DOI: 10.5061/dryad.bcc2fqz8n
title: Data from: Thermal tolerances and species interactions determine the elevational distributions of insects

description: Aim: While physiological limits to thermal extremes are often thought to
 determine the abundance and geographic distribution of species, more
 recent evidence suggests that species interactions may be equally
 important. Moreover, the relative importance of these constraints may
 shift with changing abiotic conditions, such as climate change. Here, we
 explore the relative importance of physiological tolerances to heat and
 species interactions in determining the distribution of insects along two
 elevational gradients. The gradients contrast in precipitation but not
 temperature, allowing us to separate these two climatic factors. Location:
 Montane rainforest in Costa Rica. Time period: 2015-2016. Major taxa
 studied: Bromeliad-dwelling aquatic insect larvae. Methods: We estimated
 the elevation preferences of five insect taxa by surveying 170 bromeliads
 along the moist Atlantic and the dry Pacific slopes of Monteverde, and
 experimentally determined their critical thermal maxima (CTmax). We
 determined if species-specific heat tolerances predict their elevation
 preferences, using Deming regressions, and tested if potential predators
 mediated elevation effects on species distributions, using structural
 equation models. Results: On the moist Atlantic slope, heat tolerances of
 insects explained their elevational distributions: taxa with high heat
 tolerances preferred low elevations where conditions are warmest, while
 taxa with low heat tolerances preferred high elevations where it is
 coldest. By contrast, on the drier Pacific slope, the elevational
 abundance pattern of many insects reflected negative interactions from
 cranefly larvae. These larvae are known to become predatory under drought
 conditions and were disproportionally abundant at low elevations on the
 Pacific slope. Main conclusions: We show that under drought, indirect
 effects mediated by species interactions can override any direct
 physiological effects of environmental conditions on insect distributions.
 The relative importance of limits to physiological tolerance and species
 interactions thus depends on environmental context, an important insight
 given that environmental conditions are expected to shift with climate
 change.
description: Study site: The study area extended from the
 high elevation cloud forest of Monteverde, northwestern Costa Rica
 (10°18'N 84°46'W, highest elevation ~ 1600 m), and down the
 Atlantic (10°21'N 84°40'W, lowest elevation ~ 800 m) and Pacific
 (10°17'N 84°48'W, lowest elevation ~ 1100 m) slopes of the
 Tilaran mountain range. All study sites were in the Monteverde Cloud
 Forest Reserve and the Children’s Eternal Rain Forest Reserve, except for
 the lowest elevation site on the Pacific slope (San Luis), which was on
 private land. Bromeliads occur naturally along these elevational gradients
 and exhibit variation in thermal and hydrological regimes according to
 local environmental conditions. Collection of
 “bromeliad_insect_abd.csv”: We sampled tank bromeliads at
 Monteverde [<i>Werauhia gladioliflora </i>(H. Wendl.) Antoine,
 <i>Werauhia sanguinolenta</i> Cogn. and Marchal, and
 <i>Guzmania scherzeriana</i> Mez] at nine sites along the two
 elevation gradients from late September to early December in 2015 and
 2016. Because sampling time can affect abundance patterns of insects due
 to seasonal effects, we ensured that sampling was completed in the
 shortest possible timeframe, and performed non-systematically with respect
 to mountainside and elevation. We quantified the aquatic macroinvertebrate
 communities inside bromeliads with a water-holding capacity of up to 1000
 ml (n = 14 – 25 depending on site) by rinsing the entire contents of
 bromeliads, searching bromeliad water and detritus thoroughly for insects
 and other invertebrates, and recording the identities (finest taxonomic
 level or morphospecies) and abundances of all organisms  &gt; 1 mm in
 length. We measured the water-holding capacity (in mL) of each bromeliad
 by adding a known quantity of water to the empty bromeliads until
 overflowing, and then subtracting the difference. We also quantified the
 detritus inside bromeliads as the dry weight of leaf fragments retained by
 a sieve with a mesh size of 850 µm. This data set includes the abundances
 on the focal species inside bromeliads that were used in our publication,
 as well as biophysical and geographical information on the
 bromeliads. Collection of
 “CTmax_individuals.csv”: We experimentally determined the
 heat tolerances of five common macroinvertebrate taxa [n = 10 – 12
 individuals per taxon per site of the larvae of the mosquito
 <i>Culex erethyzonfer</i> (filter-feeder), the chironomid
 <i>Polypedilum</i> spp. (collector), the marsh beetle
 <i>Scirtid</i> spp. (scraper), the crane fly
 <i>Trentepohlia</i> spp. (shredder), and the damselfly
 <i>M. modesta</i> (predator)]. Larvae were collected in 2015
 at two sites on the Atlantic slope (at 1379 m and 1221 m elevation) and
 two sites on the Pacific slope (at 1517 m and 1082 m elevation). We
 determined heat tolerances as the critical thermal maxima
 (CT<sub>max</sub>) of larvae that have been acclimatized at
 the laboratory for at least 4 days by subjecting individuals to gradually
 increasing temperatures (0.3 °C min<sup>-1</sup>) in an
 electric water bath. We estimated heat tolerance as the temperature at
 which the insect first failed to respond to a tactile stimulus.
 Collection of “Humidity.csv”: The mean daily
 humidity (±95% C.I.) was obtained from weather stations (Feb. 2 – May 15,
 2013) on the Atlantic and Pacific slopes of Monteverde Mountain, Costa
 Rica. Collection of
 "Temperature_means.csv": We placed
 iButton<sup>TM</sup> data loggers (DS1921G‐F5# Thermochron,
 4K) inside and outside (tied on and shaded by the plant) bromeliads, which
 recorded the temperature hourly. This data set consists of daily summary
 statistics from Sep. 9-30, 2016, the timeframe for which we had
 overlapping temperature data for all loggers for all sites.
description: The following files are included: 1)
 "bromeliad_insect_abd.csv"<br> This dataset contains the
 data on the bromeliad-dwelling macroinvertebrate survey.
 2) "bromeliad_insect_abd_variables.csv"<br> This
 document explains the meaning of the variables in the
 "bromeliad_insect_abd.csv" file. 3)
 "CTmax_individuals.csv"<br> This dataset contains the
 CTmax measurements of individual insects. 4)
 "CTmax_individuals_variables.csv"<br> This document
 explains the meaning of the variables in the
 "CTmax_individuals.csv" file. 5)
 "CTmax_species.csv"<br> This dataset contains the average
 CTmax for each focal species used in the analysis of the above
 publication. 6)
 "CTmax_species_variables.csv"<br> This document explains
 the meaning of the variables in the "CTmax_species.csv"
 file. 7) "Humidity.csv"<br> This
 dataset contains daily humidity statistics at the study sites with weather
 stations. 8)
 "Humidity_variables.csv"<br> This document explains the
 meaning of the variables in the "Humidity.csv" file.
 9) "Temperature_means.csv"<br> This dataset
 contains daily temperature statistics summarized from hourly temperature
 logger records. 10)
 "Temperature_means_variabless.csv"<br> This document
 explains the meaning of the variables in the
 "Temperature_means.csv"file. 11)
 "Analysis_CTmax_insects.R"<br> This R script contains the
 code to calculate average CTmax values for the focal species.
 12) "Analysis_EPs_CTmax.R"<br> This R script
 contains the code to analyse the relationship of CTmax with elevation
 preference. 13) "Analysis_SEM.R"<br>
 This R script contains the code to analyse the effect of species
 interactions on elevational distributions.