On pluralism in ecology: seeing the forest and the trees.

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 plant – plant 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 mistle-toes, 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 …