DOI: 10.5061/dryad.69p8cz93t
title: Weak effects of birds, bats and ants on their arthropod prey on pioneering tropical forest gap vegetation

description: The relative roles of plants competing for resources versus top-down
 control of vegetation by herbivores, in turn impacted by predators, during
 early stages of tropical forest succession remain poorly understood. Here
 we examine the impact of insectivorous birds, bats and ants exclusion on
 arthropods communities on replicated 5x5 m of pioneering early
 successional vegetation plots in lowland tropical forest gaps in Papua New
 Guinea. In plots from which focal taxa of predators were excluded we
 observed increased biomass of herbivorous and predatory arthropods, and
 increased density, and decreased diversity of herbivorous insects.
 However, changes in the biomass of plants, herbivores and arthropod
 predators were positively correlated or uncorrelated between these three
 trophic levels and also between individual arthropod orders. Arthropod
 abundance and biomass correlated strongly with the plant biomass
 irrespective of the arthropods’ trophic position – a signal of bottom-up
 control. Patterns in herbivore specialization confirm lack of a strong
 top-down control and were largely unaffected by the exclusion of
 insectivorous birds, bats and ants. No changes of plant-herbivore
 interaction networks were detected except for decrease in modularity of
 the exclosure plots. Our results suggest weak top-down control of
 herbivores, limited compensation between arthropod and vertebrate
 predators, and limited intra-guild predation by birds, bats and ants.
 Possible explanations are strong bottom-up control, a low activity of the
 higher order predators, especially birds, possibly also bats, in gaps, and
 continuous influx of herbivores from surrounding mature forest matrix.
description: The study was conducted in abandoned food gardens created during
 slash-and-burn subsistence agriculture practiced by indigenous
 communities. Within a radius of 3 km from the Wanang village, six
 experimental gardens (blocks) 3-5 years old were selected, each covering
 an area of approximately 400 m2, separated by a minimal distance of 200 m.
 At each garden we initiated regeneration by clearing all aboveground
 vegetation. Control and exclosure plots were randomly
 assigned within each of the block. In the exclosure plots we used a
 transparent fishing net (1.5-2.0 cm mesh size) to protect vegetation from
 foraging birds and bats. We used 50 cm plastic fence with insect glue
 applied every week, and poisonous tuna baits (permethrin 0.5%) exposed in
 monthly intervals in enclosed traps to reduce the abundance of the
 predatory ants. Exclosure plots were surrounded by a plastic fence to
 prevent ants from re-colonizing the plot. A 50 cm high, dummy plastic
 fence was built around the control plots with a 10 cm gap left open on the
 ground level to account for possible effect of fencing in the exclosure
 plots. Spontaneous regeneration was allowed to proceed
 undisturbed for 12 months. Exclosures were maintained throughout the
 entire experiment. After that time, all above-ground vegetation from all
 plots was destructively sampled. In
 each plot, all woody plant species with DBH ≥ 1 cm were sorted and identified to species and
 their leaves and stems were weighed separately with 1 g accuracy.
 Leaf area was calculated by randomly sampling leaves from the
 pool of all leaves for each species at a given plot until 50x50 cm area
 (“leaf frame”) was filled. The frame was then photographed and processed
 using ImageJ software, measuring the area of individual leaves. Leaf frame
 samples were then oven dried and weighed. Before harvesting, individual plants were
 covered with insect-net and all arthropods were sampled using knock-down
 insecticide (MORTEIN®, active ingredients: allethrin [2.09 g/kg] and
 resmethrin [0.39 g/kg]). The arthropods were preserved in 70% ethanol and
 assigned to trophic guilds (herbivore or arthropod predator), orders, and
 morpho-species (further referred to as species). We measured the body
 length of up to 20 individuals per arthropod species and used it to
 estimate individual species biomass (dry weight) with the
 help of biomass equations. Arthropod
 density was calculated as the number of individuals per square meter of
 the leaf area, estimated for woody plant individuals by multiplying their
 fresh leaf weight (in g) and specific leaf area values (in cm2/g) that
 were also based on fresh mass.