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Climatic effects on caterpillar fluctuations in northern hardwood forests

Lindsay V. Reynolds 1,2, Matthew P. Ayres 1, Thomas G. Siccama 3, and Richard T. Holmes 1
  1 Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
  2 Current address: Department of Forest, Rangeland and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523, USA
  3 Department of School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA

  Common Lepidoptera Extacting a tree core
  Common Lepidoptera of Hubbard Brook Experimental Forest Extracting a core from a yellow birch tree.
  Contact Info:
  Matthew P. Ayres
Department of Biological Sciences
Dartmouth College
Hanover, NH 03755
phone: (603) 646-2788
email: Matthew.P.Ayres@Dartmouth.Edu

LEPIDOPTERA are among the most diverse and abundant insects in temperate zone forests, such as the Hubbard Brook Experimental Forest (HBEF) (Ferguson et al. 1999). As caterpillars, Lepidoptera are important consumers of plant tissues in these ecosystems, and all life stages can be preyed upon to form the base of the grazing food web (Holmes and Schultz 1988). Thus changes in Lepidoptera abundance affect herbivory levels as well as resources for consumers, such as insectivorous birds (Nagy and Holmes 2005). Some Lepidoptera species exhibit conspicuous population fluctuations that are synchronous over large geographical areas, suggesting that climate may play a major role in the timing and occurrence of these population changes (Jones et al. 2003,Haukioja 2005). In addition, it has been noted recently that the ranges of several species of Lepidoptera have shifted in response to climate change, but the mechanisms producing these shifts are poorly understood (Parmesan et al. 1999). In general, the mechanisms by which climate influences interannual fluctuations in Lepidoptera abundance are not clear.

  Graph of interannual change in caterpillar abundance Graph of interannual change in caterpillar abundance  
  Figure 2. Mean annual caterpillar biomass from 1986 to 2005 at Hubbard Brook Experimental Forest, NH, USA. The upper panel indicates biomass of geometrids vs. other families of Lepidoptera (in log units). The lower panel shows total caterpillar biomass (back transformed from log units). Standard errors are based on four samples per year.  
  Figure 3. Interannual change in caterpillar abundance relative to minimum winter air temperature, radial tree growth, and thermal sum at four Hubbard Brook weather stations (HQ, 1, 14, and 6) during 1986- 2005.  

In this study, we examined three possible pathways through which climate might generate synchronous fluctuations of multiple caterpillar populations across a forest landscape: (Hypothesis 1) extreme winter cold might reduce abundances of many species synchronously, (Hypothesis 2) long, warm summers might enhance growth and survival in many species, and (Hypothesis 3) interannual variation in tree growth, that is correlated changes in leaf chemistry, might affect the growth and survival of many species.

To address these hypotheses, we collected data from HBEF on caterpillar population dynamics, we synthesized appropriate climate data collected at HBEF, and we collected tree cores from two dominant forest tree species sugar maple ( Acer saccharum ) and yellow birch (Betula alleghaniensis) to analyze interannual tree growth. We found that caterpillar abundance in the HBEF and the surrounding White Mountains fluctuated by >20-fold during 1986-2005 (Figure 2). Caterpillar fluctuations from one summer to the next were uncorrelated or negatively correlated with minimum air temperature during the intervening winter, leading us to reject hypothesis 1. However, caterpillar fluctuations were positively correlated with thermal sum during the summer (r = 0.49-0.56). Warmer summers led to greater caterpillar abundance in the following year, which is consistent with hypothesis 2 (Figure 3). There was little interannual variation in the radial growth of the two tree species measured and no correlation with caterpillar fluctuations occurred, refuting hypothesis 3 (Figures 3 and 5).

  Graph of interannual change in caterpillar abundance
Figure 4. From 1961 to 2004, the average thermal sum during the growing season at Hubbard Brook Forest increased by 153 degree days (5 °C base): slope ± SE = 3.5 ± 1.1 degree days / year (t = 3.29, P = 0.002, r 2 = 0.20, n = 44 years; weather stations HQ, 1, and 6). This has involved a warming trend throughout the spring, summer, and fall. The increasing slope of the warming trend with Julian date was significant at p = 0.023, r 2 = 0.95, n= 5). Plotted standard errors are based on three climate stations.

Thermal sum might influence caterpillar fluctuations through direct effects on insect development, indirect effects on susceptibility to natural enemies, and/or indirect effects on plant-insect interactions. These mechanisms are of particular interest because thermal sums have been increasing since local records of temperature began in 1957 (r = 0.41–0.45) (Figure 4). If thermal sums affect caterpillar abundance, and since caterpillars are a major component of the forest food web (Gosz et al. 1978), then directional changes in thermal sums should yield directional changes in ecosystem structure and function. Understanding the mechanisms that link caterpillar population fluctuations to thermal sums should aid in the development of general theories regarding the effects of climatic variation on ecosystems (Walther et al. 2002, Helmuth et al. 2005)

   Graph of interannual change in caterpillar abundance  
  Figure 5. Mean detrended annual radial growth of sugar maple (A. saccharum, solid line) and yellow birch (B. alleghaniensis, dotted line) from 1960-2001 in the White Mountains, NH (40 trees / species). Variation among years was significant but the coefficient of variation was only 5%.

Literature Cited
Ferguson, D.C., Harp, C.E., Opler, P.A., Peigler, R.S., Pogue, M., Powell, J.A., and Smith, M.J. 1999. Moths of North America. Northern Prairie Wildlife Research Center. Jamestown, North Dakota.

Gosz, J.R., Holmes, R.T., Likens, G.E., and Bormann, F.H. 1978. The flow of energy in a forest ecosystem. Sci. Am. 283: 92-102.

Haukioja, E. 2005. Plant defenses and population fluctuations of forest defoliators: mechanism-based scenarios. Annales Zoologica Fennici 42: 313-325.

Helmuth, B., Kingsolver, J.G., and Carrington, E. 2005. Biophysics, physiological ecology, and climate change: does mechanism matter? Annual Review of Physiology 67: 177-201.

Holmes, R.T. and Schultz, J.C. 1988. Food availability for forest birds - effects of prey distribution and abundance on bird foraging. Can. J. Zool. 66: 720-728.

Jones, J., P. J. Doran, and R. T. Holmes. 2003. Climate and food synchronize regional forest bird abundances. Ecology 84:3024-3032.

Nagy, L.R. and Holmes, R.T. 2005. Food limits annual fecundity of a migratory songbird: an experimental study. Ecology 86 : 675-681.

Parmesan, C., Ryrholm, N., Steanescu, C. Hill, J. K., Thomas, C. D., Descimon, H., Huntley, B., Kaila, L., Kullberg, J., Tammaru, T., Tennent, W. J., Thomas, J. A. and Warren, M. 1999. Poleward shifts in geographical ranges of butterfly species associated with regional warming. Nature 399: 579-583.

Reynolds, L.V., M.P. Ayres, T.G. Siccama, and R.T. Holmes.  2007.  Climatic effects on caterpillar fluctuations in northern hardwood forestsCanadian Journal of Forest Research, in press.

Walther, G.R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J.C., Fromentin, J.M., Hoegh-Guldberg, O., and Bairlein, F. 2002. Ecological responses to recent climate change. Nature 416: 389-395.

Date Prepared: October 2006