Insect size responses to climate change vary across elevations according to seasonal timing

Body size declines are a common response to warming via both plasticity and evolution, but variable size responses have been observed for terrestrial ectotherms. We investigate how temperature dependent development and growth rates in ectothermic organisms induce variation in size responses. Leveraging long-term data for six montane grasshopper species spanning 1768 to 3901m, we detect size shifts since ~1960 that depend on elevation and species’ seasonal timing. Size shifts have been concentrated at low elevations, with the early emerging species (those that overwinter as juveniles) increasing in size, while later season species are becoming smaller. Interannual temperature variation accounts for the size shifts. The earliest season species may be able to take advantage of warmer conditions accelerating growth during early spring development, whereas warm temperatures may adversely impact later season species via mechanisms such as increased rates of energy use or thermal stress. Grasshoppers tend to capitalize on warm conditions by both getting bigger and reaching adulthood earlier. Our analysis further reinforces the need to move beyond expectations of universal responses to climate change to consider how environmental exposure and sensitivity vary across elevations and life histories.

Methods
Specimens and measurements

Grasshoppers were field collected, mostly by sweep netting as part of weekly surveys, in montane or subalpine sites primarily along the 40th N parallel in Boulder County, CO. Historical collections were led by Gordon Alexander, with sampling concentrated in 1958-1960. We conducted resurveys with sampling concentrated in 2006-2015. The specimens used to measure body size are available in the University of Colorado Museum of Natural History.

Body size was measured as femur length, which is the body size metric that can best be compared between museum and recent specimens. Femur length is widely used to indicate body size in grasshoppers and other insects, due to the ease of measurement and its high correlation with body mass. We measured femurs to the nearest one hundred of a millimeter using digital calipers. Femur length was estimated as the average end to end length of both femurs, which were measured twice for most of the specimens. However, a minority of specimens were measured to a lesser resolution, had only one femur, or were measured only once. Historic specimens were dried whereas modern specimens were measured after being stored frozen or were occasionally live when part of other experiments. To confirm that femur length was not altered by drying, we measured some modern specimens fresh and again following being dried.

The focal species exhibit additional life history and functional differences. The nymphal diapausers (E. simplex and X. corallipes), along with A. clavatus and C. pellucida, primarily feed on grasses and sedges, while the other species are generalists, consuming both grasses and forbs. The species differ in dispersal ability (due to wing length differences) and lower rates of gene flow result in greater genetic differentiation and more potential for local adaptation. Melanoplus boulderensis and A. clavatus have short wings and are least dispersive. Females of E. simplex and X. corallipes have longer wings but tend to be poor fliers.

Our analysis focuses on 10 sites spanning the following elevations: 1768m, 2042m, 2134m, 2317m, 2591m, 3048m, 3414m, 3505m, 3566m, and 3901m. These sites were chosen because they offered sufficiently large historic and recent body size samples for comparison. The sites are all grassy meadows, with similar plant communities but somewhat denser vegetation at the mid elevation sites.

Climate data

Climate data were obtained for each specimen based on its site and year of sampling. We accessed daily mean temperature data for five weather stations corresponding to collection locations (A1: 2195 m, 40.01N, 105.37W; B1: 2591 m, 40.02N, 105.43W; C1: 3048 m, 40.03N, 105.55W; D1: 3739 m, 40.06N, -105.62W; Boulder: 1671 m, 39.99N, 105.27W). The Boulder data were accessed from the NOAA Physical Sciences Laboratory (https://psl.noaa.gov/boulder/data/). Data for other sites were accessed from the Niwot Ridge Long-Term Ecological Research program (LTER, https://nwt.lternet.edu/). Data from 1953 to 2008 were accessed from McGuire et al. (2012) and included some regression-based interpolation between weather stations to fill data gaps. Data from 2008 to 2015 were accessed from Buckley et al. (2021) using the same source data interpolation approach. We extended the weather record through 2022 using data from NOAA and the Niwot Ridge LTER site. Following the weather data assembly, data were missing for A1 and B1 from 1970-1986. We interpolated this weather data, again using the interpolation approach of McGuire et al. (2012). For sites without weather data, we used the available climate data most similar in elevation.

 We aggregated daily maximum, minimum, and mean temperature data into averages across days of the growing season (March through August, doy 60 to 243) and previous summer (June through August, doy 152 to 243) temperatures. We used temperatures the previous summer since they influence maternal conditions and the development of nymphal diapausing species, which overwinter in a late juvenile stage. We did not examine winter temperatures since most sites have abundant winter snow cover that covers and thermally buffers eggs or nymphs from temperature fluctuations. We examined, but ultimately did not incorporate additional climate metrics including growing degree days available for development and snow depth and timing based on model performance.

Phenology

We examined how phenology influences body size using the collection date of specimens as well as field surveys. Data from weekly surveys were analyzed for both a historic (1958-1960) and recent (2006-2016) period. We quantified phenology as the day of year when a spline fit to developmental data indicated that the average developmental stage of the sample population was 5.5 (development index ranging from 1: all first instars to 6: all adults; detailed methods in Nufio and Buckley 2019).