DOI: 10.5061/dryad.2jm63xskh
title: Data from: Effects of pasture and forest microclimatic conditions on the foraging activity of leaf-cutting ants

description: The fragmentation and transformation of land cover modify the microclimate
 of ecosystems. These changes have the potential to modify the foraging
 activity of animals, but few studies have examined this topic. In this
 paper, we investigated whether and how the foraging activity of the
 leafcutter ant Atta cephalotes is modified by microclimatic variations due
 to land cover change from forest to pasture. We characterized the
 microclimate of each habitat and identified alterations in foraging
 behavior in response to relative humidity (RH), air temperature and
 surface temperature along ant foraging trails by synchronously assessing
 foraging activity (number of ants per 5 mins including incoming laden and
 unladen and outgoing ants) and microclimatic variables (air temperature,
 RH, and maximum and minimum surface temperature along the foraging trail).
 There were climatic differences between habitats during the day but not
 throughout the night, and A. cephalotes was found to have a high tolerance
 for foraging under severe microclimatic changes. This species can forage
 at surface temperatures between 17 and 45 °C, air temperatures between 20
 and 36 °C, and an RH between 40 and 100%. We found a positive effect of
 temperature on the foraging activity of A. cephalotes in the pasture,
 where the species displayed thermophilic behavior and the ability to
 forage across a wide range of temperatures and RH. These results provide a
 mechanism to partially explain why A. cephalotes becomes highly prolific
 as anthropogenic disturbances increase and why it has turned into a key
 player of human-modified neotropical landscapes.
description: Study site This study was carried out in a
 13.4-ha remnant of tropical dry forest (Holdridge et al. 1971, GEMA 1998)
 and its surrounding matrix of managed cattle pasture at the Alejandría
 Farm (4°51´ North, 75°52´ West), a private property located in the Cauca
 Valley (Risaralda, Colombia) between the Central and Oriental Andean
 Cordilleras at 900-1000 m asl. The site receives ~1700 mm of rainfall
 annually (8-year mean, 1993-2000) concentrated in two rainy seasons
 (March-May and September-November). Sunrise is at 5:00 hours and sunset at
 17:00 hours throughout the year, and the average temperature is
 approximately 24 °C. The forest vegetation forms a closed, diverse canopy,
 the height of which varies from 20 to 40 m, and the dominant tree species
 include <i>Oxandra panamensis</i>, <i>Trophis
 caucana</i> Berg., <i>Chamaedorea linearis </i>Mart.,
 <i>Clarisia biflora</i> Ruiz., and <i>Gustavia
 speciosa</i> Kunth. This forest patch is one of the last remnants of
 the original continuous dry forest along the central Cauca River Valley
 (Ramírez et al. 2002), which has undergone large-scale deforestation to
 less than 1% of its original area due to intensive agriculture (sorghum,
 soybean, sugarcane and cotton) and cattle-ranching since 1920 (GEMA
 1998). Study species <i>Atta
 cephalotes</i> (Hymenoptera: Formicidae) is considered a “woodland
 species” and is commonly found in mature, well-conserved forest (Rockwood
 1973, Correa et al. 2005). Surprisingly, the occurrence of <i>A.
 cephalotes</i> in Colombia is not restricted to closed forests, and
 the species exhibits more disturbance tolerance than indicated by
 published observations. In fact, it is common throughout almost the entire
 country from sea level to 2500 m asl (Fernández et al. 2015). Although the
 Cauca Valley has been extensively converted from forest to pasture and
 sugarcane crops, <i>A. cephalotes</i> is broadly distributed
 and easily found, even in parks within cities such as Cali and Pereira
 (Montoya-Lerma et al. 2012) and in the grasslands, croplands and forests
 of the region (Armbrecht et al. 2001, Chacon et al. 2012).
 Study colonies We systematically surveyed
 <i>A. cephalotes</i> colonies in 13 ha of forest and in a
 similar area of the surrounding pasture. After a preliminary 15-day
 monitoring period of their foraging activity and general health status, we
 choose 12 active mature colonies as study colonies, six within the forest
 and six within the pasture. The minimum distance of each colony from the
 edge of its habitat was 50 m, a distance that should have excluded edge
 effects (Meyer et al. 2009). Because nest size can affect foraging
 activity (Lewis et al. 1974a), the size of the colonies was calculated as
 the minimum elliptical area circumscribing the central nest mound
 (Hernández et al. 1999), and although mature (e.g., presence of soldiers
 and well-established foraging trails), the colonies were consistently
 smaller in the pasture than in the forest, with mean nest sizes of 27.39
 m<sup>2 </sup>± 22.19 and 109.3 m<sup>2 </sup>±
 65.35, respectively (Welch two-sample <i>t</i>-test:
 <i>t</i> = -2.90, df = 6.14, N = 12, P = 0.026). Measurements
 were taken on the main foraging trail of each nest, which was identified
 as the trail with the highest number of ants at the time of the
 measurement. The studied foraging trails represented cleared, persistent
 trunk trails as typically described for Atta ants (Kost et al.
 2005). Microclimate Measurements
 To characterize the effects of microclimatic conditions on the
 main foraging trails of <i>A. cephalotes</i>, we assessed the
 following parameters for each nest: (1) air temperature, (2) RH, (3)
 maximum surface temperature, and (4) minimum surface temperature. The air
 temperature and RH were measured using a HOBO Pro Temperature/Relative
 Humidity sensor (Onset Computer Corp., Bourne, MA, USA ®) equipped with
 internal data loggers. The sensor was located 5 cm above the soil surface
 on the main foraging trail near the entrance hole of each colony nest, and
 it recorded measurements at hourly intervals from the
 18<sup>th</sup> of May to the 12<sup>th</sup> of
 December 2013.  The maximum and minimum surface temperature were recorded
 for all nests across the two habitats during twelve field work periods of
 eight hours: four from 6:00 to 14:00, four from 14:00 to 20:00, and four
 from 20:00 to 6:00 (totaling 96 hours of data at each nest). The surface
 temperatures were measured by
 walking slowly from the nest entrance to the end of the foraging trail
 while registering the maximum and minimum temperatures observed using an
 infrared thermometer (Fluke 62 MAX). 
 Foraging Activity To describe and compare
 the foraging activity of <i>A. cephalotes</i> between the two
 habitats and to examine relationships with microclimatic factors, the
 activity rates of all above ground workers involved in foraging-related
 tasks were assessed on the foraging trails. Particularly, we counted the
 number of outgoing and incoming unladen and laden ants at each nest.
 Outgoing ants are those that leave the colony to search and harvest
 resources for fungal substrate, to maintain the trail, e.g., by
 reinforcing the pheromone trail (Moser 1967), or to protect the foragers,
 e.g., by clearing foraged leaves to control parasites such as phorid flies
 (Yackulic & Lewis 2007). Incoming unladen ants are foragers that
 have lost their loads in the route or workers returning to the nest after
 being involved in other above ground tasks. Laden ants directly reflect
 the efficiency of foraging as they consist of ants that are transporting
 harvested leaves, flowers, seeds or other materials to feed the fungus
 garden. Foraging rates were calculated as the number of
 ants passing by a fixed point on the main foraging trail near the main
 hole of each nest in 5 minutes (Wirth et al. 2003) and were measured at
 the same time as the maximum and minimum surface temperatures. We used a
 video camera (Canon PowerShot A490) fixed to a tripod to count the ants,
 and the total number of ants, both incoming and outgoing, were determined
 using AntCounter V.1.0 software, which counts the number of ants that pass
 through a given point in two directions with respect to the nest entrance
 (Bustamante & Amarillo-Suárez 2016). Because this software does
 not distinguish between laden and unladen ants, the number of laden ants
 was counted manually by playing the recorded videos in slow motion. To
 control for differences in nest size when comparing the foraging rates
 between forest and pasture, we used the relative foraging activity (i.e.,
 the hourly foraging activity divided by the maximum foraging activity
 observed of a given nest). Data analysis
 Differences in microclimatic conditions between habitats and
 hours were determined for each microclimate parameter by a two-factorial
 ANOVA with repeated measures on one factor, where habitat and time were
 fixed factors. “Habitat” had two levels, pasture and forest, and “hour”
 had 24 levels representing each hour of the day. “Nest” corresponded to
 each of the subjects with repeated measures each hour and with each nest
 located either in the forest or the pasture.
 Additionally, to identify differences in hourly foraging between
 habitats, we performed a nested ANOVA with colony as a random factor
 nested within habitat (fixed factor) for each microclimatic condition and
 hour. Because there were 24 ANOVAs, one for each hour, we used an alpha of
 0.01 instead of 0.05 to reduce the probability of a type II statistical
 error (Zar 2010). Differences in foraging activity
 between habitats and hours were analyzed for each foraging variable
 (unladen and laden incoming and outgoing ants) in the same way as for the
 microclimate variables. To identify differences in foraging activity
 between habitats, we performed a nested ANOVA with the same parameters as
 with the microclimatic variables for each foraging activity variable and
 for each hour. The assumptions of normality and
 homoscedasticity were met in many, though not all, cases, even after the
 data were log or arcsine transformed. However, we decided to use nested
 ANOVA because there is no equivalent non-parametric approach (Dytham 2011)
 and it is robust against deviations from normality and homoscedasticity
 (Sokal & Rohlf 1995, Zar 2010). Due to unequal sample sizes, type
 III sums of squares are reported. Because the numbers
 of outgoing and unladen and laden incoming ants were expressed as
 percentages of the maximum flow, they were arcsine-transformed prior to
 ANOVAs (Doncaster & Davey 2007). To determine the relationships
 between foraging activity and microclimatic variables, we performed simple
 linear regressions between each microclimatic variable (air temperature,
 RH, and maximum and minimum temperature along the foraging trail) and the
 response variables (number of unladen or laden incoming or outgoing ants).
 All analyses were carried out using R v. 3.1.0 (R Core Team
 2015).