Ecological Succession on Rangitoto Island

Successional sequence for flora species and changes in richness and assemblage due to ecological succession on Rangitoto Island, New Zealand

Abstract:
A survey of 12 different areas of various sizes was done in the study of whether or not species richness increased with ecological succession, using the areas of vegetation growth as a benchmark for stages of succession. The results proved inconclusive as to whether or not species richness increased indefinitely as succession progressed by proved that species richness and species density increased from early to mid-successional stages.

Introduction:
Rangitoto Island is geologically a young island, formed approximately 600 to 700 years ago due to volcanic eruptions. It is because of this, the island is still in various states of ecological succession and some areas have not even begun the process and remain broken fields of lava-rocks, which is why the island was chosen as a good idea to study ecological succession. The island is very dry as no streams or fresh water reservoirs are present on the island. Therefore, the series of succession being studied could be classified as a xerosere and a lithosere. (Department of Conservation, 2012)

The study on how ecological succession changes the species richness and assemblage in the region is done to prove the hypothesis that there is a positive correlation between species richness and the progressing stages of ecological succession. Rangitoto Island was chosen due its relative young age in geographical terms providing a good comparison of different stages of succession within the same ecosystem environment. A successional sequence of plants found can be assembled using the compiled data. Figure 1 (Below): Picture shows the barren lava fields and the start of succession by herbs along the foot path and shrubs in sparse patches along with larger areas of both shrubs and trees in the background. Source: http://en.wikipedia.org/wiki/File:Rangitotolavapath.jpg Figure 2 (Above): Satellite picture of Rangitoto Island, the outlined route is the path taken during data collection, with the approximate areas for data collection highlighted as a grey box. Source: Google Earth

Figure 1 (Below): Picture shows the barren lava fields and the start of succession by herbs along the foot path and shrubs in sparse patches along with larger areas of both shrubs and trees in the background. Source: http://en.wikipedia.org/wiki/File:Rangitotolavapath.jpg Figure 2 (Above): Satellite picture of Rangitoto Island, the outlined route is the path taken during data collection, with the approximate areas for data collection highlighted as a grey box. Source: Google Earth

Method – Study Site Description:
The area on the island chosen is similar to the area pictured above in Figure 1 in the approximate area as highlighted in Figure 2. Such an area was chosen as it contains a variety of areas in various stages of succession as observed by noting that there are patches of vegetation of various sizes. This would give the study the scope it requires to test the hypothesis stated earlier. The general type of vegetation found were a combination of lichens, mosses, ferns, grasses (Astelia spp.), trees and shrubs. However, in this study, only ferns, trees and shrubs would be included in the quantitative analysis of data. The substrate of the surveyed areas seems to be composed of organic matter, such as dried leaves, sticks and soil. Qualitative observation shows the areas without vegetation show little organic matter deposits and remain barren rock, with occasional early succession by lichens evident in some areas. The nature of the vegetation patches being surveyed shows that larger patches of vegetation usually contain a larger variety of species but also plants that may belong to latter stages of succession, such as trees and shrubs.

Method Description:
The patches are selected to give a good overview of the overall area, a total of 4 patches of vegetation per team were surveyed, composed of 1 small area (<3.00m diameter), 2 medium areas (3.00 – 5.00 m) and 1 large area (>5.00 m). All observed species are visually inspected and identified via an identification key. Each team then pooled its data together, in which none of the vegetation patches should have been surveyed twice, giving a total of 12 results, 3 small areas, 6 medium areas and 3 large areas. The areas which the vegetation covers are calculated by observing the shape of the area. Circular areas calculated as: A=πr2 or A=π[(⌀/2)2]

The radius or diameter would be found by using a tape measure from as close to the center of the vegetation patch as possible. Other shapes, such as elliptical areas and rectangular areas would similarly have the appropriate measurements such as radius; length and/or width measured and area calculated if they were present. All surveyed sites were reported to be circular in shape. The number of different species and species population per location is also recorded. A brief analysis is made based on the data as is, focusing on whether or not some species are more common in areas of later succession stages. The R2 will be used to test the goodness of fit for the area surveyed against the number of species located.

Results:
It is observed that as the size of vegetation increases, the number of species found and the population of said species also increases. The coefficient of determination, R2 value will help determine if a trend-line can be adopted to predict the number of species found in a particular area. This was calculated using Microsoft Excel, function PEARSON: (R2 = 0.3013, (4 s.f.)). A graph depicting the species richness of the area against the area in m2 is also provided. (Chart 1) This shows that there is a low correlation between the survey area sizes to the number of species found. The p-value will be used to help to see if the data assembled can be used to reject the null hypothesis, with the null hypothesis being that there is no correlation between species richness and survey area size. Student’s t-test was used to find the p-value, the test was done using Microsoft Excel, function “=TTEST (Array 1, Array 2, Tail, Type)”, returning the value 0.005662 (4 s.f.) The p-value is above 0.005, meaning that the null hypothesis between species richness to survey area size cannot be rejected.

It is also observed that some species are only present in larger patches of vegetation. It is found that the species Leptrospermum scoparium was only found in small patches of vegetation. (Table 1) Qualitative observations of the sites also found that small areas usually support lichens and mosses more often that on areas of medium and large size. The species Pesudopanxax arboreus, Trichomanes reniforme, Coprosoma lucida and Hymenophyllum were only found in patches of vegetation of medium size. (Table 2) Collospermum hastaum and Astelia solandri were found in only large patches of vegetation.

Multiple species were located in either all or some patches of vegetation of various size (Metrosideros Spp.*, Astelia banksia, Griselinia lucida) some pioneer species might have been omitted due to misidentification or because the environment near the outer edges of a vegetation patch have not gone through the succession that the centre areas have gone through – and therefore, present in all areas and not counted as a pioneer species in this study. Note that Metrosideros Hybrid on the list contains 2 species, M. robusta and M. excelsa unless separately. These two species are grouped together because of inter-breeding between the two species, so they are technically neither. See Figure 2 below. Also, the information above is represented as is from other groups conducting the same survey, no attempts have been made to confirm or disprove the data in terms of species identification or population numbers.

Chart 1: Species richness in surveyed areas, showing the species richness of sampled areas against the 12 sampled areas. N = 12 A linear line of best fit is also attempted. A proposed non-linear line of best fit is provided as an alternative theory..

As Chart 1 shows, the overall species richness of the survey sites rises as survey area increases. However, if the final three survey sites are discounted, this correlation becomes more apparent. It should be noted that the line of best fit above is on a linear scale but the correlation may not be linear as the species richness of the 3 large survey sites suggest that species richness might decrease as survey area or succession continues (See dotted line, in Chart 1) In Chart 2 (next page), the overall species density is graphed against the area size. If anomalous data (such as data point “38.5, population 16” is ignored, then there is a very clear linear correlation between area size and population. (See dotted line, Chart 2) The in-depth interpretation of the data and the two charts will be explored in the Discussion.

Chart 2: Total population in surveyed areas, showing the overall species density of sampled areas against the 12 sampled areas. N = 12 A linear line of best fit is also attempted. A proposed line of best fit disregarding point 38.5 is provided as an alternative theory..

Table 4: List of species succession according to Table 1 – 3. N=12 Early Succession| Mid Succession| Late Succession|
Kunzea ericodiesGriselinia lucidaCyathodes juniperinaAstelia banksiiAstelia solandriLeptrospermum scopariumKnightia excelsa| Metrosideros spp.Geniostoma rupestreHebe strictaPesudopanxax arboreusTrichomanes reniformeMicrosorum pustulatumCoprosoma lucidaDodonaea viscoseHymenophyllum sp.Myrsine australis| Olearia furfuraceaCollospermum hastaumPteridium esculentum|

Table 4 shows a list of species succession according to their “order of first appearance” in Table 1-3, based on the assumption that a larger survey area is equal to a longer period that succession has taken place. While it is debatable on whether this is entirely accurate, qualitative observations during the survey show that some species such as ferns do not grow well on exposed surfaces and rely on early pioneer species to provide shade. Likewise, some species such as A. banksii have shown to be able to grow in all the surveyed environments but is counted as an early succession species based on its ability to grow on small survey areas.

Discussion:
Figure 2: Photo of a sign for tourists on Rangitoto. Note in 2nd paragraph, “Strange combination of plants occur, such as a northern rata crossed with a pohutukawa.” – An enlarged portion of the sentence is provided below. (Rangitoto Island Historic Conservation Trust, 2012) Figure 2: Photo of a sign for tourists on Rangitoto. Note in 2nd paragraph, “Strange combination of plants occur, such as a northern rata crossed with a pohutukawa.” – An enlarged portion of the sentence is provided below. (Rangitoto Island Historic Conservation Trust, 2012)

In theory, there should be an increase in species richness as the area being surveyed/studied increases. However, this theory was not proven by this study, as shown by the p-value shows that it cannot be said that the null hypothesis can be disproven. The reason why species richness is shown to be highest in medium areas may be due to the fact that in larger areas, some plants have already proven to be the dominant species in a particular patch, preventing competition from other species from becoming established. (Singh, 2012) Another reason maybe that as the area of the vegetation increases, the chances are that one particular larger shrub or tree will dominate the central area of the vegetation patch becomes higher; therefore with one large plant taking up the space, other plants could not be counted as accurately. The R2 value shows that a linear trend-line does not accurately apply for this study. However, if looking at the line of best fit Chart 1 and using the non-linear line of best fit, it can be seen that the general trend that species richness increases with area of vegetation may apply to a certain degree.

This study shows that areas of medium size to have the highest species richness but not in overall species density, this maybe because of the reasons explained above, where there is not dominant species yet and plants are still competition for space. However, in terms of overall flora, large areas provide a larger space for vegetation to grow and therefore a linear trend can be found correlating the flora population to that of the area size. The lack of clear early and late succession types maybe due to the fact that plant succession may take place in much smaller patches, which were not counted as in this survey as they were below 1m in diameter. Another reason is that plant succession is not limited to the surveyed list of plants as provided by the identification guide but also includes lichens and moss, which were not part of this study. (Biology Online,Life Science Reference, 24) Therefore, “small patches” in this study may already be in the process of succession that the study would place into the “medium area patches” category. Also, pioneer species will grow on the fringes of already established areas of vegetation, giving the false impression that they are present in all areas and discounted as an early succession species. (Julian, 1992)

The sample size of this survey was inadequate and no effort to factor in variables, such as seasonal shifts, rain/weather before the survey, physical locations of the surveyed vegetation in addition to systematic and random error will play a role in making this study inaccurate.

Conclusion:
In conclusion, the study as a whole is inconclusive as to whether or not there is definite positive correlation between species richness and the progressing stages of ecological succession. Further and repeated research encompassing a much wider scope of surveyed area, species accounting for environmental factors in addition to including the species density in addition to the species richness is required to give prove or disprove this hypothesis.

Bibliography

Biology Online,Life Science Reference. (24, June 2009). Primary succession. Retrieved from Life Science Reference – Biology Online: http://www.biology-online.org/dictionary/Primary_succession

Department of Conservation. (2012). Features of Rangitoto Island Scenic Reserve. Retrieved October 2012, from Department of Conservation:: http://www.doc.govt.nz/parks-and-recreation/places-to-visit/auckland/hauraki-gulf-islands/rangitoto-island-scenic-reserve/features/

Julian, A. (1992). The vegetation pattern of Rangitoto. Auckland: University of Auckland. Rangitoto Island Historic Conservation Trust. (2012). Rangitoto Island Historic Conservation Trust. Retrieved October 2012, from http://www.rangitoto.org/

Singh, B. (2012). Successional sequence of plants on Rangitoto Island. Retrieved October 2012, from www.scribd.com: http://www.scribd.com/doc/75173586/doc