DOI: 10.5061/dryad.bzkh1897m
title: Data from: Facilitation and biodiversity jointly drive mutualistic networks

description: 1. Facilitation by nurse plants increases understorey diversity and
 supports ecological communities. In turn, biodiversity shapes ecological
 networks and enhances ecosystem functioning. However, whether and how
 facilitation and increased biodiversity jointly influence community
 structure and ecosystem functioning remains unclear. 2. We performed a
 field experiment disentangling the relative contribution of nurse plants
 and increasing understorey plant diversity in driving pollination
 interactions. Both the presence of nurse shrubs as well as increased
 understorey plant diversity increased pollinator diversity and visitation
 rates. While nurse and understorey diversity effects on pollinator
 visitation rates did not interact, the effects of increasing understorey
 plant diversity on pollinator diversity were stronger in the absence than
 in the presence of shrubs, meaning that nurse shrubs attenuated the
 effects of high understorey diversity and buffered the effects of low
 understorey diversity. 3. We also found positive complementarity effects
 among understorey species as well as complementarity between nurse plants
 and understorey species at high diversity. Results also indicate negative
 selection effects, suggesting that species with generally few pollinators
 benefit the most in the polyculture, while a species (possibly the nurse
 plant) with generally lots of pollinators does not. The corresponding
 changes in pollination networks with the experimental treatments were due
 to both changes in the frequency of visits and turnover in pollinator
 community composition. 4. Synthesis Plant–plant facilitative systems,
 where a nurse plant increases understorey plant diversity, are common in
 stressful environments. Here, we show that these facilitative systems
 positively influence mutualistic interactions with pollinators via both
 direct nurse effects and indirect positive effects of increasing plant
 diversity. Conserving and supporting nurse plant systems is crucial not
 only for maintaining plant diversity but also for supporting ecosystem
 functions and services.
description: In order to disentangle the role of direct nurse effects from the
 indirect effects of the nurse plant on pollinators through increased
 understorey plant diversity, and to further examine their joint effects on
 the mutualistic interaction between plants and pollinators, a
 fully-factorial experimental design was established with two factors (Fig.
 1): (i) absence and presence of the legume shrub (open vs nurse), and (ii)
 understorey plant diversity with zero (0), one (1) and three (3) plant
 species. This design results in five treatment combinations of open–1,
 open–3, nurse–0, nurse–1 and nurse–3sp. The treatment combination open–0
 was excluded due to the inherent lack of flowers for visitation.
 A randomised block design
 was adopted by grouping together the five treatments and replicating them
 three times in each block over four blocks, for a total of <i>n
 </i>= 60 plots. Distance between plots within the same block was
 approx. 1 m. Blocks were distributed randomly over an area of about 4,800
 m<sup>2</sup>. Understorey plant species richness was
 manipulated with plants of the three species transplanted into pots and
 then the plots placed beneath <i>Retama</i> or in the open.
 The density of understorey plants and flowers was kept constant over all
 understorey diversity levels. Plants were taken from the neighbouring
 field and replaced every second day to assure fresh flowers were
 available. Pots were kept aggregated or separated
 (<i>c.</i> 30 cm apart) below the nurse or in the open. This
 additional factor (hereafter, aggregation) was replicated per block. In
 the nurse alone treatment with no understorey species, three empty pots
 with only soil were placed under the shrub to control for any potential
 effect of the pots. For the nurse plant treatments,
 <i>Retama</i> shrubs were chosen of approx. the same size
 (height 127–178 cm and width 125–220 cm). An area of 1
 m<sup>2</sup> at 1 m height was used as the pollinator
 observation area in each shrub. Flowers of the surrounding vegetation
 within at least 1 m around each shrub and open area were cleared with a
 sickle. Flower
 visitation was considered as a proxy for the ecosystem function of
 pollination (Schleuning et al., 2015; Winfree et al., 2018; IPBES, 2019)
 as it correlates well with seed production (Losapio & Schöb, 2020;
 but see Lázaro et al., 2013). Flower visits were documented by sampling,
 identifying, and recording all insects visiting the flowers of each plant
 in each plot. Observations were conducted between 9 AM and 7 PM over eight
 days between 1 May and 14 May 2017, covering the blooming phase of the
 four plant species. Each plot was observed during three surveys of 20 min
 randomly allocated over the day. Nurse plants and corresponding open plots
 were observed simultaneously, thereby reducing the potential confounding
 effects of changing weather conditions within blocks. Each block was
 sampled within two days. Pollinators were identified at the species level
 whenever possible, otherwise to the genus.
description: See the R script Losapio_JEcol.R