Introduction

Between one-third and one-half of Earth's land surface has been heavily influenced by humans [1], and 60% of tropical forestland (approximately 850 million hectares) is classified as secondary or degraded [2]. Degraded forests tend to harbour lower biological diversity, sequester less carbon, and differ in function and productivity when compared with nearby undisturbed forests [3]–[5]; however, specific restoration activities by humans in degraded forests can reverse these impacts and result in increased species diversity, ecosystem functioning, and carbon sequestration [5]–[7]. Some of this human-driven tropical forest restoration is achieved through active planting of native tree seedlings [8], but the majority of tropical forest regeneration takes place in a passive way, occurring via natural forest succession [9]. Passive regeneration is less resource and labor intensive than active restoration, but its success depends on a variety of factors, such as forest type, land-use history, and distance to nearest intact forest [10]. Thus, a key challenge for ecologists is to determine (1) the conditions under which passive regeneration can occur, and (2) which conservation measures most effectively assist passive regeneration [10].

Vertebrate frugivores assist in passive regeneration by transporting seeds to degraded areas from nearby intact forest [11]. Since approximately 90% of tropical forest tree species have fleshy fruits adapted for vertebrate dispersal [12], and vertebrate dispersers around the globe are under threat from overhunting, habitat fragmentation and invasive species [13], there is a critical need to understand the impact of frugivore loss on both the passive regeneration and active restoration of degraded forest.

Frugivores play an important role in the regeneration of degraded forests. Primates [14], [15], lizards [16], [17], bats [18], and birds [18]–[21] have all been shown to move seeds from primary to degraded habitat. Many frugivores travel long distances with seeds [22] and do not show an aversion to travelling through degraded forest areas [11], [23]; some bird species even show a preference for feeding, perching and roosting, and therefore defecating seeds, near gaps or edges [24]–[26]. In particular, vertebrate frugivores are important for moving the seeds of woody pioneer species and deep-forest, large-seeded species into degraded landscapes [27]–[31]. Several studies in regions with healthy frugivore communities have directly monitored seed rain and seedling recruitment in degraded areas adjacent to intact forests and have shown that vertebrate frugivores do move seeds, sometimes from far away, into degraded areas, often acting as the major (or only) source of native seed rain (e.g. [18], [19], [32]–[35]). The importance of vertebrate seed dispersal has been recognized by the implementation of restoration techniques which encourage dispersal, such as the planting of fruiting trees and ‘tree islands’ within degraded areas [36]–[38].

Despite the evidence that vertebrates play a role in regeneration of secondary forests, conservation or restoration of vertebrate frugivores is frequently ignored as a strategy for restoration of degraded lands (e.g. [39], [40]). The worldwide decline of vertebrate frugivore populations [41]–[43] may have negative impacts on forest regeneration [44]–[48], yet the magnitude of the effect of frugivore loss on seed dispersal to degraded forest is unknown. Here, we use a unique natural experiment to examine the impact of complete loss of vertebrate frugivores on seed dispersal from intact to degraded forests. Such natural experiments enable scientists to answer questions that cannot be addressed with a manipulative experiment for logistical or ethical reasons [49]; for example, removing all frugivores from large spatial areas is not feasible, thus a natural experiment provides a unique opportunity for understanding the magnitude of the seed dispersal provided by frugivores into degraded forest areas. While it is unlikely that forests around the world will lose all frugivores, examining the impact of such a dramatic change provides a critical view of the worst-case scenario.

The brown tree snake (Boiga irregularis) was accidentally introduced to the island of Guam in the late 1940s, causing a widespread loss of Guam's native forest bird species by the mid to late 1980s [50], [51]. Prior to the introduction of the brown tree snake, Guam's native forest avifauna consisted of 12 species, of which six were frugivorous. Predation by the brown tree snake led to the extirpation of five frugivorous species and the functional extirpation of the sixth [51]. Unlike many other places around the world that have lost native frugivores, Guam's forests have not been colonized by introduced avian frugivores that could play a similar ecological role to the extirpated species. The nearby island of Saipan provides a strong comparison to Guam, as it has similar forests, but in contrast to the silent forests of Guam, Saipan has no known snake population and healthy bird populations [52].

We investigated whether the species richness and density of bird-dispersed seeds varies between degraded forests on Saipan (with birds) and Guam (no birds). We also asked whether the proportion of seeds lacking fleshy fruit pulp (primarily due to handling by birds) differs between forests with birds (Saipan) and those without (Guam). Ours is the first study of which we are aware that utilizes a comparison between similar forests with and without avian frugivores to investigate the regeneration potential of degraded forests.

Materials and Methods

Site Description

This study was conducted on the Micronesian Islands of Guam (13°27′N, 144°46′E) and Saipan (15°11′N, 145°44′E). Both islands are at the southern end of the Mariana Island chain (Figure 1), and have climates with an average annual temperature around 27°C. The tropical climate of Guam averages 2586 mm of rainfall per year [53]. Saipan receives 1900–2300 mm of precipitation per year [54].

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 1. Map of the Mariana Islands.

The locations of Guam and Saipan, the two islands used in this study, are shown.

https://doi.org/10.1371/journal.pone.0065618.g001

The primary forest type on Guam and Saipan is karst forest, which grows on a rugged karst limestone substrate; there are approximately 40 tree and shrub species in this forest type, but 8–12 species dominate. Large swaths of karst forest were destroyed on both Guam and Saipan in the 1930s and 40s as a result of World War II, and clearing has continued for development since then. When land is cleared, the rugged karst substrate is lost, and replaced by a topographically homogenous, red dirt substrate, which is typically colonized by the non-native leguminous tree, Leucaena leucocephala (Family: Fabaceae). Today, both islands have large areas of degraded forest composed of nearly monotypic stands of L. leucocephala with a sparse understory [55], [56]. Only 13% of Guam's remaining land area is intact karst forest, while 25% is degraded forest [55]. On Saipan, a smaller island with a proportionally larger impact of WWII, less than one percent of the total land area is intact karst forest, with 61% covered by degraded forest [56].

In this study, we define degraded forests as forests dominated by L. leucocephala. Degraded forest has a low (4–8 m in height) canopy that allows significant light to penetrate to the forest floor, whereas the intact karst forest is taller (7–13 m in height), creating a multi-storied, low-light environment. Narrow bands of transitional forest, characterized by a mixture of L. leucocephala and native forest trees, form the ecotone between degraded and intact forest.

Bird-dispersed tree species

Our study focused on seeds from native, bird-dispersed tree species. We designated a tree species as ‘bird-dispersed’ if its seed is covered by fleshy fruit pulp, birds have been observed consuming either the flesh or the entire fruit, and each seed is small enough to be consumed or carried by the largest frugivorous bird species that once occurred on the island. Additionally, we have found seeds from two common species, Premna obtusifolia and Psychotria mariana, in bird scat, providing additional support for their status as ‘bird-dispersed’. Table 1 includes a summary of the bird species seen consuming fruit from each tree species. All recorded observations of avian frugivory in the Marianas come from observations of birds rather than trees, thus we do not have any quantitative estimate of the proportion of fruit crops consumed by birds.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Presence and dispersers of bird-dispersed tree species and their seeds on Guam and Saipan.

https://doi.org/10.1371/journal.pone.0065618.t001

Results

The species composition of the intact forests on Guam and Saipan were qualitatively similar as shown by transect surveys (Table 1). Similarly, seed rain in intact forest on both islands contained seeds from three native, bird-dispersed tree species: Melanolepis multiglandulosa, Premna obtusifolia, and Psychotria mariana (Table 1). In addition, nearly all traps on both islands contained seeds from the non-native species Leucaena leucocephala, although the seed rain in degraded forest differed between islands (summarized in Table 2). All seeds from native, bird-dispersed tree species found in traps in degraded forest on Saipan were from three species: Melanolepis multiglandulosa, Premna obtusifolia, and Psychotria mariana.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 2. Summary of seed trap contents on each island, and in each forest type.

https://doi.org/10.1371/journal.pone.0065618.t002

After excluding seeds captured in traps located underneath remnant native trees in degraded forest on each island, significantly more native, bird-dispersed species were captured per square meter of degraded forest on Saipan than on Guam, where none were caught. Based on coefficients from the best-fit statistical model, which incorporates the random effects of transects within sites, 1.66 seeds of native bird-dispersed tree species fall every 26 days in each square meter of degraded forest on Saipan. The species richness and density of seeds from native bird-dispersed tree species are best explained by models including island, forest type and their interaction (Table 3). Native seed species richness was greater on Saipan than on Guam, and greater in intact than in degraded forests (Figure 2A). The density of seeds from native bird-dispersed trees was higher on Saipan than on Guam and lower in degraded forests than intact karst forests (Figure 2B).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Figure 2. Mean (A) species richness and (B) density of native bird-dispersed seeds on Guam and Saipan.

Dark grey bars indicate intact karst forest, while light grey bars indicate degraded forest. Error bars represent standard error. Means were calculated using raw data on native bird-dispersed seeds per trap (0.5 m²); in degraded forests, we used only seeds found in traps with no remnant native trees nearby.

https://doi.org/10.1371/journal.pone.0065618.g002

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 3. Comparison of statistical models containing different combinations of Island and Forest type as explanatory variables for the richness of (column 1) and density (column 2) of seeds that come from native bird-dispersed species.

https://doi.org/10.1371/journal.pone.0065618.t003

The proportion of seeds from bird-dispersed tree species that were de-pulped was lower on Guam than on Saipan. On Guam, zero seeds were de-pulped; on Saipan, 68.5% of seeds in intact forest, and 76.3% of seeds in degraded forest, were de-pulped. Because there were no native bird-dispersed seeds away from remnant trees and no de-pulped seeds in degraded forest traps on Guam, we were unable to fit a full model with island, forest type, and their interaction to these data. Instead, we compared the proportion of seeds that were de-pulped out of all seeds found on each island. The model containing island as a predictor fit better than the model without island, indicating that proportion of seeds with pulp removed differs greatly between Guam and Saipan (delta AIC  = 14.47). The proportion of bird-dispersed seeds with pulp removed was slightly higher in degraded forest than intact karst forest on Saipan (0.77 vs. 0.69), but the AIC value for the model including forest type as an explanatory variable was within 1.5 AIC units of the null model (including only an intercept), indicating that forest type does not significantly improve model fit.

Discussion

Avian frugivores act as critical links between intact karst forest and degraded forest in the Mariana Islands. We found increased species richness and density in the seed rain landing in degraded forest on Saipan compared with Guam, likely due to the absence of frugivorous birds in the forests on Guam. Zero seeds/m² from bird-dispersed tree species were dispersed into degraded forests on Guam, indicating a loss of seed dispersal services on this island. In contrast, approximately 1.66 native bird-dispersed seeds/m² are dispersed into degraded forest on Saipan per 26 days. While this is a small number of seeds, these seeds provide the potential for regeneration of native trees within the degraded forest. There are few wind-dispersed tree species in the Marianas, and Mariana Fruit Bats are functionally extinct on Guam and Saipan, leaving birds and gravity as the primary modes of dispersal. This is the second study to document the loss of an ecosystem function once provided by birds on Guam; the previous demonstration focused on birds as pollinators [71].

Vertebrate dispersers play a key role in catalyzing regeneration of native forest. In systems with intact frugivore communities, vertebrate dispersal from native to degraded forest is evident from the abundance of seeds and seedlings from fleshy-fruited species found in degraded forest [20], [29], [33], [72]. A study of forests on the island of Saipan suggests that vertebrate dispersal is important for bringing native seeds to degraded forest because most of the native seeds and seedlings sampled in degraded forest had small, fleshy fruits typically dispersed by birds [73]. In our study, the native species dispersed into seed traps in degraded forest on Saipan were, in descending order of abundance, Premna obtusifolia, Psychotria mariana, and Melanolepis multiglandulosa. These species produce large fruit crops, and birds have been observed eating all three species. M. multiglandulosa is a pioneer species in this system, as it is only found in gaps and edges of forests.

Initial dispersal events from intact to degraded forests are important because they can initiate the regeneration of degraded areas. Once a native fleshy-fruited tree becomes established in degraded forest, it provides habitat, food, and perching and roosting sites for frugivores, which in turn will bring more seeds from the nearby intact forest [28], [36], [74], [75]. Without birds bringing in early pioneer species, we predict that regeneration of native species in degraded forest will progress more slowly on Guam than on Saipan.

While the focus of this study is on seed dispersal into degraded forests, we also found significant differences in seed rain in intact karst forests (Figure 2). Seed traps placed in intact forest on Guam collected a lower richness and density of seeds from native bird-dispersed species than on Saipan. Using transect surveys (Table 1), we showed that the forests on Guam and Saipan were qualitatively similar in species composition; thus, we believe that these differences in abundance and diversity reflect the lack of dispersal by birds in Guam's forests, rather than differences in species composition or inadequate sampling. Frugivorous birds often consume seeds from one tree, then perch on another and defecate, leading to a higher richness of seeds within a single seed trap. On Guam, the only seeds found in forest traps likely originated from tree species with canopies overhanging the traps since no volant dispersal agents are present; this potentially explains the lower richness and density of seeds in traps placed in Guam's intact karst forests compared to traps on Saipan (Figure 2).

In addition to moving seeds into degraded forests, birds may influence the likelihood that seeds germinate. When birds ingest fruit, they remove the fleshy fruit pulp and scarify the seed, processes that enhance germination in some species, but do not affect or even reduce seed germination in others [76]–[81]. A recent study of 15 New Zealand tree species found a statistically significant but biologically unimportant effect of birds on the proportion of seeds that germinated, suggesting that germination of most tree species would be unaffected by the loss of ingestion by birds [82]. In our study, none of the native bird-dispersed seeds found on Guam had their fleshy fruit removed, as was expected since birds are absent. On Saipan, however, nearly two-thirds of all bird-dispersed native seeds found in traps lacked fruit pulp. In a separate study, germination experiments in an outdoor nursery for two of the common bird-dispersed species, Premna obtusifolia and Psychotria mariana showed increased germination of handled seeds over whole fruit (Rogers, unpub.). Thus, the loss of birds on Guam may affect germination due to the loss of seed handling for at least these two species, regardless of whether the seeds land in degraded or intact karst forest. However, this positive effect of bird handling may not be a general phenomenon in Mariana plants, and thus requires additional study. Un-dispersed seeds may also experience lower levels of germination due to increased natural enemy attack near the parent trees (i.e. Janzen-Connell effects) [83].

There are at least two aspects of the present study that limit our capacity to predict the full impact of birds on degraded forest regeneration. First, we did not assess rates of germination from the seed bank in degraded forest. It is possible that native species were at one point deposited in the seed bank by birds, but are waiting for the right conditions to germinate [84]. However, it is unlikely that native seeds would still be viable if they fell prior to land clearing (10–80 years ago) or were brought to the degraded forests when bird dispersers were present on the island (>25 years ago), since many tropical seeds are viable for short periods of time [27], [85]. Second, we did not assess rates of germination and survival of seeds and seedlings in degraded forest, so it is possible that bird dispersal of native seeds into degraded forest does not lead to regeneration of those species. There are several lines of indirect evidence that suggest that at least some native seeds, once dispersed into degraded forests, can germinate and survive. On Saipan, native seedlings have been observed growing in the understory of many L. leucocephala-dominated forests [73], [86]. In addition, we have observed forests on Saipan that have a soil substrate similar to that indicative of cleared forest, but that support a forest cover similar to intact forest with only a few large adult L. leucocephala trees. These areas were likely bulldozed many years ago changing the substrate from karst to soil, then were initially colonized by L. leucocephala, which has since been gradually replaced by native forest species, resulting in a forest with a similar composition and diversity to intact karst forest. Finally, L. leucocephala does not appear to invade intact karst forest successfully, perhaps because it does not compete well with native trees in undisturbed forest [86]. If L. leucocephala is indeed a poor competitor with native trees, then karst forest regeneration in areas currently dominated by L. leucocephala may be successful without labor-intensive management of L. leucocephala once native seedlings get established.

Bird populations worldwide are in decline due to habitat loss, climate change, introduced disease, and invasive species [41], [47]. Nearly one-quarter of all frugivorous birds are prone to extinction [42], and the decline will impact the ecosystem services they provide [22], [47], [87]–[89]. Birds provide an important ecosystem service when they move seeds from native to degraded forest. In the Marianas, the intact karst forest harbours greater tree species diversity than degraded forest, including many species of cultural importance (e.g. Intsia bijuga, Premna obtusifolia, Eugenia spp.), and provides habitat for wildlife (e.g. coconut crabs, Mariana Fruit Bats, Micronesian Starlings). In addition, intact forests are highly productive, and often store more carbon than degraded forests due to the abundance of large, old-growth trees as opposed to small, successional trees found in degraded forests, as well as a higher canopy with understory, mid-story and canopy tree levels [6], [7], [90]; we suspect this is also the case in the Marianas. Thus, if seed dispersal by birds does speed up regeneration of degraded forest, frugivorous birds deserve recognition for providing a key ecosystem service for the people of the Mariana Islands.

Land managers should explicitly consider the role of seed dispersers in degraded forest regeneration. In locations where frugivore populations are healthy, conservation of intact forest and frugivore communities will ensure that passive regeneration of degraded forest is not limited by dispersal. In locations where frugivore communities are reduced or absent, conservation of intact forest, restoration of frugivore communities, and intensive management may all be needed to restore or replace the ecosystem services provided by frugivores. Conservation of remnant intact forest patches should be prioritized because these remnants serve as a seed source for degraded forests; forest regeneration is most likely to occur in areas where both older remnant forest stands and seed dispersers—such as avian frugivores—persist nearby [91], [92]. Restoration of frugivore communities should also restore the ecological functions they provide (e.g. seed dispersal). In cases where frugivores are absent or reduced in abundance and their restoration is impossible, management efforts will need to include active planting of seeds and seedlings in degraded forest. There has been little effort to restore native karst forest speces to areas currently covered with degraded forest in the Marianas, except for a few small-scale plantings by volunteers across the islands. Our results suggest that these efforts are necessary on Guam because birds no longer provide seed dispersal to degraded forests. In addition, managers should consider re-introducing frugivorous birds within snake exclosures in order to restore seed dispersal services. Reintroduction of native frugivores should be a top priority restoration strategy, since non-native frugivores may not replace the ecological functions of native frugivores [93]. Finally, in situations where populations of native frugivores cannot be conserved, introduction of analogue frugivores should be considered, in order to at least partially maintain the ecosystem services they provide [94]. On Guam, this could mean introducing analogue disperser species that are able to co-exist with brown tree snakes.

The island of Guam provides the worst-case scenario for the impact of frugivore loss on seed rain in degraded forest, and shows a stark view of the disruption of ecosystem services provided by frugivores, evident here as the complete cessation of seed rain into degraded forests. The unequivocal absence of adequate frugivore populations on Guam both demands immediate action, and provides an opportunity for managers to test and advance new methods for addressing seed dispersal disruptions.

Supporting Information

Acknowledgments

We thank Elizabeth Hooshiar for guidance, Kaitlin Mattos for organizational skills, Jenny Howard for field help, Eric Cook for seed identification assistance and Ross Miller at the University of Guam for providing vehicles and lab space. Dennis Hansen, Ailene Kane Ettinger, Susan Waters, and two anonymous reviewers provided helpful comments on an earlier version of the manuscript.