Downsizing of animal communities triggers stronger functional than structural decay in seed-dispersal networks

Downsizing of animal communities due to defaunation is prevalent in many ecosystems. Yet, we know little about its consequences for ecosystem functions such as seed dispersal. Here, we use eight seed-dispersal networks sampled across the Andes and simulate how downsizing of avian frugivores impacts structural network robustness and seed dispersal. We use a trait-based modeling framework to quantify the consequences of downsizing—relative to random extinctions—for the number of interactions and secondary plant extinctions (as measures of structural robustness) and for long-distance seed dispersal (as a measure of ecosystem function). We find that downsizing leads to stronger functional than structural losses. For instance, 10% size-structured loss of bird species results in almost 40% decline of long-distance seed dispersal, but in less than 10% of structural loss. Our simulations reveal that measures of the structural robustness of ecological networks underestimate the consequences of animal extinction and downsizing for ecosystem functioning.


Supplementary
in Sorensen et al. 6 for detailed information about these bird species). Body mass values in (a) and (b) were transformed to their natural logarithm. We used the density() function in R with the default smoothing bandwidth values to create both density plots. Note that the 34 species represent the full range of body mass variation recorded in the Andean communities (body mass ranges: 6.8-1770 g for the Andean species; 9-1770 g for the 34 bird species available in GPT studies, with 97% of the Andean species located in the overlapping zone).

Supplementary Note 1 | Evaluation of potential regurgitation events on the effect of downsizing on structural and functional losses in seed-dispersal networks.
In this study, we focused on avian seed dispersal events resulting from endozoochory.
However, ingested seeds might also be dispersed via regurgitation, which reduces gut passage time (GPT) in comparison to defecation time 7,8 . Regurgitated seeds are thus expected to be dispersed over shorter distances 9 . Previous studies suggested that large seeds, relative to frugivore size, are more likely to be regurgitated 10,11 , but that there is no clear allometric relationship between regurgitation time and avian body size 12,8 .
We defined defecation vs. regurgitation based on the size of fruits relative to frugivore size 8,13 and specifically used gape width as the most accurate measure of avian size in terms of seed handling 14 . To discard potential regurgitation events from the simulations, we fitted a linear quantile regression (quantile level t = 0.75) relating fruit diameter and bill width of each interaction event across the full dataset (n = 11,572 events). We assumed that 25% of these events would be due to regurgitation and removed all interaction events located above the fitted regression line (n = 2,889 events, see Supplementary Figure 6). Because regurgitation times are known to be shorter than defecation times 7,8 , we discarded these events from our simulations because regurgitated seeds would not be relevant for communitywide long-distance seed dispersal. Results of the effect of downsizing on structural and functional losses in seed-dispersal networks were qualitatively identical between simulations including all interaction events and those excluding the potential regurgitation events (Supplementary Figures 7-9).

Supplementary Figure 6 | Relationship between avian bill width and fruit diameter.
Each black dot represented a plant-frugivore interaction from the eight empirical networks recorded along the Andes. Blue line represented the t = 0.75 estimate according to the quantile regression model (log10 D = 0.6 ± 0.03 + 0.61 ± 0.014 log10BW; parameter estimate ± SE, p < 0.001, n = 11,572), which was fitted using the R package quantreg 15 , version 5.52. We assumed that fruit size in the interaction events above the fitted blue line were large relative to the bill width, so we considered these seed-dispersal events to be likely due to regurgitation and excluded them for the analysis. Results of the effect of downsizing on structural and functional changes after discarding the regurgitated seeds are shown in Supplementary Figs. 7-9. Values on the y-axis show the percentage of functional change defined by the 0.95 quantile of the community seed-dispersal distance relative to the value in the original network. Grey areas represent the 95% confidence intervals across the eight Andean seed-dispersal networks. Compare to Fig. 3 showing simulation results including all interaction events.

Long-distance seed dispersal (LDD)
Supplementary Figure 9 | Effect size of downsizing on structural and functional losses in seed-dispersal networks (potentially regurgitated seeds excluded). The effect of downsizing is defined as the difference in the proportional losses between the size-structured and random extinction scenarios. Effect sizes for the number of interactions and secondary plant extinctions (structural loss; yellow and green lines, respectively) are compared to those for long-distance seed dispersal (functional loss; blue line) along the entire extinction sequence. Yellow, green and blue areas represent the 95% confidence intervals across the eight Andean seed-dispersal networks for the effect size of structural and functional losses. Compare to Fig. 4 showing simulation results including all interaction events.