Recording species interactions is one of the main challenges in ecological studies. Frugivory has received much attention for decades as a model for mutualisms among free-living species, and a variety of methods have been designed and developed for sampling and monitoring plant–frugivore interactions. The diversity of techniques poses an important challenge when comparing, combining or replicating results from different sources with different methodologies. With the emergence of modern techniques, such as molecular analysis or multimedia remote recorders, issues when combining data from different sources have become especially relevant. We provide an overview of all the techniques used for monitoring endozoochorous primary seed dispersal, focusing on a critical appraisal of the advantages and limitations, as well as the context-dependency nature, of the different methods. We propose five data merging approaches potentially useful to combine frugivory interactions data from different methodologies. Additionally, we provide two case studies where we combine empirical data from plant–animal interactions in Mediterranean shrublands using different methodologies. Data merging resulted in a net increase in the number of distinct pairwise interactions recorded and compensated biases inherent to different methods, resulting in a more robust estimation of network topological descriptors. These case studies clarify the context-dependent character of the merging approaches, highlighting the value of collecting detailed information on the sampling effort in terms of reliable results and reproducibility. Finally, we discuss the trends with different methodological approaches used in the last decades and future perspectives in this field.

We used two empirical datasets to illustrate data merging approaches, with two different organization levels. Both case studies are focused on plant–frugivore interactions taking place in the Mediterranean shrubland of Doñana National Park, Huelva, Spain. In each case study two sampling methods were used to maximise animal–plant interactions detected.

The first case is an individual-based study on the avian frugivore assemblage of Pistacia lentiscus (Anacardiaceae) in El Puntal area, where monitoring cameras and DNA-barcoding were used to record interactions. Cameras methodology involved placing continuous-monitoring cameras (GoPro Hero® 7 model) facing individual plants. Forty individual plants were filmed for approximately 2 hours in several runs in different days (total of 84.5h). Any avian visitation was recorded as an interaction, yielding a total of 397 visitation records. Cameras were operative from sunrise for 2h and recording was set at maximum resolution. Data resulting from this sampling can be given as total number of records, or standardized by sampling time (no. records h^-1). DNA-barcoding methodology (González-Varo et al. 2014) was done to faecal samples collected in seed traps located under the same forty individual trees. Seed traps were working for 102.7±8.9 days (mean ±SD) per plant. A total of 1371 faecal samples were analyzed (mean no. per plant: 33.8±15.2). Samples were collected regardless of whether or not they had seeds, as an indicator of a visitation event. Data resulting from this sampling can be given as a total number of records with positive identification of a given frugivore species, or standardized by the sampling time with seed traps actively operating in the field (no. records/trap/day or similar)

The second case is a community-based study aiming to document species-specific plant–frugivore interactions in Hato Ratón, where analysis of fecal samples obtained with mist-netting and focal observations were used to detect interactions. Estimation of the dietary diversity of frugivore species through mist-netting, relied on seeds identification found in bird feces together with microhistological techniques to identify the non-seed remains of fruits by examining under microscope (40X, 100X) the shape, size, and structures (trichomes, glands) of exocarp tissue cells. This allowed not only the identification of fruit species when no seeds are present but also the relative volume occupied in the sample, so that an estimate of the corresponding number of fruits ingested can be derived (Jordano 1988). Between 6-10 mist nets were operated weekly for 1–2 days (for a total of 84 sampling days and 4080.5 mist-net hours), totalling 3541 fecal samples analyzed. Feeding records of frugivores visiting fruiting plants were obtained during 1.0 km-length walk censuses in the area, with 2–5 censuses carried out per month (123 sampling days), totalling 89.5 km and 2031 records. These focal observations were based on spot censuses where interactions are recorded during short stops as the observer advances along a fixed transect. In some cases (<15 % of the records) where no handling was observed but just the visit to the plant, the number of fruits was approximated from data on feeding rate (no. fruits/visit). Data resulting from this sampling can be given as total number of records, or standardized by sampling time (no. records km^-1 census or no. records h^-1 or day^-1, or similar).

For more information please refer to the Supplementary Material of our manuscript: Quintero, Isla & Jordano 2021 Methodological overview and data-merging approaches in the study of plant-frugivore interactions. Oikos 00: 1–18, 2021. doi: 10.1111/oik.08379.

Additional data, codes of analysis, final merged datasets, and supplementary figures are available in Zenodo (http://doi.org/10.5281/zenodo.4751889) or GitHub repository (https://github.com/PJordano-Lab/MS_Oikos_FSD_Monitoring_interactions).

pl_bc.csv - Observation matrix obtained with DNA-barcoding method in El Puntal case study with Pistacia lentiscus individual plants.

pl_cam.csv - Observation matrix obtained with monitoring cameras method in El Puntal case study with Pistacia lentiscus individual plants.

bc_sampling_effort.csv - Sampling effort for DNA-barcoding method in El Puntal case study at Pistacia lentiscus individual plants.

cam_sampling_effort.csv - Sampling effort for monitoring cameras method in El Puntal case study at Pistacia lentiscus individual plants.

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hr_mn.csv - Observation matrix obtained with mist-netting method in Hato Ratón case study

hr_obs.csv - Observation matrix obtained with focal observations method in Hato Ratón case study