Vertical stratification patterns of tropical forest vertebrates: a meta‐analysis

Abstract

Tropical forests harbour the highest levels of terrestrial biodiversity and represent some of the most complex ecosystems on Earth, with a significant portion of this diversity above ground. Although the vertical dimension is a central aspect of the ecology of forest communities, there is little consensus as to prominence, evenness, and consistency of community‐level stratification from ground to canopy. Here, we gather the results of 62 studies across the tropics to synthesise and assess broad patterns of vertical stratification of abundance and richness in vertebrates, the best studied taxonomic group for which results have not been collated previously. Our review of the literature yielded sufficient data for bats, small mammals, birds and amphibians. We show that variation in the stratification of abundance and richness exists within and among all taxa considered. Bat richness stratification was variable among studies, although bat abundance was weighted towards the canopy. Both bird richness and abundance stratification were variable, with no overriding pattern. On the contrary, both amphibians and small mammals showed consistent patterns of decline in abundance and richness towards the canopy. We descriptively characterise research trends in drivers of stratification cited or investigated within studies, finding local habitat structure and food distribution/foraging to be the most commonly attributed drivers. Further, we analyse the influence of macroecological variables on stratification patterns, finding latitude and elevation to be key predictors of bird stratification in particular. Prominent differences among taxa are likely due to taxon‐specific interactions with local drivers such as vertical habitat structure, food distribution, and vertical climate gradients, which may vary considerably across macroecological gradients such as elevation and biogeographic realm. Our study showcases the complexity with which animal communities organise within tropical forest ecosystems, while demonstrating the canopy as a critical niche space for tropical vertebrates, thereby highlighting the inherent vulnerability of tropical vertebrate communities to forest loss and canopy disturbance. We recognise that analyses were constrained due to variation in study designs and methods which produced a variety of abundance and richness metrics recorded across different arrangements of vertical strata. We therefore suggest the application of best practices for data reporting and highlight the significant effort required to fill research gaps in terms of under‐sampled regions, taxa, and environments.

Ecological patterns and processes in the vertical dimension of terrestrial ecosystems

“…Species richness, abundance and community composition patterns along vertical gradient are not consistent across different biomes and taxa (Figure 2). Several studies show small mammals and frogs with greater species richness and abundance on the ground than the canopy (Basham et al, 2019(Basham et al, , 2022Vieira & Monteiro-Filho, 2003). While arthropod diversity is highly variable across studies which may be due to taxon-specific requirements (Basset, 2001;Charles & Basset, 2005;Kato et al, 1995;Nadkarni, 1994;Ulyshen et al, 2010) or due in part to the difficulty in achieving consistency in sampling efforts between the canopy and the ground (Ashton et al, 2016;Basset, 1997Basset, , 2001.…”

“…• Niche plasticity/lability hypothesis: Species can adjust their vertical niche in pursuit of optimal conditions (microclimatic and/or resource; Basham et al, 2022) or as a compensatory response to disturbance (Borden et al, 2022;Neilly et al, 2018) • Arboreal trait syndrome: The amalgamation of physiological, behavioural and morphological adaptations to living above-ground generates a syndrome of traits (Scheffers & Williams, 2018). A similar hypothesis by Scheffers et al (2017) proposes that, as a consequence of this syndrome, arboreal species should also have higher dispersal potential which would increase resistance and resilience to both current and historical environmental instability, respectively.…”

“…Many studies of arthropods have focused on either Lepidoptera (butterflies; Fermon et al, 2005) or Coleoptera (beetles; Grimbacher & Stork, 2007) and few sampled the entire arthropod community (but see Basset et al, 2001; de Souza Amorim et al, 2022; Dial et al, 2006). Regardless of taxa, common explanatory factors attributed to shaping vertical diversity patterns are resource availability, microclimate and vegetation structural features (Basham et al, 2022; de Souza Amorim et al, 2022). For example, in temperate hardwood forests, Coleoptera tend to have higher species richness in strata with high branching density and occurrence of fungal fruiting bodies (Schroeder et al, 2009).…”

“…Species living towards the canopy experience higher rates of evaporative water loss; hence, stronger desiccation resistance (often combined with other adaptations such as larger body size/mass) could be an advantage (Bujan et al, 2016; Hood & Tschinkel, 1990; Tracy et al, 2010). Niche plasticity/lability hypothesis : Species can adjust their vertical niche in pursuit of optimal conditions (microclimatic and/or resource; Basham et al, 2022) or as a compensatory response to disturbance (Borden et al, 2022; Neilly et al, 2018). Here, two traits are prerequisite for a niche shift—suitable morphology to expand into a new niche (e.g.…”

“…These traits interact with climate and delineate the fundamental vertical niche of the species (Leahy et al, 2021). This niche space is further modified by the distribution of resources, natural enemies and dispersal limitation of the species, producing a realised niche within the vertical environment (Basham et al, 2022). In addition, the vertical gradient interacts with diel and seasonal changes: resources and microclimates can shift on daily to annual time‐scales and many species follow such changes through time (Basham & Scheffers, 2020).…”