DOI: 10.5061/dryad.gf1vhhmmx
title: Behavioural responses to warming differentially impact survival in introduced and native dung beetles

description: Background: Anthropogenic changes are often studied in isolation but may
 interact to affect biodiversity. For example, climate change could
 exacerbate the impacts of biological invasions if climate change
 differentially affects invasive and native species. Behavioural plasticity
 may mitigate some of the impacts of climate change, but species vary in
 their degree of behavioural plasticity. In particular, invasive species
 may have greater behavioural plasticity than native species since
 plasticity helps invasive species establish and spread in new
 environments. This plasticity could make invasives better able to cope
 with climate change. Goal: Here our goal was to examine whether
 reproductive behaviours and behavioural plasticity vary between an
 introduced and a native Onthophagus dung beetle species in response to
 warming temperatures and how differences in behaviour influence offspring
 survival. Methods: Using a repeated measures design, we exposed small
 colonies of introduced O. taurus and native O. hecate to three temperature
 treatments, including a control, low warming, and high warming treatment,
 and then measured reproductive behaviours, including the number, size, and
 burial depth of brood balls. We reared offspring in their brood balls in
 developmental temperatures that matched those of the brood ball burial
 depth to quantify survival. Results: We found that the introduced O.
 taurus produced more brood balls and larger brood balls, and buried brood
 balls deeper than the native O. hecate in all treatments. However, the two
 species did not vary in the degree of behavioural plasticity in response
 to warming. Differences in reproductive behaviours did affect survival,
 such that warming temperatures had a greater effect on survival of
 offspring of native O. hecate compared to introduced O. taurus. Broader
 implications: Overall, our results suggest that differences in behaviour
 between native and introduced species is one mechanism through which
 climate change may exacerbate negative impacts of biological
 invasions. 
description: To investigate behavioural plasticity in these species, we used a
 repeated measures design to quantify reproductive behaviours of single
 species colonies (n=18 colonies per species) in response to changes in
 their thermal environment. Each colony had five beetles (two males and
 three females). Before the start of the trials, we weighed each beetle to
 control for the effects of body size on brood ball size. We placed all
 experimental beetle colonies in plastic 2 L rectangular containers (13.5 x
 10.2 x 28.2 cm) filled to a depth of 24 cm with a 4:1 mixture of
 topsoil:sand. We mixed the soil with water to create a standardized
 moisture level across colonies, and we covered the container with aluminum
 mesh to prevent escape of the beetles.   We used 43W
 halogen light bulbs to heat experimental colonies because the bulbs
 produce a gradient of warming in the soil, mimicking soil gradients
 produced by the sun. The distance of the bulb to the soil surface
 determined the degree of warming at the surface of the soil and the
 steepness of the thermal gradient. While this set-up allowed us to produce
 soil gradients similar to field settings, we were unable to simultaneously
 measure the effect of increased temperature variation on reproductive
 behaviour; the temperature gradients produced by our warming treatments
 were thus consistent throughout the trial and did not fluctuate. The
 control treatment temperature is slightly lower than field averages over
 the breeding season but has led to high reproductive output in laboratory
 conditions for these species. The low warming treatment mimics average
 ambient high temperatures in the field throughout the summer breeding
 season (29.5°C). The high warming treatment reflects temperatures commonly
 reached during heat waves at our collection sites, and such heat waves are
 predicted to become more common due to climate change (IPCC, 2014). To
 record soil temperatures experienced by our colonies in all three
 treatments, we buried data loggers (Onset Hobo Pendant Temperature/Light
 Logger) at the surface, middle, and bottom of containers that were filled
 with soil but did not have beetles three times during the experimental
 period. To maintain warming conditions throughout the trial length, we
 kept bulbs on during the entire trial, such that all colonies experienced
 consistent light (no dark periods). We held each
 experimental colony at each of the three temperature treatments in random
 order for ten days (30 days total/colony). We fed colonies 130 ± 5g of
 autoclaved cow dung on days one, three, and six of each ten-day trial. On
 day ten, we searched through the soil in three cm sections (0-3cm, 3-6 cm,
 6-9cm, 9-12 cm, 12-15cm, 15-18cm, 18-21cm, 21-24cm) and removed any brood
 balls produced by the experimental colony. For each brood ball we recorded
 mass and soil section where it was buried (i.e. burial depth).
 To determine the effect of behavioural plasticity on offspring
 size and survival, we reared all brood balls at the average temperature of
 the soil layer in which they were buried. To approximate this burial
 temperature, we binned the container into thirds based on depth, including
 the top (0-9cm deep), middle (9-15 cm), and bottom (15-24 cm) of the
 container. We used data from the data loggers to quantify temperatures for
 each treatment and depth. We then used incubators to rear offspring in
 temperatures that reflected the brood ball location and, thus, the soil
 temperature in the containers. For the control treatment, we reared
 offspring in brood balls found in all three sections of the container at
 25°C, which reflects the lack of thermal gradient in these containers. For
 the low warming treatment, we reared offspring in brood balls found in the
 top third at 29°C, those in the middle third at 26°C, and those in the
 bottom third at 25°C. For the high warming treatment, we reared offspring
 in brood balls found in the top third at 33°C, those in the middle third
 at 27°C, and those in the bottom third at 25°C. For rearing offspring, we
 placed each brood ball in an individual, sealed 75ml plastic cup with
 holes punched in the lid. We placed each brood ball at the bottom of the
 plastic cup and packed soil around the brood ball up to the lip of the
 cup. Throughout development, we added water to the cups using a spray
 bottle to maintain soil moisture. We checked brood
 balls for beetle emergence starting four weeks after the end of the
 experimental trial. If beetles had not emerged after six weeks, we
 determined if the brood ball had an egg chamber (hollow portion of the
 brood ball). If the brood ball did not have an egg chamber, we considered
 the brood ball empty and we removed it from data analysis since it could
 be a food cache that does not reflect parental investment or reproductive
 behaviour. If the brood ball had an egg chamber, we categorized it as a
 mortality event during development.   The dataset
 contains raw data for every brood ball produced by experimental
 colonies.