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Range contraction and habitat fragmentation can cause biodiversity loss by creating conditions that directly or indirectly affect the survival of plant populations. Fragmented habitats can alter pollinator guilds and impact their behavior, which may result in pollen/pollinator limitation and selection for increased selfing as a mechanism for reproductive assurance. We used Salvia brachyodon , a narrowly distributed and endangered sage from eastern Adriatic, to test the consequences of range contraction and habitat fragmentation. Molecular data indicate a severe and relatively recent species range reduction. While one population is reproductively almost completely isolated, moderate gene flow has been detected between the remaining two populations. The high pollen-to-ovule ratio and the results of controlled hand pollination indicate that S. brachyodon has a mixed mating system. Quantitative and qualitative differences in the community and behaviour of flower visitors resulted in limited pollination services in one population where no effective pollinator other than pollen and nectar robbers were observed. In this population, self-pollination predominated over cross-pollination. Various environmental factors, in which plant-pollinator interactions play a pivotal role, have likely created selection pressures that have led to genetic and phenotypic differentiation and different resource allocation strategies among populations.

Mutualisms provide benefits to interacting species, but they also involve costs. If costs come to exceed benefits as population density or the frequency of encounters between species increases, the interaction will no longer be mutualistic. Thus curves that represent benefits and costs as functions of interaction frequency are important tools for predicting when a mutualism will tip over into antagonism. Currently, most of what we know about benefit and cost curves in pollination mutualisms comes from highly specialized pollinating seed-consumer mutualisms, such as the yucca moth-yucca interaction. There, benefits to female reproduction saturate as the number of visits to a flower increases (because the amount of pollen needed to fertilize all the flower's ovules is finite), but costs continue to increase (because pollinator offspring consume developing seeds), leading to a peak in seed production at an intermediate number of visits. But for most plant-pollinator mutualisms, costs to the plant are more subtle than consumption of seeds, and how such costs scale with interaction frequency remains largely unknown. Here, we present reasonable benefit and cost curves that are appropriate for typical pollinator-plant interactions, and we show how they can result in a wide diversity of relationships between net benefit (benefit minus cost) and interaction frequency. We then use maximum-likelihood methods to fit net-benefit curves to measures of female reproductive success for three typical pollination mutualisms from two continents, and for each system we chose the most parsimonious model using information-criterion statistics. We discuss the implications of the shape of the net-benefit curve for the ecology and evolution of plant-pollinator mutualisms, as well as the challenges that lie ahead for disentangling the underlying benefit and cost curves for typical pollination mutualisms.

Extrafloral (EF) nectaries recruit carnivorous arthropods that protect plants from herbivory, but they can also be exploited by nectar thieves. We studied the opportunistic, targeted predation (and destruction) of EF nectaries by insects, and the localized chemical defences that plants presumably use to minimize this effect. In field and laboratory experiments, we identified insects that were possibly responsible for EF nectary predation in Vicia faba (fava bean) and determined the extent and accuracy of the feeding damage done to the EF nectaries by these insects. We also performed biochemical analyses of plant tissue samples in order to detect microscale distribution patterns of chemical defences in the area of the EF nectary. We observed selective, targeted feeding on EF nectaries by several insect species, including some that are otherwise not primarily herbivorous. Biochemical analyses revealed high concentrations of l-3,4-dihydroxyphenylalanine, a non-protein amino acid that is toxic to insects, near and within the EF nectaries. These results suggest that plants allocate defences to the protection of EF nectaries from predation, consistent with expectations of optimal defence theory, and that this may not be entirely effective, as insects limit their exposure to these defences by consuming only the secreting tissue of the nectary.

Nectar robbery is usually thought to impact negatively on the reproductive success of plants, but also neutral or even positive effects have been reported. Very few studies have investigated the effects of nectar robbing on the behaviour of legitimate pollinators so far. Such behavioural changes may lead to the reduction of geitonogamy or to increased pollen movement. We simulated nectar robbing in experimental sites as well as in natural populations of Aconitum napellus ssp. lusitanicum, a rare plant pollinated by long-tongued bumblebees. In an experimental setup, we removed the nectaries of 40 % of the flowers, which is similar to rates of robbing observed in wild populations. Patches of plants with experimentally robbed flowers were compared with control patches containing plants with untreated flowers. We observed pollinator behaviour, mimicked male reproductive success (pollen dispersal) using fluorescent dye, and measured female reproductive success (seed set). The main legitimate visitors were bumblebees while honeybees were often observed robbing nectar. They did so by “base working”, i.e. sliding between tepals. Bumblebees tended to visit fewer flowers per plant and spent less time per single flower when these had been experimentally robbed. This change in behaviour consequently increased the proportion of flowers visited by bumblebees in patches with robbed flowers. Fluorescent dye mimicking pollen flow was dispersed larger distances after pollinators had visited patches with robbed flowers compared to control patches. Average seed set per plant was not affected by nectar robbing. Our results demonstrated that A. napellus does not suffer from nectar robbery but may rather benefit via improved pollen dispersal and thus, male reproductive success. Knowledge on such combined effects of behavioural changes of pollinators due to nectar robbery is important to understand the evolutionary significance of exploiters of such mutualistic relationships between plants and their pollinators.

Hummingbirds foraging in alpine meadows of central Colorado, United States, face a heterogeneous distribution of nectar rewards. This study investigated how variability in nectar resources caused by nectar-robbing bumblebees affected the foraging behavior of hummingbird pollinators and, subsequently, the reproductive success of a host plant (Ipomopsis aggregata). We presented hummingbirds with experimental arrays of I. aggregata and measured hummingbird foraging behavior as a function of known levels of nectar robbing. Hummingbirds visited significantly fewer plants with heavy nectar robbing (over 80% of available flowers robbed) and visited fewer flowers on those plants. These changes in hummingbird foraging behavior resulted in decreased percent fruit set as well as decreased total seed set in heavily robbed plants. These results indicate that hummingbird avoidance of nectar-robbed plants and flowers reduces plant fitness components. In addition, our results suggest that the mutualisms between pollinators and host plants may be affected by other species, such as nectar robbers.

The removal of floral nectar by nectar robbers may change the behavior of legitimate pollinators and, consequently, the pollination and fertilization success of flowering plants. We measured the effects of a nectar-robbing bumble bee, Bombus occidentalis, on male and female reproductive success of Ipomopsis aggregata (Polemoniaceae). We artificially varied nectar-robbing rates within 20 natural I. aggregata populations in Colorado. Using powdered fluorescent dyes as pollen analogs, we estimated the amount of pollen donated (male fitness component) and the amount of pollen received (female fitness component) by plants with different levels of nectar robbing. We also measured percentage fruit set per plant, seed set per fruit, and average seed mass as additional components of female fitness. Nectar robbing significantly decreased both male and female fitness components. Heavy nectar robbing (>80% of available flowers robbed) significantly decreased dye donation, pollen receipt, and average number of seeds per fruit at the whole-plant level. Furthermore, nectar robbing significantly decreased dye donation by individual flowers. Although nectar robbing has often been regarded as unimportant to plant reproductive success, we document significant effects of nectar robbing on components of both male and female fitness of I. aggregata. Our results suggest that interactions among nectar robbers, pollinators, and their plant hosts may have important consequences for plant fitness.

In central Chile, nectar robbing of Puya coerulea (Bromeliaceae) flowers by the austral blackbird, Curaeus curaeus, had a direct impact on the plant via removal of floral resources and, in some cases, damage or even destruction of flowers. These robbing visits also likely had an indirect impact on plant fitness due to the decreased rate of legitimate visits by the hummingbird Patagona gigas to inflorescences with many robbed flowers. The proportion of flowers within an inflorescence visited by P. gigas was inversely proportional to the ratio of robbed flowers. /// En Chile central, el robo de néctar a flores de Puya coerulea (Bromeliaceae) por Curaeus curaeus tuvo impacto directo sobre la planta por remoción de néctar y, en algunos casos, daño o destrucción de flores. El robo tuvo impacto directo en la adecuación biológica de la planta debido al decremento en la tasa de visitas del polinizador Patagona gigas a inflorescencias con flores robadas: la proporción de flores visitadas por P. gigas, dentro de una inflorescencia, fue inversamente proporcional a la proporción de flores robadas.

We conducted experimental manipulations and field observations to determine the role of members of a nectarivorous guild (pollinators and robbers) on nectar production and pollen deposition in Hamelia patens at the La Selva Biological Station (Costa Rica). Seven pollinators (hummingbirds) and four robbers (1 hummingbird and 3 perching birds) comprised the avian nectarivorous guild visiting H. patens during March 1997. In addition, two florivorous birds were observed visiting H. patens during the study. Pollinators accounted for 85.6 percent of the visits, robbers for 12.4 percent, and florivores for 2 percent of the visits. Visitation by pollinators and robbers was greatest when floral nectar was highest. No aggressive interactions between pollinators and robbers were observed during the study. Pollinators differed in their ability to carry and deposit pollen on the stigma. Territorial hummingbirds were the least effective pollinators but the most frequent visitors. Flowers were frequently robbed (71%) during the study. Flowers experimentally robbed did not increase nectar production compared to control flowers; therefore, robbery may not involve an extra energy investment in terms of nectar production. The number of pollen grains deposited on artificially robbed flowers was significantly less than the number found in flowers with extra nectar (nectar added) but did not differ from the number found in non-manipulated flowers, indicating that nectar robbers may not affect the foraging behavior of hummingbirds, and therefore pollen deposition. /// Realizamos una serie de observaciones y manipulaciones experimentales para determinar el papel que desempeñan los miembros del gremio de nectarívoros (polinizadores y robadores) en la producción de néctar y la deposición de polen en Hamelia patens, en la Estación Biológica La Selva (Costa Rica). El gremio de aves nectarívoras que visitó las flores de H. patens, estuvo compuesto por siete polinizadores (colibríes) y cuatro robadores de néctar (1 colibrí y 3 aves de percha) en marzo de 1997. Adicionalmente, se observaron dos especies de aves consumidoras de corolas de H. patens. Los polinizadores realizaron el 85.6 por ciento del total de las visitas a las flores, mientras que los robadores de néctar realizaron el 12.4 por ciento y las aves florívoras el 2 por ciento. La actividad de los polinizadores y robadores aumentó durante las horas de mayor disponibilidad de néctar en las flores sin que ocurrieran interacciones agresivas entre ambos gremios. Los polinizadores mostraron diferencias en su capacidad de transportar y depositar polen en el estigma: las especies de colibríes territoriales fueron los visitantes más frecuentes y menos efectivos. Las flores de H. patens presentaron una alta frecuencia de robos (71%) durante el estudio. Las flores robadas experimentalmente no mostraron un aumento en la producción de néctar en comparación con las flores control, lo que sugiere que el robo no representa una inversión extra de energía en términos de producción de néctar para esta planta. El número de granos de polen depositado en las flores robadas artificialmente fue significativamente menor al encontrado en flores a las que se les agregó néctar; sin embargo, resultò similar al número encontrado en flores no manipuladas. Este resultado indica que los robadores de néctar no afectan el comportamiento de forrajeo de los colibríes ni la deposición de polen.

Although mutualisms are often viewed as fragile constructs, subject to invasion by \"cheaters\" that gain from the mutualism without providing compensating benefits, few studies have explored whether or not apparent cheating behavior by one player actually denies benefits to the other species. Panicled bluebells (Mertensia paniculata) experience high rates of nectar robbery by bumble bees (Bombus mixtus and B. frigidus) in the Wrangell Mountains of southcentral Alaska. Nevertheless, experimental prevention of nectar larceny in two seasons did not enhance components of female reproductive success (nutlet initiation, seed-set, and seed mass) or male success (pollen removal). Observational data show that the absence of a negative impact of nectar robbery is a consequence of bumble bee behaviors; the two bee species that rob mature flowers buzz-pollinate the same flowers at an earlier stage in floral development. The shift in bee behavior is driven by the presentation of different rewards at different times in the flowering period; young flowers contain pollen and older flowers produce nectar. Although flowers typically receive and donate pollen before most nectar is produced, the mature flowers producing a robbed nectar reward may nonetheless contribute to the reproductive success of bluebell plants. Flowers sequestered from pollinators in the early stage were still capable of initiating nutlets, demonstrating that older flowers can enhance female reproduction if pollinators are rare (when many early-stage flowers may go unvisited). Moreover, the removal of mature flowers reduced the visitation rate to early-stage flowers on the same plant. Because individual bumble bees switch frequently between nectar robbing and pollen collection, the nectar reward in mature flowers may act as a key enticement to pollinators, which then enhance plant reproduction by legitimately visiting early-stage flowers. Rather than representing a case of cheating behavior, nectar robbery may be an integral part of this plant-pollinator mutualism.

In central Chile, nectar robbing of Puya coerulea (Bromeliaceae) flowers by the austral blackbird, Curaeus curaeus, had a direct impact on the plant via removal of floral resources and, in some cases, damage or even destruction of flowers. These robbing visits also likely had an indirect impact on plant fitness due to the decreased rate of legitimate visits by the hummingbird Patagona gigas to inflorescences with many robbed flowers. The proportion of flowers within an inflorescence visited by P. gigas was inversely proportional to the ratio of robbed flowers. RESUMEN En Chile central, el robo de néctar a flores de Puya coerulea (Bromeliaceae) por Curaeus curaeus tuvo impacto directo sobre la planta por remoción de néctar y, en algunos casos, daño o destrucción de flores. El robo tuvo impacto directo en la adecuación biológica de la planta debido al decremento en la tasa de visitas del polinizador Patagona gigas a inflorescencias con flores robadas: la proporción de flores visitadas por P. gigas, dentro de una inflorescencia, fue inversamente proporcional a la proporción de flores robadas