Plant protection services mediated by extrafloral nectaries decline with aridity but are not influenced by chronic anthropogenic disturbance in Brazilian Caatinga

Abstract

Most terrestrial species occur in human‐modified landscapes that are experiencing climate change. In addition to direct impacts on species, both anthropogenic disturbance and climate change can have important effects through changes in species interactions, including the disruption of ecological services provided by them. Here we investigate how chronic anthropogenic disturbance (CAD) and aridity affect the effectiveness of plant protection services provided by ants to plants bearing extrafloral nectaries (EFNs). The study was conducted across 13 01‐ha plots distributed along CAD and aridity gradients in Caatinga vegetation of northeastern Brazil. We focused on Pityrocarpa moniliformis, the most abundant and widely distributed EFN‐bearing tree species occurring in our study area, and we used experimental attack rates on termites as a measure of effectiveness of ant protection services. We investigated the relative roles of nectar production (volume and concentration) and ant species composition in mediating the effects of CAD and aridity on the effectiveness of protection services. Attack rates by ants declined with increasing aridity but were not related to CAD. The volume of extrafloral nectar declined with increasing CAD but was not affected by aridity, whereas the concentration was not related to either CAD or aridity. The composition of attendant ant species varied with aridity but not with CAD. Synthesis. Our findings suggest that CAD does not affect plant‐protection services mediated by EFNs in Brazilian Caatinga. However, ant‐protection services declined with increased aridity, and this occurred through changes in the composition of attendant ant species rather than by changes in the production of extrafloral nectar. Such a response to increasing aridity highlights the vulnerability of EFN‐bearing plants to climate change through decreased predation of herbivores.

Morphology of four common and phylogenetically distant ant species varies along disturbance and aridity gradients in the Caatinga dry forest

“…Previous studies in the Caatinga dry forest have reported a similar pattern, albeit with other diversity measures such as the taxonomic diversity of plants (Rito et al, 2017), functional and phylogenetic diversity of ants (Arnan, Arcoverde, et al, 2018), and ant-plant interactions (Câmara et al, 2018;Oliveira et al, 2019Oliveira et al, , 2020. In a sense, aridity seems to impose a higher selective pressure than disturbance at different levels of biological organization.…”

“…Such morphological variability is likely to represent a key asset by enabling individuals and populations to inhabit the immense variety of habitat provided by the Caatinga at the landscape and regional level. We refer to species with a high degree of generalist behavior relative to habitat use, food exploitation, and nesting (Hölldobler & Wilson, 1990;Leal et al, 2017), with potential impacts on community organization and structure, as well as the provision of ecosystem functions such as seed dispersal and plant protection against herbivores (Leal et al, 2017;Oliveira et al, 2019Oliveira et al, , 2020. This morphological variability may also represent a strategic asset for dealing…”

Drought‐induced reductions in plant defenses: Insights from extrafloral nectaries in the Caatinga dry forest

“…To characterise the rainfall gradient, mean annual rainfall within each plot was obtained from the WorldClim data base (http://www.worldclim.com.br) at a spatial resolution of 1 km. Because the rainfall and aridity indices were highly correlated ( r > .98), as observed by previous studies conducted in Catimbau NP (Arnan et al, 2018; Oliveira et al, 2021; Rito et al, 2017), we retained rainfall only as our measure of water deficit.…”

“…In this scenario, EFN traits would be favored in drier environments. On the other hand, Oliveira et al (2021) found that more aggressive ants interact more frequently with P . moniliformis plants in wetter rather than drier sites, which agree with RAH intra prediction of higher defense under higher resource availability (Figure 1b).…”

“…Because extrafloral nectar is composed of water, sugar, and amino acids, the expression of EFNs can be costly, which is, therefore, predicted to decrease in water‐stressed environments (Newman & Wagner, 2013; Yamawo et al, 2012). Maintaining extrafloral nectar production during water deficits might be disadvantageous, particularly not only if plants are under low herbivory pressure (Fagundes et al, 2017; Rios et al, 2008) as predicted by RAH intra but also if attendant‐ants are less abundant and aggressive (Câmara et al, 2018; Oliveira et al, 2021). In such environments, plants might be expected to reduce the number (Nogueira et al, 2012; Rudgers & Strauss, 2004) and size of EFNs (Rios et al, 2008; Rudgers, 2004), as well as the amount of nectar secreted (Rios et al, 2008).…”