Tropical Rainforest and Tropical Grassland1

Felicitas Asuncion C. ElagoGroup 1 Sec. U-1L

November 17, 2014

1 A scientific paper submitted in partial fulfillment of the requirements in BIO 150 (Principles of Ecology) laboratory under Ms. Rodelina Deyto, 1st semester, 2014-2015.

INTRODUCTION

Community Ecology is the sphere of Ecology that focuses on the dynamics and interactions of the populations of different species in a specific habitat. It is also the field involving the classification of communities considering factors such as the climate and species composition to which several approaches have been developed. Basic categories of communities which are defined by the species' lifestyles and physical environments depending on the governing climate are also known as Biomes (Morin, 2011). These can be categorized as Terrestrial Biomes or as Aquatic Biomes.

The tropical rainforest and the tropical grassland are examples of terrestrial biomes that predominate in the Philippines and several other countries or continents located along the equator, namely central and south America, Africa, southeast Asia, and islands in the carribean sea and pacific ocean. Tropical rainforests, the ecosystem that is said to be the most diverse in species, are characterized by the warm temperature and the frequent rainfall that it receives nearly everyday (in excess of 200 centimeters /80 inches of rain per year) which cause the rapid growth of the inhabiting plants like trees that grow tall and large providing canopy yet also result to the poor nutrition of the soil making it unfit for farming. On the other hand, the tropical grasslands, also known as Savannas, have grasses like Imperata cylindica (“cogon”) as the dominant vegetation and are occasionally with trees that are also drought- and fire-resistant. It receives an uneven distribution of 50 to 150 centimeters (20 to 60 inches) of rainfall per annum causing the ecosystem to be very seasonally structured.

An ecosystem can change as its species composition is replaced. This process is known as Succession which has two types – the Primary and Secondary succession. The former is when succession naturally happens from scratch while the latter is when succession happens through the destruction of the old ecosystem and is first occupied by the Pioneer species. Succession is also composed successional stages which will eventually lead to a stable stage called the climax community.

In tropical countries with high humidity like the Philippines, succession is affected most by the moisture and the temperature associated with the altitude. The system gains more structure during the wet season that it loses during the dry season which then creates a process that continues until it reaches maturity. Due to the higher mean sea-level temperatures, the tropics also have richer vegetation but have slower vegetation recovery due to the constant extreme weather distrurbances. The process of succession in the tropics involves a series of steps that can last 30 to 50 years for secondary succession. First step of which is when a disturbance affects a significant area and causes the invasion of short-lived forbs and vines. This is then followed by heliophytic shrubs and longer loving herbs, then by rapidly growing pioneer trees, and lastly by more shade-tolerant trees (Ewel, 1980).

A primary succession community is the result when surface or total vegetation is completely destroyed by some environmental disasters leaving no reproductive structures of previous population which is why it is much slower (1000 years) as compared to secondary succession. For instance, the forest in Mt. Makiling studied is an example of a secondary growth forest, given that this forest was never completely erased, only degenerated because of massive logging activities, fuelwood gathering and charcoal making, shifting cultivation and permanent agriculture especially during the hispanic era which means that reproductive structures of old population may have been found and that it has developed quicker than a primary growth forest would have. As mentioned by Lasco, et.al in 2001,

“Before 1992, almost all logging activities were conducted in primary forests and resulted in their transformation to post-extraction secondary forests”.

This study, conducted in November 5, 2014 at Mt. Makiling, University of the Philippines Los Banos, Laguna, aims to observe and compare the tropical rainforest and grassland biomes.The specific objectives were:

1. ) To infer stages of ecological succession after the study of grassland and forest communities;

2.) To describe the structures of tropical forest and tropical grassland communities through measures of species diversity and dominance indices.

RESULTS AND DISCUSSIONS

In order to compare the communities of the forest and grassland study sites, their respective species diversity was observed through comparing the Shannon and Simpson's indeces. The formulas for the Shannon indeces of Diversity (H') and Evenness (J) are as follows, followed by the Simpson indeces of Dominance (D), Diversity (SID), and Equitability (E):

SID = 1-D E = SID/S

where p, is the proportion of individuals belonging to species I (# or length of a species/ total # or length of all species present) and S is the species richness.“Species richness is defined as the number of species in a sample unit or other specified area” (McCune and Grace, 2002). This was determined by simply counting all the present species for both sites. The list of the species discovered for the tropical rainforest and the tropical grassland, along with the variables needed to solve for each index of species diversity, is presented in tables 4C.2 and 4C.3, respectively. The individual values of the indeces of species diversity for both ecosystems is then shown in table 4C.4 for comparison.

Based on the computed values of the Shannon and Simpson's indeces of diversity as well as the greater species richness, the tropical rainforest ecosystem has the higher species diversity with H' equal to 2. 725 and SID equal to 0.9473 as opposed to the tropical grassland's H' which is only equal to 1. 1789 and SID of 0.5903. Two possible reasons behind this are the of constant higher temperature and abundant moisture which makes the tropical rainforest a more suitable environment for many plants and animals including bacteria and other microorganisms that provide nutrients to the soil. Tropical rainforests contain the greatest biodiversity in the world with over 15 million species of plants and animals in the biome (Tropical Rainforest, 2005).

In terms of Evenness, both the values of the Shannon and Simpson's indeces show that the tropical grassland has a more even abundance of species. According to Zhang et.al in 2012, evenness affects the relative strength of interspecific and intraspecific interactions within communities, therefore causing a shift of diversity and productivity relationships both in magnitudes and form. Greater

evenness also likely increases functional trait diversity, when calculated using abundance-weighted values.

Lastly, the Simpson index of Dominance with which the tropical grassland proves to have a higher value with D equal to 0.4097 indicates that the grassland community has the higher degree to which a certain species is more numerous. In this case, the dominant species, as the biome's name suggests, are grasses like the Paspatum conjugatum or Buffalo grass and Bracharia distachya observed in the sample transect which dominate because they are able to grow in this environment with long periods of drought. Common plants in grasslands also have long tap roots that can reach the deep water table, thick bark to resist annual fires, trunks that can store water, as well as leaves that fall off during the winter to save water. What is rare in this biome are therefore plants that require much hydration like forest trees and shrubs. In fact, no trees were observed in the grassland study site.

On the other hand, the tropical forest has a very low dominance value of 0.0257 as evidenced by the fact that it is not dominated by any of the inhabiting tree species in particular given that its species composition is greatly diverse. Trees of the same species are rarely found near each other. This biodiversity and separation prevents contamination and die-off from disease or insect infestations.

REFERENCES

Banerjee, P. (2013, July 9). 7 major Differences between Primary & Secondary Succession | DNA2LIFE. Retrieved November 15, 2014, from http://www.dna2life.com/environment/7-major-differences-between-primary-secondary-succession

Enger, E. D., & Smith, B. F. (2002). Kinds of Ecosystems and Communities. In Environmental science: A study of interrelationships (8th ed., pp. 109-136). Boston: McGraw-Hill Higher Education.

Ewel, J. (1980). Tropical Succession: Manifold Routes to Maturity. Biotropics, 12(2), 2-7.

Lasco, R. D., Visco, R. G., & Pulhin, J. M. (2001). Secondary Forests In The Philippines: Formation And Transformation In The 20th Century. Journal of Tropical Forest Science, 13(4), 652–670.

McCune, B., Grace, J. B., & Urban, D. L. (2002). Species Diversity. In Analysis of ecological communities (pp. 25-34). Gleneden Beach, OR: MjM Software Design.

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Wheeling Jesuit University. (2005, April 28). Earth Floor: Biomes. Retrieved from http://www.cotf.edu/ete/modules/msese/earthsysflr/savannah.html

Zhang, Y., Chen, H. Y., & Reich, P. B. (2012). Forest productivity increases with evenness, species richness and trait variation: a global meta-analysis. Journal of Ecology, 100(3), 742–749. doi: 10.1111/j.1365-2745.2011.01944.x