Species interactions in an Andean bird–flowering plant network: phenology is more important than abundance or morphology

Abstract

Biological constraints and neutral processes have been proposed to explain the properties of plant–pollinator networks. Using interactions between nectarivorous birds (hummingbirds and flowerpiercers) and flowering plants in high elevation forests (i.e., “elfin” forests) of the Andes, we explore the importance of biological constraints and neutral processes (random interactions) to explain the observed species interactions and network metrics, such as connectance, specialization, nestedness and asymmetry. In cold environments of elfin forests, which are located at the top of the tropical montane forest zone, many plants are adapted for pollination by birds, making this an ideal system to study plant–pollinator networks. To build the network of interactions between birds and plants, we used direct field observations. We measured abundance of birds using mist-nets and flower abundance using transects, and phenology by scoring presence of birds and flowers over time. We compared the length of birds’ bills to flower length to identify “forbidden interactions”—those interactions that could not result in legitimate floral visits based on mis-match in morphology. Diglossa flowerpiercers, which are characterized as “illegitimate” flower visitors, were relatively abundant. We found that the elfin forest network was nested with phenology being the factor that best explained interaction frequencies and nestedness, providing support for biological constraints hypothesis. We did not find morphological constraints to be important in explaining observed interaction frequencies and network metrics. Other network metrics (connectance, evenness and asymmetry), however, were better predicted by abundance (neutral process) models. Flowerpiercers, which cut holes and access flowers at their base and, consequently, facilitate nectar access for other hummingbirds, explain why morphological mis-matches were relatively unimportant in this system. Future work should focus on how changes in abundance and phenology, likely results of climate change and habitat fragmentation, and the role of nectar robbers impact ecological and evolutionary dynamics of plant–pollinator (or flower-visitor) interactions.

Plant and habitat use by Black-breasted Pufflegs (Eriocnemis nigrivestis), a critically endangered hummingbird

“…Because hummingbirds use their extensible tongues to extract nectar from flowers, we added 33% of the bill length to the total functional bill length (Vizentin‐Bugoni et al. , González and Loiselle ). We used the difference between corolla length and the two measures of bill length (bill length alone and with tongues) to explore patterns of trait matching.…”

Assemblage of Nectarivorous Birds and Their Floral Resources in an Elfin Forest of the Central Andes of Peru

“…Unchog forest was visited during different periods from May 2011 to August 2014, over a total of 134 days; 17 days within May-July of 2011, 24 days within February, July-November of 2012, 41 days in January-July, September, November of 2013 and 52 days in January and May-August of 2014. For more details in sampling effort, see Gonzalez & Loiselle (2016). This forest has a dry and rainy season, and some bird species are more prone to be present or plants blossoming depending on the season.…”

“…The assemblages, or networks of nectarivorous birds and their resources provide an important framework to study ecological principles such as diversity, distribution, adaptation and energy flow (Rojas-Nossa, 2013;Bennett et al, 2014;Abrahamczyk & Kessler, 2015;Gonzalez & Loiselle, 2016). The information gathered from nine sources on nectarivorous birds that visited plants in the Andes gave us a limited view of the real interactions in the elfin forest, because there were fewer taxa than in the observed network.…”

“…Metallura odomae, sister taxa of Metallura theresiae in high elevation montane forest in the north of Peru feeds mainly in Brachyotum and in Berberis (Hesse, 2010). Small-billed hummingbirds rely on Brachyotum, its importance in the network is explained in Gonzalez & Loiselle (2016). Schuchmann (1999) mentions that the Shining Sunbeam Aglaeactis cupripennis does local migrations, Heliangelus prefers shrubs in the ecotone between paramo and forest and Coeligena forages in the forest itself or forest edges.…”

Complementary Effects of Species Abundances and Ecological Neighborhood on the Occurrence of Fruit-Frugivore Interactions

“…Secondly, there is a deterministic process driven by a matching of traits (e.g., phenology, morphology, behavior, etc.) that finally permits the interaction between cooccurring species (Maglianesi et al, 2014;Gonzalez and Loiselle, 2016;Donoso et al, 2017). Although both mechanisms are not mutually exclusive, neutral processes are usually expected from systems dominated by generalist species (e.g., González-Castro et al, 2015), whereas niche-driven processes are expected to be more relevant in those systems where high biodiversity involves a large trait variation among species (e.g., in tropical contexts, Bender et al, 2017).…”