Dynamics of dipteran and orthopteran populations in response to wet-dry cycles within a rapidly suburbanizing zone of Accra, Ghana

Introduction

The global acceleration of suburban sprawl has been particularly pronounced in sub-Saharan Africa, where the intersection of rapid population growth, economic development, and infrastructure expansion has generated extensive zones of landscape transition at city margins [1] . Along the Gulf of Guinea coastline, where major metropolitan areas like Accra have experienced near-continuous built-up expansion, the ecological consequences for insect communities inhabiting peri-urban landscapes have received inadequate scientific attention [2] . Diptera and Orthoptera represent two ecologically distinct insect orders that fulfill complementary functional roles in terrestrial ecosystems. Diptera contribute to decomposition, pollination, and nutrient cycling, while simultaneously including medically significant pest species [3] . Orthoptera, as primarily herbivorous insects with acoustic signaling behaviors, serve as keystone prey items for numerous vertebrate predators and function as indicators of grassland habitat quality [4] . The population dynamics of both orders are strongly influenced by seasonal moisture availability, temperature fluctuations, and vegetation phenology, yet their specific responses to the combination of seasonal climatic variation and suburban habitat modification have not been jointly investigated in West African settings. The Accra-Tema metropolitan corridor has expanded its built-up area by approximately 156 km² between 2015 and 2025, converting substantial areas of coastal savanna, agricultural land, and wetland margins into residential and commercial developments [5] . This transformation creates a spatially heterogeneous landscape where remnant natural habitats, abandoned farmland at various successional stages, and actively managed suburban plots coexist within a complex mosaic. This research aimed to simultaneously track dipteran and orthopteran population dynamics across this heterogeneous suburban landscape throughout a complete annual cycle, with specific objectives to quantify seasonal abundance patterns, identify environmental drivers of population variation, and evaluate the relative importance of land-cover type and seasonal timing in structuring these two insect communities.

Material and methods

Material

The research area encompassed a 34 km² suburban expansion zone situated between Accra (5°33'N, 0°12'W) and Tema (5°40'N, 0°01'W) along the coastal plains of the Greater Accra Region, Ghana. The climate is characterized as tropical savanna (Aw) with a bimodal rainfall pattern: major rains (April-July, mean monthly rainfall 134 mm), minor rains (September-November, 56 mm), and two dry periods (December-March and August). Mean annual temperature is 26.8°C with minimal seasonal variation (range: 24.2-29.4°C). Native vegetation comprises coastal thicket and Imperata cylindrica grassland interspersed with Azadirachta indica and Leucaena leucocephala in modified areas. Six land-cover categories were delineated using droneacquired ortho-imagery: (1) active residential plots with managed gardens, (2) construction sites with cleared ground, (3) fallow agricultural plots (1-5 years abandoned), (4) actively farmed small-holdings, (5) remnant coastal scrubland, and (6) riparian buffer strips along seasonal streams. Four replicate sampling stations were established within each land-cover category, totaling 24 stations.

Methods

Population monitoring was conducted monthly from April 2025 through March 2026, yielding 12 sampling rounds. Diptera sampling employed yellow sticky traps (20 × 25 cm, Seabright Laboratories) deployed at 1.0 m height (three per station, 72-hour exposure) and sweep net surveys (100 sweeps per station along standardized 50-m transects). Orthoptera sampling utilized belt transect surveys (50 m × 2 m, walking pace with visual counting and targeted netting) during morning hours (07:00-10:00) and acoustic monitoring using autonomous recording units (AudioMoth devices, 44.1 kHz sampling rate, 18:00-22:00) for species identification through stridulation analysis. Diptera specimens were identified to species level using Crosskey's Afrotropical keys and the Catalogue of Diptera of the Afrotropical Region. Orthoptera identification followed Dirsh's classification framework for West African Acridoidea and Naskrecki's keys for Tettigoniidae. Acoustic species identification was validated against voucher recordings from the British Library Sound Archive. Environmental measurements at each station during every sampling event included air temperature and relative humidity (Kestrel 4500 weather meter), soil moisture (tensiometer at 10 cm depth), vegetation height (graduated pole measurements at 10 random points), herbaceous cover percentage (1 m² quadrat estimation at 5 points), and flowering plant density (count of flowering individuals within 5 m radius of each trap). Statistical analyses employed generalized linear mixed models (GLMMs) with negative binomial distribution to examine effects of land-cover type, season, and their interaction on abundance and species richness. Random effects included sampling station nested within land-cover type. Environmental correlates were assessed using distancebased redundancy analysis (dbRDA). All analyses used R version 4.4.1 with glmmTMB, vegan, and lme4 packages. Significance was set at α = 0.05 with Bonferroni correction for multiple comparisons.

Results

Across the 12-month sampling period, 3,847 dipteran specimens representing 67 species from 14 families and 2,156 orthopteran specimens representing 43 species from 8 families were collected and identified.

Discussion

The temporal asynchrony between dipteran and orthopteran population peaks represents a noteworthy ecological pattern with functional significance for suburban food web dynamics. Dipteran populations, driven primarily by moisture availability and peaking during the major rains, provide an early-season pulse of invertebrate biomass, while the delayed orthopteran peak extends the period of abundant prey availability into the transitional and early dry period [6] . This complementary temporal pattern may buffer insectivorous vertebrate populations against seasonal food shortages, a consideration of particular importance for bird communities inhabiting suburban landscapes [7] . The dominance of Muscidae among Diptera reflects the association of this family with anthropogenic environments, where domestic waste, livestock presence, and managed organic substrates provide abundant larval development sites [8] . The strong humidity-driven seasonality of overall dipteran abundance is consistent with the known dependence of many fly species on moist conditions for oviposition and larval survival, particularly in the families Muscidae, Calliphoridae, and Sarcophagidae [3] . The vegetation height dependence of orthopteran abundance aligns with established understanding of grasshopper and cricket habitat requirements, where taller vegetation provides enhanced structural complexity for thermoregulation, predator avoidance, and acoustic signaling [9] . The particularly high orthopteran diversity on fallow agricultural plots likely reflects the development of dense herbaceous vegetation during the abandonment process, creating favorable microhabitat conditions that are absent from actively managed or recently developed sites [10] . Kumari and Priya [11] documented comparable patterns of seasonal variation in insect biodiversity across different functional groups in suburban transitional habitats, supporting the generality of seasonally mediated community dynamics in peri-urban settings. The present findings extend this understanding by demonstrating order-specific responses to distinct environmental drivers within the same landscape matrix. The significant land-cover × season interaction for both orders indicates that seasonal effects on insect populations are modulated by habitat context, consistent with the hypothesis that habitat quality buffers or amplifies the intensity of seasonal population fluctuations [12] . Construction sites showed the most extreme seasonal amplitudes, likely due to their minimal vegetation cover and limited capacity to buffer microclimate extremes during dry periods.

Limitations

Acoustic monitoring for Orthoptera was limited to evening hours, potentially underrepresenting diurnal species with quiet or infrequent stridulation. The sweep net methodology may have undersampled arboreal Diptera associated with tree canopies. Monthly sampling intervals may have missed shortduration population peaks occurring between sampling events. The categorization of land-cover types, while based on objective criteria, involved some transitional areas that did not fit neatly into single categories.

Safety considerations

Field personnel working near riparian areas were briefed on risks associated with Naja melanoleuca (forest cobra) and Bitis arietans (puff adder), both documented in the research area. Sampling near construction sites required compliance with Ghana Labour Commission safety regulations for nonconstruction personnel. Personal protective equipment including closed-toe boots, long trousers, and high-visibility vests was mandatory during all field activities. Insecticidetreated bed nets were provided for overnight field stays during malaria season.

Acknowledgement

AcknowledgementsThis research was supported by the University of Cape Coast Research Fund (UCC-RF/2024/ENT/019). The Ghana Environmental Protection Agency provided land-use data for the Accra-Tema corridor. The authors acknowledge the technical expertise of Isaac Kwame Asante in dipteran taxonomy and Akosua Dankwa in acoustic orthoptera monitoring. Local community leaders in the Kpone-Katamanso district facilitated access to private land for sampling.

References

1. Cobbinah PB, Erdiaw-Kwasie MO, Amoateng P. Rethinking sustainable development within the framework of poverty and urbanisation in developing countries. Environ Dev. 2015; 13:18-32.
2. Appiah DO, Bugri JT, Forkuor EK, Boateng PK. Determinants of peri-urbanization and land use change patterns in peri-urban Ghana. J Sustain Dev. 2014;7(6):95-109.
3. Courtney GW, Pape T, Skevington JH, Sinclair BJ. Biodiversity of Diptera. In: Foottit RG, Adler PH, editors. Insect Biodiversity: Science and Society. Oxford: Wiley-Blackwell; 2017. p. 229-278.
4. Gardiner T, Hill J, Chesmore D. Review of the methods frequently used to estimate the abundance of Orthoptera in grassland ecosystems. J Insect Conserv. 2005;9(3):151-173.
5. Stow DA, Weeks JR, Toure S, Coulter LL, Lippitt CD, Ashcroft E. Urban vegetation cover and vegetation change in Accra, Ghana. Prof Geogr. 2013;65(2):296312.
6. Poulin B, Lefebvre G, McNeil R. Variations in bird abundance in tropical arid and semi-arid habitats. Ibis. 1993;135(4):432-441.
7. Shochat E, Warren PS, Faeth SH, McIntyre NE, Hope D. From patterns to emerging processes in mechanistic urban ecology. Trends Ecol Evol. 2006;21(4):186-191.
8. Keiding J. Review of the global status and recent development of insecticide resistance in field populations of the housefly, Musca domestica. Bull Entomol Res. 1999;89(Suppl 1):S9-S67.
9. Uvarov B. Grasshoppers and Locusts. Vol. 2. Behaviour, Ecology, Biogeography, Population Dynamics. London: Centre for Overseas Pest Research; 1977. p. 234-267.
10. Öckinger E, Dannestam Å, Smith HG. The importance of fragmentation area and habitat quality for arthropod diversity in non-crop habitat fragments. Biol Conserv. 2009;142(3):547-556.
11. Kumari B, Priya A. Seasonal variation in insect biodiversity in a transitioning sub-urban area. Zool Entomol Lett. 2022;2(1):42-49.
12. Kotiaho JS, Kaitala V, Komonen A, Päivinen J. Predicting the risk of extinction from shared ecological characteristics. Proc Natl Acad Sci U S A. 2005;102(6):1963-1967.