Vertical stratification collapses under seasonal shifts in climate

Abstract

Aim Tropical forests are vertically complex, and offer unique niche opportunities in the form of climate, habitat and resource gradients from ground to canopy. Rainforest species organize within this vertical spatial gradient and recent macroecological research suggests that the highest levels of vertical stratification occur in structurally complex and climatically stable tropical rainforests. However, although the classical view of the tropics is that of aseasonality, particularly in temperature, there is strong seasonality in rainfall. Thus, we predict considerable variation in vertical stratification in time, characterized by a seasonal restructuring of communities across vertical space. Location Sierra Llorona, Colón Province, Panama. Taxon Amphibians. Methods We performed 121 ground‐to‐canopy surveys across the wet and dry seasons for amphibians. Using a bootstrap simulation method we calculated species‐specific and community‐wide vertical height and abundance shifts between seasons. Separately, we tested the importance of vertical height and season on the vertical distribution of species using a redundancy analysis, and employed linear models to explore turnover in species composition across vertical height between seasons. Results Our results show a clear downward shift of 5 m in height in amphibian communities from the wet season to dry season. We also observe significant changes in species composition across vertical strata in both seasons, driven primarily by nestedness in the dry season (loss of species over height) and genuine turnover in the wet season (loss and addition of species over height). Main conclusions The exploitation of canopy microhabitats and resources in the wet season resulted in complex patterns of stratification, whereas drying flattened the distribution and simplified the composition of arboreal communities. As such, pattern and process in the vertical dimension is not static in time but rather exists as a dichotomy with inverse patterns between wet and dry seasons. Complex, multidimensional distributions of diverse rainforest communities can be simplified by climatic constraints – an important consideration as anthropogenic climate change increases the magnitude of seasonal swings in temperature and precipitation.

Chemical defenses shift with the seasonal vertical migration of a Panamanian poison frog

“…We surveyed for A. fulguritus during the dry (mid‐December–March) and wet seasons (April–mid‐December) (Comita & Engelbrecht 2009, Basham & Scheffers 2020). Specifically, in the dry season frogs were captured between February 4, 2019 and March 9, 2019, and in the wet season between July 2, 2019 and July 27, 2019 (Figure S2).…”

“…Thus, we attempted to standardize the amount of time experienced by individuals in both seasons before sampling, with individuals sampled between 2 and 3 months after the beginning of either season (Figure S2). Using survey methods in Scheffers et al (2013) and Basham and Scheffers (2020), we conducted vertical, ground to canopy surveys for A. fulguritus , with each survey centered on a single Espavé tree, which is the principal habitat for A. fulguritus in this area (EB; unpublished data, Figure 1). A second researcher surveyed the ground in a 15 m radius around the tree for 1 hour searching through leaf litter, logs, and other microhabitats (Heyer et al 1994).…”

“…Thus, we produced 50,000 randomly derived mean differences, which we used to generate a z‐score by subtracting the mean of the distribution from the observed value and dividing by the standard deviation of the distribution. We hypothesized that a decrease in activity in the dry season (Born et al 2010, Basham & Scheffers 2020) will cause a decline in quantity and richness of alkaloid. Thus, we calculated a p value from the z‐score under a one‐tailed hypothesis, testing an effect of a lower mean in the dry season.…”

“…Here, we explore alkaloid profiles in a small neotropical poison frog Andinobates fulguritus (the yellow‐bellied poison frog, Silverstone 1975) between the forest canopy during the wet season and the forest ground during the dry season in a Panamanian rainforest. Basham and Scheffers (2020) show that A. fulguritus vertically migrates 25 m annually between the wet and dry seasons. In the wet season, A. fulguritus inhabits arboreal epiphyte masses in the canopy, whereas in the dry season it inhabits leaf litter on the ground.…”

“…As frogs are able to retain alkaloids over multiple seasons (Mebs et al 2018), the arboreality of A. fulguritus may increase the defensive capability in these frogs. However, as climate change is expected to lengthen and strengthen dry seasons across many parts of the tropics, arboreal poison frogs such as A. fulguritus may be forced to the ground for longer periods (Basham and Scheffers 2020), inhibiting their activity and possibly leading to a reduction or shift in their defensive potential.…”