DOI: 10.5061/dryad.q573n5tnd
title: Predation risk estimated on live and artificial insect prey follows different patterns

description: Models mimicking prey organisms are increasingly used in ecological
 studies including testing fundamental ecological and evolutionary
 theories. The general consensus is that predation risk estimated on
 artificial models may not quantitatively correspond to predation pressure
 on live prey, but it still can be used in various comparisons. We tested
 whether the use of live and artificial prey reveals the same patterns of
 variation in predation risk. We exposed live prey (blowfly larvae and
 puparia) and plasticine models of blowfly puparia in two boreal forest
 sites, both openly and in ant- and bird-exclusion treatments, and we
 quantified attacks by both avian and invertebrate predators. Bird attack
 rates were always higher on live puparia than on their plasticine models,
 but the magnitude of this difference declined from 8.4-fold in early
 summer to 2-fold in mid- and late summer. We attribute these changes to
 different responses to prey by experienced adult birds that dominate the
 bird communities in early summer versus explorative juvenile birds that
 are abundant later in the season. Invertebrate daily predation rates on
 maggots decreased from 56% in early summer to 28% in late summer, but
 invertebrate attacks on plasticine models showed no seasonal changes.
 Overall, invertebrate predation on maggots was 67-fold greater than their
 predation on models. Observations showed that wood ants did not attack
 plasticine models and did not leave on them any damage marks. Estimates
 based on artificial prey indicate a much greater role of bird predation
 than invertebrate predation, while estimates based on live prey suggest
 the opposite pattern. Thus, using live and artificial prey may lead to
 different conclusions about relative importance of different predator
 groups in a locality. Moreover, for both avian and invertebrate predators,
 predation risk based on artificial and live prey shows different seasonal
 changes and may potentially demonstrate different spatial patterns.
description: In our study, we used live
 larvae (maggots hereafter) and puparia of the blowfly Calliphora sp.
 (Diptera: Calliphoridae) and plasticine models with similar appearance to
 fly puparia in size, colour and shape. Maggots, obtained in a local
 fishing bait shop, were 10–13 mm long. They were divided into two equal
 parts: one part was placed in a refrigerator (at 4°C) to prevent pupation
 and the other part was kept at room temperature (22°C) and allowed to
 pupate. We mixed red and black plasticine (advertised as non-toxic and
 odourless; Chemical plant “Luch”, Yaroslavl, Russia) in the proportion 4:1
 to obtain a colour (brown) similar to that of puparia, and we moulded
 models from this plasticine. Three individual models were attached in a
 line along a stick (about 10 cm long, cut from thin birch branches) using
 universal glue (Yleisliima; Biltema Suomi Oy, Helsinki, Finland). In the
 same fashion we glued three puparia. The live and plasticine puparia set
 in this way mimicked some caterpillar-like prey.
 The data were obtained from
 three different experiments. Experiment 1 was designed to compare bird and
 invertebrate predation on three different prey items. For this experiment,
 at each site, we selected 10 pairs (blocks) of young (2–3 m tall) downy
 birches (Betula pubescens). The distance between the paired trees was
 0.5–2 m, and the distance between blocks was about 20 m. One tree in each
 block was isolated from non-flying invertebrate predators (mostly ants) by
 applying a ring of non-drying glue (Sticky-Trap, Vilofarm A/S, Hobro,
 Denmark) at the base of the trunk (ant exclusion hereafter). All ants and
 other invertebrates were removed by shaking these isolated trees and then
 manually. We ensured that the branches and foliage of any ant-exclusion
 plants did not touch other trees to prevent any migration of non-flying
 invertebrate predators (with possible exception of spiders) from
 neighbouring trees. The second tree in each block remained accessible to
 ants and other predators (control hereafter).
 On each of 20 trees, we placed
 three kinds of prey items: an open transparent plastic vial (40 mm height
 and 30 mm diameter) containing three maggots, one stick with three puparia
 and one stick with three models. The vial was attached with tape to the
 tree trunk 80–100 cm above the ground. Each vial also contained a small
 stick to ease the movement of predatory invertebrates (we had previously
 verified that the maggots were incapable of climbing on the stick). Sticks
 with puparia and models were attached with small pieces of wire to birch
 branches at 100–150 cm above the ground at the outer part of the crown, so
 that the distance between them was as far as possible. Experiment 1 was
 repeated six times in Kustavi and seven times in Turku using the same
 trees from 28 May to 11 September 2021. At 24 h from the start of the
 experiment, we recorded (i) the number of maggots that had disappeared
 from each vial, (ii) the number of dead maggots in each vial; (iii) the
 number of damaged puparia; and (iv) the presence of attack marks by both
 invertebrates and birds on each of the three plasticine models. At 4–7
 days from the start of the experiment, we repeated records on live puparia
 and plasticine models. Experiment 2 was conducted to explore whether
 birds attacked the maggots exposed in vials. At the beginning of June
 2021, at both study sites, we selected 20 young birches and attached a
 vial with five maggots to the trunk, as described above. To exclude bird
 predation, we covered the openings of the vials on 10 of these birches
 with bird netting. The number of maggots that disappeared from vials was
 recorded at 2 days from the start of the
 experiment. Experiment 3 was conducted to
 compare invertebrate attacks on maggots and puparia. In early June of
 2021, we placed two vials (as in experiment 1) on each of 20 birch trees
 in each of two sites. Half of the trees were isolated from ants by glue
 rings, as described above. We placed three maggots in one of these vials
 and three puparia into the other. The number of prey individuals that
 disappeared from vials was recorded at 4 days from the start of the
 experiment.