On pluralism in ecology: seeing the forest and the trees

Abstract

In response to our paper testing whether mistletoe represents a keystone resource [1], Thomsen & Wernberg (TW) [2] integrated our experiment with previous work and introduced a novel hypothesis to explain how structural complexity influences community composition. While we share their enthusiasm for unveiling generalized determinants of diversity, we caution that their approach may overlook some of the complexity our research has uncovered-complexity essential for understanding how and where these patterns arise. Here, we clarify the functional distinction between parasites and epibionts, highlighting that structural similarities belie different underlying mechanisms and pathways. Rather than purely a structural response, we posit that our findings are indicative of fundamental changes to nutrient dynamics and suggest that many cases of presumed facilitation are likely mediated by hitherto-overlooked indirect interactions. Reflecting on the search for generality in ecology, we note how the cascading habitat-formation hypothesis [2] exemplifies the balance between big-picture, hypothesis-generating synthesis and mechanistic, hypothesis-testing experimentation.TW [2] were dismayed that we did not compare our findings with marine research and, whereas we concur that greater integration between research (and researchers) in terrestrial and aquatic systems is essential to inform ecological theory, this was not an oversight. Our objective was to quantify the influence of a parasitic plant on community composition and compare the magnitude of the effect and the underlying mechanisms with work on parasites more broadly and other entities deemed ecological keystones. There are no aquatic parasitic plants (freshwater or marine [3]), so all studies of plantplant interactions are necessarily restricted to structural aspects. While it is useful to integrate research findings on both terrestrial and aquatic organisms that use other organisms for structural support (e.g. corals and strangler figs; macroalgae and vines), this framework may not be applicable to parasitic groups. Although the structural effects of mistletoe on canopy complexity were predicted to be significant [4], this prediction was not supported by our experiment, with mistletoe-nesting birds not exhibiting significantly greater declines once all mistletoe plants were removed. Indeed, the most marked response to mistletoe removal was displayed by ground-foraging insectivores, a response considered to relate to the abundant enriched litter shed by mistletoes, promoting understorey complexity, microbial activity and prey availability on the forest floor [1]. So, rather than structural complexity driving diversity, it is nutrient enrichment driving small-scale heterogeneity and thereby promoting coexistence, consistent with work on other parasitic groups [5]. These nutrients are partly reallocated from the host by the parasite but are also imported from further afield, deposited by visiting animals (the dryad hypothesis [6]).This interplay between small-scale he...

Genetic Differentiation and Fragmentation in Response to Climate Change of the Narrow Endemic Psittacanthus auriculatus

“…Despite their positive role as ecological keystones in many forests and woodlands (Watson, 2001; Watson & Herring, 2014) and facilitative effects of hemiparasitic mistletoes on community structure (March & Watson, 2010; Ndagurwa, Dube, & Mlambo, 2014; Watson, 2015; Watson & Herring, 2012), the loss of mistletoe species should not represent intuitively a threat to associated hosts and could even provide benefit (Strona, 2015; Strona, Galli, & Fattorini, 2013). For instance, the extinction risk of P. auriculatus is low parasitizing widely distributed host species with low vulnerability to extinction like A. schaffneri (86% prevalence; Díaz Infante et al., 2016).…”

“…Mistletoes are considered ecologically important key resources as they provide high-quality nutritional resources for a variety of obligate and partially dependent species, especially during periods of seasonal scarcity (Watson, 2001), and influence indirectly community structure in low productivity systems on bottom-up processes (e.g., Watson, 2009; Watson & Herring, 2014). Most of these parasitic plants depend on avian vectors for pollen and seed dispersal and obtain all of their water and minerals from the host (obligate hemiparasites) through a vascular connection termed haustorium (Calder & Bernhardt, 1983).…”

On tropical mistletoes: tractable models for evolutionary ecology, ecosystem function, and phytochemistry

“…The final set of opportunities for research on tropical mistletoes transcends individual interactions and explores the overall influence of these plants on diversity and ecosystem function. Through a series of studies in temperate Australian woodlands, the effects of mistletoe on diversity have been quantified, unravelling the mechanisms underlying their role as ecological keystones (Watson and Herring 2014). Compared with otherwise similar control woodlands, those from which mistletoe was experimentally removed lost more than a third of their woodland resident bird species (Watson and Herring 2012), the community-wide response driven by groundforaging insectivores (Watson 2015).…”

Hybridization and differential introgression associated with environmental shifts in a mistletoe species complex

“…119 species) are characteristic stem parasites (hemiparasites) throughout the Neotropics 21 . Most species are host generalists and ecologically very important because they provide food resources (e.g., fruits and nectar) to many animals 13 and indirectly influence community structure in low productivity systems 33 , 57 . The recently diverged species complex of Psittacanthus calyculatus and P .…”