DOI: 10.5061/dryad.c59zw3rk0
title: Human disturbance and aridity influence biomass harvesting by leaf-cutting ants with impacts on nutrient dynamics in a Caatinga dry forest

description: Human activities have converted mature forests into mosaics of
 successional vegetation and chronically disturbed habitats, altering the
 patterns of populations distribution, foraging ecology, and thus, the flow
 of matter and nutrients through ecosystems. Although the effects of human
 disturbance are mostly harmful, hyperabundant native generalist species
 can emerge and increase their populations under disturbance, such as
 leaf-cutting ants (LCA), prominent herbivores that are considered
 ecosystem engineers. Here, we examined the population response of two LCA
 species of the Caatinga dry forest (Acromyrmex balzani and A. rugosus) to
 increasing levels chronic anthropogenic disturbance and aridity, and
 assessed the foraging activity, biomass and nutrients harvested by their
 colonies. We found that colony densities increased at more disturbed
 habitats, varying considerably from 0 to 81 nests/ha, but aridity had no
 effect. The two species exhibited markedly different foraging activities
 (44.66 ± 28.76 and 294.6 ± 260.53 ants foraging daily), with the foraging
 rate increasing in more arid conditions for a species with smaller nests,
 but with no response to disturbance. Biomass consumption varied distinctly
 between species, ranging from 0 to 4.81 g (7.24 kg ha.yr-1, in A. balzani)
 and from 5.6 to 74 g (174.39 kg ha.yr-1, in A. rugosus). Furthermore,
 there was no effect of disturbance and aridity on the biomass harvesting
 of individual colonies. However, there was a considerable increase in the
 biomass harvested by the populations of colonies in the plots (i.e.
 accounting for colony densities). Moreover, the species A. balzani foraged
 upon more nutrient-rich material at more disturbed and arid habitats, with
 plant material containing higher concentrations of N, Ca, S, Sr, Fe, and
 Mn, as well as lower C:N ratio in these areas. Our results suggest that
 Acromyrmex species (1) can achieve larger populations in more disturbed
 habitats, though not directly associated with aridity, (2) operate as a
 key herbivore able to fit harvesting/diet through the entire environmental
 gradient and forage complementary (monocot vs. dicot), (3) reallocate
 expressive amount of forest biomass, resulting into temporary nutrient
 sinks with potential impacts on Caatinga resilience.
description: <strong>Colony density</strong> To
 address the colony density of the two <em>Acromyrmex</em>
 species, we surveyed four transects of 4 × 100 m (1600
 m<sup>2</sup>) in each of the 19 study plots, totalizing
 30,400 m<sup>2</sup>. We started at the centre of the plot and
 moved straight ahead in four directions separated by a 90º angle. To
 ensure the transect length and direction we used the GPS tracking function
 (Global Positioning System, Garmin e-Trex 20), with an estimated
 resolution &lt; 3 m. Using the GPS device, we marked every colony
 encountered along the transects. When crossing an active foraging trail,
 the associated colony was detected by tracing the ants back to their nest
 entrance. The colony density was calculated by dividing the number of
 nests found in the total area sampled per plot (1600
 m<sup>2</sup>) by 0.16, thus obtaining colony density per
 hectare. To fit the data into a poison distribution, decimal values of
 colony density were rounded to the nearest integer.
 <strong>Foraging activity</strong>
 To assess the foraging activity of LCA colonies, we accompanied
 12 colonies of <em>A. balzani</em> and five colonies of
 <em>A. rugosus </em>during a series of 24-hour field
 campaigns. We conducted 5-min counts of foraging ants (returning to their
 nest carrying food fragments) passing an imaginary line at hourly
 intervals (Siqueira et al., 2018; Urbas et al., 2007; Wirth et al., 1997).
 The field campaigns were conducted twice for all colonies, once during the
 wet season (March–July) and once again during the dry season
 (August–February). The objective was to determine the seasonal foraging
 activity of both LCA species, with a total of four census hours per colony
 totalling 68 h of field observations. The 12 <em>A. balzani
 </em>colonies were located in six different plots, each area
 containing two colonies. We thus used the mean number of foraging ants of
 the two colonies at each plot. The five <em>A. rugosus
 </em>were situated in five different plots, and thus each colony
 constituted a single sample unit. Finally, we determined the peak of daily
 foraging activity of each colony from both species, which is the time of
 the day when colonies have the highest number of workers passing by the
 imaginary line during the 5-min intervals (Wirth et al., 1997, Urbas et
 al., 2007, Siqueira et al., 2018).
 <strong>Biomass consumption rates</strong>
 To quantify the effects of disturbance and water deficit on the
 colony biomass consumption rates, we marked 32 active colonies of
 <em>A. balzani</em> in a subset of seven plots, and 21 active
 colonies of <em>A. rugosus</em> in a subset of eight plots. We
 sampled each focal colony twice, one time during the wet season (March –
 July) and a second time at the dry season (August – February). To
 calculate biomass consumption, we captured all ants carrying plant
 fragments (leaves, flowers, stalks, and seeds) at 15-min intervals on the
 colony trail during the peak of daily activity (see the previous section).
 Thus, each colony was sampled for a total period of 30 minutes, totalling
 26 h and 30 minutes of field sampling. Plant fragments were packed in
 plastic bags for transportation, then transferred to paper bags and dried
 in a stove to a constant weight at approximately 60°C for dry weight
 determination. All ants were carefully released back on the trail. Based
 on fragments weight, we calculated the biomass-based consumption rate per
 colony by summing daily totals from the 5-minutes hourly observations
 described above (see also Wirth et al. 1997, Urbas et al. 2007, Siqueira
 et al. 2018 for more details on methodology). We also determined the
 biomass harvested per hectare per year (i.e. considering the colony
 density in the area). Firstly, we calculated the average biomass collected
 by each colony between the two seasons. Secondly, the biomass collected by
 all colonies sampled within the same area was added together and the
 proportion corresponding to one hectare was determined, considering the
 density of colonies in the area for each LCA species, obtaining the
 biomass harvested by the colonies per hectare per year for each
 plot. <strong>Nutrient concentration in plant
 material harvested by colonies</strong> To
 determine the nutrient concentrations of plant material harvested by the
 ants, we collected at least 1 g of dry weight per colony from 21 colonies
 of <em>A. balzani</em> and 20 colonies of <em>A.
 rugosus</em>, using the same sampling method as for the
 above-described biomass harvest at 15-min intervals on the colony’s
 foraging trails during the peak of daily activity. The harvested plant
 fragments collected were packed in paper bags and dried at 40-50 °C by 48
 h, identified and stored in clean plastic bags. The dry samples were first
 ground using a planetary ball mill (PM 200, Retsche ®) for 10, 15 or 20
 minutes depending on the hardness of the material. We obtained a loose
 plant powder that was weighed to one gram per sample and packed into a
 polyethylene pill of 20 mm internal diameter and covered with 6-µm-thick
 polypropylene film (Mylar®). We placed the polyethylene pills in an
 energy-dispersive X-ray fluorescence spectrometer, model EDX 720 from
 Shimadzu, which consists of a rhodium tube for generating X-rays, a sealed
 chamber for sample analysis in a vacuum atmosphere, and a Si(Li) detector
 to measure the induced radiation. We selected magnesium, phosphorus,
 chlorine, potassium, calcium, manganese, iron, zinc, strontium, and
 sulphur as nutrients to assess their concentrations in the plant material
 harvested by the focal colonies. We also quantified total carbon and
 nitrogen in the plant material with an elemental analyser EuroVector
 (EA3000) coupled to an isotopic ratio mass spectrometer Denta V Advantage
 (Thermo Scientific). We used the elemental analyser configured with a CHN
 reactor filled with chromium oxide, reduced copper wires and silver cobalt
 oxide, a water adsorption trap (magnesium cobalt) and a chromatographic
 separation column.
description: # Human disturbance and aridity influence biomass harvesting by
 leaf-cutting ants with impacts on nutrient dynamics in a Caatinga dry
 forest
 [https://doi.org/10.5061/dryad.c59zw3rk0](https://doi.org/10.5061/dryad.c59zw3rk0) ## Description of the data and file structure ##### Colony density (refer to xlsx. file "colony\_density") To address the colony density of the two *Acromyrmex* species we marked every colony encoutered starting in the centre of the plot and moved straight ahead in four directions using the GPS tracking function. The colony density was calculated by dividing the number of colonies found in the total area sampled per plot (1600 m2) by 0.16, thus obtaining colony density per hectare ("density_rug" and "density_bal" columm titles). To fit the data into a poison distribution, decimal values of colony density were rounded to the nearest integer ("density rug integer" and "density_bal integer" columm titles) ##### \*\*Biomass harvesting and foraring activity \*\*(refer to xlsx. file "Biomass\_foraging") To calculate biomass consumption, we captured all ants carrying plant fragments (leaves, flowers, stalks, and seeds) at 15-min intervals on the colony trail during the peak of daily activity. The load carried by each ant was calculated by dividing the biomass collected at 1 and 5 minutes by the number of ants passing through the trail during each period. This procedure was conducted for each colony. We also calculated the average biomass collected by each colony between the two seasons. Also, the biomass collected by all colonies sampled within the same area was added together and the proportion corresponding to one hectare was determined, considering the density of colonies in the area for each LCA species, obtaining the biomass harvested by the colonies per hectare per year for each plot.  **Nutrient concentration in plant material harvested by colonies (refer to xlsx. file "plant_nutrients") ** We selected magnesium, phosphorus, chlorine, potassium, calcium, manganese, iron, zinc, strontium, and sulphur as nutrients to assess their concentrations in the plant material harvested by the focal colonies. The concentration of each element is displayed in different measurement unit (ppm and g/kg). We calculated the stock of each nutrient by multiplying the biomass that each colony collects by the concentration of each nutrient as a percentage. ### Files and variables #### File: colony\_density.xlsx **Description:**  ##### Variables * rugosus - number of colonies of *Acromyrmex rugosus* found within a 1600 m2 area. * balzani - number of colonies of *Acromyrmex balzani *found within a 1600 m2 area. * density_rug - density of colonies of A. rugosus (rugosus/0.16) as colonies per hectare * density_bal - density of colonies of A. balzani (balzani/0.16) as colonies per hectare * density_*rug*_integer - density of colonies of A. rugosus (balzani/0.16) as colonies per hectare ignoring decimals * density_*bal_*integer - density of colonies of A. balzani (balzani/0.16) as colonies per hectare ignoring decimals * GMDI - Global Multimetric Disturbance Index * DH - Water Deficit Index  #### File: Biomass\_foraging.xlsx **Description (The descriptions of the variables named here are valid for all the nested data sheets):**  ##### Variables * GMDI - Global Multimetric Disturbance Index * DH - Water Deficit Index  * nest - colony identification  * biomass_15min - Biomass collected by colony during 15 minutes (g) * biomass_5min - Biomass collected by the colony during 5 minutes (g) * season - the season of the year in which the biomass was collected. * n_*frag*_5min - Number of fragments by the colony collected during 5 minutes  * biomass_day - Biomass collected by the colony per day  * biomass_1min - the plant biomass loaded per minute by each colony  * n.ants_1min - number of ants passing the imaginary line per minute * each_ant_load - average biomass (g) loaded by each ant passing the imaginary line during 1 minute of observation * ants_day - number os ants going outside to forage daily  * fragments_1min - average number of fragments carried by ants during 1 minute of observation  #### File: Plant\_nutrients.xlsx **Description:**  ##### Variables * **codigo** - The identification code of the laboratory analysing the sample * **area** - Identification of the area where the sampling was made * **nest** - colony identification * **GMDI** - Global Multimetric Disturbance Index * **DH** - Water Deficit Index  * **Especies** - The ant species studied * **from Mg to S** - concentration of nutrients in the plant material harvested by colonies in ppm (parts per million) * **from C.percent to S.percent** - nutrient concentrations in the plant material harvested by colonies expressed as a percentage * **C por N** - Carbon/Nitrogen ratio  * **from "Mg.g_kg" to "N.g_Kg"** - nutrient concentrations in the plant material harvested by colonies expressed in grams per kilogram * **biomass_day** - Biomass collected by each colony per day considering the colonies which had samples analyzed (g) * **biomass day kg** -  Biomass collected by each colony per day considering the colonies which had samples analyzed (kg) * **from "Stock_Mg" to "Stock_N" -** nutrient stocks in the plant material harvested by colonies expressed in grams (g) Observations:  * *NA* cells are data not available due to insufficient amount of the nutrient in the sample to be detected in the chemical analysis. ## Code/software * Microsoft Excel (2010 +) * Excel online * Google sheets * libre office * R