Trophic interactions and feedbacks maintain intact and degraded states of Hawaiian tropical forests

Feedbacks within ecosystems can lead to internal reinforcement of the current state providing ecosystem resilience. Often, multiple biotic interactions across trophic levels play a role in such feedbacks, yet these are generally studied independently, obscuring relative importance among different factors. We look at various potential feedbacks intact and degraded mesic forests on Hawai?i Island where managers have planted native Acacia koa (koa) trees an attempt jumpstart succession former cattle pastures. These restoration forests, however, not undergone secondary succession, instead maintaining overstory with exotic pasture grass understory. contrasted level processes that influence capacity for natural understory regeneration: between bird-mediated seed rain fruiting (“top-down”), as well links composition microhabitats germination (“bottom-up”). quantified under canopy along transects spanning intact, fragmented, forests. Along transects, we established plots around focal measure abundance species, ground cover (e.g., grass, bryophyte), obtained estimates bird density evaluate contribution each factors seedling abundance. also used factorial addition/grass removal experiment directly compare substrate. found evidence both top-down bottom-up reinforced forest type. In fragmented combination comparatively high ample substrate is likely critical diverse system. contrast, grasses exhibit priority effects inhibiting effectively negating any benefits could be derived from rain. Such suggests active, rather than passive, would beneficial increase diversity areas.