In subtropical and tropical forests up to 90% of woody plant species depend on fruit eating animals for the dispersal of their seeds. Birds are the most diverse and abundant animal group that acts as seed dispersers. Yet, many birds are threatened by the ongoing deforestation and the introduction of alien invasive (non-native and ecosystem-transforming) plant species. It remains a challenge for ecologists to predict how different frugivorous bird species respond to these environmental changes with ultimate consequence for the dispersal service they provide.
Fig 1 Large, undisturbed subtropical forests nowadays are generally confined to protected areas and gorges. On the plains forest extent is heavily reduced by intensive sugar cane farming.
Factors that may drive bird responses to habitat disturbance comprise different dependencies on forested habitat and fruits as resources. Whereas forests specialists only occur in large, undisturbed forests, forest generalists often prevail in smaller fragments, and forest visitors generally dwell in open habitat such as grassland. Similarly, obligate frugivores exclusively feed on fruits to meet their dietary demands, whereas partial frugivores supplement their diets with insects or floral nectar, and opportunistic frugivores only rarely pick some of their favorite fruits.
Fig 2 Forest island within sugar cane. Forest specialists and specialized frugivores are practically absent from such islands.
In our study “Guild-specific shifts in visitation rates of frugivores with habitat loss and plant invasion”, now on Early View in Oikos, we investigated whether changes in visitation rates of bird species (to plants for foraging on fruits) with forest loss and plant invasion can be predicted by their different dependencies on forested habitat and fruits as resources. To do so, we conducted extensive observations of plant-frugivore interactions in a subtropical South African forest landscape. These forests are highly diverse, and more than 700 bird species can be found in the region! However, South African forests are also under increasing pressure from intensive agriculture and urban sprawl, and many invasive plant species have replaced the natural vegetation. Fieldwork was fun but could be tough – sometimes we observed no visitor in 18-h of observation! Further, we wore military-style ‘ghillie suits’ for camouflage, a very effective way to hide in dense vegetation and minimize disturbance of birds. However, a woolen suit is a rather poor choice in South African summer…
Fig 3 Trumpeter hornbills (Bycanistes bucinator) are among the largest avian frugivores in South Africa. This individual feeds on fruits of Ficus glumosa.
Fig 5 Speckled mousebirds (Colius striatus) are generalized frugivores which are able to persist in a wide array of differently disturbed habitats. Unfortunately, few fruits on this Tassel-berry (Antidemsa venosum) shrub are fully ripe yet.
Still, the African bird life was totally worth it, and we found highly interesting results. As expected forest specialists were most negatively affected by habitat loss. However, interestingly, obligate frugivores were overall least affected by habitat loss and plant invasion. Fully depending on fruits requires a generalized fruit choice, which seems to make obligate frugivores more robust to changes in habitat conditions. In contrast, visitation of partial and opportunistic frugivores declined – a pattern that can be explained by the comparably more specialized or ‘picky’ foraging behavior of non-obligate frugivores. Specialist foragers were particularly rare when high degrees of habitat loss and plant invasion interacted in synergy.
Fig 4 Testing the ghillie suit in a farmhouse garden. When hiding in even denser vegetation, the observer becomes practically invisible to the avian (and human) eye.
In summary, our study shows that forest loss and plant invasion may especially negatively affect forest specialists and specialized frugivores. This is worrying, as it is those ‘unusual’ species, which by their diverging ecological and behavioral differences from generalized species, considerably contribute to the astonishing diversity of subtropical and tropical forests. Not to forget that most often, they are also wonderful to look at.
Fig 6 These fruits of invasive Bugweed (Solanum mauritianum) show clear marks of pecking frugivores. The plant flowers (background) and fruits at the same time and is able to do so year-round, a significant advantage over many native plant species.
Ingo Grass, Dana G. Berens and Nina Farwig