Ecology
Montipora digitata is found in shallow reef environments including areas with extreme tidal conditions or shallow mud flats (AIMS 2011). Most corals prefer to live in areas with limited to no sedimentation, low wave action and low disturbance levels. However, M.digitata can also survive in a strong wave action environment (Harpeni and David 2011).
M. digitata has an obligate endosymbiotic relationship with dinoflagellates called zooxanthallae. The zooxanthallae are transmitted from parents to the eggs, unlike other corals that have to gain their endosymbionts from the environment (Van Oppen et al 2004). The zooxanthellae receives excess CO2 and protection from the coral, while the coral receives N, C, and up to 50% of its energy needs from the zooxanthellae (Ruppert et al 2004). M. digitata also has external associations with other species. Many species of fish and crustaceans use corals as a form of protection from predation or hiding from the sun. Some species use corals as a food source.
The spatial ecology of the abundance and distribution of Montipora Digitata on Heron Island has not been well studied. In the below project we aim to fill this gap in the literature.
PROJECT:
The Abundance and Distribution of Montipora Digitata on Heron Island.
Introduction:
Heron Island is home to many species of corals. Most corals prefer areas of clear water and low levels of sedimentation (Fabricius 2005). However, according to Harpeni and David 2011, Montipora digitata is a hardy coral that can withstand high sedimentation and high concentrations of suspended solids (Harpeni and David 2011). M. digitata is considered native to Australia and is regularly found on Heron Island (IUCN 2012). It is usually found in shallow reef environments and can grow in the forms of digitate (finger like) and arborescent (tree like) (AIMS 2012). M.digitata is usually found abundant in areas of high sedimentation and wave action which is unusual for many corals (Harpeni and David 2011). This is what makes this species so interesting to study over different zones of the reef. M. digitata has not been specifically studied on Heron Island before. Our aim was to determine the abundance and distribution of M. digitata in the reef surrounding the island. In particular, we aimed to compare the presence of this species in the inner reef flat and the outer reef flat. We also compared the abundance of M.digitata on the north and south of the island.
Questions:
Is there variation in the abundance of M. Digitata between the inner reef flat and the outer reef flat?
Is there variation in the abundance of M. Digitata between the northern and southern locations on Heron reef?
Does the abundance of M. Digitata vary depending on the surrounding species in the area?
Methods:
We compared the abundance of Montipora Digitata at four locations on Heron Island including two south side locations and two north side locations as seen in the map below (figure 1). In each of the four locations, we sampled at two different habitats which consisted of the inner reef flat and the outer reef flat.
Figure 1: A map of Heron Island indicating where data was collected. Figure is modified from the University of Queensland Properties and Facilities Division 2012, The University of Queensland, <http://www.pf.uq.edu.au/aerials.html>
At each location and habitat 10 quadrates were randomly placed by using a random number generator to determine the angle of degree to face and the number of steps to take. Abundance was determined by estimating the area of each 1x1m quadrate occupied by M. Digitata. Records were also taken of the size and distribution of other species that occurred near M. Digitata within each quadrate. Statistical analysis was conducted using student t-tests and ANOVA tests on the computer program ‘R’.
Results:
Is there variation in the abundance of M. digitata between the inner reef flat and the outer reef flat?
We found a trend of a higher abundance of M. digitata occurring on the outer reef flat compared to the inner reef flat. However the statistical analysis resulted in a p value of 0.1994. Therefore, this result was not deemed to be statistically significant to the 5 per cent level.
Is there variation in the abundance of M. digitata between the northern and southern locations on Heron Island reef?
M. digitata was found to be more abundant on the south side of Heron Island. This was deemed to be significant to the five per cent level (p = 0.01).
We also found that on the southside of the island there was a higher abundance of M. digitata at the west location compared to the east location (p=0.06). This was not considered significant, however a trend was observed. There was no correlation between the abundance of M.digitata on the north-east and north-west project sites (p=0.26).
Does the abundance of M. digitata vary depending on the surrounding species in the area?
There were no observational data to suggest that M. digitata is affected by the surrounding species in the area. M. digitata was found to have a variety of neighbouring corals on the reef including Anacropora and Porieties. However, no specific species were regularly found to occur next to M. digitata.
Discussion:
A trend was found of M. digitata having a greater abundance in the outer reef flat compared to the inner reef flat. Most journal articles do not differentiate between the inner reef flat and the outer reef flat. Instead they position both under the same category of reef flat which makes it hard for comparison. Interestingly, not all corals on Heron Island were found to show the same trend as M. digitata. For example, Poriets Demsa was observed to have an increased abundance in the inner reef flat (for more information see Poriets demsa in the species list of this website). The higher abundance of M. digitata in the outer reef flat may be due to this species being able to survive in higher energy environments, whereas, Poriets demsa may prefer lower energy environments.
The south side of Heron Island was found to have a much greater abundance of M. digitata compared to the northern side (p = 0.01). The south side of Heron Island has a deep channel which separates it from Wistari Reef. This side also experiences more wave and turbidity action from incoming currents and south-east winds. M. digitata is unusual compared to most other corals because it can become abundant in areas of high wave action and sedimentation (Harpeni and David 2011). Many coral species cannot live in these conditions which may mean that M. digitata has less competition on the southern side of the island. On the northern side of the island, M. digitata may have more competition because more corals can grow in the calmer environment. Harpeni and David 2011 found that M. digitata regularly loses competition to other species such as Porites and Mussidae. This may explain why M. digitata was found to be more dominant on the southern side of Heron Island.
Interestingly, we also found a trend of the western side on the south of the island having a larger abundance of M.digitata than the eastern side. The eastern side of the south was found to have a large proportion of algae dominating the area which may have affected the M. digitata abundance. However, we are unsure if this is the cause in the decline of M. digitata in this area. More studies would be beneficial to understand and determine why this trend occurred.
The possibility of whether the abundance of M. digitata varied with its neighbouring corals was also investigated. We thought it would be interesting to ask this question because some combinations of corals have been found to affect the reef community structure (Bradbury and Young 1983). However, we discovered that M. digitata did not have any common coral neighbours. Specific coral species surrounding M. digitata in the reef flat were found to not have an effect on the abundance of M. digitata. |