Does a Species’ Extinction–Proneness Predict Its Contribution to Nestedness? A Test Using a Sunbird-Tree Visitation Network

Abstract

Animal pollinators and the plants they pollinate depend on networks of mutualistic partnerships and more broadly on the stability of such networks. Based mainly on insect-plant visitation networks, theory predicts that species that are most prone to extinction contribute the most to nestedness, however empirical tests are rare. We used a sunbird-tree visitation network within which were both extinction prone vs non extinction prone sunbird species to test the idea. We predicted that the extinction prone species would contribute the most to nestedness. Using local abundance as a proxy for extinction risk we considered that locally rare sunbird species, by virtue of their small population size and associated demographic stochasticity to be more at risk of extinction than the common species. Our network was not strongly nested and all sunbird species made similar contributions to nestedness, so that in our empirical test, extinction proneness did not predict contribution to nestedness. The consequences of this finding remain unclear. It may be that network theory based on plant-insect mutualisms is not widely applicable and does not work for tree- sunbird mutualistic networks. Alternatively it may be that our network was too small to provide results with any statistical power. Without doubt our study highlights the problems faced when testing network theory in the field; a plethora of ecological considerations can variously impact on results.

Promiscuous pollinators—Evidence from an Afromontane sunbird–plant pollen transport network

“…The study was conducted at Ngel Nyaki Forest Reserve (NNFR) (07 ° 05′N 11 ° 04′E) on the Mambilla Plateau in southeast Nigeria at an elevation of approximately 1650 m a.s.l. NNFR covers approximately 4600 ha and comprises ~ 7.5 ha of montane forest as well as degraded riparian fragments and grassland (Nsor et al., ). The mean annual rainfall is 1800 mm, and mean monthly maximum and minimum temperature for the rainy and dry seasons are 26, 13, 23, and 16°C, respectively (Chapman, Goldson & Beck, ).…”

“…Sunbirds occurring in and around Ngel Nyaki Forest belong to four genera, Cinnyris, Cyanomitra, Hedydipna , and Anthreptes (Nsor, ). Twelve species have been recorded from the area and eight from the genus Cinnyris, and these species are more abundant than species from the other three genera (Nsor et al., ). Tree species comprise a high proportion of Afromontane endemics or near‐endemics and are described in Chapman and Chapman ().…”

“…The flower visitation network (FVN) was generated from observations of sunbird visits to five individuals from each of 16 tree species as described in Nsor et al. (). Observations were made between November and February 2012–2014.…”

“…One of the sunbird networks cited in Zanata et al. () is a flower visitation network (FVN) from our study site (Nsor, Chapman & Godsoe, ). Here, we build on Nsor et al.…”

“…Here, we build on Nsor et al. () by creating the first sunbird pollen transport network (PTN) that we are aware of and use it in conjunction with the FVN to explore specialization within potentially generalist networks.…”