The effects of seed predators on the recruitment of mangroves

Abstract

Summary1 Propagule (diaspore) predation by crabs has been shown to be a major source of mortality for mangroves. We measured predation by crabs on seeds of nine tropical mangrove species in multifactorial experiments by following the fates of tethered propagules. 2 We tested whether planting, intertidal position and canopy gaps influenced predation of propagules and whether the predation of propagules was reduced in the presence of conspecifics. We also tested if predation influenced patterns of propagule establishment. 3 Mortality due to predation ranged from 22 to 100%, with Aegiceras corniculatum > Avicennia marina > Bruguiera parviflora > Aegialitis annulata > B. exaristata > Ceriops australis > C. decandra = B. gymnorrhiza > Rhizophora stylosa . Initial predator preference was correlated with the size of propagules. 4 Propagule planting (prone vs. implanted) and canopy type had the largest magnitude of effects across all species for treatment effects. Propagules dispersed in the prone position had more mortality while those dispersed into canopy gaps were generally less preyed upon. Three species were tested for dominance-predation by regression of stand relative density with final predation by crabs for canopy treatments. No species had significant effects that supported the hypothesis. 5 Predation by crabs often changed with intertidal position but showed no consistent pattern among species or gap treatments. Interactions of canopy treatment and tidal position showed that predation by crabs did not have a major influence on the zonation of mangroves in our study sites. 6 Analyses of covariance of predation and establishment showed that establishment success is strongly controlled by predation in six of the nine species tested. This suggests that herbivores have a greater impact on recruitment than do microhabitat effects on resources. The combined effects of predator refuge and growth preference enhance recruitment in large canopy gaps. Crab predators appear to maintain the floristic similarity between canopy gaps and surrounding forests in tropical mangrove forests of northern Australia by removing allopatric species from gaps.

Propagule size and predispersal damage by insects affect establishment and early growth of mangrove seedlings

“…Factors know to affect these processes include: flooding depth and period (Rabinowitz 1978;Jimenez and Sauter 1991;McGuinness 1997a;Delgado et al 2001), soil characteristics (Jimenez and Sauter 1991;McKee 1995a), desiccation stress (Jimenez and Sauter 1991;McKee 1995a;McGuinness 1997a), and predation by crabs (Smith 1987;Smith et al 1989;McKee 1995b;McGuinness 1997b;Sousa and Mitchell 1999;Delgado et al 2001;Clarke and Kerrigan 2002). Two less well-studied factors that may influence the success of dispersing propagules and young seedlings are propagule size and predispersal damage by insects (Onuf et al 1977;Robertson et al 1990;Minchinton and Dalby-Ball 2001).…”

Local positive feedback and the persistence and recovery of fringe Avicennia marina (Forssk.) vierh. mangroves

“…The high growth rates and the relative insensitivity to changes in survival and establishment of propagules and seedlings seem to indicate that mangroves' strategy of reproduction is adapted to less favourable conditions with higher losses, especially for the young growth stages. Indeed, studies have reported quite a high variability in propagule and seedling survival (Bhat et al 2004;Clarke and Kerrigan 2002;Clarke et al 2001;Hogarth 1999;Kitaya et al 2002). Given the life-history parameters in our model, reforestation should present no problem in most areas.…”

“…The influence of positive feedbacks in mangroves, however, is not dependent on the specific mechanism involved. Given that positive feedbacks have been found to be present in various systems (Scheffer et al 2001) (Clarke and Kerrigan 2002). Studying feedback mechanisms in mangroves could follow studies in other systems.…”

“…For a propagule to establish, it needs to form roots. The period to root initiation for A. marina propagules is 2-4 days (Clarke and Kerrigan 2002). The time to ''settle firmly'' or ''anchoring time'' (T anc ) however is 6-10 days (Shigeyasu 2000).…”

Reproductive ecology of Aegialitis rotundifolia Roxb., A crypto-viviparous mangrove plant species in Krishna mangrove forest, Andra Pradesh

“…But, field studies indicate that only a few of those hypocotyls which have fallen at the parental sites, settled well and showed further growth. Clarke and Kerrigan (2002) reported that the small-sized hypocotyls contain high nutrient content and low fibber content. The crabs prefer to consume such hypocotyls and feed on them prior to establishment.…”