The Denser Canopy of Mangrove Drives the Structure of Insect Communities

Mangrove restoration in Trenggalek, East Java has resulted an age variation of mangrove ecosystem. Diverse species of insects predominantly found in mangroves were collected using yellow pan traps, swipe nets and by direct picking from three different sites. This research was conducted from April until August 2015. There are 9,181 individual insects associated with mangroves comprised of 42 species from 31 families and eight orders. The first site or the 15 years old mangrove (66.22% canopy cover) indicated the highest Shannon diversity index at 2.54, Evenness index of 0.32 and Margalef richness index of 4.84. The lowest diversity was recorded in the third site or the five years old mangrove (19.65% canopy cover), with the Shannon diversity index at 2.28, Evenness index at 0.26 and Margalef richness index at 4.59. The most abundant species located was the Eristena mangalis, with 1,724 individuals (relative abundance of 18.78%), followed by Monolepta sp. with 1,649 individuals (relative abundance of 17.96%). These are the phytophagous insects associated with mangrove leaves. This study concluded that the older mangrove ecosystem have a denser canopy that supports insect life.


INTRODUCTION
Since mangrove destruction started, efforts are being made to conserve mangrove as they are valuable for social life and extremely beneficial for ecosystems (Brander et al. 2012;Kerry 2017;Kusmana 2018). Mangroves can occupy the intertidal area, they interact with aquatic and terrestrial ecosystems, which helps to support diverse flora and fauna under mangrove vegetation 78 (Macintosh & Ashton 2002;Romanach et al. 2018;Martin et al. 2019), one of which is insects. Insects play a crucial role in ecology and provide a strong linkage between the mangrove ecosystem and round ecosystems (Anneboina & Kavi 2017).
Insects can be permanent residents or temporary visitors to the mangrove ecosystem and might play as a pest (Faridah-Hanum et al. 2014;Srinivasan et al. 2014). They are herbivores that feed on leaves, flowers, seeds, stem or mangrove propagules (Macintosh & Ashton 2002;Ong et al. 2010). Honeybees are also associated with mangrove as they can be helpful, since the mangrove apiculture is an activity in economic productivity (Hill & Webster 1995;Ong et al. 2010;Fernandes et al. 2012;Kusmana 2018). Furthermore, some insects play crucial roles as pollinators that can maintain the ecosystem by ensuring reproduction (Macintosh & Ashton 2002;Chen et al. 2018).
Insects play an important role in nutrient flow and biochemical energy in the mangrove ecosystem. The high abundance of insects in mangroves confirms mangrove litter detritus formation and ecosystem function (Rahman 2002;Cannicci et al. 2008;Nagelkerken et al. 2008). This insect plays a significant role in detritus production processes and energy flow in the inshore mangrove system (Fleming et al. 1990;Rahman 2002). Furthermore, the turnover rate of nutrients and organic sedimentation are also increased by the attack of mangrove insects on the leaves and stems (Macintosh 1991;Rahman 2002;Krauss et al. 2008). The abundance of insects in the colonies of dead mangrove tree trunks and fallen timber increase the detritus formation (Ng & Sivasothi 2002;Rahman 2002;Nagelkerken et al. 2008).
Mangrove insects have been studied at some locations in Southeast Asia (Rahman 2002;Rahmawaty et al. 2005;Ong et al. 2010;Yatin et al. 2015). Further study in other locations need to be performed to provide necessary information related to the insect in the mangrove ecosystem. Mangrove ecosystems in Trenggalek, East Java, Indonesia are constantly under pressure due to various human activities, such as farming and agriculture. They have an impact on the exploitation of existing biological resources, which is the destruction of the mangrove ecosystem. The activities of preserving mangrove area have been carried out in the last few years by planting some species of mangrove.
The planting of mangroves in several locations has resulted in age variations of the mangrove ecosystem in Trenggalek. Therefore, it is necessary to conduct a study that focused on the insect structure in a different age of mangrove. The parameters used to describe the insect structure are abundance and diversity of insects. It will be useful for gaining basic information for future research focused on the diversity of insects in mangrove and further conservation management of the mangrove ecosystem. Furthermore, it will be the evidence of the relationship between insect diversity and canopy percentage.The results of abundance and diversity of insects should be different depending on the age of mangrove and canopy density.

Study Site and Determination of Sampling Unit
This study was conducted from April until August 2015 at the mangrove area in Trenggalek region (-8.3068715, 111.7062334) which is under the management of Marine and Fishery Office of Trenggalek, East Java, Indonesia. There were three different mangrove ages, 15 years old (first site), eight years old (second site) and five years old (third site) (Syaifudin 2010). Tree height is also measured as a support data for mangrove age (Kalliovirta & Tokola 2005;Altman et al. 2016). Each site is separated by 5 km distance. A total number of 15 mangrove species exist in this location: Rhizophora apiculata, Rhizophora mucronate, Terminalia catappa, Scyphyphora hydrophyllacea, Heritiera littoralis, Bruguiera cylindrical, Sonneratia alba, Lumnitzera racemosa, Xilocarpus muloccensis, Lumnitzera littorea, Avicennia lanata, Avicennia marina, Ceriops tagal, Ceriops decandra and Exocaria agalocha (Syaifudin 2010).
A total of 15 plots (10 m × 10 m for each plot) were assigned in each site. The highest tree from each plot will be used as a centre tree. The average distance among plots was 50 m. These plots are placed by using purposive random sampling and arranged to cover each site. The tree height, canopy density and insect sampling will also be measured in the same site.

Tree Height and Canopy Measurement
Thirty (30) trees selected from each site (two trees from each plot) will be used for tree height and canopy measurement. The tree height is measured using indirect techniques. An indirect technique is the most commonly used for standing trees (Larsen et al. 1987) because the tip is often inaccessible. Using a range finder calibrated in meters, the distance from a squatting position to the highest point on the tree crown was measured. The distance from the same spot to the tree base was also measured with a measuring tape (Leverett 2010;Hunter et al. 2013). Based on the Pythagoras Theorem of right-angled triangles, the tree height was finally computed.
where; a = the distance to the highest point on the tree (hypotenuse) b = the distance to the tree base C = the tree height The canopy is measured using densiometer for 30 trees (the same tree used for tree height measurement). Four observations are made for each tree and the accurate duplication of the observer procedure must be assured (Williams et al. 2003). The densiometer holding it at breast height. The canopy density is estimated by counting how many of 24 squares on a mirrored grid are covered by image of the canopy. The curved mirror reflects the canopy above and canopy closure. During each observation, the observer should try to: (i) hold the instrument level, (ii) keep it oriented in the desired direction, and (iii) hold a certain viewing angle so as to prevent parallax of the reflection angle between the eye and the mirror (Nuttle 1997;Korhonen et al. 2006).

Sampling Techniques and Insect Identification
Insects are collected by using a yellow pan trap, swipe net and direct picking in all sites twice in a month. The yellow pan trap (Ø 20 cm) are installed into each plot by 50 cm from ground (15 pan traps per site). Each pan trap filled by water and detergent to break the water surface. All pan traps are modified with a netting on the site to prevent overflow during heavy rainfall. The traps installation started at 07:00 am and collected three days later. Insects explorations are also conducted by using swipe net every 3 h (started 07:00 until 16:00) in the same plot (15 plots 10 m × 10 m per site explored by swipe net and direct picking). Sometimes the plant material needed to be sampled by cutting the stems and branches with plant cutters because some insects such as wood-borers insect can only be found in the stems or branches (Kandasamy 2003;Ong et al. 2010). The insect captured in this study will be placed in ethanol 70% and brought to Ecology laboratory at State University of Malang for further identification. Samples were identified from the lowest taxonomic level following some insect identification key based on morphological character (Murphy 1989; Commonwealth Scientific and Industrial Research Organisation (CSIRO) 1991; Ping et al. 2003;Braby 2011;Yen 2012;Schnitzler et al. 2014).

Data Analysis
All the data were calculated using Microsoft Excel. The difference of tree height and canopy cover was tested using analysis of variance (ANOVA). Insect diversity for each site was analysed using Shannon index (H'), Evenness index (E') and Richness index (R) and calculated using Margalef Index in R statistical software. The estimated curve to visualise the collected species number is calculated using PAST version 3 software.

Tree Height and Canopy Cover of Mangrove
Mangrove ecosystem in Trenggalek has different tree height and canopy cover based on mangrove age (Table 1). In this study, the older mangrove has the highest tree (F 2, 87 = 646.74, P < 0.001) and denser canopy compared to the youngest mangrove (F 2, 87 = 402.76, P < 0.001). Mangrove in the first site (15 years old) has the highest tree (8.36 m) and denser canopy (66.22%) followed by the second site or 8 years old mangrove with 4.68 m tree height and 30.51% for canopy cover and 5 years old mangrove has the lowest tree height (2.26 m) and canopy cover (19.65%).

The Insect Diversity and Abundance
This study found 9,181 individual of insects associated with mangrove, consisting of 42 species from 31 families and 8 orders (Table 2). Lepidoptera was the dominant species (44.43%), followed by Coleoptera (43.03%), Hymenoptera (7%), Diptera (2.86%), Orthoptera (1.33%), Hemiptera (1.23%), Blatodea (0.08%) and Mantodea (0.03%) (Fig. 1). Species with the most abundant insects was Eristena mangalis (Lepidoptera: Pyralidae) with 1,724 individuals (18.78%), followed by Monolepta sp. (Coleoptera: Chrysomelidae) with 1,649 insects (17.96%) and Rhyparida wallacei (Coleoptera: Chrysomelidae) with 1,262 insects (13.75%). The number of species and individual that were found in the first site (40 species and 3,169 individuals) were higher compared to the second (39 species and 2,889 individuals) and the third site (37 species and 9,181 individuals). The three sites showed high insect diversity (H' > 2.0); first site (H' = 2.54), second site (H' = 2.34), and third site (H' = 2.29). Additionally, the first site accounts for the highest value for Shannon diversity index (H' = 2.54), Evenness index (E' = 0.32), and Margalef richness index (R' = 4.84) (Fig. 2). The estimation of insect species demonstrate an increase in the number of species in plot 1 to plot 4. This shows that a large number of insect species were found during the sampling period. Then, in the fifth plot, the estimated curve reached the asymptote point, and there was no curve increase until the end of the plot (Fig. 3). Achieving the estimator curve at the asymptote point shows that the observed mangrove insects are well collected. In addition, the estimation curve that reaches the asymptote point is suspected because there are no rare species (Rubio et al. 2008

DISCUSSIONS
The mangroves ecosystem provides habitat and resource that supports a large number of insects at different trophic levels (Faridah-Hanum et al. 2014). Insects perform many vital functions. They aerate the soil, pollinate blossoms and control insect and plant pests. The primary trophic groups are herbivorous insects that feed on leaves, wood borer, and flower/fruit/seed-feeding (Cannicci et al. 2008;Doydee et al. 2010); saproxylic and saprophagous insects feed on dead and decaying organic material (Fleming et al. 1990), and parasitic and predatory insects feed or prey on other animals (Nagelkerken et al. 2008). Most of the insects in this study are Lepidopteran and Coleopteran. Most of them are herbivore insects that feed on leaves and stem borer. Other studies also mention that both orders become dominant insect at mangrove ecosystem in Alas Purwo National Park, East Java (Astri et al. 2018) andGili Meno, North Lombok (Yatin et al. 2015). The larvae of common aquatic moth, Eristena mangalis -a moth of the Pyralidae family, feed on young leaves (Macintosh 1991;Nagelkerken et al. 2008). The larvae of Odites sp., known as mangrove moth, are found on the foliage of various species of Rhizophoraceae (Kandasamy 2003). One of the most common leaf-eating beetles in the mangrove is Rhyparida wallacei (Macintosh 1991). Another beetle that is dominant in mangrove is Monolepta sp. beetle, which can cause extensive leave damage, especially on Avicennia mangrove (Kandasamy 2003;Faridah-Hanum et al. 2014). Among the insects, ants play an important ecological role. Their high abundance and multitude of interactions are engaged in making them essential for ecosystem functioning (Wilson 1959;Cannicci et al. 2008). Ants are able to protect plants against herbivores via their predatory and territorial behaviour (Bronstein 1998;Styrsky & Eubanks 2007).
The species found in this study were more diverse compared to different locations in Indonesia. Astri et al. (2018) reported that 12 species from 9 families were associated with mangrove in Alas Purwo National Park, East Java. Eighty arboreal insects were found in Kupang bay, East Nusa Tenggara (Tokan et al. 2018). Ashuri et al. (2016) also reported that 32 insects were found in the ecosystem of mangrove in Wonorejo, Surabaya. Another result from outside Indonesia, the study in Singapore reported that 16 species found and Monolepta sp. beetle becomes a serious pest in mangrove in Singapore (Lim et al. 2001). The bark beetles, Dendroctonus sp. found in this study is D. frontalis while the bark beetle found in Tamil Nadu, India is D. mican (Srinivasan et al. 2014). Furthermore, the leaf webbing caterpillar, Odites sp. were also reported attacking all the species of Aviciennia spp. in Tamil Nadu, India (Kandasamy 2003).
This research has found that the first site has a greater diversity index compared to others. This may be because the first site, which is the oldest of the three ecosystems, has a denser canopy typical (Cannicci et al. 2008), resulting in a greater contrast in light levels (Tokan et al. 2018). Vertical structure complexity increases with successional age (Humphrey et al. 1999). In addition, Intachat et al. (1999) also reported that mangroves could be expected to have lower herbivore diversity than other types as a result of their lower dense canopy.
Some studies have mentioned that the tree canopy supports the highest diversity of insect. The study from Simmon and Linsenmair (2001) in Malaysia mentions that the density of herbivorous insects associated with the canopy of Qurcus subsericea is greater in the dense canopy compared to the lower canopy, suggesting that these microenvironments were appropriate with the insect communities. On the other hand, the number of herbivorous insect was higher in the understory than in canopy (Murakami et al. 2007;Levey et al. 2016), meaning that the environmental factors such as light can explain this result (Rego et al. 2019).

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The results from this study indicate that diverse species of insects are found in a mangrove in Trenggalek, East Java. Mangroves provide a habitat that supports a large number of insects at different trophic levels (Nadkarni 2001;Valencia-Cuevas & Tovar-Sánchez 2015). Further study will help us to identify the new species of coastal environmental insects and analyse the same factors that can support insect life. On the other hand, insects and plants are becoming extinct because of habitat loss, over-exploitation, pollution, overpopulation and global climate change (Macintosh & Ashton 2002;Kannan & Padmanaban 2013;Yatin et al. 2015). Hence it will be necessary to conduct a further detailed study, including seasonal surveys and other methods, to counteract these challenges by investigating the insects and other factors in this area for biodiversity conservation and management.

CONCLUSION
Mangrove ecosystem is approved as a suitable ecosystem for insect. The diverse species of insects found in this study area, where 9,181 individual insects of 42 species from 31 families and 8 orders associated with mangroves. The mangrove ecosystem on the first site (15 years old) has higher canopy cover percentage (66.22%) that resulting height insect diversity compared to second site (8 years old) and third site (5 years old). It becomes the evidence that mangrove ecosystems have a relatively old age and dense canopy that can support more variety of insects.