Competition for water and rapid exclusion of an island endemic by a pantropical species in a tropical climate

Water availability has major effects on community structure and dynamics globally, yet our understanding of competition for water in the tropics is limited. On the tropical Trindade Island, we explored competition for water in the context of the rapid exclusion of an endemic sedge, Cyperus atlanticus (Cyperaceae), by a pantropical, N-fixing shrub, Guilandina bonduc (Fabaceae). Guilandina patches were generally surrounded by rings of bare soil, and dead Cyperus halos commonly surrounded these bare zones. With geo-referenced measurements, we showed that Guilandina patches and bare soil zones rapidly expanded and replaced adjacent Cyperus populations. We found that soil water potentials were much lower in bare soils than soils under Guilandina or Cyperus, and that leaf water potentials of Cyperus plants were lower when co-occurring with Guilandina than when alone. When Guilandina was removed experimentally, Cyperus populations expanded and largely covered the bare soil zones. Our results indicate that when Guilandina establishes, its root systems expand beyond its canopies and these roots pull water from soils beneath Cyperus and kill it, creating bare zone halos, and then Guilandina expands and repeats the process. This scenario indicates rapid competitive exclusion and displacement of an endemic by a common pantropical species, at least in part through competition for water.

Automated summary

Water competition in the tropics was explored in this study, with a focus on the rapid exclusion of an endemic sedge by a pantropical shrub. The shrub's root systems expanded beyond its canopy, pulling water from the soil beneath the sedge and leading to its displacement and extinction. This scenario highlights the importance of water availability in shaping community structure and dynamics in tropical ecosystems.