Forest degradation limits the complementarity and quality of animal seed dispersal

Abstract

Forest degradation changes the structural heterogeneity of forests and species communities, with potential consequences for ecosystem functions including seed dispersal by frugivorous animals. While the quantity of seed dispersal may be robust towards forest degradation, changes in the effectiveness of seed dispersal through qualitative changes are poorly understood. Here, we carried out extensive field sampling on the structure of forest microhabitats, seed deposition sites and plant recruitment along three characteristics of forest microhabitats (canopy cover, ground vegetation and deadwood) in Europe's last lowland primeval forest (Białowieża, Poland). We then applied niche modelling to study forest degradation effects on multi-dimensional seed deposition by frugivores and recruitment of fleshy-fruited plants. Forest degradation was shown to (i) reduce the niche volume of forest microhabitat characteristics by half, (ii) homogenize the spatial seed deposition within and among frugivore species, and (iii) limit the regeneration of plants via changes in seed deposition and recruitment. Our study shows that the loss of frugivores in degraded forests is accompanied by a reduction in the complementarity and quality of seed dispersal by remaining frugivores. By contrast, structure-rich habitats, such as old-growth forests, safeguard the diversity of species interactions, forming the basis for high-quality ecosystem functions.

Common seed dispersers contribute most to the persistence of a fleshy-fruited tree

“…3 , 4 ). Our results revealed that seed dispersal by several important disperser species in the Białowieża Forest is non-random not only for the plant community as a whole 15 , but also for the seeds of individual species. Non-random seed dispersal by animals seems to be common 12 , 14 and has been observed, for instance, for bellbirds 56 , muntjacs 26 , and lemurs 57 along canopy gradients in tropical forests.…”

“…The seed dispersal mutualism is an important ecosystem process that contributes to animal nutrition and the plant’s regeneration cycle 6 – 8 . In return for fruit pulp 9 , 10 , animals deposit seeds in favorable microhabitats, improve seed germination, and help plants to colonize new locations 11 – 15 . At larger scales, seed dispersal maintains meta-community dynamics 16 and helps plants to migrate 17 , 18 .…”

“…However, there are no studies on the seed dispersal loop in species-rich and complex ecosystems 36 . Thus, it remains unclear whether the functional role of animals is context-dependent 3 , 27 , 37 , 38 , can be predicted based on the interaction frequency of animals with plants 4 or based on ecological traits 39 , 40 , and whether the loss of interactions due to the extinction of individual species can be compensated by other animals 12 , 15 , 41 . Even when all mutualistic dispersers are lost simultaneously, plants may still persist, if seed dispersal by gravity, wind or water is sufficient 42 , 43 .…”

“…In a second step, we determined animal-specific deposition patterns of seeds by collecting and DNA barcoding 1729 scat samples with 9590 seeds of F. alnus and other fleshy-fruited plants along the forest canopy gradient (Fig. 1c ) 15 , 53 . In a third step, we included an estimate of seed germination after gut passage from a recent meta-analysis in the models 11 .…”

“…In a third step, we included an estimate of seed germination after gut passage from a recent meta-analysis in the models 11 . In a fourth step, we investigated seedling recruitment by experimentally sowing 2500 seeds in the forest 15 . These comprehensive datasets were extended in this study by investigating the growth, survival, and reproduction of 938 individuals of F. alnus along the forest canopy gradient over three years (Fig.…”