Night fever at the Pantanal wetlands: night activity of anuran assemblages

“The nighttime environment provides the ecological theater for the nocturnal play”

(Gaston 2019, American Naturalist)

This blog post is provided by Dr Larissa Sugai and tells the #StoryBehindThePaper for the article “Drivers of assemblage-wide calling activity in tropical anurans and the role of temporal resolution”, which was recently published in Journal of Animal Ecology.

As ecologists, our theoretical background has profound historical roots. Sometimes, we’re not aware of the biases attached to these roots. Many have been perpetuated through generations of ecologists, and thus some approaches in ecology remain similar to how it was in the past. As pointed out by Wines (1989), our perceptual range of ecological phenomena has long been limited to anthropocentric scales because we lacked technological tools to expand our viewpoint. For example, there is an anecdotal association of most ecological processes to daylight duration. If we look back in time, studying nocturnal ecological phenomena was challenging without tools taken for granted nowadays, such as a flashlight.

This legacy manifests to this day, as only a small fraction of published articles in ecology journals address nocturnal phenomena (Gaston 2019). It’s 2020, and the ecologists’ toolbox is packed with technologies to survey organisms. In part, I’m only writing these words in here thanks to advances applied in acoustic monitoring. Using automated acoustic devices, I, Diego Llusia, Thiago Silva, and Tadeu Siqueira were able to address nighttime activity of anuran assemblages (frogs and toads) in a tropical wetland, namely the Pantanal. In our paper “Drivers of assemblage-wide calling activity in tropical anurans and the role of temporal resolution”, we show that signaling anuran assemblages not only have a wide variation in night activity but also that the ecological context modulates assemblage-wide activity patterns. And these findings were only clearer when we looked at time with a fine lens, specifically, by determining composition at the hourly resolution.

The rainy season is a frenzy period for pond-breeding anurans in the Pantanal. Beginning at dusk, males of diverse species start calling to attract mates, creating intriguing mixed-species choruses that inundate the soundscape. Each species has a species-specific advertisement call, which enables researchers to identify signaling individuals. With automated acoustic devices, the presence of a species can be determined for long periods with a fine temporal resolution, and allows, for instance, the description of diel activity patterns and their abiotic drivers. But here, we were interested if changes in diel species activity could be detected at the community scale and what the potential drivers of such changes were.

Some anuran species found in the area. First row, from left to right: Dendropsophus elianeae, Boana raniceps, Pseudis paradoxa, all from Hylidae. Second row: Boana punctata (Hylidae), Lysapsus limelum (Hylidae), Physalaemus albomarginatus (Leptodactylidae). Third row: Elachistocleis matogrosso (Microhylidae), Densdropsophus nanus (Hylidae), Leptodactylus fuscus (Leptodactylidae). (Photo credits: Henrique Folly and Larissa Sugai)

For that, we monitored 39 assemblages over a gradient of habitat heterogeneity representing variation in vegetation at the breeding sites (local) and on vegetation types from forested to open areas (landscape). For 3-5 days, we identified the signaling anuran species on 2-minute recordings taken at 20-minute intervals from approximately sunset to sunrise. We used two resolutions to describe the signaling assemblages: summing up all species at each hour, or every 3 hours corresponding to the early, mid, and late-night periods. To check how the composition (i.e., the identity of species) of signaling assemblages changes over the night, we calculated the degree of similarity between signaling assemblages from the early and late-night periods (within-night), using the 1- and 3-hour resolution.

Larissa making directional recordings of anuran individuals perched in aquatic vegetation. (Photo credit: Raul Costa Pereira)

We thought of two potential mechanisms that could promote changes in the composition of vocalizing species (i.e. which species were present) between night periods. First, we hypothesized that the exclusion of inferior competitors would underlie a pattern of changes in the composition of vocalizing species between early-late night periods. Secondly, an increase in habitat heterogeneity would promote more chances for species to explore distinct micro-habitats and reduce competition for space, leading to a pattern of few changes in anuran assemblages at night periods.

First of all, the main findings were only true when we looked at the 1-hour resolution. We found the composition of signaling anuran assemblages to be more different between early-late night periods when they (i) were composed predominantly of species from the Leptodactylidae family (white-lipped frogs) in respect to Hylidae (treefrogs), and (ii) were located at low-heterogeneous breeding habitats. Leptodactylidaeans and Hylidaeans have fundamental differences in habitat-use, with the first being mainly terrestrial species, and the former are arboreal and use the vertical stratum of vegetation. Differences in microhabitat-use between species from these two families and variation in habitat heterogeneity may reduce competitive interactions, which might underlie the pattern of changes between early and late night periods.

Some distinct types of ponds with different vegetation types in Barranco Alto farm, Aquiauana municipality, Mato Grosso do Sul state, Brazil. (Photo credit: Raul Costa Pereira and Larissa Sugai)

            Our second finding is that nightly variation in species activity may differ across days, meaning that while some assemblages have consistent within-night variation across days, others may undergo different trajectories on different days. Specifically, those assemblages that had larger variation in nightly activity across days were located at landscapes with increasing vegetation heterogeneity (e.g. tall trees and shrubs). We suggest that the more heterogeneous landscapes may favor spatial dynamics (e.g. dispersal) and combined with priority effects (i.e, depending on the identity of the early-arrivals), assemblages may undergo distinct nightly dynamics.

Tuiuiú, or jaburu (Jabiru mycteria) can be easility be spotten in this region. (Photo credit: Larissa Sugai)

These findings made a lot of sense in light of a memory. When I was 17-ish and looking for underground bands’ concerts, options were limited in my agribusiness-town in central Brazil. “Bar Fly” was the option. Starting from Thursday, each day was music themed. Being an early-arrival type of the “indie“ public, it was clear that nighttime activity had distinct phases. Early-night was characterized by a few, mostly unsociable people enjoying a playlist on their own. Middle-night saw the peak of activity, with people arriving from other parties and live music on in the bar. At the end of the night, quoting The Doors “some are born to the endless night”. Interestingly, whenever you arrived at Bar Fly, you would notice a car across the street bounded by few headbangers, namely the “metaleiros”. They were always there, all night long. The low variation in night activity of the metaleiros was akin to the Hylidaeans, while the surroundings of that loud car provided the greatest opportunity for their coexistence at night. Conversely, Bar Fly was spatially limited and the indie Leptodactylidaeans did not endure as long as the metaleiros, resulting in a larger variation in night activity. Additionally, Bar Fly had heterogeneous music genres across days, attracting different publics that underwent distinct trajectories at night, so that between-night variation was high. Metaleiros however, were at the same parking spot no matter the day, with their same nightly activity rhythm, so that between-night variation was low.

Larissa and an automated acoustic recorder. Photo credit: Raul Costa Pereira

From Bar Fly to the Pantanal wetlands, patterns of night activity can provide details on the functioning of organisms and animal communities. This finding contrasts with traditional approaches in community ecology that consider communities as static entities at fine temporal resolutions and put emphasis on larger temporal dimensions. In this sense, fine temporal resolution should be more often explicitly incorporated in community ecology approaches.

Read the paper:

Read the full paper here: Sugai, L.S., Silva, T.S., Llusia, D. and Siqueira, T. (2020), Drivers of assemblage‐wide calling activity in tropical anurans and the role of temporal resolution. J Anim Ecol. Accepted Author Manuscript.

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