Review Article, Geoinfor Geostat An Overview Vol: 13 Issue: 3

Seasonal Variation in Insect Pollinator Diversity in Two Different Vegetation Zones in Nigeria

Abstract

Keywords: Bees; Flowers; Rainforest; Savanna; Dry season; Rainy season

Keywords

Bees; Flowers; Rainforest; Savanna; Dry season; Rainy season

Introduction

Pollinators provide critical ecosystem services to crops and wild plants in both agricultural and natural landscapes [1,2]. Insects are among the most diverse and dominant group of pollinators. The diversity and success of insect pollinators is however prone to various anthropogenic stressors/pressures such as land use change, pesticides and global climate change. Furthermore, their populations are prone to seasonal fluctuations in biotic and abiotic factors that are important drivers of insect biology and ecology [3].

The response of insect pollinators to seasonality in tropical rainforests may be different from the tropical dry region. As against migration and dormancy in the tropical dry region, seasonality is seen in the tropical rainforest [4,5]. This simply means that insect species are present and active throughout the year, and abundance peaks are not concentrated at any particular period [6-8]. However, there are two types of seasonal patterns in abundance that can be recognized for the active developmental stages in the tropical dry regions. One type peaks during the wet season [9,10], and the other type peaks during the dry season [9,11,12]. Insects in the tropical dry regions enter diapause in the wet season or in the dry season depending on species [9]. Some may migrate looking for suitable conditions [13].

In Nigeria, the weather pattern is typically distinguished by wet and dry seasons which last from February to September and October to January respectively. The wet season is a period of high rainfall and humidity whereas the dry season is a period of low rainfall and low humidity resulting in a dry and dusty environment. Drought is usually prominent most especially in the northern part of the country. In the dry season, there is low diversity of flowering plants and its resources while in the wet season there is high diversity of flowering plants and its resources [3]. Some plants have their main flowering season at some point during the dry season; this may provide some resources for insect pollinators [3]. Reduction in flowering plants and floral resources which are essential for survival and reproduction of flower-visiting insects would affect the reproductive output [14] and population densities [15] of these insects of economic importance.

Although, the effect of seasonal patterns on the life cycles and temporal diversity patterns can be readily seen in the abundance of most temperate insect species; many become dormant during winter season as a means to cope. But in tropical regions, the situation may be different because there is no winter season [9]. The tropic region is defined as the region where the minimum monthly temperature is greater than 18°C [16]. That is, the tropic region cannot be as cold as the temperate region during winter. This brings about questions like; can seasons affect abundance of insects? And in what ways are they affected in the tropics? [9].

In this study, we determined the seasonal variation in the abundance and species richness of dominant insect pollinator taxa such as bees, butterflies, flies and wasps. We also determined if the effect of season was different in two vegetation zones; rainforest and savanna in Nigeria.

Literature Review

Study area

The study was conducted in the savanna and rain forest vegetation zones of Nigeria. The climate of the savanna is characterized by relatively high temperatures throughout the year with an average annual maximum of 33°C and minimum of 21°C. The average annual rainfall of savanna is 1600 mm [17]. In the rain forest, the average annual maximum temperature is 30°C and minimum temperature is 21°C [18], while average annual rainfall is above 2000 mm [19].

In the rain forest vegetation zone, we selected Osun-Osogbo grove, Osogbo (7°45′ and 7°75´ N; 4°33′ and 4°55′ E) and Obafemi Awolowo University, Ile-Ife Nigeria (7°26´ and 7°32´ N; 4°31´ and 4°35´ E). For the savanna vegetation zone, we selected Old Oyo National Park (8°27′ and 8°28´ N; 3°46′ and 3°47′ E) and Old Oyo camp base, Sepeteri Nigeria (8°35′ and 8°36´ N; 3°39′ and 3°40′ E). Eight 25 m × 25 m sampling plots were established in each study location, within which insect and flowering plant sampling was carried out.

Sampling of insect pollinators and flowering plants

Sampling of insect pollinators was carried out four times between August 2018 and July 2019 on each sampling plot. Sampling was carried out twice during rainy and dry seasons within the one-year sampling period. For the insect pollinators, two sampling methods were combined, which included use of colored pan traps as well as sweep netting within the sampling plots. Nine coloured bowls (1 L capacity), three each in white, blue and yellow were placed randomly at three different sampling positions on each sampling plot. At each sampling position, one each of white, blue and yellow pan traps were placed in a triangle shaped arrangement following the approach described by Potts et al., Wilson et al., and Brittain et al., [20]. The bowls were half-filled with water and few drops of liquid detergent were added to break the surface tension of water and enhance insect trapping. The bowls were left at the site for 24 hr., after which trapped insects were removed, rinsed and stored in 70% ethanol until they were sorted in the laboratory.

Additionally, insect pollinators were sampled by sweep netting within the study plots. This was undertaken between 9:00 and 14:00, under favorable weather conditions with mild wind on each sampling day. Sampling was carried out for 15 min on each plot. Insects visiting the floral parts of flowering plants on each plot were collected with sweep net and recorded. Flowering plants visited by the insects were also collected for later identification. Insect identification was undertaken with the aid of appropriate identification keys such as bees of the world, butterflies of West Africa and common butterflies of IITA and with the help of expert taxonomists. Identification was done to species level, but where species level identification was not possible, it was done to family or generic levels and labeled as morphospecies. Reference specimens of the insects were deposited in the entomology collection of department of zoology, Obafemi Awolowo University, Ile-Ife, Nigeria.

The sampling of flowering plants (shrubs and herbs) in the study plots was carried out only after insect sampling to avoid disturbing the lower visiting insects. A 1 m × 1 m quadrat was randomly thrown ten times on each 25 m × 25 m plot. Species richness and abundance of the flowering plants were estimated within the quadrats. Specimens of flowering plants were later identified at Ife Herbarium, botany department, Obafemi Awolowo University, Ile-Ife.

Statistical analysis

The abundance and species richness of the main taxa (bees, wasps, beetles, butterflies, and flies) recorded from all sampling periods were pooled for the analyses. Generalized linear model (Poisson error distribution) was used to determine the effects of season and vegetation on the abundance and species richness of insect pollinators.

Results

A total of 1588 insect individuals, belonging to four orders (Hymenoptera, Coleoptera, Lepidoptera, and Diptera), 228 morphospecies and five taxonomic groups (bees, wasps, beetles, butterflies, and flies) were collected over the sampling period. 655 insect individuals were collected in rainforest, and 933 individuals were collected in savanna. In turn, 879 insect individuals were collected during the rainy season and 709 insect individuals were collected during the dry season.

The overall mean abundance of insect pollinators was significantly higher in savanna (14.578 ± 1.0712) compared to rainforest (10.234 ± 0.908) but no significant difference in overall mean abundance of insect pollinators between the two seasons. Also, there was no significant difference observed in overall mean species richness of insect pollinators between the two seasons and vegetation zones.

There was significant difference in the abundance of wasps as well as the abundance and species richness of bees and butterflies between the seasons. While the abundance of wasps and butterflies were higher in rainy season, bees on the contrary had higher abundance in dry season. Similarly, bee species richness was significantly higher in dry season; however, butterfly species richness was significantly lower in dry season compared to rainy season. There was no significant difference in species richness and abundance of other taxa between seasons.

Furthermore, there was a significant difference in the abundance of beetles as well as the abundance and species richness of bees, butterflies and wasps across vegetation zones. While the abundance of butterflies was higher, there was lower abundance of bees, beetles and wasps in the rainforest compared to savanna vegetation zones, similarly, savanna vegetation zone had higher species richness of bees and wasps but lower butterfly species richness compared to the rainforest vegetation zone.

Discussion

This study showed higher abundance of insect pollinators in savanna compared to rainforest vegetation zone. The canopy cover of trees in the savanna vegetation zone is more open compared to the dense and closed canopy cover which characterizes a typical rainforest vegetation zone. Open canopy may provide suitable microclimatic conditions in the understory vegetation in savanna vegetation zone which may aid the growth of flowering shrubs and herbs in the understory and may subsequently promote the diversity of insect pollinators. Since pollinators often track flowering plant diversity, the flower-rich savannah ecosystem may be crucial for the maintenance of high pollinator diversity in this region compared to the rainforest. Thus, in this era of increasing anthropogenic threats aiding the transformation of natural habitats, management actions necessary for promoting flower diversity in the savannah may be critical in ameliorating pollinator loss in threatened ecosystems.

Furthermore, this study showed that the effect of season on insect pollinator diversity is taxa specific. While the effect of season was reported for bees, butterflies and wasps, no effect was found for other insect pollinator taxa. Bees and other flower-visiting insects are important insect pollinators collecting floral rewards and contributing significantly to pollination of crops and native vegetation globally. This may be significant to the pollination of food crops in Nigeria especially during rainy season where more food resources and nesting sites are usually available for these important pollinator groups. In this study, mean abundance and species richness of bees were higher significantly in the dry season compared to rainy season, however, this contrast the pattern observed in the abundance and species richness of butterflies and wasps which were significantly higher during the rainy season. These findings call for further investigation to ascertain if this pattern also occurs in agricultural landscapes. This will ensure that pollination is maximized for pollinator dependent crops which flower and require the visit of these insect pollinators in different seasons.

We recorded higher abundance and species richness of butterflies in rainforest compared to savannah. Butterflies depend on forest resources for breeding of larvae which feed on leaves and other plant tissues. This aids the suitability of butterflies as indicators of habitat quality particularly in forest ecosystems. Also, it has been reported that butterflies thrive more in areas where there are heterogeneous vegetation’s, which provide varied source of foods.

Conclusion

This study showed taxa-specific response of insect pollinators to season and vegetation in the study area. This underscores the potential importance of these pollinators as indicators of fluctuations in season and vegetation dependent resources. This has important implications for the diversity of these pollinators and the associated ecosystem service of pollination in both natural and agricultural landscapes.