Alessandro Filazzola

Functional assessment of animal interactions with shrub‐facilitation complexes: a formal synthesis and conceptual framework

Summary Facilitation studies focus primarily on plants often neglecting the extended effects that cascade through ecological networks. Plants interact with other organisms through consumptive effects and a myriad of non-trophic effects such as habitat amelioration or pollination. Shrubs are a dominant benefactor species frequent in plant-facilitation studies but can also have direct and indirect interactions with animals. Herein, we use a systematic review to address the following two objectives: (i) to propose a conceptual framework that explores these interactions including the functional roles of the interacting species, and (ii) to quantitatively summarize the current state of this field examining effects beyond plant–plant interactions. To date, a relatively limited number of studies have examined the importance of coupled benefactor-subordinate plant positive interactions with animals (79 studies in total). From this set of studies, 36 studies documented positive plant interactions generating a total of 53 independent instances of either shrub–plant–animal or shrub–animal–plant interactions. These interaction pathways were evenly split between direct (49%) and indirect (51%) interactions of shrubs with animals. Hypotheses frequently tested included seed trapping, herbivore protection, magnet pollination and facilitation-mediated secondary seed dispersal. The most common functional role of shrubs was protection from herbivory, and the most common animal role associated with plant-facilitation complexes was that of a consumer. None of these studies explored bidirectional plant–animal interactions, used a network approach to describe the interaction sets, nor contrasted interaction strengths. Multitrophic, integrated sets of experiments incorporating plant facilitation into community dynamics are thus critical in advancing management of high-stress ecosystems wherein positive interactions are commonly reported.

A cost–benefit model for plant–plant interactions: a density-series tool to detect facilitation

Generally, only the net outcome of plant–plant interactions is measured in population and community ecology research, with few attempts to determine the relative importance of negative (competition) and positive (facilitation) interactions between subordinate species. Changes in the intensity of interactions along gradients, between life-stages, or with changing densities, and the use of selective removals enhance our capacity to infer positive and negative interactions. However, the most powerful examples at least in detecting facilitation typically involve measuring changes with or without a nurse-plant or benefactor species and often involve only a very limited numbers of species. In plant competition studies, however, greater number of species are commonly tested and density-dependent series are not an uncommon tool to test for net negative interactions. Here, we develop a cost–benefit model that can be used to comprehensively calculate the average expected net gain per individual at every point in a density series provided several response variables are recorded at each density. The utility of this model is demonstrated using both hypothetical data and several empirical data sets, and it is used to infer either both positive and negative net effects. Expected net gain can also serve as an accurate estimate of mean fitness per individual at a given density provided appropriate performance measures were recorded within the primary study. Within a single density series, both facilitation and competition can occur and were detectable using this method. This approach emphasizes the current view that both negative and positive interactions play a role in shaping plant communities. Furthermore, it is evident that facilitation can be detected using the manipulative density series typically associated with competition studies and not just using the typical target nurse-plant methodology. Finally, this method is a significant advance over the current practice of tallying up single responses within a study to estimate outcomes by providing a single, synthetic measure of the net gain or cost of interactions.

The effect of consumer pressure and abiotic stress on positive plant interactions are mediated by extreme climatic events

Environmental extremes resulting from a changing climate can have profound implications for plant interactions in desert communities. Positive interactions can buffer plant communities from abiotic stress and consumer pressure caused by climatic extremes, but limited research has explored this empirically. We tested the hypothesis that the mechanism of shrub facilitation on an annual plant community can change with precipitation extremes in deserts. During years of extreme drought and above-average rainfall in a desert, we measured plant interactions and biomass while manipulating a soil moisture gradient and reducing consumer pressure. Shrubs facilitated the annual plant community at all levels of soil moisture through reductions in microclimatic stress in both years and herbivore protection in the wet year only. Shrub facilitation and the high rainfall year contributed to the dominance of a competitive annual species in the plant community. Precipitation patterns in deserts determine the magnitude and type of facilitation mechanisms. Moreover, shrub facilitation mediates the interspecific competition within the associated annual community between years with different rainfall amounts. Examining multiple drivers during extreme climate events is a challenging area of research, but it is a necessary consideration given forecasts predicting that these events will increase in frequency and magnitude.