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Forest nutrient dynamics’ influence on interannual variation of
caterpillar community abundance

 

  Contact Info:
  Erik Stange
Dartmouth College
Department of Biological Sciences
Hanover, NH 03755
phone: (603) 646–2788
email: Erik.Stange@Dartmouth.EDU

TWENTY YEARS of systematic sampling have shown that the abundance of Lepidoptera larvae (caterpillars) in the White Mountain National Forest can be quite variable from one year to the next, producing 50-fold variation in the aggregate biomass of these herbivores. Through the use black light traps to capture night-flying adult Lepidoptera (moths), we have established that these interannual fluctuations in caterpillar abundance and biomass result from the near-synchronous population dynamics of scores of species. This result suggests that the forces generating the caterpillar fluctuations must be general such that they are capable of simultaneously affecting many species in a similar fashion.

Most of the many hundreds of caterpillar species living in the White Mountain National Forests surrounding Hubbard Brook feed on tree foliage (leaves). Because the forests are comprised of approximately a dozen dominant tree species, large numbers of caterpillar species share host plants. Therefore, interannual changes in the nutritional quality in the leaves of even one host plant might be capable of generating the correlated population dynamics of many species of Lepidoptera. Our work tested the hypothesis that interannual variation in the availability of nutrients in the soil can influence the nutritional quality of tree leaves for caterpillars and ultimately the total biomass of the forest’s caterpillar community.

 
  Measuring forewing length using digital calipers.
   

Over a period of four years, our measurements of the foliar chemistry of the forest's four dominant understory tree species (Acer saccharum, Acer pensylvanicum, and Viburnum alnifolium) showed the nutritional quality of these leaves varied interannually in a way that was consistent throughout the landscape and was linked to changes in the availability of nitrogen (N) and phosphorus (P) in the soil. Measurements of the forewing lengths of many Lepidoptera species that feed in the early season (May and early June) showed that moths were both bigger and more abundant in years when foliage contained more N—suggesting that groups of the forests’ caterpillar species experience correlated variations in larval growth performance. Because interannual climatic variation is the principal driver of soil nutrient dynamics, projected climate changes may increase the abundance of these dominant forests herbivores.

Other Methods Used
We measure soil nutrient availability with bags of resin that attracts both positive and negatively charged ions, placed in the soil for up to 6 months at a time. We used whole tree fertilization with NH4NO3 and PO4 to test whether tree leaf chemistry responds to increased soil nutrient availability. We used elemental analyzers to measure the foliar concentration of N and P, as well as colorimetric tests of extracts to test for interannual variation in C-based defense compounds (condensed tannins and total phenolics).

Date Prepared: December 2009