A network analysis of plant–pollinator interactions in temperate rain ...

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704 Oecologia (2009) 160:697–706Fig. 5 Species contribution to topological shifts driven by selectiveremovals. Left-hand panel Change in network temperature afterdeleting species i (ordered in the ordinate). Right-hand panel Changein the value of the fitted exponent of the power law degreedistribution. Dashed vertical lines ±95th percentile of the absolutevalue of deviations from the whole network. Tree species are (frombottom to top) Amomyrtus luna, Amomyrtus meli, Caldcluviapaniculata, Eucryphia cordifolia, Embothrium coccineum, Gevuinaavellana, Luma apiculata, Myrceugenia ovata, Myrceugenia planipes.D.g. Diphaglossa gayi, B.d. Bombus dahlbomii, A.m. Apismellifera, M.p. Mesograpta philippi, M.c. Mitraria coccinea, A.o.Asteranthera ovata, U.m. Ugni molinae, T.s. Tepualia stipularis; forother abbreviations, see Figs. 3 and 4the Chiloé rain forest network is consistent with manywell-resolved pollination and seed dispersal bipartite networks(Bascompte et al. 2003; Vázquez and Aizen 2004)and gives quantitative support to earlier community characterizations(Smith-Ramírez et al. 2005). On the otherhand, the Chiloé rain forest network exhibited a connectivitypattern best described by a power law (scale-free)cumulative degree distribution. This pattern is widespreadacross many complex networks including ecologicalinteraction webs, as well as protein, metabolic, social andinformation networks (Albert and Barabási 2002; Newman2003; but see Dunne et al. 2002a). Scale-free degree distributionsand nestedness are associated with networkrobustness against random loss of species, but also withhigh sensitivity to deletion of hub species (Albert et al.2000). This indicates that the studied community exhibits astructure of interactions well suited for facing unselectiveextinctions.What is the relative importance of defined groups ofplant and insect species for the maintenance of the Chiloérain forest network? There were numerous shrubs speciesand they were more specialized than trees in terms ofpollinator visits, although they were in turn more generalistthan the fewer vine species. Consequently, their removalwas predicted to decrease the steepness of the degree distributionof plants, although it would not change itsexponential shape. As a result, the network without shrubsretained the core generalist tree species and increased theirpersistence. Nevertheless, the loss of the more specializedshrubs, together with the supergeneralist Tepualia stipularis(degree = 47), would cause a decrease in the level ofnestedness of the network because most of the remnantspecies were weakly nested generalists. Because in nestednetworks specialist pollinators are mostly linked to generalistplants (Bascompte et al. 2003), the loss of shrubswould cause a decrease in the degree of generalist pollinators.The loss of shrubs, therefore, changed the power lawdistribution of the animals’ degree distribution in the originalnetwork to exponential. On the other hand, becausetrees were more generalist in their pollinators, theirremoval made the exponential degree distribution of plantssteeper, and increased the extinction probability of bothplants and pollinators.Hymenopterans were the most generalist group of speciesamong pollinators visiting large numbers of plants and,unlike trees, they included the supergeneralist bumblebeeBombus dahlbomii (degree = 21). This caused the animals’degree distribution to shift from power law toexponential when hymenopterans were deleted from thesystem. On the other hand, this also caused the loss ofspecialist plants, shifting the plants’ degree distributionfrom exponential to long-tailed power law. Both theremoval of generalist animals and the loss of specialistplants explain the decrease in network nestedness. Associatedwith these topological changes, we found that theprobability of extinction largely increased after loss ofhymenopterans from the system.The removal of dipterans had an appreciable quantitativeeffect on the degree distribution of rain forest plants inthe system, which became steeper due to a decrease in thedegree of generalist forest plants. This shift also caused thedegree distribution of the whole network to shift frompower to a truncated power. Although nestedness remained123
Oecologia (2009) 160:697–706 705unchanged after deleting flies, the dynamic output didchange. At high extinction/colonization rates, removingdipterans did not differ from random pollinator speciesdeletion. At low and moderate k, species extinctions werelower after removing dipterans compared to randomremoval, which is explained by the strong effect ofhymenopterans. Hence, random hymenopteran removalwould constitute a more severe perturbation than dipteranremoval. The exception was found for plants at k = 0.25,where deletion of dipterans predicted the extinction of twoplant species (Gaultheria mucronata and Myrteola nummularia)that depend exclusively on them.Removing coleopterans from the web exerted only amarginal effect on network persistence, because extinctionof pollinators was entirely explained by decreased animalspecies richness. Plant extinction probability was lowerwhen removing beetles relative to random species loss, andnot different from the dynamics of the unmanipulatednetwork (except at k = 0.75). Removal of coleopterans didnot change the shape of the degree distributions, either forthe whole network or for plants and animals separately.The same occurred when vines, mostly related to specializedvisitors, were removed from the mutualistic web.The effect of vine removal on extinction probability canbe considered negligible at low and moderate k because ofthe low number of extinct species (mean = 0.99 plantspecies, range 0–3). At k = 0.75, random removal exerteda more severe effect on species persistence than removal ofvines due to the greater relative weight of trees.Overall, this work highlights the importance of floweringcanopy trees and hymenopterans as the core groups thatmaintain the species richness, structure and dynamics ofthe Chiloé rain forest pollination network. This wasrevealed by the changes detected in topological anddynamic features of the rain forest network after modelingthe removal of each group of species. Furthermore, discerningwhich species are mostly responsible for the effectsobserved after removal of multi-species groups can be ofgreat value from a conservation perspective, becauseresearch, conservation and management efforts can bedirected to few pivotal species in cases where protectingthe entire community is not possible. By assessing thesensitivity of degree distribution and nestedness to theremoval of individual nodes we obtained a measure ofthe relative contribution of each species to the maintenanceof community structure and diversity. In this way, weidentified a number of species that deserve special attentionfor the conservation of the entire species assemblage. Thehymenopterans Bombus dahlbomii and Diphaglossa gayi,the shrubs Tepualia stipularis and Ugni molinae, the vinesMitraria coccinea and Asteranthera ovata, and the wholeset of trees were predicted to exert a disproportionatelylarge influence on the preservation of the structure of thenetwork under study. Their extinction would seriouslyharm the reproductive success of plants in this temperaterain forest. Some tree species with the richest sets ofpollinators, such as the emergent Eucryphia cordilfolia, arecurrently under severe and rapid decline due to selectivelogging and land clearing, and their loss may promotecascade extinctions of network participants (Armesto et al.1996). In particular, the woody species Tepualia stipularisand Eucryphia cordifolia were shown in this study to playan important structural role within the Chiloé network andthey are also relevant for habitat provision (Díaz et al.2005). Unfortunately, they both are highly threatenedspecies in Chiloé Island, representing respectively 55 and11% of the total commercialized firewood (unpublisheddata).It is accepted that the stability and robustness of pollinationnetworks to species loss are sensitive to structuralattributes of the network. More specifically, nestedness andlong-tailed degree distribution have been postulated as keytopological properties that prevent species loss (Memmottet al. 2004; Fortuna and Bascompte 2006; Bascompte andJordano 2007). Here we explored the possible interplaybetween structure and dynamics of a pollination systemthat supports a highly valued and remarkably endangeredforest ecosystem. A better understanding of the functioningand evolution of target ecological systems would be gainedif further efforts were made to ensure that most of theparticipant species were recorded. In addition, it is necessaryto identify them at the best level of taxonomic resolution,recording their temporal and spatial variation, and todevelop better analytical tools to manage, organize andtake advantage of the data.Acknowledgements The authors thank J. D. Flores for his valuablehelp in scientific computing, M. A. Fortuna for his help with thedynamic model, M. A. Rodríguez-Gironés for specially implementinga batch version of his software BINMATNEST, and M. Franco forimproving the readability of this paper. All the experiments andsampling comply with the current laws of the country (Chile) inwhich they were performed. Work started by an Endowed PresidentialChair in Sciences to J. J. A. We acknowledge support from FON-DAP–FONDECYT 1501-0001, P05-002 ICM and PFB-23 CONI-CYT, Chile. This is a contribution to the research program of SendaDarwin Biological Station, Chiloé, Chile. We appreciate the companyand assistance in the field of M. Nuñez-Ávila, L. Suárez, P. Martínezand el Negro.ReferencesAizen MA, Ezcurra C (1998) High incidence of plant-animalmutualisms in the woody flora of the temperate forest ofsouthern South America: biogeographical origin and presentecological significance. Ecol Aust (Argentina) 8:217–236Aizen MA, Vasquez DP, Smith-Ramírez C (2002) Historia natural yconservación de los mutualismos planta-animal del bosquetemplado de Sudamérica austral. Rev Chil Hist Nat 75:79–97123
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