DOI: 10.5061/dryad.12jm63z1g
title: Distribution of ant assemblage, microclimate and microhabitat along vertical gradients

description: Abiotic and biotic factors structure species assembly in ecosystems both
 horizontally and vertically. However, the way community composition
 changes along comparable horizontal and vertical distances in complex
 three-dimensional habitats, and the factors driving these patterns,
 remains poorly understood. By sampling ant assemblages at comparable
 vertical and horizontal spatial scales in a tropical rain forest, we
 tested hypotheses that predicted differences in vertical and horizontal
 turnover explained by different drivers in vertical and horizontal space.
 These drivers included environmental filtering, such as microclimate
 (temperature, humidity, and photosynthetic photon flux density) and
 microhabitat connectivity (leaf area) which are structured differently
 across vertical and horizontal space. We found that both ant abundance and
 richness decreased significantly with increasing vertical height. Although
 dissimilarity between ant assemblages increased with vertical distance,
 indicating a clear distance-decay pattern, the dissimilarity was higher
 horizontally where it appeared independent of distance. The pronounced
 horizontal and vertical structuring of ant assemblages across short
 distances is likely explained by a combination of microclimate and
 microhabitat connectivity. Our results demonstrate the importance of
 considering three-dimensional spatial variation in local assemblages and
 reveal how highly diverse communities can be supported by complex
 habitats.
description: Ant assemblages were sampled using insecticide fogging between
 0700 and 0930 on 12-24 May 2002 from within the canopy by a climber
 rappelling down using a Swing-Fog model SN50 (Phoenix Fogger, Dallas, TX,
 USA) near each of seven vertical sample transects. These vertical
 transects were suspended from a 130 m horizontal traverse line secured in
 the upper canopy and were arranged at 20-25 m intervals horizontally (Fig.
 2). Each of these transects supported multiple individual circular fogging
 trays (1 m<sup>2</sup>) (n=86) suspended in the air with
 attached ethanol-filled collecting bottles spaced at approximately 5 m
 vertical intervals beginning at 1 m above the ground (Fig. 2). These trays
 collected knocked-down arthropods that were between trays at the time of
 fogging. A 1.6% aqueous solution of the synthetic pyrethrum (Cypermethrin)
 was used. Arthropods were collected into 80% ethanol 1-2 hours after
 fogging, and ants were separated as part of arthropod ordinal sorting (see
 Dial et al. 2006 for results on ordinal arthropod assemblages). The ants
 sampled using fogging are mainly diurnal foraging species active during
 the sampling period (0700 to 0930), and therefore likely present a subset
 of the total local ant diversity. In total, we obtained and identified ant
 assemblage samples for 61 out of 86 sampling points, with 14 samples
 having no ants, nine samples having been lost between sampling and
 analysis, and two samples in Transect 5 (two individual ants discovered)
 belonging to an emergent forest layer without horizontal positions for
 comparison. To quantify microclimate and habitat
 structure, air temperature (˚C) and relative humidity (%) were measured at
 0.5 h intervals over 24 hours using Hobo Pro RH/Temperature Data Logger
 (Onset Computer Corporation, Pocassest, MA, USA). Data loggers were placed
 at 3 m intervals along each vertical transect, starting 1 m above the
 ground. At the data logger locations, photosynthetic photon flux density
 (PPFD) was recorded using a handheld light meter (Quantum Lightmeter,
 Spectrum Technologies, Plainfield, 1L, USA) and normalized by dividing by
 maximum light value within each vertical transect to account for
 between-day variation in lighting. The intent was to identify the relative
 (not absolute) light environment of the forest canopy (Dial et al 2006).
 We estimated one-sided total leaf area between sampling trays as a measure
 of microhabitat connectivity at different sampling points for transects 1
 to 6 (T1-T6; Fig 2c). The leaf area within a sampling interval was
 calculated by multiplying the number of leaf intersections by the size of
 the base area of the interval which was 1 m<sup>2</sup> (the
 area of the sample tray). We then used
 these data to estimate leaf area index (LAI) over vertical intervals
 (sampling methods described in Dial et al. 2004, 2006, and 2011;
 estimation methods in Dial et al. 2006 and 2011). Conceptually, LAI refers
 to the number of leaf layers above the ground surface that would be
 pierced by a vertical line. For example, if LAI = 7, then there are, on
 average seven leaf layers above a random point on the ground within that
 height range; or 7 m<sup>2</sup> of leaf area per
 m<sup>2</sup> of ground surface. We assumed (following
 MacArthur and Horn 1969) that for any sample point in the canopy located
 at height <em>z </em>above the ground, the foliage density was
 approximately equal in all directions. Following this assumption at each
 height<em> z</em>, we systematically measured horizontal
 distances (<em>d<sub>i</sub></em>) with a laser
 range finder to the nearest canopy element (foliage and stems) in 12
 uniformly distributed azimuths every 2 m vertically from the ground to the
 height of the horizontal traverse line supporting the vertical transect.
 Using the <em>n</em> ≤ 12 distances to foliage at each sample
 point, we found the mean distance (d) to foliage, doubled the mean (assuming that
 the observer was on the average midway between foliage elements), then
 inverted it to find leaf intersections per vertical meter at height
 <em>z</em> as LAI<sub><em>z</em></sub>
 = 1/(2d). By multiplying
 the LAI<sub><em>z</em></sub> by collection area
 (1m<sup>2</sup>) we estimated the leaf area sampled within the
 interval.