Disperser limitation and recruitment of an endemic African tree in a fragmented landscape

Abstract

Forest fragmentation may have positive or negative effects on tropical tree populations. Our earlier study of an endemic African tree, Leptonychia usambarensis (Sterculiaceae), in the East Usambara Mountains of Tanzania, found poorer recruitment of seedlings and juveniles in small fragments compared to continuous forest, and concomitant reduction of seed-dispersal agents and seed dispersal. However, the possibility that other biotic or abiotic consequences of the fragmentation process contribute to diminished recruitment in fragments was left open. Here we test whether excessive seed predation, diminished fecundity, low seed quality, or adverse abiotic effects acted independently or in concert with reduced seed dispersal to limit seedling and juvenile recruitment in fragments. Extended observations of disperser activity, a seed placement experiment, seed predator censuses, and reciprocal seedling transplants from forest and fragment sources failed to support the alternative hypotheses for poorer seedling and juvenile recruitment in fragments, leaving reduced seed dispersal as the most plausible mechanism. Poorer recruitment of this species in forest fragments, where high edge-to-area ratios admit more light than in continuous forest, is particularly striking because the tree is an early successional species that might be expected to thrive in disturbed microhabitats.

Seed dispersal increases local species richness and reduces spatial turnover of tropical tree seedlings

“…Our findings highlight that global declines in vertebrate populations (14-17) could significantly impact forest regeneration through reducing seed dispersal. While Guam represents an extreme case of complete disperser loss, leading to gap seedling richness being approximately halved and seedling turnover among gaps being doubled, other studies have demonstrated that partial loss of dispersers can reduce seedling diversity and disproportionally alter species abundances (20,23,45), particularly of light-demanding pioneer species (22,46). While we have to wait to see how gap dynamics play out in the presence and absence of dispersers on these islands, our findings show that global vertebrate losses have the potential to profoundly alter tropical forest composition.…”

“…Key issues here are that even high hunting pressure has not led to the complete loss of vertebrate dispersers (e.g., ref. 14); hunting also removes vertebrate seed predators and seedling browsers (20,22,23), making it difficult to isolate an effect of seed disperser loss alone; and the treatment of interest (removal of vertebrate dispersers) can be confounded with other site differences. Hunting pressure, for example, is often higher in areas without formal conservation protection, meaning these areas experience additional human pressures that affect tree populations, such as forest fragmentation and tree harvesting (14,20,24,25).…”

Effects of forest fragmentation on the seedling recruitment of a tropical herb: assessing seed vs. safe‐site limitation

“…However, we believe it is essential to conduct similar studies in a diversity of fragmented landscapes, as well as with a broader diversity of plant functional groups, to better predict the consequences of fragmentation on plant recruitment (Flinn and Vellend 2005). To date dispersal mechanisms have received the most attention, with studies suggesting less dispersal limitation for wind-and vertebrate-dispersed species (Flinn and Vellend 2005, Muscarella and Fleming 2007, Cordeiro et al 2009). Furthermore, most of the attention in tropical systems has focused on tree species (e.g., Norden et al 2009), despite the fact that understory plants represent up to 25% of the plant diversity in tropical forests (Gentry and Emmons 1987).…”

Does Atta laevigata (Smith, 1858) act as Solanum lycocarpum seed dispersers?

“…Zelikova & Breed (2008) stated that, when mainly a small number of species in an environment perform the removal of seeds, the removal rate might decrease, by the loss of these species or their density reduction. According to Cordeiro et al (2009), the loss of seed disperser organisms can endanger the plant species that depend on these for the dispersion process, or for the seeds germination, hence the establishment of new individuals and populations of this species. Thus the A. laevigata foraging becomes important for S. lycocarpum seed dispersal, even in small proportions and the short distances.…”