Artificial perches produced a significant increase in the abundance in the seed rain that reach deforested areas surrounding the Espinal woodland remnants. The abundance of seeds found under artificial perches indicates that these structures act as a focus of attraction for birds and promote their arrival, facilitating the entry of plant propagules and giving rise to seed nucleation points in areas that have lost woody cover (Zwiener et al. 2014). These results agree with those obtained by most restoration programs that have used artificial perches around the world (Guidetti et al. 2016). In the few studies that did not find a significant increase in seeds under artificial perches, the duration of sampling may have been insufficient, considering that birds require a certain period of adaptation to start using these structures (Shiels and Walker 2003). Predation of dispersed seeds by other animals was also considered another possible cause (Holl 1998). Increasing abundance in seed rain in deforested areas is crucial, especially given that having a nearby source of propagules does not always imply a bigger seed rain and, in addition to that, a large proportion of the seeds arriving in the area do not survive until germination (Graham and Page 2012; Reid and Holl 2013). Therefore, the use of artificial perches then results in higher plant densities than projects based on passive restorations (Tres and Reis 2009; Schorn et al. 2010).
The installation of artificial perches also produced a significant increase in species richness compared to seed rain in deforested areas without perches. Perches give temporal and spatial continuity to the diverse interactions between frugivorous birds and plants in the forest, enabling, for example, the dispersal of rare species seeds by anti-apostatic selection, an important mechanism that structures the diversity of forests in recovery (Karubian et al. 2012; Carlo and Morales 2016). Thus, the seed rain under perches reflects in part the reproductive potential of endozochoric plants (Guimarães et al. 2008). Nevertheless, the process of seed dispersal from the woodland to the open area involves not only fruit consumption but also the movement of the birds. In this sense, there are some forest visitors or habitat generalists species more adapted to the conditions in deforested areas, but forest specialist species may refuse to cross the edge of the woodland, dispersing seeds mostly in the forest interior or flying straight ahead directly to another forest remnant (Bennun et al. 1996; García et al. 2010; Pizo and dos Santos 2010; Carlo et al. 2013). Generally, frugivorous bird species with non-specialized diets (species that consume fruits based on their availability in the environment) are the ones that are habitat generalists, making the most important seed dispersal among different environments (Purificação et al. 2014). This characteristic of the frugivorous bird assemblage can be the reason for the diminution of the seed species richness observed in the artificial perches in comparison to the woodland treatment. It is expected that as the development of secondary forest patches progresses in the deforested areas, more forest specialists and obligate frugivorous species will begin to move within the forest, increasing much more the species richness in the seed rain also there (Schleuning et al. 2011a; García and Martínez 2012; Bomfim et al. 2018).
Seed composition under artificial perches was similar to that of the woodland. Since seed flow is established from the neighboring woodland remnants, perches can promote vegetation similar to the original ecosystem (Schorn et al. 2010). Considering that the identity of the seed rain species has decisive effects on the natural succession of the sites, the control in the seeds’ dispersal of non-native species could be one of the factors that seriously condition the use of artificial perches. It is quite common for invasive species to produce large amounts of fruits (Gleditsch and Carlo 2011). Here, the dispersal of non-native species was high in all treatments, even within the woodland. The noticeable difference between native and non-native species abundance was strongly influenced by the quantity of Morus alba (Common Mulberry; Moraceae) dispersed seeds. This species has a large number of seeds per fruit (56.8 ± 15.7) and a large number of fruits per adult tree (1000-10000, SD, pers. obs.), ripen at the studied area from October to early December (SD, pers. obs.). But, while the abundance of native seeds did not decrease significantly outside the woodland, the richness of native species did. Again, the reason for the difference observed between the species richness in the artificial perches and the woodland treatment could be the movement of occasional or opportunistic frugivorous being attracted to the woodland when the amount of exotic fruits increases abruptly or tracking the presence of others across the foraging landscape (Carlo et al. 2007; Purificação et al. 2014). Instead, obligate frugivorous species get involved in much more interactions to sustain their basic metabolic requirements and have more selective foraging behaviors (Schleuning et al. 2011a; Sebastian-Gonzalez 2017; Bomfim et al. 2018), but do not leave the forest as frequently (Purificação et al. 2014), increasing the seed rain species richness of native plants inside the woodland. However, thinking about the possibilities of ecological restoration is interesting to state that several non-native species differ on the fruiting season with native (Gosper 2004; White and Vivian-Smith 2011). In the study area, the fruiting peak for non-native fleshy fruit species was detected from October to December, while for the native species the peak is between January and February and another occurs between May and June (Scarpa 2013). So, installation of artificial perches could be carried out if species with fleshy fruits in nearby woodland remnants are well recognized, avoiding sites where non-native species are present or placing them only at the time of the year when the invasive species have fewer fruits (Prather et al. 2017).
A large proportion of the seeds that reached the site favored by the artificial perches in the experimental area were woody plant species, as it has also been found by Prather et al. (2017) in Houston, USA. In this sense, artificial perches act in a similar way to isolated trees and remnant shrubs that persist to disturbance, accelerating the colonization of woody species below and around their canopy (Slocum 2001; Schlawin and Zahawi 2008). After developing, these trees or erect shrubs could act as natural perches for birds and successively improve conditions in the deforested area (providing shade, food, and nesting sites). Regeneration in deforested areas appears to be irregular and dependent on the formation of vegetation groups or nuclei that facilitate the recruitment of seeds from the forest, the establishment of new seedlings, and the expansion of woody plant cover (Carlo and Morales 2016). The influence of these nuclei continues even in later successional stages, showing a higher density of trees, basal area, and species richness in their vicinity (Slocum 2001; Schlawin and Zahawi 2008).
The experience states that artificial perches for birds are easy to implement, require little labor for manufacture and installation, and have substantially low costs, taking into account that they can be manufactured from materials recycled or found in the site (Holl 1998; Reis et al. 2010; Graham and Page 2012). In the future, it would be interesting to analyze what happens with more complex perches, or with a grouped arrangement of perches, since large nuclei are proven to be more attractive to birds (Holl 1998; Toh et al. 1999; Corbin and Holl 2012) and here only the simplest model has been studied. Perches seem to be a possible alternative when there are remnants of woodland in the proximity of the open area. It would also be interesting to study the optimal distances relative to the edge of the remnant for which perches are most effective in the Espinal (Pizo and dos Santos 2010). Nevertheless, while a big challenge of restoration in deforested areas is to attract birds to deposit seeds, there can be other barriers to plant regeneration (Holl et al. 2000; Reid and Holl 2013). For that reason, we considered that artificial perches may be much more successful in ecosystems that are sufficiently resilient and just need minimal intervention. Subtropical and temperate environments with disturbance matrices similar to the ones considered here have shown a significant increase in seedlings established under artificial perches (Guidetti et al. 2016). The soil and environmental conditions (low elevation, warm temperatures, and abundant rainfall) during and immediately after the period of highest native seeds dispersal at the experimental sites are expected to favor germination and seedling growth (Holl 2013). Further, both sites were located in protected areas, making the commitment of stopped disturbances and ensuring that care can be prolonged in time. In other contexts, additional measures and multiple approaches (such as soil transposition or livestock exclusion) may be required (Bevilacqua Marcuzzo et al. 2013).
In conclusion, artificial perches function as a nucleation technique to increase seeds dispersal by birds in deforested areas around Espinal woodland remnants. Artificial perches increased the abundance and species richness in the seed rain, with a species composition similar to that of the woodland. Using artificial perches should be avoided in sites with potentially invasive non-native species. However, in places where native and non-native species show different fructification peaks, artificial perches could be used in certain periods of the year, avoiding the dispersal of undesirable seeds of invasive species. In addition, artificial perches principally facilitate the arrival of woody species that then also serve as natural perches; this can progressively improve the conditions in the deforested area, having a positive impact even in later successional stages.