Opposed latitudinal patterns of network‐derived and dietary specialization in avian plant–frugivore interaction systems

Abstract

Latitudinal patterns of biodiversity have been studied for centuries, but it is only during the last decades that species interaction networks have been used to examine the proposed latitudinal gradient of biotic specialization. These studies have given idiosyncratic results, which may either be because of genuine biological differences between systems, different concepts and scales used to quantify biotic specialization or because the methodological approaches used to compare interaction networks were inappropriate. Here we carefully examine the latitudinal specialization gradient using a global dataset of avian plant–frugivore assemblages and interaction networks. In particular, we test whether network‐derived specialization patterns differ from patterns based on assemblage‐level information on avian dietary preferences on specific food types. We found that network‐derived measures of specialization (complementary specialization H2′ and < d’>, modularity Q) increased with latitude, i.e. frugivorous birds divide the niche of fruiting plants most finely at high latitudes where they also formed more modular interaction networks than at tropical latitudes. However, the strength and significance of the relationship between specialization metrics and latitude was influenced by the methodological approach. On the other hand, assemblage‐level information on avian specialization on fruit diet (i.e. the proportion of obligate frugivorous bird species feeding primarily on fruit) revealed an opposed latitudinal pattern as more bird species were specialized on fruit diet in tropical than in temperate assemblages. This difference in the latitudinal specialization gradient reflects that obligate frugivores require a high diversity of fruit plants, as observed in tropical systems, and fulfil more generalized roles in plant–frugivore networks than bird species feeding on different food types. Future research should focus on revealing the underlying ecological, historical and evolutionary mechanisms shaping these patterns. Our results highlight the necessity of comparing different scales of biotic specialization for a better understanding of geographical patterns of specialization in resource–consumer interactions.

The geographical variation of network structure is scale dependent: understanding the biotic specialization of host–parasitoid networks

“…Dalsgaard and colleagues (Dalsgaard et al 2017) analysed the latitudinal gradient in biotic specialization of avian plant-frugivore networks from two different perspectives, namely niche partitioning in the network and consumers’ dietary specialization, and found opposite results. They showed that network-derived specialization increases with latitude while bird species were more specialized on specific fruit diet in the Tropics.…”

“…While some authors showed an increase in network specialization and higher predation risk towards the tropics (Olesen and Jordano 2002, Dalsgaard et al 2011, Trøjelsgaard and Olesen 2013, Roslin et al 2017), others found the opposite (Schleuning et al 2012). Yet some others found distinct trends for each hemisphere (Pauw and Stanway 2015) and for each measure of biotic specialization considered (Dalsgaard et al 2017), or no latitudinal trend at all (Ollerton and Cranmer 2002, Morris et al 2014). …”

“…The idea that the degree of specialization increases towards the tropics has been rarely explored beyond niches becoming narrower, that is, beyond specialization from the consumers’ perspective (MacArthur 1955, Vázquez and Simberloff 2002, Vázquez and Stevens 2004,; but see Schleuning et al 2012, Moles and Ollerton 2016, Dalsgaard et al 2017). However, specialization is a multifaceted concept.…”

“…However, specialization is a multifaceted concept. There are other metrics of biotic specialization beyond diet breadth, such as network-based metrics that reflect the general level of specialization of the community (Devictor et al 2010, Poisot et al 2012b, Dalsgaard et al 2017). This is important since different facets of specialization can reveal different patterns across environmental gradients (Dalsgaard et al 2017).…”

Partitioning wild bee and hoverfly contributions to plant–pollinator network structure in fragmented habitats

“…We estimated richness of pairwise interactions (links) using the Chao 1 estimator of asymptotic richness (Chao ) with links as “species” and interaction frequencies as “abundances” (Dalsgaard et al. ). Estimated sampling completeness of each network (i.e., observed richness of links divided by the estimated link richness; Dalsgaard et al.…”