The Pollination Relations of Melastoma

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Abstract

While flowers are often seen as one nature's more beautiful creations, they are also key components of the life cycle of numerous plant species. Flowers are unique to anigiosperms, their primary purpose being reproduction, as they are the reproductive organs of all angiosperms. Melastomataceae is a family of dicotyledonous flowering plants that are widely distributed throughout the Pan-Tropical region of the world. Like all other angiosperms, the plants of this family utilize flowers as a unique reproductive mechanism. Melastoma malabathricum and Clidemia hirta are two species of said family that are widely distributed throughout Southeast Asia. While these plants belong to the same family they vary greatly in morphology, and natural distribution. These species differ in color, size, number of petals, anthers and stamen. And Melastoma malabathricum is a native species to Southeast Asia while Clidemia hirta is an invasive species. Within all flowering species the composition of pollinators depends on various biological and chemical characters of each flowers, and because these two species differ so greatly the species composition differs greatly as well. This study focuses on the differences in species composition and abundance between Melastoma malabathricum and Clidemia hirta located along the roadside of Maliau Basin, located on the island of Borneo. Striking differences in total insect abundance, and species diversity were observed, Melastoma malabathricum having both more individuals and species associated with it's flowers compared to Clidemia hirta. While further study is required, these differences could possibly attributed to the behavioral/preferential differences between species, or the difference in plant adaptation attributed to one species occurring naturally and one species being introduced.

Introduction

Angiosperms are unique in that they produce flowering bodies in order reproduce. To achieve pollination the flowers of angiosperms take on numerous forms to best suit the mechanism by which they will be pollinated, whether it be by wind, water, or animal pollinators. In this study we focused insect and arachnid pollinators. While it is well understood that several morphological and chemical characteristics of flowers attract specific pollinators, there is still much to learn in regards to which pollinators associate with which plants. Melastomataceae is a taxon of dicotyledonous flowering plants widely distributed throughout tropical regions. Plants of this family tend to be brightly colored and seem to be adapted for insect pollination. Melastoma malabathricum and Clidemia hirta are two common species of the family Melastomataceae found along the trails throughout Maliau Basin, located on the island of Borneo, Malaysia. While these plants belong to the same family they differ greatly in flower morphology, varying in color, size, number of petals, number of anthers, and number of stamen. These species also differ in that Melastoma malabathricum is native to the Southeast Asia, while Clidemia hirta is recognized as an aggressive invasive species. In this study we tested whether or not there is a significant difference between the pollinator composition of each plant species. This experiment also served as an analysis of the reproductive success of native species compared to invasive species. And adding to the results of this experiment, the data collected will serve as a preliminary catalog of the pollinator composition of both Melastoma malabathricum and Clidemia hirta to the family level.

Melastoma malabathricum
Clidemia hirta

Methods

Field

In this study 15 specimens of both Melastoma malabathricum and Clidemia hirta were randomly sampled for pollinators between 9:00 – 11:00 hr and 17:00 – 19:00 hr, during a 2 day period (1-2 July 2010). Insects that were directly in contact with the flowers of each species were collected with aspirators, nets and by hand. The specimens collected were then sorted by hand and microscope to quantify the number of individuals of each pollinator species as well as identify each up to the family level in order to compare pollinator composition between the two flowering species.

Analysis

R environment (R version 2.11.0 (2010-04-22) Copyright (C) 2010 The R Foundation for Statistical Computing) was used to analyze the data collected. The specimens were organized by flower type, time collected, family, and morphotype (species). R was then used in order to calculate the species diversity of each plant, the total abundance of pollinators of each plant, the total number of morphotypes associated with each plant, and the total number of families associated with each plant. Using outside sources, research was then conducted on those families that had the highest total abundance in each flower in order to assess the correlation between familial behavior and high abundance, as well as the adaptability of invasive species compared to native species.

Results

62 flowers of Melastoma malabathricum (purple flower) and 59 flowers of Clidemia hirta (white flower) were observed in this study. After sampling and sorting pollinators of both species over a 2 day period, the total abundance of specimens, number of families and number of morphotypes (species) were all found to be greater in Melastoma malabathricum than in Clidemia hirta. 113 individuals, 17 families, and 30 morphotypes were found to associate with Melastoma malabathricum in the morning and 104 individuals, 13 families, and 21 morphotypes found in the evening, while 31 individuals, 8 families, and 13 morphotypes found to associate with Clidemia hirta in the morning and the 7 individuals, 2 families, and 3 morphotypes found in the evening (Figure 1). The total species diversity of both species of plants was found to be highest during the morning, Melastoma malabathricum with a diversity value of 2.72 and Clidemia hirta with a value of 2.32. The diversity of pollinators seemed to decrease significantly in the evening, Melastoma malabathricum with a value of 2.22 and Clidemia hirta with a value of 1.00.

Clear clusters formed by species

Discussion

The results of our experiment show that Melastoma malabathricum not only attract a higher abundance of insects to their flowers compared to Clidemia hirta, but also attract a greater variety of species to their flowers, and therefore have a greater familial and species diversity than Clidemia hirta. The families that had the highest abundance in both species of plant were (in order from highest to lowest) Formicidae (ants), Curculiondae (weevils), and Drosophilidae (fruit flies). While ants are rather poor pollinators, they tend to be anthophilous (attracted to flowers). Myrecophily (ant pollination) is rare because ants are poorly adapted for pollination. The majority of ants are flightless, small, and smooth bodied, thus they are unlikely to facilitate cross-pollination. Flightlessness confines ants to a single plant, small size reduces the chance of contact with both anthers and stigmas, and pollen does not easily adhere to their smooth exterior (Gullan, Cranston 2010). The fact that Formicidae were found in the highest abundance, with 73 total individuals in Melastoma malabathricum, and 20 total in Clidemia hirta, could be attributed to ants' attraction to nectaries. Ants have been regarded as essential consumers of extrafloral nectar. Many flowers seem to produce nectar in order to gain protection against herbivores from ants. When given the option, ants tend to associate with plants that produce extra floral nectar, and while their presence does not aid the plant in reproduction, it does provide essential protection against predators (Heil 2004). Fruit flies more simply utilize fermenting materials, including decaying flowers and flower parts to both feed and breed (Carson 2001). In rain forest particularly, plants are almost always visited by beetles that feed on nectar and pollen (Momose, 2006). Cuculionid beetles, are thought to be the primary pollinators of and are completely host specific to Encephalartos firiderici-guilielmi (Downie & Williams, 2009). Cyclanthura flower weevils have also recently been identified as pollinators of multiple species of Anthurium (Araceae) in Costa Rica. These weevils are present on the inflorescences of these flowers in small numbers and consume plant tissues as well as pollen. These weevils also utilize these flowers for reproduction , utilizing them as sites to oviposit (Franz, 2007). While these weevils tend to associate with flowers of a different family than Melastomataceae, their behavior may apply to said family. While both Melastoma malabathricum and Clidemia hirta belong to the family Melastomataceae, Clidemia hirta is universally recognize as an aggressive and invasive species in Hawaii, Samoa, Fiji, Mauritius, Seyechelles, Southeast Asia (Borneo, Sigapore, and Sri Lanka), and Tanzania (DeWalt 2003) . The species is native to the lowland forests of Central and South America, but has recently spread globally. Naturally Clidemia hirta is found in disturbed areas, such as riversides and roadsides. In this study this species was found to be widely distributed along the roadsides of Maliau Basin. And while the distribution of this species along the roads could be attributed to it's natural adaptation to this niche, it's total abundance is less understood.

Most species in the family Melastomataceae have poricidal anthers, meaning pollen can only be dispersed through a vector. When compared to Melastoma malabathricum, Clidemia hirta is visited by significantly fewer pollinators, but surprisingly this invasive species is just as, if not more abundant than the native. While our study looked at the composition and abundance of insect pollinators of both species, it did not take into account other dispersal methods. Clidemia hirta produces fruits that seem to be bird dispersed, and has been know to reproduce through apomixis (DeWalt 2003). Therefore while the lack of insect species diversity and abundance in Clidemia hirta suggests that this invasive species is poorly adapted for insect pollination compared to the native species Melastoma malabathricum, the abundance of this invasive species may be attributed to it's alternative mechanisms for reproduction.

References

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