding, especially from the angle that caves may be sheltered from the immediate effects of climate change. A nice replicated mesocosm study within the caves and near the entrances. 'X' amount of replicates sampled throughout the year at various time points to gauge the effect of seasonal nutrient pulses. How about adding various mixtures of different quality leaves to test the hypothesis that maximum food quality results in highest species richness.
------------------------------------------------------------------------------------------------
HILLS, N., HOSE, G.C., CANTLAY, A.J., MURRAY, B.R. (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