Predation risk and floral rewards: How pollinators balance these conflicts and the consequences on plant fitness.

Foraging behavior of pollinators is shaped by, among other factors, the conflict between maximizing resource intake and minimizing predation risk; yet, empirical studies quantifying variation in both forces are rare, compared to those investigating each separately. Here, we discuss the importance of simultaneously assessing bottom-up and top-down forces in the study of plant-pollinator interactions, and propose a conceptual and testable graphical hypothesis for pollinator foraging behavior and plant fitness outcomes as a function of varying floral rewards and predation risk. In low predation risk scenarios, no noticeable changes in pollinator foraging behavior are expected, with reward levels affecting only the activity threshold. However, as predation risk increases we propose that there is a decrease in foraging behavior, with a steeper decline as plants are more rewarding and profitable. Lastly, in high predation risk scenarios, we expect foraging to approach zero, regardless of floral rewards. Thus, we propose that pollinator foraging behavior follows an inverse S-shape curve, with more pronounced changes in foraging activity at intermediate levels of predation risk, especially in high reward systems. We present empirical evidence that is consistent with this hypothesis. In terms of the consequences for plant fitness, we propose that specialized plant-pollinator systems should be more vulnerable to increased predation risk, with a steeper and faster decline in plant fitness, compared with generalist systems, in which pollinator redundancy can delay or buffer the effect of predators. Moreover, whereas we expect that specialist systems follows a similar inverse S-shape curve, in generalist systems we propose three different scenarios as a function not only of reward level but also compatibility, mating-system, and the interplay between growth form and floral display. The incorporation of trade-offs in pollinator behavior balancing the conflicting demands between feeding and predation risk has a promising future as a key feature enabling the development of more complex foraging models.

Sexual dimorphism and female advantage hypothesis in the gynomonoecious-gynodioecious Dianthus plumarius (Caryophyllaceae).

To explain the co-existence and maintenance of females along with hermaphrodite plants, the female advantage hypothesis has been proposed where females should show greater fecundity compared to their conspecific hermaphrodites. On the other hand, greater attraction would be selected in the hermaphrodites to increase their male function, potentially leading to larger showier flowers, with more rewards. Here, I tested the sexual dimorphism trade-off hypothesis with the gynomonoecious-gynodioecious Dianthus plumarius (Caryophyllaceae), in the gardens of Bariloche (Patagonia, Argentina). I measured in female and hermaphrodite plants: flower size, nectar volume and concentration, flower lifespan, ovule production, seed number, seed set and seed weight. Additionally, bagging and pollen supplementation experiments were carried out to evaluate pollen limitation, probability of apomixis, if spontaneous autogamy is possible, and to examine the importance of pollen origin. I found that hermaphrodite flowers are more attractive, with larger-sized flowers and higher nectar volume, whereas female flowers compensate with longer lifespan of stigmatic receptivity and more concentrated nectar. Despite ovule number was lower in female flowers, these showed higher seed set and produced more and heavier seeds than hermaphrodites under open pollination. No evidence of apomixis was found in females, but spontaneous autogamy may occur in hermaphrodites. Hand-pollination experiments showed first that both flower types suffered pollen limitation, but it was higher on hermaphrodite flowers. Finally, despite self-compatibility, pollen origin is important because hand self-pollination decreases seed weight. These findings provide strong evidence in support of the mechanisms and underlying conditions that would allow the co-existence and maintenance of female and hermaphrodite individuals within populations.

RecruitNet: A global database of plant recruitment networks.

Plant recruitment interactions (i.e., what recruits under what) shape the composition, diversity, and structure of plant communities. Despite the huge body of knowledge on the mechanisms underlying recruitment interactions among species, we still know little about the structure of the recruitment networks emerging in ecological communities. Modeling and analyzing the community-level structure of plant recruitment interactions as a complex network can provide relevant information on ecological and evolutionary processes acting both at the species and ecosystem levels. We report a data set containing 143 plant recruitment networks in 23 countries across five continents, including temperate and tropical ecosystems. Each network identifies the species under which another species recruits. All networks report the number of recruits (i.e., individuals) per species. The data set includes >850,000 recruiting individuals involved in 118,411 paired interactions among 3318 vascular plant species across the globe. The cover of canopy species and open ground is also provided. Three sampling protocols were used: (1) The Recruitment Network (RN) protocol (106 networks) focuses on interactions among established plants ("canopy species") and plants in their early stages of recruitment ("recruit species"). A series of plots was delimited within a locality, and all the individuals recruiting and their canopy species were identified; (2) The paired Canopy-Open (pCO) protocol (26 networks) consists in locating a potential canopy plant and identifying recruiting individuals under the canopy and in a nearby open space of the same area; (3) The Georeferenced plot (GP) protocol (11 networks) consists in using information from georeferenced individual plants in large plots to infer canopy-recruit interactions. Some networks incorporate data for both herbs and woody species, whereas others focus exclusively on woody species. The location of each study site, geographical coordinates, country, locality, responsible author, sampling dates, sampling method, and life habits of both canopy and recruit species are provided. This database will allow researchers to test ecological, biogeographical, and evolutionary hypotheses related to plant recruitment interactions. There are no copyright restrictions on the data set; please cite this data paper when using these data in publications.

Pollination success increases with plant diversity in high-Andean communities.

Pollinator-mediated plant-plant interactions have traditionally been viewed within the competition paradigm. However, facilitation via pollinator sharing might be the rule rather than the exception in harsh environments. Moreover, plant diversity could be playing a key role in fostering pollinator-mediated facilitation. Yet, the facilitative effect of plant diversity on pollination remains poorly understood, especially under natural conditions. By examining a total of 9371 stigmas of 88 species from nine high-Andean communities in NW Patagonia, we explored the prevalent sign of the relation between conspecific pollen receipt and heterospecific pollen diversity, and assessed whether the incidence of different outcomes varies with altitude and whether pollen receipt relates to plant diversity. Conspecific pollen receipt increased with heterospecific pollen diversity on stigmas. In all communities, species showed either positive or neutral but never negative relations between the number of heterospecific pollen donor species and conspecific pollen receipt. The incidence of species showing positive relations increased with altitude. Finally, stigmas collected from communities with more co-flowering species had richer heterospecific pollen loads and higher abundance of conspecific pollen grains. Our findings suggest that plant diversity enhances pollination success in high-Andean plant communities. This study emphasizes the importance of plant diversity in fostering indirect plant-plant facilitative interactions in alpine environments, which could promote species coexistence and biodiversity maintenance.

Intraspecific variation in body size of bumblebee workers influences anti-predator behaviour.

Flower-dwelling predators make flowers dangerous foraging sites for pollinators, potentially affecting their anti-predator behaviour. Moreover, predation vulnerability often varies among pollinators' body sizes with interspecific comparisons showing that smaller species are more vulnerable than larger ones. However, how intraspecific body size variation influences pollinator behaviour under predation risk is still unknown, especially under natural conditions. We hypothesized that bumblebee workers of different sizes will exhibit different foraging strategies under predation risk. We predict that (a) small workers should more often exhibit anti-predator behaviours than larger workers. We also hypothesized that the anti-predator behaviour should be influenced by predator size and reward availability; therefore, we expect (b) higher avoidance behaviour towards larger predator sizes and (c) more and longer visits to inflorescences with high nectar availability. Finally, we expect that (d) nectar availability should overcome the anti-predator behaviour in less vulnerable, large, workers. We recorded flower visitation, time spent and rejection behaviours of different sizes of Bombus terrestris (Apidae) workers (large, medium and small) to inflorescences of Alstroemeria aurea (Alstroemeriaceae) with different treatments of artificial spiders (small and large) and nectar availability (with, without). Anti-predator and foraging behaviour of bumblebees was affected by the size of the worker, the presence of artificial spiders and nectar availability. Large and medium size bumblebees strongly reduced flower visitation and time spent in the presence of artificial spiders, consistently avoiding flowers with spiders, regardless of spider size or nectar availability. Instead, small bumblebees seldom modified their behaviour when facing artificial spiders, only increasing their avoidance or decreasing their foraging time in nectarless flowers hosting large artificial spiders. This pattern of larger workers being more sensitive to predation risk than smaller ones at the intraspecific level in B. terrestris is contrary to the expected and acknowledged trend based on previous interspecific comparisons, but partially consistent with predictions of models of optimal foraging theory. Intraspecific behavioural variability was uncovered only when nectar was available, whereas artificial predator size rarely modified bumblebee anti-predator and foraging behaviour. Therefore, our findings suggest that the trade-off between maximizing resource intake and minimizing predation risk strongly varies across bumblebee worker body sizes.

Resumen Los depredadores que cazan sobre flores hacen que éstas sean sitios peligrosos de forrajeo para los polinizadores, pudiendo afectar su comportamiento. En general, la vulnerabilidad a la depredación varía con el tamaño del polinizador, siendo en comparaciones interespecíficas las especies más pequeñas las más vulnerables. Sin embargo, aún se desconoce cómo la variación intraespecífica del tamaño corporal influye en el comportamiento del polinizador bajo riesgo de depredación, especialmente en condiciones naturales.. Esperamos que abejorros de distinto tamaño exhiban diferentes estrategias de alimentación ante el riesgo de depredación. Predecimos que (i) obreras pequeñas manifiesten con mayor frecuencia comportamientos anti depredadores que obreras más grandes. Conjuntamente, esperamos (ii) una mayor evasión hacia tamaños de depredadores más grandes, y (iii) más visitas y mayor permanencia en inflorescencias con néctar. Finalmente, esperamos que (iv) la disponibilidad de néctar prevalezca a la manifestación del comportamiento anti depredador en obreras grandes, supuestamente menos vulnerables. Registramos a campo el número de visitas, tiempo de permanencia y el comportamiento de evasión de diferentes tamaños de obreras de Bombus terrestris (Apidae) (grandes, medianas y pequeñas) a inflorescencias de Alstroemeria aurea (Alstroemeriaceae) con arañas artificiales (pequeñas, grandes) y distinta disponibilidad de néctar (con, sin). El comportamiento de los abejorros se vio afectado por su tamaño corporal, la presencia de arañas artificiales y la disponibilidad de néctar. Abejorros grandes y medianos disminuyeron considerablemente las visitas y el tiempo de permanencia en inflorescencias, evadiendo consistentemente las flores que albergaron arañas artificiales, independientemente del tamaño del modelo o la cantidad de néctar. En cambio, abejorros pequeños rara vez modificaron su comportamiento, aumentando la evasión y disminuyendo el tiempo de permanencia solo en flores sin néctar y con arañas artificiales grandes. El patrón encontrado a nivel intraespecífico en B. terrestris resultó ser opuesto al observado en comparaciones interespecíficas, pero parcialmente consistente con modelos de teoría de forrajeo óptimo. Esta variabilidad intraespecífica se manifestó solo cuando había néctar disponible, mientras el tamaño del depredador apenas modificó el comportamiento. Nuestros hallazgos sugieren que el balance entre maximizar la ingesta de recursos y minimizar el riesgo de depredación varía fuertemente con el tamaño corporal de los abejorros.