Live under strong power: A third plant species alters interspecific interactions between two plant species

Organisms from higher tropical levels (e.g., herbivores) can mediate indirect competitive interactions between two target plant species, but such apparent competition may also be mediated by a third plant species and thus may vary depending on the size and competitive ability of the target plant species and the third plant species. We set up an outdoor experiment with four aquatic plant species with similar niches but differing greatly in size, i.e., Lemna minor (the smallest), Spirodela polyrhiza (the second smallest), Salvinia natans (the second largest) and Pistia stratiotes (the largest). We grew L. minor and S. polyrhiza alone or in mixture and in the absence of any third plant species, in the presence of a small, third species S. natans or in the presence of a large, third species P. stratiotes. In the absence of the third species or in the presence of S. natans, the growth of L. minor was greatly inhibited by S. polyrhiza, but in the presence of P. stratiotes, the inhibiting effects of S. polyrhiza disappeared completely and the growth of L. minor greatly increased. By contrast, the presence of L. minor had no effect on the growth of S. polyrhiza, and this effect did not depend on the presence or absence of the third plant species, although the presence of the third species, particularly P. stratiotes, decreased its growth. We conclude that the presence of a third plant species can regulate plant-plant interactions, but such an impact depends on the size and competitive ability of the competing species as well as the third species. Our findings highlight the role of complexity in plant-plant interactions, and suggest that apparent competition between plants can also occur at the same trophic level. These results have important implications for the explanations of species coexistence and biodiversity maintenance.