One unresolved question is whether communities harbouring different diversity levels might respond differently. In other words, does biodiversity have a modulating role in these responses to changing species composition? We need to remove the same species from different background communities to see how responses differ across a diversity gradient. Given the complexity of these experiments, it is easy to see why such studies are not commonly done. Migratory species and the communities that harbour them represent an ideal natural experiment in which the same species temporarily leaves ecosystems across latitudinal and diversity gradients. We surveyed communities of plants and the hummingbirds that pollinate their flowers along a latitudinal gradient from central Mexico to southern Alaska. We focused on communities that harbour the migratory rufous hummingbird Selasphorus rufus, and evaluated the foraging niches and the functional role of hummingbirds in the presence and the absence of the migratory species. We further explored the consequences of the removal of this species for plant reproductive success. Our study aims to answer three main questions. How does the role of a single pollinator species change throughout its distribution range?
How do the roles of the rest of the hummingbird pollinators in the community change in response to a temporary species loss? What are the consequences for the functions they perform? Given its widespread distribution, we expect that the migratory hummingbird will be a generalist species feeding on a diverse set of resources across all the range. Further, 10 liter pot we expect resident species to change their feeding preferences when the migratory species is present or absent. In particular, we expect them to specialize on a small set of resources in the presence of the migratory competitor and then to expand their diet in its absence. Finally, we expect there to be changes on reproductive success particularly for those species of plants on which resident species specialized during the period in which the migratory species was present, but that will potentially receive heterospecific pollen during the period in which the migrant is absent as a consequence of an expansion in the diets of resident hummingbirds following.Surveys were carried out during the year 2019. At each site, we selected six 1 km transects separated by at least 500 m from each other. Each transect was surveyed seven times per period during two periods: when the migrant species was present and again after the migrant departed . Between three and four transects were surveyed per day. No surveys were done in Alaska during the absence period as no resident hummingbirds occur in this area. Along each transect, we recorded all the feeding interactions observed between every species of hummingbird present in the study area and every flowering plant species.
In addition, we recorded flower availability of the different hummingbird visited plants once for each period as the total sum of flowers observed per species. In addition, in order to evaluate the impact of species removal on plant reproductive success, we further recorded several measures of plant reproductive success for a subset of plant species that were flowering in both periods: four in Mexico , and three in California . These plants were selected because they were preferred resources by hummingbirds in the area and because they had two flowering periods, one when the migratory species was present and one in its absence. In Mexico, we counted the total number of seeds per fruit and measured fruit length and width in 30 fruits from five individuals per plant species. In California, we recorded fruit set for 10 individuals per plant species . For two of the species , given the large number of flowers produced, we marked three branches within each of the 10 individuals per species and counted the number of flowers and three weeks later the number of fruits produced at each of the two periods . For the third species, Castilleja affinis, we recorded fruit set for the whole plant. In addition, we measured fruit length, width and wet weight for a subsetof 10 fruits per branch for each of the two Ribes species and for all fruits for C. affinis individuals.To determine species functional roles, we extracted a series of metrics from plant–hummingbird interaction networks. For each transect within each period, we constructed a weighted bipartite interaction network by pooling the data for the seven rounds of sampling.
We then calculated a series of relevant metrics for the whole hummingbird community and for each of the hummingbird species in particular.At the community level, we focused on two metrics that provide parallel information on niche occupancy for the hummingbird guild. First, we calculated niche overlap using Horn’s index, which estimates the similarity in interaction partners between hummingbird species. Second, we calculated functional complementarity. This metric, not correlated with the previous one , presents additional information on the diversity of roles or niches occupied by hummingbird species. It is calculated as the total branch length of a functional dendrogram based on the Euclidean distance between hummingbirds in plant assemblages visited.At the species level, we focused on metrics that provide information on the functional role of each hummingbird species within the community. In particular, we calculated normalized degree, species-level specialization and strength for each species at each transect and period. Normalized degree gives an idea of the diversity of plant species visited by each hummingbird species. It is calculated by dividing a hummingbird species’ degree by the total number of plant species. In this case, we used the total number of flowering plant species found within our independent flower availability surveys as the denominator. d0 provides information on the level of specialization of each hummingbird species based on discrimination from a random selection of partners. In calculating this metric, we included abundance data as the flower availability for each plant species from our independent surveys. Strength provides complementary information on the dependence of plant species on a particular hummingbird species. It is calculated as the sum of the dependencies on that species of the plant species visited by a focal hummingbird. All network metrics were calculated using package bipartite.First, we evaluated sampling completeness within our study by estimating the asymptotic number of plant and pollinator species present, as well as plant–pollinator links. This is a non-parametric estimator of species richness for abundance data which includes non-detected species , allowing us to calculate the proportion of species detected with our original sampling data. We used Chao 1 asymptotic species richness estimators and estimated the richness of pollinators, plants and plant–pollinator links accumulated as sampling effort increased up to 100% sampling coverage using package iNEXT within the R environment. Then, to answer our first question related to how the role of a single species changes throughout its distribution range, we evaluated how the migratory species’s normalized degree changed along its migratory pathway by fitting a general linear model including site as the explanatory variable.In addition, 10 liter drainage collection pot we focused on changes in the role of migratory species through the indirect interactions it engages in by using motif analyses. To this end, we compared the motif role signatures of S. rufus in Mexico and California . Additionally, we compared the signatures of S. rufus to those of two other abundant species, Hylocharis leucotis in Mexico and Calypte anna in California. Motif role signatures were calculated using package bmotif . Motif frequencies were normalized by dividing the position counts for each node by the total number of times that node appears in any of the positions. Statistical comparisons were done using permutational multivariate analysis of variance using Bray–Curtis as the dissimilarity distance. Visual comparisons were done by means of non-metric multidimensional scaling plots. To answer our second question, related to how the roles of the other hummingbird species in the community shift when the migratory species is removed, we ran analyses at the whole community and at the species level. At the community-level, we evaluated whether niche overlap and functional complementarity varied across periods by running general linear mixed models with period, plant species richness and floral resource availability as explanatory variables, and transect nested within site as a random factor.
At the species level, we evaluated whether normalized degree, d0 , and strength varied across periods by running GLMMs that included period and its interaction with hummingbird species as explanatory variables. Transect nested within site was included as a random factor. For these species-level analyses, we removed the migratory species to focus on how the roles of the rest of the species change. Finally, to answer our last question related to how changes in community structure and species functional roles might affect plant reproductive success, we ran separate GLMMs for the two sites as we were able to collect different measures of reproductive success. In the case of Mexico, response variables were fruit length, fruit weight and the number of seeds per fruit, scaled prior to analysis to allow meaningful comparisons across species with contrasting life histories. For California, response variables were fruit set, fruit length, width and weight. In both cases, models included period and its interaction with plant species as well as the plant’s normalized degree, a measure of the diversity of pollinators visiting it, as explanatory variables. Plant individual was included as a random factor. We used a normal distribution to fit all models except in the case of fruit set where data were fitted to a binomial distribution. All statistical analyses were done using R and all GLMMs were fitted using package lme4.An analysis of the completeness of our sampling revealed that with our survey we were able to capture very high levels of hummingbird and plant species diversity at all sites ranging from 88 to 100% in hummingbird species, 56–100% in plant species and 55–100% in plant–hummingbird interactions . As demonstrated by our analysis of foraging niches for the migratory hummingbird, the role of the migratory species S. rufus changed substantially throughout its distribution range. In particular, normalized degree, a measure of the number of plant species visited divided by all possible plant species, is much smaller in California than in either Alaska or Mexico . However, the difference is only significant between California and Alaska , because inter-transect variability is high in Mexico. Despite there being a similar number of available plant resources in California and Alaska , in California the migratory species feeds almost exclusively on one resource, Ribes sanguineum. Our motif approach analyses to species roles shows that S. rufus has very different roles at two of the locations sampled . While the species occupies mostly specialist roles in California, in Mexico it engages in interactions involving more than one plant species . When comparing the role of the migrant species with that of two abundant species at each of the sites, Hylocharis leucotis in Mexico and Calypte anna in California, our results show a certain overlap between S. rufus and each of the two species when they co-occur , while the overlap of their motif signatures is smaller in the absence of the migratory species. In relation to the response of the hummingbird community to species removal, we focused our analyses on two different levels: at the level of the whole community and at the level of each of the hummingbird species. At the community level, when the migratory species leaves the area the niche overlap between different resident species decreases . This effect is greatest in California . In addition, we find that functional complementarity decreases in Mexico when the migrant leaves . At the species level, all three variables evaluated change between periods before and after the removal of the migratory species. However, resident hummingbird species vary strongly in the magnitude and direction of these changes . In the case of California, we see particularly large changes in these variables for the species Calypte anna, which becomes more generalized and more important for the plant species that depend on it in the absence of the migratory species. In Mexico, the species whose role changes most is Hylocharis leucotis, which also diversiflies the number of plant species it visits and becomes more important for plant species in the community in the absence of the migratory species .