Habitat loss is a global threat to biodiversity with pervasive effects on species and populations. These impacts may generate cascading effects on ecological processes propagating across ecological networks. Thus, understanding how habitat loss affects ecological networks is fundamental for conservation. We used a database of 25 plant-frugivore networks distributed across the whole Brazilian Atlantic Forest to understand how landscape-scale habitat loss shapes network structure, robustness, species role and traits related to seed dispersal. We compared whether these network properties have linear or non-linear relationships and used centrality metrics and indirect effects to evaluate if habitat loss change the role of species in plant-frugivore networks. We found linear and non-linear relationships with negative effects of habitat loss on the network structure. As a consequence of shifts in species richness and number of links, the number of interactions and the proportion of possible interactions observed (connectance) were negatively associated with habitat loss. In contrast, nestedness increased with habitat loss. Network robustness, mean bill width and mean seed size were not significantly related to habitat loss. In addition to changes in interaction patterns at network level, habitat loss also favors changes in interaction among species, shifting the species playing central roles in network organization or contributing to indirect effects in the networks. In forested landscapes, obligate frugivores are the main central species in the network, and the ones potentially contributing to indirect effects, while in deforested landscapes these roles are fulfilled by occasional frugivores. Thus, our results emphasize the widespread effect of habitat loss on plant-frugivore systems, adding evidence that its pervasive effects on biodiversity also proliferate on mutualistic interactions with negative consequences for seed dispersal that potentially go beyond the direct pairs of interacting species. 

We gathered information from published studies on frugivory networks in the Brazilian Atlantic Forest. This biodiversity hotspot has suffered from intense deforestation, and currently less than 30% of its forest still remains (Rezende et al. 2018, Galetti et al. 2021). The Atlantic Forest is considered a hotspot of biodiversity in the world due to the high diversity, levels of endemism and the presence of threatened species (Myers et al. 2000). In the Atlantic Rainforest, seed dispersal by animals is mainly performed by a diverse assemblage of bird species, especially in areas in which large mammals died out due to multiple forms of human impact. In this paper, we have focused in the seed dispersal by birds.

Data on plant-bird networks were obtained in the largest published database on interactions available for the Brazilian Atlantic Forest, the ATLANTIC FRUGIVORY (Bello et al. 2017). This database compiled more than 8000 frugivory interactions from 331 vertebrates and 788 plant species recorded in 166 studies. From the database we selected only network studies focusing on birds. Our dataset includes only community-level studies (i.e., those that recorded the interactions between bird and plant assemblages in a forest fragment, using different methodologies such as focal observations, feeding bouts, etc). Thus, we excluded from the database studies focusing on plant or animal populations. We completed the data searching for additional studies on plant-bird interactions in the Web of Science and Scopus using the following search terms: ("ecological network*" OR "mutualistic network*" OR "frugivor*") AND ("bird*") AND (“Atlantic Forest”) in TOPIC. From this search, we included only network studies that have the coordinates of the sampling fragment and provided the matrix of interactions. Our final database is composed of 25 qualitative plant-bird frugivory networks from different forest fragments, 12 from the ATLANTIC FRUGIVORY and 13 from the literature search.