Revisiting the drivers of acoustic similarities in tropical anuran assemblages.

Acoustic signaling is key in mediating mate choice, which directly impacts individual fitness. Because background noise and habitat structure can impair signal transmission, the acoustic space of mixed-species assemblages has long been hypothesized to reflect selective pressures against signal interference and degradation. However, other potential drivers that received far less attention can drive similar outputs on the acoustic space. Phylogenetic niche conservatism and allometric constraints may also modulate species acoustic features, and the acoustic space of communities could be a side-effect of ecological assembly processes involving other traits (e.g., environmental filtering). Additionally, the acoustic space can also reflect the sorting of species relying on public information through extended communication networks. Using an integrative approach, we revisit the potential drivers of the acoustic space by addressing the distribution of acoustic traits, body size, and phylogenetic relatedness in tropical anuran assemblages across gradients of environmental heterogeneity in the Pantanal wetlands. We found the overall acoustic space to be aggregated compared with null expectations, even when accounting for confounding effects of body size. Across assemblages, acoustic and phylogenetic differences were positively related, while acoustic and body size similarities were negatively related, although to a minor extent. We suggest that acoustic partitioning, acoustic adaptation, and allometric constraints play a minor role in shaping the acoustic output of tropical anuran assemblages and that phylogenetic niche conservatism and public information use would influence between-assemblage variation. Our findings highlight an overlooked multivariate nature of the acoustic dimension and underscore the importance of including the ecological context of communities to understand drivers of the acoustic space.

Drivers of assemblage-wide calling activity in tropical anurans and the role of temporal resolution.

Temporal scale in animal communities is often associated with seasonality, despite the large variation in species activity during a diel cycle. A gap thus remains in understanding the dynamics of short-term activity in animal communities. Here we assessed calling activity of tropical anurans and addressed how species composition varied during night activity in assemblages along gradients of local and landscape environmental heterogeneity. We investigated 39 anuran assemblages in the Pantanal wetlands (Brazil) with passive acoustic monitoring during the peak of one breeding season, and first determined changes in species composition between night periods (early, mid and late) using two temporal resolutions (1- and 3-hr intervals). Then, we addressed the role of habitat structure (local and landscape heterogeneity variables from field-based and remote sensing metrics) and ecological context (species richness and phylogenetic relatedness) in determining changes in species composition (a) between night periods and (b) across days. Nocturnal calling activity of anuran assemblages varied more within the 1-hr resolution than the 3-hr resolution. Differences in species composition between early- and late-night periods were related to local habitat structure and phylogenetic relatedness, while a low variation in compositional changes across days was associated with low-heterogeneous landscapes. None of these relationships were observed using the coarser temporal resolution (3 hr). Our findings on the variation of calling activity in tropical anuran assemblages suggest potential trade-offs mediated by fine-temporal partitioning. Local and landscape heterogeneity may provide conditions for spatial partitioning, while the relatedness among co-signalling species provides cues on the ecological overlap of species with similar requirements. These relationships suggest a role of niche dimensional complementarity on the structuring of these anuran assemblages over fine-temporal scales. We argue that fine-temporal differences between species in breeding activity can influence the outcome of species interaction and thus, addressing temporal scaling issues can improve our understanding of the dynamics of animal communities.

Responses of aquatic macrophyte cover and productivity to flooding variability on the Amazon floodplain.

Macrophyte net primary productivity (NPP) is a significant but understudied component of the carbon budget in large Amazonian floodplains. Annual NPP is determined by the interaction between stem elongation (vertical growth) and plant cover changes (horizontal expansion), each affected differently by flood duration and amplitude. Therefore, hydrological changes as predicted for the Amazon basin could result in significant changes in annual macrophyte NPP. This study investigates the responses of macrophyte horizontal expansion and vertical growth to flooding variability, and its possible effects on the contribution of macrophytes to the carbon budget of Amazonian floodplains. Monthly macrophyte cover was estimated using satellite imagery for the 2003-2004 and 2004-2005 hydrological years, and biomass was measured in situ between 2003 and 2004. Regression models between macrophyte variables and river-stage data were used to build a semiempirical model of macrophyte NPP as a function of water level. Historical river-stage records (1970-2011) were used to simulate variations in NPP, as a function of annual flooding. Vertical growth varied by a factor of ca. 2 over the simulated years, whereas minimum and maximum annual cover varied by ca. 3.5 and 1.5, respectively. Results suggest that these processes act in opposite directions to determine macrophyte NPP, with larger sensitivity to changes in vertical growth, and thus maximum flooding levels. Years with uncommonly large flooding amplitude resulted in the highest NPP values, as both horizontal expansion and vertical growth were enhanced under these conditions. Over the simulated period, annual NPP varied by ca. 1.5 (1.06-1.63 TgC yr(-1) ). A small increasing trend in flooding amplitude, and by extension NPP, was observed for the studied period. Variability in growth rates caused by local biotic and abiotic factors, and the lack of knowledge on macrophyte physiological responses to extreme hydrological conditions remain the major sources of uncertainty.