Variety of Nighttime Insect Communities in Two Mulu Forest Habitats

Gunung Mulu National Park is home to a variety of forest inhabitants that are crucial to the environment. They are made up of many invertebrate and vertebrate species. Insects are the most diverse animal group in the ecosystem, making insect research essential to understanding ecological variety and evolutionary processes. Because they serve as decomposers of organic matter, prey for vertebrate species, pollinators for plants, and lastly as a source of food for other insects and seeds, invertebrate species, like insects, play a crucial part in ecology. The main determinants of the presence and abundance of insects are the canopy cover and the amount of light. Since insects are very much diverse and abundant in the ecosystem, several studies have been widely conducted to examine their respond to changes in forest habitants. However, more research needs to be conducted to determine the impact of insect prosperity and abundance in cases where there are disturbances in tropical rainforest. Occurrence natural disturbance in ecosystem makes it difficult to interpret as well predict the diversity of insects. According to Hamer (2001), there a quite a number of natural disturbances that occur in ecological systems such as river ridges, natural disasters which result into changes in the canopy, falling of trees, though most species have developed resistance mechanisms to such natural changes in environment and disturbances. However, there is an emerging concern of the negatives effects human activities, such as selective logging of trees in tropical rainforest, render on the biodiverse of insects (Willott, Liam, Compton, & Sutton 2000). Notably, human activities result into changes in specific habitat characteristics and dynamics thereby influencing the population of insects depending on the type, degree and magnitude of the disturbance (Perry& Herms 2017). Thus, in overall, population of nocturnal insects and abundance of nocturnal insects is different depending on the impacts of natural and human disturbances. Different pattern of their distribution is also attributed to differences in characteristics such as body size and shape, capability to fly and accessibility of food (Karim& Abang, 2002)

Aims

The focus of the present study is on the influence of anthropogenic (human) disturbances on the abundance and diversity of nocturnal insects by comparing insect richness and diversity between undisturbed and disturbed sites in a tropical rainforest. Thus, the aim of the study was to observe whether human disturbances had a significant effect on the total number of nocturnal insects, the total number of orders and the total number of morphospecies. Also the study was aimed at determining whether vegetation (leaf litter, understory and canopy cover) were significantly different between the undisturbed and disturbed sites observed.

Hypotheses

Hypothesizing that undisturbed sites would have a significantly higher number of insects, orders and morphospecies compared to the disturbed sites and that undisturbed sites would have a significantly greater degree of vegetation (leaf litter, understory and canopy cover

Methods

Study Sites

This research was carried out in two different habitats in Gunung Mulu National Park, Sarawak, Malaysian Borneo. (Fig. 1). The park was founded in 1974 and covers an area of approximately 528 sq. km. The forest cover of the park is about 90% and is largely a rainforest. The dominant plant species include Nepenthes pitcher plants and strangler figs. Three sites in areas away from anthropogenic (human) activities or disturbance were chosen( fig. a) as well other equal number of sites in an area that is greatly disturbed by human activities, that is, at the centre of residential premises (fig. b). The site selection was based on the availability of several nocturnal insects in which moth species are the most abundant species.





Figure a. Aerial map of undisturbed sites at Gunung Mulu National Park, with sampling sites and distance between marked. Stars indicate the three undisturbed sites (U1, U2 and U3). Boardwalk (blue) and path (orange). Figure b. Aerial map of disturbed sites at Gunung Mulu National Park, with sampling sites and distance between marked. Stars indicate the three disturbed sites (D1, D2 and D3). Boardwalk marked in blue.

Sampling Methods

Insects were captured with the help of six light traps whereby a single trap was placed at each sample site. The light traps used are capable of attracting nocturnal insects. They are made of a bucket (~20cm in diameter), a small contain in the bottom of the bucket half filled with ethanol, plastic sheets made into funnels to direct insects towards the ethanol and a UV light attached to the outside of the bucket with electrical tape directing light towards the container at the bottom of the bucket(fig. 2). Sample were collected on 25th of September 2017 where by traps were laid throughout the night and stopped in case of heavy downpour. The containers were marked with specific site numbers (U1, U2, U3, D1, D2 or D3) in order to identify them and left in operation several hours of night. The security measures of the traps were installed to uphold the integrity of the experiment. The following morning, all six traps and samples were collected and taken to research centre for sorting. The sites were noted to facilitate further surveys.



Figure 2. Labelled diagram and photograph of the light traps constructed for sample collection. One trap was placed at each site (a total of six).

Site Surveys

The distance between each site to the closet path as well as the distance between the sampling sites was measured and noted down. Haphazard sampling method was used to survey each of the six sites. This was conducted through an object from the sampling and tracing a 5m quadrant from where it landed. An observational survey was done on each quadrant to identify canopy cover (fig. 3), leaf litter and understory (Fig. 4)



Figure 3. Photographs taken from each of the six sample sites, taken from the sites looking directly upwards. Displays the canopy cover at each site

Figure 4. Photographs taken from each of the six sample sites, taken standing in the 5m haphazard quadrat facing where the trap was positioned.

































Insects Sorting

The sorting of insects was done in the research centre at Mulu National Park using microscopes where by each replicate was sorted separate times. They were at first grouped into orders and later classified into morphospecies. In the process of classifying into morphospecies, a reference collection was made and used throughout the classification of samples from the six sample sites. Each order was given a letter to identify and a number used to represent each of the individual morphospecies.

Statistical Analysis

Pearson’s Chi-squared test was conducted for the relationship between site and the number of orders per site. Shannon-Weiner Diversity Index was used to determine the average diversity and evenness in all sites. Histograms and mosaic plots were used for visual representation of data.

Results

Throughout the study a total of 374 species and 5790 individuals were identified. The highest number of species was detected in the undisturbed forest Habitat than the disturbed Habitats. The number of individuals found in each forest habitat did correlate with the number of species. The highest number of registered individuals was in the forest undisturbed Habitat, followed by the disturbed forest Habitat. Results from the study generally depict that more nocturnal insects were found in the undisturbed sites as compared to disturbed sites.

Table 1: Ecological populations of nocturnal insect communities in the different forest Habitats in Mulu





Undisturbed forest Habitat

disturbed Forest Habitat

No. of species

270

104

No. of Individuals

3288

2562

Shannon Index

5.765

4.235

Simpson Index

0.9897

0.9003

Pielou's Equitableness Index

0.7

0.7008

Data Analysis

Ecological parameters of nocturnal insect’s communities were examined in each of the sampling sites. The parameters obtained were calculated by Past software to show the distribution patterns in different ecological systems (Hammer, 2001). In some species, identification was not possible by macro-morphological features. The total numbers of specimens in these genera were used for the data analysis. The measure of diversity was determined by the Shannon and Simpson indices. These indices are composed of species richness and evenness components (Jost, 2010), which were also calculated (J=H’/lnS- where S is species richness). The equitableness was established using the Pielou formula (1969). The level of abundance in each sampling site was displayed using a rank-abundance plot. To compare Shannon’s diversity values, a t-test was carried out (p=0.01) (Peter, 1974). Diversity profiles are a graphical display of a collection of diversity indices; values are calculated from the frequencies of each component species and an alpha scale parameter, which ranks from zero to infinity. A collection of higher diversity has a profile that is above the profiles of other collections (Colwell, R.., Mao, C., & Chang, J.,( 2004).). The calculated diversity indices (Shannon and Simpson) do not show outrageous results. The Shannon diversity index value was highest in the undisturbed Forest Habitat, while the Simpson diversity index produced different results (Table 1). These variations in data can be explained by the dissimilar sensitivity of the diversity formulas to dominant and rare species, and the equitableness. The Shannon diversity formula calculates using the degree of evenness of species abundances, while the Simpson index is heavily weighted towards the most abundant species in the sample (Peter, 1974).

Discussion

This study is intended to be part of a project with the goal of establishing a long-term intercontinental collaboration based on a sampling procedure using standardized repeated measures at permanent sites to document nocturnal insect species richness and abundance through time and across the landscape. In reference various researches on fauna conducted at Mulu National Park, several comparatives studies have been published depicting that nocturnal insects are found in abundance in undisturbed forest habitats. The experiment was conducted overnight because studies have shown that on average, nocturnal insects are most active at night compared to daytime. As a result, more insects were able to be captured thereby facilitating the success of the study. Light traps used in the study, have the capability of attracting and capturing insects as a result of presence of both light and ethanol. Insect’s populations in ecological systems are closely related to environmental factors such as light, humidity, temperature and moistures among others. Light intensity is direct proportional to growth of vegetation and thus control insect’s populations. The higher the light intensity, the denser the vegetation. And this definitely will lead to high population of the insects.

The hypothesis that they would be a higher diversity of nocturnal insects in the undisturbed habitats sites is very much true. The lower number of species was expected in residential premises because of extensive human activities that are detrimental to nocturnal insects. Such activities are most importantly agricultural that not only kill the insects and their respective developmental stages but also destroy their breeding sites. A similar study was performed by Summerville & Crist (2003) in which they found a significant relationship between community composition of moth, which is an example of nocturnal insect, and forest structure, especially the floristic composition. Forest management plays a very vital in maintenance of favorable forest structure and nocturnal insect communities. Maintenance of forest structure mainly depends on the logging method used. . Unlogged or selectively cut forest stands are more favorable for forest moth assemblages (Summerville & Crist, 2003). Logging determines the vegetation beneath the forest canopy, which is a causal factor for the nocturnal insect community structure in forested ecosystems (Usher & Keiller 1998, Ober & Hayes, 2009). The final results of this study also emphasize the key role played by the number of plant species and the vegetation structure. To verify the role of the mixture rate and diversity of vegetation on species requires further investigation. Despite the lower diversity found in the disturbed forest habitat, the forests play an important role in the populations of nocturnal insects communities of the Mulu National Park. This is due to the fact that forests are important for the proper climate and species composition and the high production of biomass. The two study sites (disturbed and undisturbed) provided an excellent comparison of the influences of climate on nocturnal insects abundance/species richness. Both sites show a parallel pattern in temperature trends throughout the year. While the peak insect abundance/species richness corresponds to peak temperatures (summer months) at The Northern undisturbed habitat, the same is not shown for peak moth abundance/species richness in the south disturbed habitats, corresponding to relatively cooler early summer conditions (Magurran, 1988).

The temperature alone does not seem to provide a steady explanation regarding the pattern in nocturnal insect’s abundance as well as species richness at the two different sites of study. The observations that the high or low range in abundance and species richness occurred at the similar elevation and forest types suggested that the diversity pattern is resulted from similar ecological driving force across the globe. An understanding of spatial patterns in nocturnal insect’s abundance/species richness also creates a basis for long-term analysis on the geographic width of each species. Documentation of the local pattern distribution of each species is pertinent to assessing the gauge of geographical range in each species in the context of the current trapping transects and proposing which species may be at risk given rapid environmental changes (Wilson, 1988).

Conclusion and Recommendations

Nocturnal insects captured for study were only those that can fly hence rendering the study ineffective as not all nocturnal insects are capable of flying. Notably, the sample collection was only conducted in one night thus providing results that are not sufficient enough for scientific analysis. Moreover, the study shows a small distance between the disturbed and undisturbed sampling sites and thus very minimal differences are expected to be displayed between the insect’s species. Finally, it is recommendable to conduct more sampling in different seasons and increase the number of buckets so as to come up with more replicates for a commendable scientific research on the diversity of nocturnal insects in different ecological habitats.

References

Colwell, R., Mao, C., & Chang, J. (2004). Interpolating, extrapolating, and comparing incidence‐based species accumulation curves. Ecology, 85(10), 2717-2727.

Hammer, R., Harper, D., & Ryan, P. (2001). PAST – Paleontological statistics software package for education and data analysis.

Jost, L. (2010). The relation between evenness and diversity. Diversity, 2, 207−232.

Karim, A., & Abang, F. (2002). The Larger Moths (Lepidoptera: Heterocera) of the Croker Range National Park, Sabah: A Preliminary Checklist.

Magurran, A. (1988). Ecological diversity and its measurement. Biodiversity, National Academy Press, Washington.

Peet, K. (1974). The measurements of species diversity. Ann. Rev. Eeal. System, 5, p. 285-307.

Perry, J., & Herms, C. (2017). First record of the genus Pygiopachymerus Pic (Coleoptera: Chrysomelidae: Bruchinae) for Mexico. Retrieved from http://www.insectbiodiversity.org/index.php/jib/article

Peter, H., & Karlien, S. (1974). Introduction to the Study of Meiofauna. Smithsonian Institution Press, Washington D.C., London Publication.

Pielou, C. (1969). An introduction to mathematical ecology. New York: Wiley.

Summerville, K., & Crist, T. (2003). Spatial variation in insect community and species responses to habitat loss and plant community composition. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/16222547

Willott, S., Liam, D., Compton, S., & Sutton. S. (2000). Effects of selective logging on the butterflies of a Bornean Rainforest. Conservation Biology 14(4), 1055-1065.

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