Closing the seed dispersal loop: quantitatively linking plant-frugivore interactions to the recruitment cycle of plant communities
Final Report Abstract
Seed dispersal by animals is a key process for the functioning of a forest ecosystem, also in temperate forests. The disruption of seed dispersal interactions between frugivores and plants has been discussed to challenge plant regeneration and ecosystem functioning in the long-term. Understanding the effects of plant-frugivore interactions for plant population dynamics as well as consequences of forest degradation for these processes is thus essential to safeguard intact forest communities. In our project, we linked plant-frugivore interactions to recruitment of associated plant populations in the light of forest degradation. Using the Białowieza Forest as a unique study site, we applied molecular techniques to determine the identity of seed dispersers for >9,000 deposited seeds over three consecutive years. We complemented these data by data on recruitment and survival of >6,000 plant individuals. Throughout the study period, we identified at least 15 woody, fleshy-fruited plant species that are dispersed by 42 frugivorous animals in ash-alder forests. The seed-dispersing animals differed in the range of their sizes from small common birds such as the European robin or forest mice, to intermedia-sized mammals such as foxes and martens, to the largest European herbivores moose and bison. Although the frugivore community is highly diverse, the majority of seeds were dispersed by only four common songbirds. These four species can be considered key dispersers in Bialowieza Forest, which highlights the value of common bird species for ecosystem functioning. Based on interaction networks created for this community, we could show that seed deposition networks identify the same most important frugivore disperser species as networks compiled from fugivore observations. Yet, single elusive species are better represented using the indirect deposition networks. We further identified fruit availability as a key feature to determine network characteristics. Integrating frugivore effects into plant regeneration for a model species, we found strong microhabitat-dependent survival of the species. Animal seed dispersal increased the population growth rate of the dependent plant species and was especially beneficial when the plant relied on animal-dispersers to reach favorable recruitment sites. Surprisingly, we could show that due to a high degree of functional redundancy in interactions, the loss of single disperser species could be functionally buffered by the remaining community. Investigating trait-matching in our frugivore and plant community, we found that, interestingly, the variation in fruit sizes was higher within plant individuals than between individuals of the same species. This subindividual variation in fruit size presumably improves the connectedness of plant individuals with their seed dispersers, so that key dispersers of temperate plant communities should be able to disperse fruits of all plant individuals in a community. Last, we found that forest degradation halved the niche space of forest microhabitats seeds were deposited in and especially led to a disappearance of microhabitats important for recruitment. As a result, the quality of seed deposition was reduced in degraded forest, as deposition sites were very homogeneous and lacked complementarity. Our results show that the higher habitat heterogeneity in old-growth forests in comparison to degraded forests safeguards the diversity of plant-frugivore interactions, which translates in more robust recruitment patterns of the plant community. Overall, our project revealed new insights into the structure and functioning of seed dispersal interactions in temperate forest ecosystems and highlights the value of old-growth forests to preserve ecological processes in the long-term.
Publications
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(2021) Observing frugivores or collecting scats: a method comparison to construct quantitative seed dispersal networks. Oikos 130 (8): 1359-1369
Schlautmann, J., Rehling, F., Albrecht, J., Jaroszewicz, B., Schabo, D. G., Farwig, N.
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(2021) Within-species trait variation can lead to size limitations in seed dispersal of small-fruited plants. Frontiers in Ecology and Evolution, 9
Rehling, F., Jaroszewicz, B., Braasch, L. V., Albrecht, J., Jordano, P., Schlautmann, J., Farwig, N., Schabo, D. G.
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(2022) Forest degradation limits the complementarity and quality of animal seed dispersal. Proceedings of the Royal Society: Biological Sciences
Rehling, F., Schlautmann, J., Jaroszewicz, B., Schabo, D.G., Farwig, N.
