THE EFFECT OF NUTRIENT ENRICHMENT ON HERBIVORY IN THE MANGROVE AVICENNIA MARINA  OVER FOUR SITES WITHIN AUSTRALIA.

Abstract

Mangrove forest ecosystems are under major threat from urban development and increasing eutrophication due to run-off of fertilizers used in agriculture.  Little is known of the effects of excessive eutrophication at a whole system level.  Not much is known of how plant productivity and herbivory is affected with increased eutrofication within mangrove forest systems.  Many studies have shown that the leaf area removal by herbivores is an important ecological factor in determining plant productivity.  In this study I wished to understand the effect of nutrient enrichment on the amount of leaf area removed by herbivores in mangrove forests.

At four sites that spanned 22° in latitude, individual trees of the mangrove Avicennia marina (Forsk.) Vierh. sensu lato (Tomlinson 1986) were fertilized with nitrogen (400 g urea per tree), phosphorus (400 g triple phosphate) or unfertilized (left as controls). Sites were located at Batemans Bay, NSW (27 trees), Exmouth, WA (21 trees), Cape Cleveland, QLD (18 trees), and Port Douglas, QLD (24 trees).  Digital images were taken of all leaves from representative twigs, and both total leaf area and proportion leaf area lost to herbivory were measured under the different nutrient treatments.

The proportion of canopy leaf area removed by herbivores in A. marina was high (10-25%), suggesting that herbivores may have a significant negative impact on the productivity of mangrove forests. I found no effect of nutrient enrichment on the proportion of leaf area removed by herbivores at all sites. However, addition of nitrogen fertilizer increased growth, and thus the total amount of leaf area removed by herbivores was increased with fertilization with nitrogen, Over sites there was an increase in the proportion of leaf area removed by herbivores with increasing latitude. Sites with higher average annual humidity had higher levels of herbivory than sites with lower humidity.