Competition induces allelopathy but suppresses growth and anti-herbivore defence in a chemically rich seaweed

Abstract

Many seaweeds and terrestrial plants induce chemical defences in response to herbivory, but whether they induce chemical defences against competitors (allelopathy) remains poorly understood. We evaluated whether two tropical seaweeds induce allelopathy in response to competition with a reef-building coral. We also assessed the effects of competition on seaweed growth and seaweed chemical defence against herbivores. Following 8 days of competition with the coral Porites cylindrica, the chemically rich seaweed Galaxaura filamentosa induced increased allelochemicals and became nearly twice as damaging to the coral. However, it also experienced significantly reduced growth and increased palatability to herbivores (because of reduced chemical defences). Under the same conditions, the seaweed Sargassum polycystum did not induce allelopathy and did not experience a change in growth or palatability. This is the first demonstration of induced allelopathy in a seaweed, or of competitors reducing seaweed chemical defences against herbivores. Our results suggest that the chemical ecology of coral-seaweed-herbivore interactions can be complex and nuanced, highlighting the need to incorporate greater ecological complexity into the study of chemical defence.

Testing for allelopathy in invasive plants: it all depends on the substrate!

“…This finding is consistent with the idea that allelochemicals-just as herbivore defense (Herms and Mattson 1992;Karban 2011)-are metabolically costly, and that there exists a tradeoff between resource allocation to growth versus allelopathy (see also Rasher and Hay 2014). Our results are in line with Kovarova et al (2010Kovarova et al ( , 2011 who also found that the production of secondary compounds in knotweed depends on nutrient supply.…”

Soil microbes alleviate allelopathy of invasive plants

“…The roots of Centaurea diffusa increase release of 8-hydroxyquinoline under iron limitation [10]. Eight days of competition induce the seaweed Galaxaura filamentosa to increase the release of allelochemicals [43]. Thus, future studies with more realistic allelochemical inputs and more ecologically relevant interactions between plants are required to enhance our understanding of the complex allelopathy between invasive and native plants.…”

Competitors as accomplices: seaweed competitors hide corals from predatory sea stars

“…shading, abrasion, sediment trapping) and/or chemically or microbially mediated [23,24,26,[46][47][48][49]. Prior studiesconducted on these same reefs detected no evidence that Sargassum, its lipid-soluble extracts or inert Sargassum mimics caused bleaching or visible damage to common corals [24,25,38,44], but previous assays did not evaluate effects on coral growth. Our findings are consistent with earlier studies that report reductions in coral growth because of competition with Sargassum sp.…”

“…shading, abrasion, overgrowth) and/ or chemical (i.e. allelopathy) mechanisms to directly reduce coral recruitment, growth, survival and fecundity [22][23][24][25]. Coral-algal interactions may also suppress corals indirectly by promoting virulent bacteria [26,27], or by stressing corals in ways that attract corallivores that further damage competing corals [28].…”