Odonata (Insecta) Communities in a Lowland Mixed Mosaic Forest in Central Kalimantan, Indonesia

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Odonata (Insecta) Communities in a Lowland Mixed Mosaic Forest in Central Kalimantan, Indonesia


4.1. Diversity, Abundance, and Communities

Our results of the present study indicate that Odonata diversity, abundance, and distribution patterns, and composition observed are shaped by a number of interplaying factors such as habitat characteristics, undergrowth vegetation, ecosystem productivity, body size and body size induced ecophysiological constraints, and the presence of habitat specialists.

The divergence observed between trends in species diversity and abundance was unexpected, i.e., species diversity from greatest to lowest was in the order: mixed peat swamp > kerangas > low pole, whilst abundance, from greatest to lowest, was in the order: low pole > mixed peat swamp > kerangas. Given the importance of vegetation to Odonata and its role as a structural constituent [18,31,32], we expected species diversity to be positively related with increased undergrowth vegetation. Instead, based on data from a previous study [44], undergrowth vegetation was most abundant in low pole, which showed the lowest species diversity, but the highest abundance in both suborders. According to Hykel et al. (2020) [32], only certain habitats with specific physical structure can provide suitable conditions for perching. Building on this, we can suggest that the abiotic conditions (or habitats structural features) in low pole were suitable in providing sufficient perching structures, given that 62% of the captures in low pole can be described as characteristic perchers, which are typically small odonates [14,64]. This is coherent with our results in that the Anisoptera assemblage in low pole showed significantly smaller body, hind wing, and thorax lengths, as compared to the Anisopteran assemblages in kerangas and mixed peat swamp habitat.
Another factor potentially limiting species diversity and abundance is that of ecosystem productivity. Typically, there exists a relationship between species diversity and ecosystem productivity, with more productive ecosystems supporting a higher species diversity and abundance [65]. Based on previous studies in the region, ecosystem productivity was found to be the highest for mixed peat swamp and the lowest for low pole [44,66]; a trend consistent with that of Odonata species diversity observed herein. However, the dissonance observed between species diversity and abundance implies that other factors in addition to vegetation and productivity are at play in determining the species diversity and abundance between the habitat types in the study area.
Body size is the nexus that links shade and thermoregulatory behavior in odonates. The difference in size between dragonflies and damselflies, with, generally speaking, the former being larger than the latter [14,64], leads to contrasting ecophysiological requirements related to thermoregulation and varying propensities for dispersal [23,26,35]. This results in different distribution patterns and habitat choice between the suborders. Most damselflies are small “thermal conformers” that exchange their heat with their environment via convective heat exchange [64], which is dependent on their surface-to-volume ratio and leaves them susceptible to overheating and dehydration. Therefore, in our lowland site close to the equator, lower temperatures, such as in areas covered by dense forest canopy, as observed in mixed peat swamp habitat, are likely to favour several species of Zygoptera [23,67,68], with over 80% of damselfly species confined to and living around streams covered by dense vegetation [68]. Furthermore, the restriction of species of this suborder to one area or habitat is not uncommon, as several species show high degrees of habitat specialization [14,69,70]. Thus, ecophysiological constraints and the tendency of the damselfly species found in mixed peat swamp to remain confined to mixed peat swamp habitat (i.e., showing high degrees of habitat specialization), likely explained the low homogenization in species assemblages observed between mixed peat swamp and low pole, and the five-fold increase in damselfly species recorded in mixed peat swamp habitat.
On the contrary, despite the ecophysiological constraints several species of the suborder Zygoptera face [23], low pole supported two species—Amphicnemis triplex and Ceriagrion sp., which were abundantly present. This is because within the suborder Zygoptera, there exists a continuum in families that are true habitat specialists to families that are true habitat generalists. Both A. triplex and Ceriagrion sp. represent the family Coenagrionidae, known to include the most ubiquitous species that are highly abundant and dominant in open, highly illuminated habitats with stagnant water [71]. Interestingly, although both species belong to the same ubiquitous family, A. triplex was captured across all habitat types, whereas Ceriagrion sp. was exclusively confined in its distribution to low pole peat swamp habitat. Therefore, it is likely that other species-specific factors such as resource preference [23], may explain the observed distribution pattern in Ceriagrion sp. Unlike Anisoptera, there was no consistent patterns across morphological traits in Zygopteran species assemblages between habitat-types. However, it is uncertain what explains the significant differences found in the hindwing-to-body ratio between the Zygopteran assemblages found in low pole and mixed peat swamp; with this not being further investigated in this study.
Anisoptera are largely endotherms or heliotherms [14,64]. Heliotherms exchange heat with their environment via irradiation heat exchange, which is dependent only on the surface area exposed to the sun [64]: a mechanism characteristic of several percher species [64]. Increased luminosity favors species of this suborder, as they require high ambient temperatures to become active, while shaded environments are found to restrict and reduce the occurrence and abundance of several species of dragonfly [27,72,73]. The greater relative abundance of Anisoptera in low pole is thus likely to be attributed to the accentuated luminosity that arises from the characteristic low forest canopy [44,74], and also likely explains the reduced abundance of Anisoptera in the more shaded mixed peat swamp and kerangas habitats. Additionally, the uneven peaty forest floor and permanently high water-table in low pole [14,74] leads to the formation of forest pools of varying depths and sizes, a habitat characteristic ideal for several species of Anisoptera [18,75].
The degree of similarity observed between Odonata species assemblages in kerangas and low pole was due to the overlap in Anisoptera species, with all species sampled in kerangas also being found in low pole. This is perhaps surprising, considering how different kerangas and low pole habitats are in terms of their biotic and abiotic characteristics [44,48,74]. However, the ability to tolerate a wide range of environmental conditions is not uncommon within the suborder Anisoptera, as several species are generalists [23,67,73]. The dissimilarity in species assemblages found between low pole and mixed peat swamp is likely to be primarily due to the presence of habitat specialists. Specialists increase the diversity and abundance found within a habitat type as they show distinct habitat preferences [76]. The most abundant species in low pole—Brachygonia oculata and Brachygonia ophelia (Libellulidae)—are peat swamp specialists, along with Brachygonia puella (Libellulidae), another species that favors open and marshy low pH habitat [38]. These species accounted for ~62% of the total captures of Odonata (Anisoptera and Zygoptera) in low pole, a substantial amount, considering that nine species were found all together. This argument is further substantiated given that both habitat types – mixed peat swamp and low pole, differed greatly in their suborder ratio, in line with the well-established thesis that both Anisoptera and Zygoptera show contrasting ecophysiological requirements [23]. The significantly smaller Anisoptera in low pole compared to mixed peat swamp likely indicates that Anisoptera species between these two habitat-types exhibit differing thermoregulatory abilities, and thus different habitat selection strategies [23,26,35]. These results align with those of several studies showing variations in community structure and composition between suborders because of their contrasting ecophysiological requirements [14,23,69,72].

4.2. Morphology, Dispersal, and Habitat Selection

Here we find that morphological traits associated with increased dispersal and larger range size were in accordance with suborder habitat selection, with Odonata that showed larger morphological traits selecting for more lentic habitats, while Odonata that showed smaller morphological traits selecting for more lotic habitats.

Odonata species are either associated with lentic or lotic habitats, and those species in unpredictable habitats need a greater ability to disperse to ensure the survival of populations [79,80]. Therefore, Odonata species adapted to lentic habitats might require larger thorax and wing morphology, which might allow them to disperse better than species adapted to more lotic (reliable) habitats [37,80]. Within the Odonata communities in KHDTK, Anisoptera showed a greater use of forest pools (lentic sources), while Zygoptera showed a greater use of flowing water (lotic sources). As lentic sources are considered more uncertain habitats than lotic sources [80], this was in accordance with the differences in morphological traits observed between the suborders in our study. An increased thorax size implies better dispersal capacity as a larger thorax has more space available for thoracic musculature, and therefore greater power output [81]. This is also coherent with hindwing lengths observed, in which Anisoptera showed larger hindwings relative to Zygoptera, given that wing morphology is also an important determinant of dispersal capacity [35]. However, Rundle et al. [82] suggested that when hindwing length relative to body length was used (HWL/BL), it was more indicative of range size (i.e., distributional range or site occupancy), as a greater HWL/BL ratio is positively associated with enhanced flight performance and dispersal [37,82]. This, we also found, in that Anisoptera exhibited significantly larger HWL/BL ratios as opposed to Zygoptera, which was in line with suborder habitat selection patterns observed.
There is very little published information available on the migration patterns of southern Asian Odonata [83] and on the ecology of Borneo Odonata species. Due to this we have been unable to consider the potential influence of (seasonal) migrations in our study. Although, we expect this to be relatively low, given Borneo’s relatively aseasonal climate and that several Borneo Odonata species are endemic to the island [38]; we did, however, notice over the duration of the study, potential community shifts which may be an indication of migration. Thus, we might nevertheless recommend that future Odonata researcher in the area attempt to address this topic.

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