Facilitation In Plant Communities Research Articles

The Pteridophytes of Adams, Northern Luzon, Philippines and their Ecosystem Services

The Philippines has approximately 1,100 species of lycophytes and ferns, and this represents approximately 9% of the worldwide fern flora. Botanical explorations and local documentations continue to expand the list. Local documentations increase our knowledge on species distribution and conservation status, which are essential in biodiversity conservation. This paper reports on a survey of the pteridophytes of Adams, Ilocos Norte, one of the remaining floristic sites in Luzon Island, Philippines. A series of floristic surveys conducted at selected sites in Adams recorded and vouchered 47 species, 34 genera and 21 families of pteridophytes. Among all these pteridophytes, six are threatened Philippine plant species. These are Platycerium coronarium, Ceratopteris thalictroides, Tectaria psomiocarpa, Psilotum nudum, Blechnum egregium, and Angiopteris evecta. This study contributes to the baseline data on Philippine pteridophytes especially on their distribution, and can serve as reference for biodiversity conservation and restoration efforts as ferns may help in the stabilization of degraded lands and facilitation in plant communities.

Living close to your neighbors: the importance of both competition and facilitation in plant communities.

Recent work has demonstrated that competition and facilitation likely operate jointly in plant communities, but teasing out the relative role of each has proven difficult. Here we address how competition and facilitation vary with seasonal fluctuations in environmental conditions, and how the effects of these fluctuations change with plant ontogeny. We planted three sizes of pine seedlings (Pinus strobus) into an herbaceous diversity experiment and measured pine growth every two weeks for two growing seasons. Both competition and facilitation occurred at different times of year between pines and their neighbors. Facilitation was important for the smallest pines when environmental conditions were severe. This effect decreased as pines got larger. Competition was stronger than facilitation overall and outweighed facilitative effects at annual time scales. Our data suggest that both competition and the counter-directional effects of facilitation may be more common and more intense than previously considered.

Moving forward on facilitation research: response to changing environments and effects on the diversity, functioning and evolution of plant communities.

Once seen as anomalous, facilitative interactions among plants and their importance for community structure and functioning are now widely recognized. The growing body of modelling, descriptive and experimental studies on facilitation covers a wide variety of terrestrial and aquatic systems throughout the globe. However, the lack of a general body of theory linking facilitation among different types of organisms and biomes and their responses to environmental changes prevents further advances in our knowledge regarding the evolutionary and ecological implications of facilitation in plant communities. Moreover, insights gathered from alternative lines of inquiry may substantially improve our understanding of facilitation, but these have been largely neglected thus far. Despite over 15 years of research and debate on this topic, there is no consensus on the degree to which plant-plant interactions change predictably along environmental gradients (i.e. the stress-gradient hypothesis), and this hinders our ability to predict how plant-plant interactions may affect the response of plant communities to ongoing global environmental change. The existing controversies regarding the response of plant-plant interactions across environmental gradients can be reconciled when clearly considering and determining the species-specificity of the response, the functional or individual stress type, and the scale of interest (pairwise interactions or community-level response). Here, we introduce a theoretical framework to do this, supported by multiple lines of empirical evidence. We also discuss current gaps in our knowledge regarding how plant-plant interactions change along environmental gradients. These include the existence of thresholds in the amount of species-specific stress that a benefactor can alleviate, the linearity or non-linearity of the response of pairwise interactions across distance from the ecological optimum of the beneficiary, and the need to explore further how frequent interactions among multiple species are and how they change across different environments. We review the latest advances in these topics and provide new approaches to fill current gaps in our knowledge. We also apply our theoretical framework to advance our knowledge on the evolutionary aspects of plant facilitation, and the relative importance of facilitation, in comparison with other ecological processes, for maintaining ecosystem structure, functioning and dynamics. We build links between these topics and related fields, such as ecological restoration, woody encroachment, invasion ecology, ecological modelling and biodiversity-ecosystem-functioning relationships. By identifying commonalities and insights from alternative lines of research, we further advance our understanding of facilitation and provide testable hypotheses regarding the role of (positive) biotic interactions in the maintenance of biodiversity and the response of ecological communities to ongoing environmental changes.

Do indirect interactions always contribute to net indirect facilitation?

Indirect facilitation theory assumes that the addition of species in complex communities should decrease competitive interactions through the emergence of positive indirect effects among competitors. However, experiments conducted in communities have rarely observed a net indirect facilitation. We hypothesize that this may be due to the likely occurrence of negative indirect interactions that were overlooked in indirect facilitation theory. We used a spatially explicit modeling approach to quantify indirect effects and net interactions occurring within a system of three competitors. In contrast to field experiments our modeling approach allows quantifying positive indirect effects since their calculation needs to be done in absence of direct negative effects. We showed the existence in a system of three competitors of two negative indirect interactions, in addition to the positive one emphasized in indirect facilitation theory. We also made precise the conditions under which the balance of these three indirect interactions becomes negative and induces a competition enhancement and in turn no net indirect facilitation. The existence of negative indirect effects among competitors contributes to explain the rare occurrence of indirect facilitation in plant communities.

The importance of plant life form on spatial associations along a subtropical coastal dune gradient

AbstractQuestionsDoes the spatial association between isolated adult trees and understorey plants change along a gradient of sand dunes? Does this association depend on the life form of the understorey plant?LocationCoastal sand dunes, southeast Brazil.MethodsWe recorded the occurrence of understorey plant species in 100 paired 0.25 m2 plots under adult trees and in adjacent treeless sites along an environmental gradient from beach to inland. Occurrence probabilities were modelled as a function of the fixed variables of the presence of a neighbour, distance from the seashore and life form, and a random variable, the block (i.e. the pair of plots). Generalized linear mixed models (GLMM) were fitted in a backward step‐wise procedure using Akaike's information criterion (AIC) for model selection.ResultsThe occurrence of understorey plants was affected by the presence of an adult tree neighbour, but the effect varied with the life form of the understorey species. Positive spatial association was found between isolated adult neighbour and young trees, whereas a negative association was found for shrubs. Moreover, a neutral association was found for lianas, whereas for herbs the effect of the presence of an adult neighbour ranged from neutral to negative, depended on the subgroup considered. The strength of the negative association with forbs increased with distance from the seashore. However, for the other life forms, the associational pattern with adult trees did not change along the gradient.ConclusionsFor most of the understorey life forms there is no evidence that the spatial association between isolated adult trees and understorey plants changes with the distance from the seashore, as predicted by the stress gradient hypothesis, a common hypothesis in the literature about facilitation in plant communities. Furthermore, the positive spatial association between isolated adult trees and young trees identified along the entire gradient studied indicates a positive feedback that explains the transition from open vegetation to forest in subtropical coastal dune environments.

Positive Interactions between Desert Granivores: Localized Facilitation of Harvester Ants by Kangaroo Rats

Facilitation, when one species enhances the environment or performance of another species, can be highly localized in space. While facilitation in plant communities has been intensely studied, the role of facilitation in shaping animal communities is less well understood. In the Chihuahuan Desert, both kangaroo rats and harvester ants depend on the abundant seeds of annual plants. Kangaroo rats, however, are hypothesized to facilitate harvester ants through soil disturbance and selective seed predation rather than competing with them. I used a spatially explicit approach to examine whether a positive or negative interaction exists between banner-tailed kangaroo rat (Dipodomys spectabilis) mounds and rough harvester ant (Pogonomyrmex rugosus) colonies. The presence of a scale-dependent interaction between mounds and colonies was tested by comparing fitted spatial point process models with and without interspecific effects. Also, the effect of proximity to a mound on colony mortality and spatial patterns of surviving colonies was examined. The spatial pattern of kangaroo rat mounds and harvester ant colonies was consistent with a positive interspecific interaction at small scales (<10 m). Mortality risk of vulnerable, recently founded harvester ant colonies was lower when located close to a kangaroo rat mound and proximity to a mound partly predicted the spatial pattern of surviving colonies. My findings support localized facilitation of harvester ants by kangaroo rats, likely mediated through ecosystem engineering and foraging effects on plant cover and composition. The scale-dependent effect of kangaroo rats on abiotic and biotic factors appears to result in greater founding and survivorship of young colonies near mounds. These results suggest that soil disturbance and foraging by rodents can have subtle impacts on the distribution and demography of other species.

Towards a more exact definition of the importance of competition - a reply to Freckleton et al. (2009)

Summary 1. Freckleton, Watkinson & Rees (2009) criticize a recent paper by ourselves in this journal (Brooker & Kikvidze 2008) as well as our earlier work on competition importance (Brooker et al. 2005). In response, here we clarify our ideas with the aim of defining more clearly the key points of scientific debate, specifically (i) the definition of the importance of competition and (ii) its measurement. 2. Freckleton, Watkinson & Rees (2009) interpret the classic paper by Welden & Slauson (1986) such that importance as a concept relates to long-term, population-level consequences of competition. However, we consider competition importance to be the proportional impact of competition relative to the overall impact of the environment, and our index Cimp expresses changes in competition importance – as defined by ourselves – along productivity gradients. We argue that our definition more accurately reflects the work of Welden & Slauson, as well as a more recent use of the concept (Grace 1991), which precedes the work of Freckleton & Watkinson (2001). 3. We highlight that Cimp was never proposed as a general index of competition importance, but is readily applicable in certain circumstances. Notably, our index and the approaches to measuring competition importance as set out by Freckleton, Watkinson & Rees (2009) are not unrelated. 4. We also discuss some recent additional responses to both our (2008) paper and that by Freckleton, Watkinson & Rees (2009), including applications of the concept of competition importance. Although the authors of these papers may not have used our index Cimp, they follow the same definitions for the overall concept of competition importance as ourselves. 5. Synthesis. We conclude that the complex topic of biotic interactions, including the specific issue of the importance of competition, invites a range of approaches. Importantly, these approaches can be complementary and not conflicting. Here, we propose what we see as a sensible resolution to the current debate concerning the definition of competition importance, a resolution which is backed by the original source article, literature precedent and current usage.

A multitrophic perspective on functioning and evolution of facilitation in plant communities

Summary 1. Plant facilitation has been studied mostly in the context of plant–plant interactions, whereas multitrophic interactions including those that occur below ground have not yet received much attention. Here, I will discuss how above‐ground and below‐ground natural enemies and their predators influence plant facilitation and its evolution. 2. Specific above‐ground and below‐ground plant enemies and their predators play a major role in structuring the composition and dynamics of plant communities. In successional sequences, above‐ground and below‐ground multitrophic level interactions may tip the balance from competitive to facilitative states and vice versa. 3. Little is known about how above‐ground and below‐ground multitrophic interactions develop along resource or stress gradients and how the outcomes of above‐ground–below‐ground interactions depend on variations in these environmental conditions. 4. Facilitated plants need to fit into the above‐ground–below‐ground multitrophic communities of their facilitators. 5. Little is known also about the evolution of plant facilitation. The observed distance in phylogeny between facilitators and facilitated plants suggests that host‐specific enemies may very well co‐determine which species become facilitated by which facilitators. 6. Further, very little attention has been given to how plant strategies (allelopathy, accumulation of enemies, monopolization of symbionts) may be the result of selection against being facilitative. 7. Synthesis. Plant facilitation cannot be understood without considering a plant’s natural enemies and also its enemies’ enemies. Plant enemies can turn competitive interactions into facilitative interactions, whereas the enemies’ enemies can turn facilitation back into competition. Below‐ground interactions will have longer‐lasting effects on facilitation than those above ground, because many organisms can persist in the soil, even when the host plants have disappeared.

Facilitation in the conceptual melting pot

Summary 1. Here we present an introduction to this issue’s Special Feature arising from the British Ecological Society Symposium: Facilitation in Plant Communities (20–22 April 2009). 2. Papers in the Special Feature demonstrate the benefits that arise from cross‐system application of general concepts, for example, the well‐known stress gradient hypothesis. Such comparisons challenge our definition of facilitation, as well as our pre‐conceptions on the nature of intermediary organisms. 3. We suggest that under some circumstances a clear definition of the two‐way nature of interactions is essential, e.g. when considering the evolutionary implications of facilitation. In other cases, however, we can perhaps be more relaxed, e.g. when facilitation is a component of conservation ecology. 4. Synthesis. Overall we believe that establishing facilitation as an independent concept has driven substantial progress towards a clearer understanding of how ecological systems work. Through the links established by work such as that presented in this Special Feature, we believe this field will continue to make rapid progress and aid ecological understanding in general.

Is the cask of facilitation ready for bottling? A symposium on the connectivity and future directions of positive plant interactions

The 2009 British Ecological Society's Annual Symposium entitled 'Facilitation in Plant Communities' was held at the University of Aberdeen, Scotland, from 20 to 22 April 2009. This was the first ever international meeting dedicated to the rapidly expanding field of facilitation. The aim of the symposium was to assess the current 'state-of-play' by contrasting findings from different systems and by looking outwards in an attempt to integrate this field with other related fields. It was also aimed at understanding how knowledge of facilitation can help understand community dynamics and be applied to ecosystem restoration. The symposium identified several key areas where future work is likely to be most profitable.

Refining the stress‐gradient hypothesis for competition and facilitation in plant communities

Summary The stress‐gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common in conditions of high abiotic stress relative to more benign abiotic conditions. With notable exceptions, most tests of the SGH have studied the interaction between a single pair or a few pairs of species, and thus have evaluated shifts in the magnitude and direction of pair‐wise interactions along stress gradients, rather than shifts in the general frequency of interactions. The SGH has been supported by numerous studies in many ecosystems, has provided a crucial foundation for studying the interplay between facilitation and competition in plant communities, and has a high heuristic value. However, recent empirical research indicates that factors like the variation among species and the nature of the stress gradient studied add complexity not considered in the SGH, creating an opportunity to extend the SGH's general conceptual framework. We suggest that one approach for extending the SGH framework is to differentiate between the original idea of how ‘common’ interactions might be along stress gradients and the ubiquitous empirical approach of studying shifts in the strength of pair‐wise interactions. Furthermore, by explicitly considering the life history of the interacting species (relative tolerance to stress vs. competitive ability) and the characteristics of the stress factor (resource vs. non‐resource) we may be able to greatly refine specific predictions relevant to the SGH. We propose that the general pattern predicted by the SGH would hold more frequently for some combinations of life histories and stress factor, particularly when the benefactor and beneficiary species are mostly competitive and stress‐tolerant, respectively. However, we also predict that other combinations are likely to yield different results. For example, the effect of neighbours can be negative at both ends of the stress gradient when both interacting species have similar ‘competitive’ or ‘stress‐tolerant’ life histories and the abiotic stress gradient is driven by a resource (e.g. water). Synthesis. The extension of the SGH presented here provides specific and testable hypotheses to foster research and helps to reconcile potential discrepancies among previous studies. It represents an important step in incorporating the complexity and species‐specificity of potential outcomes into models and theories addressing how plant–plant interactions change along stress gradients.

Journal of Ecology News

<i>Journal of Ecology</i> News

Facilitation in plant communities: the past, the present, and the future

1 Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field. 2 We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant–plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research. 3 With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies. 4 There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity. 5 Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes. 6 Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms. 7 Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers. 8 Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plant–plant interactions in the design and interpretation of studies from the fields of restoration and global change ecology. 9 Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists.

COMPETITION AND FACILITATION: A SYNTHETIC APPROACH TO INTERACTIONS IN PLANT COMMUNITIES

Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities.