The following is a guest post written by Santiago Soliveres.
Once seen as anomalies, positive (facilitative) interactions among plants and their importance for ecosystem structure and functioning are now fully recognized (see Brooker et al. 2008 for a review). Research focused on facilitation has received increasing attention from the most important ecological and interdisciplinary journals during recent years, and the Journal of Ecology stands above them in the concerted effort to advance our knowledge of this important topic.
Researchers agree that facilitative interactions have important implications for community structure (albeit their relative importance as drivers of ecosystem structure and functioning regarding other processes remains poorly understood; Kikvidze et al. 2005; Mitchell, Cahill & Hik 2009; Maestre et al. 2010). However, they disagree about what the different drivers of such interactions are, and how these interactions behave across environmental gradients. The main aim of this guest post is to briefly review the currently hot debate regarding the relationship between the outcome of plant–plant interactions and environmental conditions. The study of this relationship has been by far the most active line of research in facilitation literature, championed by the Stress Gradient Hypothesis (hereafter SGH; Bertness & Callaway 1994), which predicts a monotonic increase in the frequency of positive interactions with increasing environmental stress. The SGH has been revisited many times; for example, it has been argued that the shape of the relationship between the outcomes of plant–plant interactions and environmental stress gradients is unimodal instead of linear (e.g. Michalet et al. 2006; Holmgren & Scheffer 2010). This relationship also depends on the type and number of stress factors involved (Maestre et al. 2009; Smit, Rietkerk & Wassen 2009), making the shape of plant–plant interaction outcomes vs environmental stress difficult to predict. It becomes even harder to predict these relationships when we account for the fact that they are highly species specific, with positive interactions more readily detectable among woody species than among grasses (Gómez-Aparicio 2009), tertiary than quaternary species (Valiente-Banuet et al. 2006), competitive than stress-tolerant species (Liancourt, Michalet & Callaway 2005), or juveniles than adult individuals (Miriti 2006). Overall, it seems that facilitative interactions occur among certain suites of ecological traits (Butterfield & Callaway 2013; Schöb et al. 2013), with a secondary influence of the environmental conditions. One may think that with these many exceptions and refinements, the SGH as it was originally formulated should be abandoned. However, in a recent meta-analysis, He, Bertness & Altieri (2013) claim that their results strongly support the SGH as originally formulated, proposing to go “back to the basics” in this debate. Empirical support for the original SGH has recently also been found in approaches at the community level (e.g. Dohn et al. 2013).
It seems evident that species that hold different functional traits also have different ecological requirements. Consequently, different species are going to react differently to their environment, also in their interactions with their neighbours, (e.g. Greyner-La Peire et al. 2001; Liancourt, Callaway & Michalet 2005). This may explain the high species-specificity in the shape of the relationships between the outcomes of plant–plant interactions and environmental conditions when taking into account all the species in a given community (e.g. Soliveres, Torices & Maestre 2012) and the difficulty to predict such shapes. How does this species-specificity in the outcome of pairwise interactions translate to the community level and the response of such communities to changes in the environment? Different researchers have given different answers to this same question. Holmgren & Scheffer (2010) predict facilitation to be stronger under moderate environments. Soliveres et al. (2011) proposed the “individual-based stress concept”. This hypothesis predicts that the shape of the frequency of positive plant–plant interactions at the community level will respond differently to environmental gradients depending on the nature of the gradient and the variability in environmental requirements covered by the species forming a given community. If the gradient is formed by a single stress factor or multiple related ones, the shape of the relationship will be unimodal, following the same rationale as in Holmgren & Scheffer (2010). However, if the gradient is formed by different unrelated stress factors, or the variability in environmental requirements covered by the species forming the community is high, no relationship should be expected between the frequency of positive plant-plant interactions and environmental stress. As with the original SGH, empirical support exists for both views (e.g. Hacker & Gaines 1997; López et al. 2013) and the debate seems far from conclusion.
Further investigation into the relationship between plant-plant interactions and environmental conditions is crucial, since it will undoubtedly help to improve our understanding of the responses of plant communities to global climate change. However, there are other active lines of research on the horizon that cannot be forgotten. To further discuss and report on key advancements that have been made in the field of facilitation during the last years, Chris Smit, Santiago Soliveres, Fernando Maestre and Johannes Metz are organizing a special session within the 43rd Annual Meeting of the Ecological Society of Germany, Austria and Switzerland, which will take place from September 9 to 13, 2013 in Potsdam, Germany. Our session will discuss the progress that has been made in facilitation research since the last international meeting focused on facilitation took place in 2009. The session will cover the following themes: the relative importance of different (a)biotic drivers of facilitation, the importance of facilitation as a process driving plant communities and their response to global change, facilitation networks, and the evolutionary aspects of plant–plant facilitation. Contributions to this session are welcomed. Visit the webpage of the meeting for information on abstract submission and deadlines.
Santiago Soliveres –
Universidad Rey Juan Carlos
Bertness, M.D. & Callaway, R.M. (1994) Positive interactions in communities. Trends in Ecology and Evolution, 9, 191–193.
Brooker, R.W., et al. (2008) Facilitation in plant communities: the past, the present and the future. Journal of Ecology, 96, 18-34.
Butterfield, B.J. & Callaway, R.M. (2013) A functional comparative approach to facilitation and its context dependence. Functional Ecology. doi: 10.1111/1365-2435.12019
Dohn, J., Dembélé, F., Karembé, M., Moustakas, A., Amévor, K.A. & Hanan, N.P. (2013) Tree effects on grass growth in savannas: competition, facilitation and the stress-gradient hypothesis. Journal of Ecology, 101, 202-209.
Gómez-Aparicio, L. (2009) The role of plant interactions in the restoration of degraded ecosystems: a meta-analysis across life-forms and ecosystems. Journal of Ecology, 97, 1202–1214
Greiner La Peyre, M.K., Grace, J.B., Hahn, E. & Mendelssohn, I.A. (2001) The importance of competition in regulating plant species abundance along a salinity gradient. Ecology, 82, 62-69
Hacker, S.D. & Gaines, S.D. (1997) Some implications of direct positive interactions for community species diversity. Ecology, 78, 1990-2003
He, Q., Bertness, M.D. & Altieri, A.H. (2013) Global shifts towards positive species interactions with increasing environmental stress. Ecology Letters, DOI: 10.1111/ele.12080.
Holmgren, M. & Scheffer, M. (2010) Strong facilitation in mild environments: the stress gradient hypothesis revisited. Journal of Ecology, 98, 1269-1275.
Kikvidze, Z., Pugnaire, F.I., Choler, P., Lortie, C.J., Michalet, R. & Callaway, R.M. (2005) Linking patterns and processes in alpine plant communities: a global study. Ecology, 86, 1395-1400.
Liancourt P., Callaway R.M. & Michalet R. (2005) Stress tolerance abilities and competitive responses determine the outcome of biotic interactions. Ecology, 86, 1611-1618.
López, R.P., Valdivia, S., Rivera-Milanovic, M. & Rios, R.S. (2013) Co-occurrence patterns along a regional aridity gradient of the subtropical Andes do not support stress gradient hypothesis. PLoS ONE, 8(3), e58518
Maestre, F. T. et al. (2010) Do biotic interactions modulate ecosystem functioning along abiotic stress gradients? Insights from semi-arid Mediterranean plant and biological soil crust communities. Philosophical Transactions of the Royal Society B, 365, 2057-2070
Maestre, F.T., Callaway, R.M., Valladares, F. & Lortie, C.J. (2009) Refining the stress-gradient hypothesis for competition and facilitation in plant communities. Journal of Ecology, 97, 199–205.
Michalet, R., Brooker, R.W., Cavieres, L.A., Kikvidze, Z., Lortie, C.J. & Pugnaire, F.I. (2006) Do biotic interactions shape both sides of the humped-back model of species richness in plant communities? Ecology Letters, 9, 767–773.
Miriti, M.N. (2006) Ontogenetic shift from facilitation to competition in a desert shrub. Journal of Ecology, 94, 973–979.
Mitchell, M.G., Cahill, J.F. & Hik, D.S. (2009) Plant interactions are unimportant in a subarctic-alpine plant community. Ecology, 90, 2360-2367.
Schöb, C., Armas, C., Guler, M., Prieto, I. & Pugnaire, F.I. (2013) Variability in functional traits mediates plant interactions along stress gradients. Journal of Ecology doi: 10.1111/1365-2745.12062
Smit, C., Rietkerk, M. & Wassen, M.J. (2009) Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesis. Journal of Ecology, 97, 1215–1219
Soliveres, S., Eldridge, D., Maestre, F.T., Bowker, M.A., Tighe, M. & Escudero, A. (2011) Microhabitat amelioration and reduced competition among understorey plants as drivers of facilitation across environmental gradients: towards a unifying framework. Perspectives in Plant Ecology, Evolution and Systematics, 13, 247-258
Soliveres, S., Torices, R. & Maestre, F.T. (2012) Evolutionary relationships can be more important than abiotic conditions in defining plant-plant interactions. Oikos, 121, 1638-1648
Valiente-Banuet, A., Vital, A., Verdú, M. & Callaway, R. (2006) Modern Quaternary plant lineages promote diversity through facilitation of ancient Tertiary lineages. Proceedings of the National Academy of Sciences USA, 103, 16812-16817.