Seed Dispersal and Establishment of Endangered Plants on Oceanic Islands: The Janzen-Connell Model, and the Use of Ecological Analogues

Abstract

BackgroundThe Janzen-Connell model states that plant-specific natural enemies may have a disproportionately large negative effect on progeny close to maternal trees. The majority of experimental and theoretical studies addressing the Janzen-Connell model have explored how it can explain existing patterns of species diversity in tropical mainland areas. Very few studies have investigated how the model's predictions apply to isolated oceanic islands, or to the conservation management of endangered plants. Here, we provide the first experimental investigation of the predictions of the Janzen-Connell model on an oceanic island, in a conservation context. In addition, we experimentally evaluate the use of ecological analogue animals to resurrect the functional component of extinct frugivores that could have dispersed seeds away from maternal trees.Methodology/Principal FindingsIn Mauritius, we investigated seed germination and seedling survival patterns of the critically endangered endemic plant Syzygium mamillatum (Myrtaceae) in relation to proximity to maternal trees. We found strong negative effects of proximity to maternal trees on growth and survival of seedlings. We successfully used giant Aldabran tortoises as ecological analogues for extinct Mauritian frugivores. Effects of gut-passage were negative at the seed germination stage, but seedlings from gut-passed seeds grew taller, had more leaves, and suffered less damage from natural enemies than any of the other seedlings.Conclusions/SignificanceWe provide the first experimental evidence of a distance-dependent Janzen-Connell effect on an oceanic island. Our results potentially have serious implications for the conservation management of rare plant species on oceanic islands, which harbour a disproportionately large fraction of the world's endemic and endangered plants. Furthermore, in contrast to recent controversy about the use of non-indigenous extant megafauna for re-wilding projects in North America and elsewhere, we argue that Mauritius and other oceanic islands are ideal study systems in which to empirically explore the use of ecological analogue species in restoration ecology.

Potential Stressors Leading to Seedling Mortality in the Endemic Håyun Lågu Tree (Serianthes Nelsonii Merr.) in the Island of Guam

“…Experimental testing of the Janzen-Connell model's spatial effects has not been accomplished adequately for tropical forests [5], and even less so for endangered species on oceanic islands [8]. Here, we employed in situ experimental techniques commonly used in Janzen-Connell research protocols to determine the influence of potential agents of mortality on the limited seedling longevity beneath Guam's only known naturally occurring S. nelsonii tree.…”

Conservation of oceanic island floras: Present and future global challenges

“…There is increasing evidence that island plants suffer from the extinction or decline of seed dispersers (e.g. Hansen and Galetti, 2009;Hansen et al, 2008;Whittaker and Ferná ndez-Palacios, 2007;McConkey and Drake, 2006;Cox et al, 1991), and pollinators (e.g. Mortensen et al, 2008;Cox and Elmqvist, 2000).…”

The Use of Extant Non-Indigenous Tortoises as a Restoration Tool to Replace Extinct Ecosystem Engineers

“…Many of the introduced vertebrates are seed predators (Cheke & Hume 2008) and so fail to compensate for the loss of native dispersers. Limited seed-dispersal and poor seedling recruitment is widespread in Mauritian habitats and hampers natural regeneration (Hansen et al 2008). …”

“…The Mascarene Islands, for instance, have lost the majority of their endemic large-bodied vertebrates (Cheke & Hume 2008). There is increasing evidence that this has led to the disruption of important interactions and ecosystem processes, such as herbivory and seed dispersal (Maunder et al 2002;Cheke & Hume 2008;Hansen et al 2008). Due to their isolation and relatively simple ecosystems, the Mascarenes are an ideal system in which to study both the effects of extinction and disruption, and also to explore how lost interactions and habitat dynamics can at least partly be resurrected.…”