Ecology

This particular species of coral is found mainly in the inner reef.  General observations show that colonies prefer to grow in the absence of other species, although it can be found in the company of others on occasion.  When found with other species, the neighbour tended to be either algae or another coral, but never anything like a sea cucumber or other animal not attached to the substrate.  It can still be found in the much denser outer reef, although in much lower numbers.  This probably is due to the solitary nature of the colony, preferring to be without other organisms around. 

Abstract

The coral Poritesdensa is a fairly sparse coral found on the Great Barrier Reef.  Its massive growth form should lead it togrow in all wave energy environments; and this is generally reflected in theresults. There was one area of the reef where no coral growth of that specieswas found, although the factors influencing this are currently notunderstood. 

Introduction

There are many factors, both abiotic and biotic, that affect dispersal of a larval organism.  For example, it is known that chemical cues in the water interact with the larvae and can induce metamorphosis if the signature is right (Ruppert et al, 2004).  Adding to this signal is the environmental status of the water – the larvae are equipped with many sensory cells to detect such things and make a decision based on the responses from these cells (Mayorova et al, 2012).  Water quality can be affected by higher energy environments and this again would impact on the survival of a coral –the more turbid the water, the less sunlight can get through (Stanley et al, 2012).  Biotic factors such as presence of superior competitors (in this instance the resource would be space availability) (Stanely et al, 2012).  The importance of understanding factors affecting abundance and distribution will help in the conservation of this species, as well as helping to further understand the ecology of the area in which it inhabits.  

Questions

1.      Is there variation in abundance distribution for Porites densa between the inner reef flat and the outer reef flat surrounding Heron Island?

2.      Is there variation in abundance distribution for Porites densa between the different wave energies experienced on the reefs surrounding Heron Island?

Method

At low tide the inner reef and outer reef were surveyed at two separate sites for each side of the island. For example, the south side was characterised as being quite low energy and the north was characterised as the high energy area of the island.  Ten timed searches lasting two minutes each were conducted within the inner reef and the outer reef of each site.  Number of colonies, and also approximate size of colonies were measured if found during this time.  Each search was conducted an appropriate distance away from the last (randomly selected amount of steps, with a minimum of 20 steps between each search). Searches were also conducted in a spiral pattern circling out for the duration of the two minutes.  This also ensures all colonies are counted, and the same search pattern is employed to ensure standardisation.  Organisms surrounding the colonies were also identified and recorded to genus level, as an added level to the data collection.  A tube with a flat glass bottom was used to view the colonies under water and a standard 30cm ruler was used to measure the colonies. 

Results

There are no significant differences between any of the different areas, except for the outer reef on the north side, where absolutely no corals were found.  There were significant differences in coral abundances between the inner reef and outer reef on the north side (P=0.03).  The outer reefs on the north and south side were also comparably different(p=0.01).  The inner reefs on the north and south side has similar abundance distributions (p=0.15), as did the inner and outer reefs on the south side of the island (p=0.18). 

Figure 4: graph showing average biomass of coral for eacharea and standard deviation bars. 

Discussion

The lack of significance between many of the sites suggests that either these sites are very environmentally similar, or the P.densa is a very adaptable organism that is suited to a wide range of environmental stimuli.  The total lack of any colonies in the outer reef on the northern side of the island suggests there is an exclusion happening in this area, although it is currently unknown whether this is due to competition, resource availability or other environmental stimuli.  Further studies could explore why this is.  The lack of significance could also be due to experimental error or too small a sample size.  It was observed in the field that these organisms prefer to grow away from other coral species, surrounded by either sand or algae.  There was no data collected on this so no significant inferences can be made, however.  This potential preference could also explain the absence of this species on the north side outer reef.