Vertical niche usage and trait associations in Gabonese amphibians

Tropical forests are vertically complex, and offer unique niche opportunities in the form of resource, climate, and habitat-gradients from the forest floor to the canopy. Rainforest amphibians organize within this vertical space and the highest levels of vertical stratification occur in structurally complex and climatically stable tropical rainforests. Amphibians have diversified into numerous habitat and climatic niches, which has led to the development of a wide variety of morphological, behavioural, physiological, and reproductive traits. However, a lack of data regarding the vertical niche space used by amphibian species has prevented a nuanced analysis of traits and vertical height. We performed 74 ground-to-canopy surveys for amphibians at Baposso Village, Ngounie Province, Gabon, and describe the vertical stratification patterns of the assemblage in terms of richness, abundance, and species specific vertical niche usage. We analyse the relationships between amphibian traits with vertical height using linear mixed effects models, finding strong support that frogs with bigger toes in relation to their length access greater height in the canopy. We also see differences in the vertical heights of species according to their reproductive modes, highlighting the importance of reproductive mode diversity for the vertical stratification of amphibian assemblages.

Methods
In southern Gabon, we surveyed a Dense Moist Forest (Verhegghen et al. 2012) located around Baposso Village, Ngounie Province, which borders the Monts Birougou Ramsar site (Lat 20°85′00″ N: Lon 12°13′50″; 650 – 800 m a.s.l, Figure 1). We focussed our sampling in older growth areas with mature trees, with primary canopy tree species including Mutombo, Wapaka, Posso, and Paduc. Temperature and humidity were measured at the site using climate loggers deployed in two arrays. One array comprised three temperature loggers (HOBO Temperature Pendant); the soil (10 cm below the surface), 2 m height in the understory, and 25 m height in the canopy. The second array utilised climate loggers (HOBO U23 Pro v2) in the understory and canopy which record humidity in addition to temperature. Loggers were protected from sunlight and weather by installation within a PVC T-Joint pipe which allowed airflow.

We surveyed for frogs during the wet season of 2022 (October – November). We conducted vertical, ground-to-canopy surveys for amphibians, with each survey centred on a single canopy tree. Tree selection was randomized; however, trees were required to be a minimum of 20 m apart and had to meet safety standards for arborist single-rope climbing (Jepson 2000). We did not limit our selection by tree species and were only limited in selection by safety concerns. We surveyed 74 trees at night for amphibians. For each tree we surveyed for 10 minutes at the ground, understory (2-4m), sub-canopy (approx. half the maximum height climbed), and canopy (maximum height climbed), with a 20 minute roaming survey between the strata totalling to 60 minutes. The 10 minute ground survey consisted of searching through leaf litter and ground habitat in a 4 x 4 m area to standardise to the same search area available during climbing. Above-ground, we searched for arboreal amphibians in tree holes, moss, epiphytes and other microhabitat structures (Heyer et al. 1994), and measured the height above ground of each individual using a laser distance metre (Leica Geosystems, Leica Disto D2; http://www.leica-geosystems.ca).

Encountered amphibians were photographed, weighed, and measured for snout-vent length (SVL), mouth width (hereafter: mouth), tibia length (hereafter: tibia), and toe disc width (3rd digit from inside thumb).