Effects of seed predators and light level on the distribution of Avicennia marina (Forsk.) Vierh. in tropical, tidal forests

Avicennia marina has an unusual distributional pattern across the intertidal region in mangrove forests of north-eastern Australia. It is often abundant in the low intertidal and high intertidal but is very rare in mid-intertidal areas. The influences of shade intolerance and seed predators on the distribution of Avicennia were investigated. Field observations of 391 light gaps indicated that while seedlings were equally abundant in gaps versus under the neighboring forest canopy, saplings were much more abundant in gaps. Subsequent field studies showed that seedlings survived and grew best in light gaps as opposed to under the forest canopy. Further field experiments revealed that 96·0±3·4% of the post-dispersal propagules of Avicennia are consumed by seed predators, primarily grapsid crabs. Predation on propagules was less in low and high intertidal regions where conspecific adults were common and was highest in the mid intertidal where Avicennia is rarest. Predator exclusion experiments indicated that Avicennia was capable of growing in mid-intertidal areas. It is concluded that the combination of shade intolerance and extensive predation on its' propagules effectively limits the distribution of Avicennia marina across the intertidal and may account for the dominance by members of the Rhizophoraceae in the mangrove forests of northern Australia.