Sunbird pollination and the fate of strong contributors to a mutualistic network in a West African Montane Forest
Thesis DisciplineBiological Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The survival of most species depends on their network of mutualistic partnerships with other species. Network structure - the pattern and arrangement of species in a given interaction, can reveal predictable outcomes on the fate of species and network stability. Therefore understanding what makes networks stable is extremely important. However current investigations into network stability have mainly been through theoretical and simulation studies and often based on data from plant-insect visitation networks. Empirical testing is now imperative and networks other than just plant-insect ones need to be incorporated into such studies. Moreover, the use of visitation frequency as a proxy for pollination frequency in plant-pollination network studies needs to be evaluated. In this thesis, I tested theoretical views and models on network structure and species survival using empirical data from a sunbird-tree pollination network in a remote montane forest reserve in Nigeria. First I investigated how changing landscapes and habitats affected sunbird distribution in the reserve. Secondly I compared a sunbird-tree visitation with a sunbird-tree pollination network to determine how good a proxy bird visitation is for bird pollination. Two network properties which affect network stability and underpin this work are i) connectance/ interaction strength and ii) nestedness. I examined how species relative abundance influences their interaction strength in the network and whether a species contribution to nestedness determines its survival probability. This latter point was of special interest as theoretical studies on networks have suggested that species which contribute the most to nestedness are most prone to extinction, which seems counterintuitive. The visitation frequencies of sunbird species on flowers of as many tree species as was logistically possible were observed, and sunbird species were trapped to determine the amount of pollen they transported. Using these data, I developed the flower visitation (FVN) and pollen transport networks (PTN). To determine how FVN reflects PTN, I compared the two networks using null models that controlled for species’ degree (number of links) and network size. Differences in observed nestedness differed significantly from null model expectations. I worked out an extinction proneness based on IUCN criteria for determining species at risk of extinction using rarity as a measure of vulnerability. An assessment of species abundance and diversity in the reserve and nearby fragments provided the standard for risk categorisation and evaluation of species’ robustness to changing landscapes. FVN correlated positively with PTN, despite 64 % dissimilarity in species composition. The PTN had fewer species than FVN, but was more nested and specialised than the FVN, indicating that analyses of FVN often overestimate pollination through the inclusion of interactions with variable effects such as nectar robbing and insectivory. Although some species were relatively stronger interactors in both networks, the strongest contributor for FVN was not the strongest for PTN. FVN is therefore an inadequate predictor of efficient pollinators and a poor reflection of PTN. Abundant species had a higher interaction strength overall. Strong contributors to nestedness were the rare species, which explains why they are more prone to extinction. In my empirical network, nestedness will decrease overall through the loss of rare species, but in accordance with network theory, this will not collapse the network, because it is the abundant species with the most links which maintain network stability. Although fragmentation is not yet a challenge to sunbird distribution, anthropogenic disturbance such as indiscriminate burning of grassland to stimulate re-growth, may alter crucial habitats for sunbird survival.