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

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 greater than 350 000 km² of southeastern Australia and conducted standardized bird surveys between 2016 and 2021, spanning a major drought event in 2018-2019. 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 2018/2019 summer temperatures, low 2019/2020 summer rainfall and the interaction between summer temperatures and rainfall in 2019/2020. The positive association between surviving mistletoes and woodland birds was greatest in the peak drought breeding seasons of 2018/2019 and 2019/2020, 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.

Keywords: climate change; ecosystem resillience; food webs; global change; phenology; population monitoring.

Figure 1.

(a,b) Annual rainfall and mean monthly maximum temperature data for weather stations spanning the spatial and temporal extent of the bird monitoring dataset. (c) Distribution of both woodland bird and mistletoe (red) and mistletoe only (light blue) monitoring sites in southeastern Australia. Top left inset: box Amyema miquelii, long-flowered Dendrophthoe vitellina and needle-leaf Amyema cambageii mistletoe species included in the study. Bottom right inset: study range on a national scale. Place labels (excluding Sydney) show the location of the summary climate data presented in (a,b). Rainfall and temperature data are shown to summarize annual variation in the climate surface data used in mistletoe models, sourced from the Australian Bureau of Meteorology http://www.bom.gov.au/climate/data/, accessed 9/3/2021. (Online version in colour.)

Figure 2.

(a) Fixed effect estimates of the associations between environmental, biotic and climatic effects and mistletoe abundance and mortality. Land use factorial effects are relative to land use: national park/nature reserve. Points denote the posterior means and the error bars denote the 95% credibility intervals for the effects. Only significant fixed effects or factor levels (where estimates ± 95% credibility intervals do not overlap zero) from the top model, based on lowest DIC, are shown. See electronic supplementary material, figure S7 for the full model. (b–d) Spatial fields for the SPDE random effect of response variables of live mistletoe abundance (b), live mistletoe abundance including bird data (c), and dead mistletoe abundance (d), based on habitat (b,d) or bird (c) monitoring point locations (figure 1). Predictions are derived using the ggField function from the PointPolygon package v. 0.1.0 [47]. Table 1 and electronic supplementary material, file S1 for further information on the fixed effects and factor levels. (Online version in colour.)

Figure 3.

(a) Fixed effect estimates of the association between environmental, biotic and climatic effects and woodland bird abundance. Factorial effects are relative to the following levels: blossom = absent; water distance = 0 (i.e. water present); land use: national park/nature reserve; breeding season = 2016/2017. Points denote the posterior means and the error bars denote the 95% credibility intervals for the effects. Only significant fixed effects or effects with significant factor levels (where estimates ± 95% credibility intervals do not overlap zero) from the top models, based on lowest DIC, are shown. See electronic supplementary material, figure S8 for the full model summary; (b–e) spatial fields for the SPDE random effect of response variables of total (b), small resident (c), nectarivorous (d) and insectivorous (e) woodland bird abundance based on bird monitoring point locations (figure 1). (Online version in colour.)

Figure 4.

Fixed effect estimates of the interaction between breeding season × live mistletoe abundance and woodland bird abundance. Points denote the posterior means and the error bars denote the 95% credibility intervals for the effects. Estimates are derived from the same model as shown in figure 3 and electronic supplementary material, figure S8. (Online version in colour.)