Dana G. Berens

Logging and forest edges reduce redundancy in plant-frugivore networks in an old-growth European forest

Summary1. Seed dispersal by frugivores is the basis for regeneration of fleshy-fruited plants in forest ecosys-tems. Previous studies have reported a decrease in forest specialist frugivores due to logging andforest edges. Forest generalists appear less sensitive and may even increase at forest edges. Suchchanges in the abundance of frugivores may have consequences for consumer/resource ratios andcompetition in plant–frugivore networks.2. Optimal foraging theory predicts an increase in dietary specialization of animals at low consumer/resource ratios due to reduced competition. A decrease in forest specialists in logged forests shouldcause decreased consumer/resource ratios, increased dietary specialization and reduced redundancy,whereas an increased abundance of forest generalists at edges may compensate for a loss of specialists.3. In Europe’s last old-growth lowland forest (Bialowieza, Eastern Poland), we recorded fruit_removal by frugivores from fleshy-fruited plant species in the interior and at edges of logged andold-growth forests for 2 consecutive years.4. The abundance of forest generalists increased at forest edges, whereas specialists were unaffected.Conversely, logging resulted in a decrease in abundance of forest specialists but had no effect onthe abundance of generalists. Accordingly, consumer/resource ratios increased from interior to edgesand were reduced in the interior of logged forests compared with the interior of old-growth forests.As predicted by optimal foraging theory, a decrease in consumer/resource ratios coincided withincreased dietary specialization and a loss of redundancy in the interior of logged forests. Despitelow dietary specialization, redundancy was reduced at forest edges as forest generalists dominatedplant–frugivore interactions.5. Synthesis. We show that a shift in frugivore assemblages at forest edges and increased dietaryspecialization of frugivores in the interior of logged forests involved a loss of redundancy comparedwith continuous old-growth forests. This suggests that seed dispersal services in secondary foresthabitats depend on an impoverished subset of dispersal vectors and may suffer reduced adaptivepotential to changing environmental conditions. Thus, our study highlights the value of old-growthforests for the conservation of frugivore-mediated seed dispersal processes.Key-words: Bialowieza Forest, ecosystem services, functional niche, mutualistic networks, optimal_foraging, plant–animal interactions, resource specialization, seed dispersalIntroduction

Additive effects of exotic plant abundance and land-use intensity on plant–pollinator interactions

The continuing spread of exotic plants and increasing human land-use are two major drivers of global change threatening ecosystems, species and their interactions. Separate effects of these two drivers on plant–pollinator interactions have been thoroughly studied, but we still lack an understanding of combined and potential interactive effects. In a subtropical South African landscape, we studied 17 plant–pollinator networks along two gradients of relative abundance of exotics and land-use intensity. In general, pollinator visitation rates were lower on exotic plants than on native ones. Surprisingly, while visitation rates on native plants increased with relative abundance of exotics and land-use intensity, pollinator visitation on exotic plants decreased along the same gradients. There was a decrease in the specialization of plants on pollinators and vice versa with both drivers, regardless of plant origin. Decreases in pollinator specialization thereby seemed to be mediated by a species turnover towards habitat generalists. However, contrary to expectations, we detected no interactive effects between the two drivers. Our results suggest that exotic plants and land-use promote generalist plants and pollinators, while negatively affecting specialized plant–pollinator interactions. Weak integration and high specialization of exotic plants may have prevented interactive effects between exotic plants and land-use. Still, the additive effects of exotic plants and land-use on specialized plant–pollinator interactions would have been overlooked in a single-factor study. We therefore highlight the need to consider multiple drivers of global change in ecological research and conservation management.

Correlated loss of ecosystem services in coupled mutualistic networks.

Networks of species interactions promote biodiversity and provide important ecosystem services. These networks have traditionally been studied in isolation, but species are commonly involved in multiple, diverse types of interaction. Therefore, whether different types of species interaction networks coupled through shared species show idiosyncratic or correlated responses to habitat degradation is unresolved. Here we study the collective response of coupled mutualistic networks of plants and their pollinators and seed dispersers to the degradation of Europes last relict of old-growth lowland forest (Białowieża, Poland). We show that logging of old-growth forests has correlated effects on the number of partners and interactions of plants in both mutualisms, and that these effects are mediated by shifts in plant densities on logged sites. These results suggest bottom-up-controlled effects of habitat degradation on plant-animal mutualistic networks, and predict that the conversion of primary old-growth forests to secondary habitats may cause a parallel loss of multiple animal-mediated ecosystem services.

Habitat Characteristics of Forest Fragments Determine Specialisation of Plant-Frugivore Networks in a Mosaic Forest Landscape

Plant-frugivore networks play a key role in the regeneration of sub-tropical forest ecosystems. However, information about the impact of habitat characteristics on plant-frugivore networks in fragmented forests is scarce. We investigated the importance of fruit abundance, fruiting plant species richness and canopy cover within habitat fragments for the structure and robustness of plant-frugivore networks in a mosaic forest landscape of South Africa. In total, 53 avian species were involved in fruit removal of 31 fleshy-fruiting plant species. Species specialisation was always higher for plants than for frugivores. Both species and network-level specialisation increased with increasing fruit abundance and decreased with increasing fruiting plant species richness and canopy cover within fragments. Interaction diversity was unaffected by fruit abundance and canopy cover, but increased slightly with increasing fruiting plant species richness. These findings suggest that especially the availability of resources is an important determinant of the structure of plant-frugivore networks in a fragmented forest landscape.

Natural habitat loss and exotic plants reduce the functional diversity of flower visitors in a heterogeneous subtropical landscape

Summary 1. Functional diversity (FD) of pollinators can increase plant reproductive output and the stability of plant-pollinator communities. Yet, in times of world-wide pollinator declines, effects of global change on pollinator FD remain poorly understood. Loss of natural habitat and exotic plant invasions are two major drivers of global change that particularly threaten pollinator diversity. 2. In a subtropical South African landscape, we investigated changes in the FD of flower visitor assemblages on native and exotic plants along gradients of natural habitat loss and relative abundance of exotic plants. We used a data set of 1434 flower visitor individuals sampled on 131 focal plants and calculated the FD in three flower visitor traits that are strongly related to plant–flower visitor interactions and pollination processes: proboscis length, proboscis diameter and body length. 3. Multivariate FD of flower visitors decreased with both increasing natural habitat loss and relative exotic abundance. Importantly, changes in FD went beyond those in flower visitor richness. Furthermore, flower visitor richness was not related to either natural habitat loss or relative exotic abundance. Loss in multivariate FD seemed to be mediated by complementary reductions of FD in proboscis length with natural habitat loss and of FD in body length with both global change drivers. Correspondingly, we recorded lower abundances of long-tongued flower visitors with natural habitat loss and reduced variance in body size with both drivers. In contrast, FD in proboscis diameter was unaffected by either driver. All effects of the two global change drivers were non-interactive. 4. Our results show that both natural habitat loss and exotic plants negatively affect flower visitor FD, which may imperil pollination of specialized plant species in degraded habitats. In contrast, flower visitor richness may not cover all facets of flower visitor FD that are relevant to pollination processes. Distinct responses of visitor traits to the two drivers suggest limited options to infer relations of one trait to another. Finally, additive effects of natural habitat loss and exotic plant invasions highlight the need to consider multiple drivers of global change when investigating ecosystem processes at a community scale.

Variation in neighbourhood context shapes frugivore-mediated facilitation and competition among co-dispersed plant species

Summary1. Co-occurring and simultaneously fruiting plant species may either compete for dispersal byshared frugivores or enhance each other’s dispersal through joint attraction of frugivores. Whilecompetitive plant–plant interactions are expected to cause the evolutionary divergence of fruit phe-nologies, facilitative interactions are assumed to promote their convergence. To which extent com-petitive and facilitative interactions among plant species with similar phenological niches arecontrolled by spatial variation in their local abundance and co-occurrence is poorly understood.2. Here, we test the hypotheses that when a plant species fruits in high densities, large phenologicaloverlap with other plant species causes competition for seed dispersers owing to frugivore satiation.Conversely, we expect large phenological overlap to enhance the dispersal of a plant species fruitingin low densities through attraction of frugivores by other species in its local neighbourhood.3. We test these predictions on plant–frugivore networks based on seed removal from 15 woody,fleshy-fruited plant species by 30 avian and 4 mammalian frugivore species across 13 study sites inBialowieza Forest, Poland._4. A null model indicated that fruit phenologies of the regional plant assemblage were more differ-entiated than expected by chance. In the local networks, the tendency of plants to share frugivoresincreased with phenological overlap. High phenological overlap reduced the seed removal rates,interaction strength (proportion of interactions) and the number of partners of plant species fruitingin high densities. Conversely, plant species fruiting in low densities mainly profited from high phe-nological overlap with other species. Importantly, the sharing of mutualistic partners among co-fruiting plant species was also reflected in their co-occurrence.5. Synthesis. Our study highlights that, in spite of the overall signal of competition, frugivore-medi-ated interactions among cofruiting plant species may consistently promote the establishment and per-sistence of rare species through facilitation. In addition, our results suggest that, among otherfactors, indirect coupling of species through shared mutualistic partners might be an important deter-minant of plant community assembly. The coupling through shared mutualists may cause the forma-tion of associations among co-dispersed plant species and might contribute to the coexistence ofspecies in plant–animal mutualistic communities.Key-words: Bialowieza Forest, interaction networks, limiting similarity, niche differentiation, phe-_nological overlap, plant–animal mutualism, plant–plant interactions, seed dispersalIntroduction

Effects of local tree diversity on herbivore communities diminish with increasing forest fragmentation on the landscape scale.

Forest fragmentation and plant diversity have been shown to play a crucial role for herbivorous insects (herbivores, hereafter). In turn, herbivory-induced leaf area loss is known to have direct implications for plant growth and reproduction as well as long-term consequences for ecosystem functioning and forest regeneration. So far, previous studies determined diverging responses of herbivores to forest fragmentation and plant diversity. Those inconsistent results may be owed to complex interactive effects of both co-occurring environmental factors albeit they act on different spatial scales. In this study, we investigated whether forest fragmentation on the landscape scale and tree diversity on the local habitat scale show interactive effects on the herbivore community and leaf area loss in subtropical forests in South Africa. We applied standardized beating samples and a community-based approach to estimate changes in herbivore community composition, herbivore abundance, and the effective number of herbivore species on the tree species-level. We further monitored leaf area loss to link changes in the herbivore community to the associated process of herbivory. Forest fragmentation and tree diversity interactively affected the herbivore community composition, mainly by a species turnover within the family of Curculionidae. Furthermore, herbivore abundance increased and the number of herbivore species decreased with increasing tree diversity in slightly fragmented forests whereas the effects diminished with increasing forest fragmentation. Surprisingly, leaf area loss was neither affected by forest fragmentation or tree diversity, nor by changes in the herbivore community. Our study highlights the need to consider interactive effects of environmental changes across spatial scales in order to draw reliable conclusions for community and interaction patterns. Moreover, forest fragmentation seems to alter the effect of tree diversity on the herbivore community, and thus, has the potential to jeopardize ecosystem functioning and forest regeneration.