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Mistletoes could moderate drought impacts on woodland birds, but are themselves susceptible to drought-induced dieback

Citation

Crates, Ross (2022), Mistletoes could moderate drought impacts on woodland birds, but are themselves susceptible to drought-induced dieback, Dryad, Dataset, https://doi.org/10.5061/dryad.76hdr7sxp

Abstract

Mistletoes are hemiparasitic plants and keystone species in many ecosystems globally. Given predicted increases in drought frequency and intensity, mistletoes may be crucial for moderating drought impacts on community structure. Dependent on host vascular flows, mistletoes can succumb to stress when water availability falls, making them susceptible to mortality during drought. We counted mistletoe across >350,000 km2 of south-eastern Australia and conducted standardised bird surveys between 2016 and 2021, spanning a major drought event in 2018-19. We aimed to identify predictors of mistletoe abundance and mortality and determine whether mistletoes might moderate drought impacts on woodland birds. Live mistletoe abundance varied with tree species composition, land use and presence of mistletoebirds. Mistletoe mortality was widespread, consistent with high 2018/19 summer temperatures, low 2019/20 summer rainfall and the interaction between summer temperatures and rainfall in 2019/20. The positive association between surviving mistletoes and woodland birds was greatest in the peak drought breeding seasons of 2018/19 and 2019/20, particularly for small residents and insectivores. Paradoxically, mistletoes could moderate drought impacts on birds, but are themselves vulnerable to drought-induced mortality. An improved understanding of the drivers and dynamics of mistletoe mortality is needed to address potential cascading trophic impacts associated with mistletoe die-off.

Methods

During 2019/20, we conducted habitat assessments at 2,111 monitoring sites spanning over 300,000 km2 of south-eastern Australia (Figure 1). We selected monitoring sites in areas of woodland habitat deemed suitable for two Critically Endangered bird species: the regent honeyeater and swift parrot Lathamus discolor. We used a combination of MaxEnt habitat suitability models (Figures S1 and S2), expert field searches and the location of previous sightings to inform site locations. Since both bird species are habitat specialists29, our sampling encompasses the highest quality remaining woodlands in south-eastern Australia.

Each monitoring site was a 50m radius (0.79 hectares) around a fixed GPS location to two metres accuracy. During one visit to each site in 2019/20, we recorded the habitat fixed effects detailed in Table 1. Our mistletoe counts focussed on the three most abundant mistletoe species (Family Loranthaceae) in the study range: box, needle-leaf and long-flowered. We conducted a 360° search of the canopy from each site centroid, deviating from this point where necessary to count accurately the number of live and dead mistletoe clumps present.

Table 1: Site-level and visit-level fixed effects obtained for identifying predictors of mistletoe abundance and health and to model the effect of mistletoe abundance on woodland bird abundance. For further information on the fixed effects, see Table S1.

Level

Fixed effect

 

Description

Site-level

Spatial location

WGS84 decimal latitude longitude to 2m accuracy.

Region

10-level factor defining regional clusters of monitoring sites. Included as a random term in mistletoe and bird models.

Land use

9-level factor: Primary land use.

Canopy cover

Percentage canopy cover to the nearest 5%.

Tree species PC1 & PC2

Principal component axes 1 & 2 of tree species composition (see Figure S3).

Tree age

Proportion of trees present with a diameter at breast height >80cm.

Tree health

Proportion of trees in the site that are healthy or only mildly stressed per Briggs & Taws30.

Shrub cover

Percentage shrub cover (vegetation 30cm to 2m) to the nearest 5%.

Live mistletoe

Total number of clumps of live mistletoe across all three species.

Dead mistletoe

Total number of clumps of dead mistletoe across all three species.

 

Distance to permanent or semi-permanent water source

5-level factor: 1 = water present within site, 2 = water within 100m, 3 = water within 300m, 4 = water >300m away, 5 = distance to water unknown.

 

Mistletoebird presence

Presence/absence of mistletoebirds detected during ≥ 1 bird survey per site.

 

Noisy miner abundance

Mean abundance of noisy miners (a hyperabundant and colonial native bird that excludes other songbirds from habitats they occupy) detected during repeat bird surveys at each site (mistletoe models), or abundance per site visit (bird models).

Visit-level

Breeding Season

Annual Austral breeding season August to January.

Hours since dawn/to dusk

Hours from 6am (morning) or hours to 7pm (afternoon).

 

Observer

7-level factor: bird surveyor / habitat assessor. Random effect in bird models.

 

Blossom

5- level factor: site-level blossom abundance (including both eucalypts and mistletoes): 0 = no blossom; 1 = light blossom- few flowers in a small number of trees; 2 = moderate blossom- few flowers in many trees or moderate flowering in a few trees; 3 = heavy blossom- profuse flowering in few trees or moderate flowering in multiple trees; 4 = very heavy blossom- multiple profusely-flowering canopies.

 

Max summer temperature

Mean monthly maximum summer temperature November to Feb.

 

Max summer rain

Mean maximum monthly summer rainfall November to Feb.

Bird surveys

We conducted 9,012 point-count surveys at a total of 1,218 monitoring sites in the Austral spring/summer breeding season (August to January) between 2016 and 2021 (Table S2). Each survey was conducted by one of 15 professional ornithologists, with 86% of surveys completed by seven observers. Our rapid (5-minute) census, involving one minute of regent honeyeater song broadcast, was designed to maximise the detectability of such rare, nomadic habitat specialists by increasing the spatial extent of surveys without compromising detectability31. We recorded the maximum count of all bird species detected visually or aurally within a 50m radius of the fixed-point location during each site visit, along with a blossom score for each site. Observers remained at the site centroid as much as possible, but deviated where necessary to identify individual birds to species level or to obtain accurate counts of birds occupying heavily flowering trees near site boundaries. We did not include transient birds flying through or over study sites in the counts. The blossom score was a five-level factor (Table 1); a simple way of modelling variation in blossom abundance on nomadic species occupancy patterns32. To account for intra-seasonal variation in flowering phenology and associated changes in woodland bird distribution/abundance33, we surveyed as many sites as possible (77%) twice; once in spring between August and October and again in early summer between November and January.

Climate data

We sourced climate data from the Australian National University Climate surface database (ANUCLIM v6.134). We obtained national monthly maximum temperature and rainfall measures between 2017 and 2020, and derived these measures for each of our monitoring site locations from a 250m national raster. See Supplementary file S1 for further information on derivation of the climate data. For mistletoe analysis, we calculated annual mean maximum rainfall and temperature measures averaged across the summer months of November to February, when mistletoes are most susceptible to drought impacts35.

Statistical analysis

We used R v3.4.336 for all statistical analyses. We first checked for spatial autocorrelation in mistletoe abundance and mortality data using correlograms of Moran’s I via package ncf v1.2-537. To check for cross-correlation between covariates, we used GGally v1.4.038, but no covariates showed consistent strong positive or negative correlation with others.

To account for interspecific variation in the suitability of tree species as mistletoe hosts39, we ran a centred and scaled Principal Component Analysis (PCA) of the proportional contribution of each tree species to canopy cover across sites using stats v3.6.2 in base R. The first two Principal Component axes, which we included in subsequent mistletoe and bird models (Tree species PC1 & PC2), together explained 9% of the total variation in tree species composition. Because the proportion of total variation explained by the PCA was relatively low, we also conducted a separate analysis of the association between individual tree species and live mistletoe abundance (Supplementary file S2).

To account for spatial autocorrelation in the mistletoe and bird data, we fitted a series of Integrated Nested Laplace Approximation (INLA) Generalised Linear Mixed Models (GLMMs) via package INLA v21.02-2340. The INLA models included a Stochastic Partial Differentiation Equation (SPDE) random term that calculates the distances between the spatial location of monitoring sites using Matern covariance41,42. We selected the best models as those with the lowest Deviance Information Criterion (DIC) value and assessed their goodness of fit based on conditional R2 values43.

To answer question 1 What are the predictors of mistletoe abundance and the drivers of mistletoe mortality? we first used live mistletoe counts as the response in a GLMM with a negative binomial error structure. The model included as fixed effects: Land use, canopy cover, tree species composition, tree health, tree age and distance to standing water, with region included as a random term (Table 1). To assess the association between mistletoebird presence (the key disperser of mistletoe fruits15) and noisy miner abundance (a key driver of mistletoebird distribution44) on mistletoe abundance, we re-ran the model on the subset of sites where we conducted bird surveys (Figure 1) and included mistletoebird presence and noisy miner abundance as fixed effects in the saturated model.

We replaced live mistletoe abundance with dead mistletoe abundance as the response measure to identify the predictors of mistletoe mortality. To the fixed effects included in the live mistletoe model described above, we added live mistletoe abundance, mean maximum monthly summer (Nov-Feb) temperature and summer rainfall for 2017/18, 2018/19 and 2019/20, as well as the annual interaction between these temperature and rainfall measures.

To answer question two What is the relationship between live mistletoe abundance and woodland bird abundance, and how does this relationship change during drought? we calculated four woodland bird abundance response measures, based on overall bird abundance, body size, residency status and feeding guild (Tables 2 and S2). We excluded noisy miners from bird abundance measures due to their impacts on woodland bird abundance44, and instead included noisy miner abundance as a fixed effect in the woodland bird models (Table 1). The response measures were counts of birds described in Table 2. Fixed effects included overall blossom score, breeding season, noisy miner abundance, canopy and shrub cover extent, tree species composition, land use, distance to standing water, survey time, and live mistletoe abundance (log + 1 transformed). We included observer and region as random terms. To examine how the relationship between woodland birds and mistletoes changed during drought, we included in the woodland bird models the interaction term live mistletoe abundance × breeding season. We also conducted supplementary analyses to further explore the relative importance of mistletoe and eucalypt blossom during the drought for woodland birds (Supplementary file S3). For all bird models we used a Poisson error structure.

Table 2: Bird response measures used in models to answer question 2: What is the relationship between live mistletoe abundance and woodland bird abundance, and how does this relationship change during drought? Note the species composition of bird response measures are not mutually exclusive. See table S3 and the raw dataset available via the Dryad digital repository.

Bird response

 

Description

 

Justification

Total bird abundance

Total abundance of all bird species detected, excluding noisy miners.

 

Overall bird abundance is the ultimate measure of bird community response to mistletoe health and abundance45.

 

Small resident bird abundance

Abundance of all birds with mean body mass less than 60g considered not to be migratory or nomadic.

 

60g is the mean mass of noisy miners, which exclude smaller birds from habitats they occupy. Excluding migratory and nomadic species accounts for high spatio-temporal variability of such species, independent of any effects of mistletoe abundance on bird abundance27,44.

 

Nectarivores

 

 

 

 

Insectivores

Total abundance of all nectarivorous birds.

 

 

 

Total abundance of all insectivorous birds.

Feeding guilds will differ in the extent to which they depend on mistletoe abundance. Nectarivores predicted to be most dependent on mistletoes as a direct feeding substrate46.

 

Insectivores predicted to be less dependent on mistletoes than nectarivores, but through potential impacts of mistletoe on insectivore abundance, insectivores may be more dependent on mistletoe than granivores14,15.

Usage Notes

The files contained in this sumbmission are excel.csv files and can be opened and run with the accompanying R-script

Funding

Mach energy

NSW Department of Planning,Industry and Environment

CWP Renewables

NSW Local Land Services