Photos from the Field: Lee at al. 2014. Seasonality & Canopy Beetles
Christina J. Lee, Alec Baxt, Suzette Castillo, and Amy Berkov. 2014. Stratification in French Guiana: Cerambycid Beetles Go Up When Rains Come Down. Biotropica 46(3): 302-311
When tropical rain forest insect species are associated with a particular season or forest stratum, it can imply tolerance for heat and drought—or moisture dependence; attributes that may predict their responses to global climate change. In French Guiana (1995–1996), wood-boring cerambycid beetles made a seasonal shift in stratum. During the dry season, ground stratum bait branches were densely colonized, but during the rainy season almost all cerambycids emerged from canopy stratum branches. Because the same substrate was available at both levels, abiotic factors probably influenced branch selection. In this study, cerambycids were reared at the same site (2007–2008) to determine if the seasonal shift recurred. Microclimate data (temperature, relative humidity, and wind speed) were collected with portable weather meters to test the hypothesis that microclimate would be similar at ground stratum during the dry season, and at canopy stratum during the rainy season. The seasonal shift in stratum did recur; many cerambycid individuals (56%) belonged to species classified as ‘seasonal shifters’. Temperatures in the preferred microhabitats were intermediate, but relative humidity remained high during the rainy season (regardless of stratum) and it was windier in the canopy (regardless of season). The shifters preferentially colonized branches at moderate mean temperatures (23.0–24.3°C) and high mean relative humidities (91.3–100%). Shifters were considered season and stratum generalists because they were reared at both strata and were present in both seasons, but they may actually track a narrow microclimate window. Should the regional climate become warmer and drier, it would probably favor species currently restricted to the dry season or canopy stratum.
To test our hypotheses about seasonality, stratification, and cerambycid stratification, arborist Alec Baxt placed portable Kestrel weather meters at ground and canopy stratum to measure temperature, relative humidity, and wind speed. He experimented with several different methods in the canopy: first fixed placement under a shade cloth, then constructing rigs that enabled him to raise and lower the meters for data download without disturbing the rest of our experimental set-up (suspended bait branches). For subsequent projects we set the meters to collect data less frequently—requiring less frequent download—and simply attached the meters to the lines used to suspend bait branches. This is less labor intensive and safer, but shade cloths might extend the short life-span of the meters during the rainy season.