1. Range expansion in plant populations, especially at the colonization front, can be either limited by disproportionately large effects of antagonistic interactions or facilitated by their release. How the strength of antagonistic interactions changes along successional gradients during range expansion is still poorly documented, especially when diverse assemblages of plant antagonists (rodents, invertebrates, and birds) combine within interaction networks.

2. We study the changes in individual-based, predispersal seed-pulp predator networks along a colonization gradient in a rapidly-expanding Juniperus phoenicea population in Doñana National Park (SW Spain). Additionally, we analysed the role of individual plant traits and neighbourhood attributes in network configuration by using Exponential Random Graph Models.

3. Seven seed-pulp consumer animal species varied significantly in their frequency of interaction and prevalence. While invertebrate species were well established in old and intermediately mature stands, greenfinch (Chloris chloris) was dominant at the colonization front. Variable species roles and spread of interactions among individual plants generated changes in the configuration of interactions during plant expansion.

4. Individual plant traits strongly determined the topology of these networks, although with differences between stands. Increasing individual crop size and seeds per cone increased the interaction odds of individual plants, while seed viability showed the opposite effect. The network topology at the colonization front appeared less driven by individual traits, possibly because of the short interaction history of this recently established area. The disproportionately large effect of C. chloris in these recently established stands, potentially resulted in large seed losses during range expansion.

5. Synthesis.  Turnover of antagonistic interactions, characterized the colonization front, resulting in more heterogeneous interaction strengths among individual plants. We found no evidence for a complete or sizeable antagonistic release of J. phoenicea at the colonization front promoting this rapid expansion. It becomes necessary to explore interactions with seed dispersers to understand how antagonistic and mutualistic plant-animal interactions balance during range expansion. Our study highlights the importance of an individual-based approach in understanding how interactions are structured and driven in natural changing landscapes.

Sampling interactions

We monitored each of the 105 focal plants during the two consecutive study seasons, using different methods (see Online Suppl. Mat. of published paper for details). To collect data on rodent interactions, we combined two sampling methods: camera-trap survey and live-trapping. We conducted 8 direct night-time trapping campaigns and the photo-trapping sampling effort involved a total of 201,600 hours of recording, both methods evenly distributed among focal plants. For both methods we consider the presence of a species under a focal plant as a potential interaction. We quantified invertebrate interactions by dissecting 50 mature cones from each focal plant per season (number of analysed cones = 7,906 cones). We identified antagonistic species and their markings based on Roques et al., (1984). When a parasitoid was recorded, we assigned the event as a detection of the host species, and indicative of one interaction of the plant with the corresponding host pulp-seed predator species. Finally, C. chloris interactions were quantified by seed traps placed under focal plants to collect the remnants of cones falling during their visits. Based on video records of foraging finches during full visits to the plants (mean [±1SD] of 4,5 ± 2,6 cones consumed per visit), we estimated that every four damaged cones found in the seed traps conservatively indicate one visit event by C. chloris in an individual plant. We carried out 35 checks of the seed-trap during the entire sampling, in which we counted the number of cones attacked, and emptied the trap. To handle the same interaction units for all data, we calculated the frequency of occurrence of each animal interaction for its survey. For example, how frequently an insect species was present in a cone relative to all the analysed cones, or how frequently we detected at least one C. chloris visit during weekly seed-traps checks.

Plant characteristics 

We thoroughly sampled a set of intrinsic and extrinsic plant traits known to influence animal preferences across the 105 focal plants. We considered plant traits that could drive interactions with pre-dispersal seed-pulp predators hierarchically (Sallabanks, 1993): first, the individual neighbourhood context, then general plant traits, and finally at a higher scale of detail, cone quality traits. To characterize the juniper neighbourhood density and productivity of each plant we used the georeferenced location of all the individuals in each stand and estimated by direct count the cone production in all the juniper individuals growing in a buffer area of 100 m² surrounding each focal individual. For each focal individual, we recorded its height, the two maximum diameters of its canopy projection, canopy area, and total cone crop size. Direct counts of cones were carried out by scanning the whole plant canopy area and counting the cones with a hand-counter. We used the harvested cones (50 cones per plant) to measure cone traits. For each plant, we measured the average values of maximum length and diameter of the cones, total fresh mass, pulp mass, seed mass, one-seed mass, number of seeds per cone and seed viability was estimated by flotation procedure.

Specific code for this research is available in our gitHub Repository: https://github.com/PJordano-Lab/Juniperus_antagonists_2022