Seed mass equalises the strength of positive and negative plant–plant interactions in a semi-arid grassland
The interplay and balance between positive and negative interactions are important and recurrent topics in plant ecology. If facilitation occurs because benefactors ameliorate stress, such balance may be driven by seed mass because stress tolerance and competitive ability of plants, particularly seedlings, are positively correlated with seed mass. Thus, small-seeded, stress-intolerant species may require facilitation, but not large-seeded ones. This would equalise the magnitudes of opposite-signed interactions because in small-seeded species, positive effects of facilitators and negative effects of competitors should be strong, while both effects should be weak in large-seeded species. To test this idea, we assessed the effects of interactions with four associated species on different components of the performance of ten focal species. As expected, the largest facilitative and competitive effects were recorded in small-seeded species, and positive interactions had similar magnitudes to negative ones for any given seed mass, especially when performance was integrated into lifelong fitness. Furthermore, the fact that small-seeded species seem to be strongly facilitated may explain why they are not outcompeted by large-seeded species. This is an alternative to other hypotheses explaining the coexistence of plants with different-sized seeds. The close balance between opposite-signed interactions in the presence of stress may also explain why interactions have strong effects on individuals (that interact with only a few species), but seemingly weaker effects on populations, where interactions between many species would cancel out.
KeywordsCompetition Facilitation Semi-arid grassland Seed size Stress tolerance
L. F. V. V. Boullosa, D. García-Meza, E. García-Morales, F. Herce, A. Martínez, M. Martínez, E. Montoya, A. Navarrete, E. Lezama, M. Ramos, B. Santini, A. Torres and B. Vázquez helped us during fieldwork. M. A. Romero provided computational support, O. Briones, Z. Cano-Santana, C. Montaña, P. L. Valverde, M. T. Valverde, W. Dawson and three anonymous reviewers provided valuable comments. Consejo Nacional de Ciencia y Tecnología (CONACyT) provided VZ a fellowship during her master’s degree. She also thanks Posgrado en Ciencias Biológicas. This project was funded by Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica de la Universidad Nacional Autónoma de México (PAPIIT-UNAM IN225511). Agradecemos a la comunidad de Concepción Buenavista por su invaluable apoyo.
Author contribution statement
CM and VZ conceived the research, and VZ collected and analysed the data. Both authors contributed to preparing the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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