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 (CT_max). 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.

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 [Werauhia gladioliflora (H. Wendl.) Antoine, Werauhia sanguinolenta Cogn. and Marchal, and Guzmania scherzeriana 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  > 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 Culex erethyzonfer (filter-feeder), the chironomid Polypedilum spp. (collector), the marsh beetle Scirtid spp. (scraper), the crane fly Trentepohlia spp. (shredder), and the damselfly M. modesta (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_max) 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^-1) 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^TM 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.

The following files are included:

1) "bromeliad_insect_abd.csv"
This dataset contains the data on the bromeliad-dwelling macroinvertebrate survey.

2) "bromeliad_insect_abd_variables.csv"
This document explains the meaning of the variables in the "bromeliad_insect_abd.csv" file.

3) "CTmax_individuals.csv"
This dataset contains the CTmax measurements of individual insects.

4) "CTmax_individuals_variables.csv"
This document explains the meaning of the variables in the "CTmax_individuals.csv" file.

5) "CTmax_species.csv"
This dataset contains the average CTmax for each focal species used in the analysis of the above publication.

6) "CTmax_species_variables.csv"
This document explains the meaning of the variables in the "CTmax_species.csv" file.

7) "Humidity.csv"
This dataset contains daily humidity statistics at the study sites with weather stations.

8) "Humidity_variables.csv"
This document explains the meaning of the variables in the "Humidity.csv" file.

9) "Temperature_means.csv"
This dataset contains daily temperature statistics summarized from hourly temperature logger records.

10) "Temperature_means_variabless.csv"
This document explains the meaning of the variables in the "Temperature_means.csv"file.

11) "Analysis_CTmax_insects.R"
This R script contains the code to calculate average CTmax values for the focal species.

12) "Analysis_EPs_CTmax.R"
This R script contains the code to analyse the relationship of CTmax with elevation preference.

13) "Analysis_SEM.R"
This R script contains the code to analyse the effect of species interactions on elevational distributions.