Extreme rainfall events alter the trophic structure in bromeliad tanks across the Neotropics

Changes in global and regional precipitation regimes are among the most pervasive components of climate change. Intensification of rainfall cycles, ranging from frequent downpours to severe droughts, could cause widespread, but largely unknown, alterations to trophic structure and ecosystem function. We conducted multi-site coordinated experiments to show how variation in the quantity and evenness of rainfall modulates trophic structure in 210 natural freshwater microcosms (tank bromeliads) across Central and South America (18°N to 29°S). The biomass of smaller organisms (detritivores) was higher under more stable hydrological conditions. Conversely, the biomass of predators was highest when rainfall was uneven, resulting in top-heavy biomass pyramids. These results illustrate how extremes of precipitation, resulting in localized droughts or flooding, can erode the base of freshwater food webs, with negative implications for the stability of trophic dynamics.


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Policy information about availability of data All manuscripts must include a data availability statement. This statement should provide the following information, where applicable: -Accession codes, unique identifiers, or web links for publicly available datasets -A list of figures that have associated raw data -A description of any restrictions on data availability Gustavo Romero Apr 13, 2020 The database is originated from replicated field experiments. No software was used for data collection.
All the analyses were conducted using the free R software and language. We used R version 3.6.0 (2019-04-26) This is a multi-site coordinated experiments to show how variation in the quantity and evenness of rainfall modulates trophic structure in 210 natural freshwater ecosystems (tank bromeliads) across Central and South America (18°N to 29°S).
We used natural, detritus-based microcosms (bromeliad phytotelmata) as model systems due to their widespread distribution and ease of manipulation. Bromeliad aquatic ecosystems are inhabited by a diverse fauna of macroinvertebrates (insects, crustaceans), comprising top predators, mesopredators, and detritivores.
At the end of the experiment (60th day), we dissected each bromeliad by removing and washing each leaf separately in running water and then filtered this water through 125 and 850 µm sieves. We recorded the morphospecies and abundance of all aquatic macroinvertebrates (body size larger than 0.5 mm). We determined the body size and trophic position of each individual organism surveyed. Trophic position was determined from our own feeding trials, gut contents and from the literature. To calculate invertebrate body mass, we used allometric equations between the body length and dry mass, or mean of dry mass for very small insects.
For each experimental site, we used 30 bromeliads (i.e., 30 sample units), which were randomly selected to receive one of the treatments described in the item "Reproducibility" below. Sample size (n=30) is predefined because of the combinations of treatments (10 levels of treatment µ and 3 levels of the treatment k) The field experiments lasted 60 days. We replicated the experiment in seven sites across the Neotropics. Within each site, the 30 experimental bromeliads were at least 2 m apart each other.
No data were excluded from the analyses.
We contrasted rainfall-mediated changes in hydrological stability of the study system with the effects of two main rainfall components: (i) the mean daily amount of rainfall, !; and (ii) distribution of rainfall events around this mean through time, k (i.e., a measure of rainfall frequency). Sites with many dry days and infrequent rainfall resulted in low ! and low k respectively. Ambient levels of the rainfall components were first determined using recent meteorological data from each site (see Methods). We applied a negative binomial distribution to these data to estimate the parameters ! and k. We randomly applied ten levels of ! (ranging from 0.1 to 3.0) and three levels of k ranging from 0.5 to 2.0 in a fully factorial experimental design at each of our seven sites for a total of 10 × 3 × 7 = 210 food webs in individual bromeliads. This allowed us to compare ambient, baseline conditions (! = 1, k = 1) and extreme fluctuations of rainfall quantity (10-300%) and frequency (50-200%) to average historical levels of daily variability for each site.
Each of 210 experimental bromeliads (30 bromeliads x 7 sites) represented an independent sample unit. The experiment was replicated in seven distinct sites. All attempts at replication were successful.
In each site, we randomly selected thirty bromeliads of the most abundant species and with the most common size. We used bromeliads that had more than 100 ml of tank capacity and thus can be colonized by the large predators. To remove any residual invertebrates, we hung the bromeliads upside down, and let them dry for seven days. For each site we homogenized detritus and Blinding Did the study involve field work? Field-collected samples invertebrates collected from these bromeliads, then used equal aliquots of the mixture to initiate the community assembly in the experimental ecosystems27. We employed individual transparent plastic shelters above each bromeliad to prevent natural rainfall into the plants.
Each experiment was replicated seven times across the Neotropics. The PIs and their research groups conducted the experiment in these different sites following a standardized protocol. Typically the researchers cannot see what organisms are living into the bromeliad wells before dessicating them in the laboratory.
Each field site presented particular environmental conditions. These variations were controlled in our experiments. All the 7 replicated experiments were conducted during the rainy season.
We replicated the experiment at seven sites across Central and South America (from 29°S to 18°N), including Las Gamas All the experiments were conducted following the rules and laws of the related coutries. No material was necessary to import or export. All the materials were analysed in local laboratories, following local rules and laws. When necessary (depending on local rules), permits for conducting the experiments were requested and approved.
In each site, we selected thirty bromeliads of the most abundant species and with the most common size. We used bromeliads that had more than 100 ml of tank capacity and thus can be colonized by the large predators. We washed each bromeliad with spring water to remove detritus and organisms. To remove any residual invertebrates, we hung the bromeliads upside down, and let them dry for seven days. For each site we homogenized detritus and invertebrates collected from these bromeliads, then used equal aliquots of the mixture to initiate the community assembly in the experimental ecosystems. We employed individual transparent plastic shelters above each bromeliad to prevent natural rainfall into the plants. The rain shelters were settled high