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My Research: How might climatic changes affect both insect population dynamics and ecosystem processes in northern hardwood forests?

For over 20 years, Richard T. Holmes and his colleagues have documented the abundance and available biomass of caterpillars (or Lepidoptera) in the Hubbard Brook Experimental Forest in West Thornton, New Hampshire. Caterpillars are the primary food source for Neotropical migratory songbirds, and bird reproductive success is closely linked to food availability. The aggregated caterpillar community is surprisingly dynamic interannually (year-to-year): annual measures of caterpillar abundance and total biomass both varied more than 20-fold over the past two decades, as illustrated in the following figure from Reynolds et al. (in press).

Other interesting points :

• Peak caterpillar years were not associated with noticeable outbreaks of any one species and appeared to involve correlated responses of dozens of Lepidoptera species to factors influencing their abundance (grouped dynamics).
• Caterpillar fluctuations were correlated across three other sites located up to 30 km away (Jones et al. 2003)
• The fluctuations were correlated with the El Nino/ Southern Oscillation Index (Sillett et al. 2000).

This evidence suggests that large collections of species within this caterpillar community respond somewhat synchronously to climate effects. Northern New England's climate is remarkably variable from one year to the next, and we also have strong evidence that the region is gradually warming. Both types of climatic variation may be responsible for the caterpillar community dynamics witnessed over the past 20 years, but we don't understand how.

Specific Questions:

Are population fluctuations in the Hubbard Brook Lepidoptera community truly the result of grouped dynamics?

Do species' population growth rates vary according to criteria defined by aspects of species' ecologies and life histories, thereby providing structure to the community's population dynamics?

Can climatic variability generate changes in the ecosystem's nutrient dynamics, producing interannual variation in foliar nutritional quality that influences Lepidoptera community abundance?

 

Is there evidence of grouped dynamics?

Starting in June 2004, we have used six black light traps to collect weekly samples of the night-flying moth populations at the three study sites, ending in mid October (approximately one month beyond the end of the growing season). We then calculated species’ interannual per capita growth rate (R_t) for over 120 species in the Lepidoptera community:

R_t = Ln(N_t+1) - Ln(N_t) .

Where N_t is the total abundance of that species in year t and N_t+1 is the same species’ total abundance the following year. If the mean R_t for all identified species differs significantly from zero over a given year, it supports the hypothesis that the HBEF Lepidoptera community has grouped dynamics.

We identified 105 species that had 5 of more captures in either of the first two years. Of those, a significant proportion displayed negative interannual per capita growth rates
(mean R_t = - 0.801 ind. • ind.^-1 • yr^-1). We will expand the analysis once trap capture data is ready from the 2006 season, and eventually add data from 2007 as well.

What mechanisms cause group dynamics (and caterpillar fluctuations)?

We organize species into functional groups according to criteria that are potentially relevant to variation in species’ interannual per capita growth rates. Criteria include:

Taxonomic: at family, subfamily, tribe and genus levels
Phenology: timing and duration of larval and adult life stages, overwintering strategies, and whether or not adults feed
Host plants: preferred host plants and breadth within species’ diets
Feeding style: external feeder or leaf roller
General expected abundance
Range: relative position of study sites within range and species’ range size

These criteria represent hypotheses for how factors acting at broad spatial scales may influence subgroups within the Lepidoptera community differently. If species' R_t varies according to any of these criteria, it would suggest the mechanism responsible for grouped dynamics within the community.

 

Is leaf nutritional value influenced by climatic variation, and is the result great enough to influence caterpillar populations?

Although there are as many as 1000 (or more) Lepidoptera species in northern hardwood forests, the forests actually have very few different types of trees. It seems likely, therefore, that changes in food quality of one or more of the three or four dominant hardwood tree species would affect a large number of Lepidoptera species. Pursuing this question actually involves testing three hypotheses:

1. Soil mineral nutrients show meaningful variation that is caused by the climate.
2. Variation in soil mineral nutrients influences variation in foliar chemistry.
3. The interannual variation in foliar chemistry is sufficient enough to influence Lepidoptera larval growth performance.

We are using a variety of methods to test each sub-hypothesis, including quanitying soil nutrient mobility (availability) with resin bags, chemical analyses of leaf foliage from the four dominant hardwood tree species throughout the growing season, and growth trials (bioassays) to assess whether possible changes in leaf nutrient content influence growth performance of Leidoptera larvae. If tests support all three sub-hypotheses, it would suggest that nutrient limitation (a "bottom-up" effect) is a major driver affecting this community.

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