Effects of habitat modification in mangroves on the structure of mollusc and crab assemblages

Abstract

The abundance and species richness of mollusc and crab assemblages were examined in a subtropical mangrove forest in Moreton Bay, Queensland, Australia, which has been disturbed and damaged by the construction of a wooden boardwalk and a path. Sections of the forest immediately adjacent to the boardwalk and path were compared with reference areas to determine whether changes to the small-scale structural complexity within the forest affected the benthic fauna. The disturbed area was characterised by having 65-80% fewer pneumatophores, significantly fewer species and individuals of molluscs, but significantly more species and individuals of crabs than the reference areas.The abundance of mangrove pneumatophores and the attached epiphytic algae were manipulated at two sites to determine whether observed differences in these features could account for the differences in the assemblage of molluscs in the disturbed area of the forest compared with reference areas. Five experimental treatments were used: undisturbed controls, pneumatophore removals (abundance reduced by ca. 65%), epiphytic algal removals (algae removed from ca. 65% of pneumatophores), pneumatophore disturbance controls and algal disturbance controls. The experimental reduction of the abundance of mangrove pneumatophores and the associated epiphytic algae led to significant declines (by as much as 83%) in the number of molluscs utilising the substratum in the modified plots. There was no significant difference in the abundance of molluscs in the pneumatophore and algal removal plots suggesting any effect was primarily related to removal of the epiphytic algae from the surface of the pneumatophores.The responses by the biota to the changes in the physical environment demonstrate that even relatively small-scale modifications to the physical structure of subtropical mangrove forests can lead to significant effects on the diversity and abundance of macrobenthic organisms in these habitats. Such modifications have the potential to cause cascading effects at higher trophic levels with a deterioration in the value of these habitats as nursery and feeding grounds. Future efforts at conservation of these estuarine environments must focus on the prevention or reduction of modifications to the physical structure and integrity of the system, rather than just on the prevention of loss of entire patches of habitat.

Ecosystem-Wide Impacts of Deforestation in Mangroves: The Urabá Gulf (Colombian Caribbean) Case Study

“…Gastropods in the Caribbean and elsewhere have been pointed as key detritivores and sediment grazers, even outweighing the role of crabs [80][81][82]. Available assessments on the impacts of mangrove deforestation on benthos report that vegetation provides support and physical habitat that may reduce predation and desiccation (Asia: [83,84]; Africa: [85,86]; Australia: [87]). For instance, clearing pneumatophores in small-scale deforestations for building walkways and trails was correlated with a decline in density and species richness in the entire community, particularly on gastropods [87].…”

“…Available assessments on the impacts of mangrove deforestation on benthos report that vegetation provides support and physical habitat that may reduce predation and desiccation (Asia: [83,84]; Africa: [85,86]; Australia: [87]). For instance, clearing pneumatophores in small-scale deforestations for building walkways and trails was correlated with a decline in density and species richness in the entire community, particularly on gastropods [87]. In addition, community and population metrics, otherwise uncorrelated with physicochemical variables under natural conditions [84], became significantly explained by temperature and pH in deforested mangroves [86].…”

The significance and vulnerability of Australian saltmarshes: implications for management in a changing climate

“…The most commonly encountered and wide-ranging saltmarsh crab is Helograpsus haswellianus (Grapsidae), being common in Tasmania (Richardson et al 1998), South Australia (Imgraben and Dittman 2008), New South Wales (Mazumder and Saintilan 2003;Morrisey 1995), and southeast Queensland (Skilleter and Warren 2000). H. haswellianus was primarily responsible for larval export from the saltmarsh at Towra Point, NSW (Mazumder et al 2006).…”

“…Three other species of burrowing crab are common in temperate Australia: two Grapsidae (Paragrapsus laevis and Parasesarma erythrodactyla) and one Ocypodidae (Heloecius cordiformis). In southeast Queensland, Australoplax tridentata replaces H. cordiformis as the common Ocypodid in the saltmarsh (Skilleter and Warren 2000).…”

Ecological relations between mangrove leaf litter and the spatial distribution of the gastropod Melampus coffeus in a fringe mangrove forest

“…However, they observed more abundance of species in vegetated microhabitats. In addition, microhabitats also provide many mangrove animals, such as molluscs, crustaceans (Odum & Heald, 1972) and polychaeta (SKilleter & warren, 2000), with food resources.…”