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Phenological distributions are characterized by their central tendency, breadth, and shape, and all three determine the extent to which interacting species overlap in time. Pollination mutualisms rely on temporal co-occurrence of pollinators and their floral resources, and although much work has been done to characterize the shapes of flower phenological distributions, similar studies that include pollinators are lacking. Here, we provide the first broad assessment of skewness, a component of distribution shape, for a bee community. We compare skewness in bees to that in flowers, relate bee and flower skewness to other properties of their phenology, and quantify the potential consequences of differences in skewness between bees and flowers. Both bee and flower phenologies tend to be right-skewed, with a more exaggerated asymmetry in bees. Early-season species tend to be the most skewed, and this relationship is also stronger in bees than in flowers. Based on a simulation experiment, differences in bee and flower skewness could account for up to 14% of pairwise overlap differences. Given the potential for interaction loss, we argue that difference in skewness of interacting species is an underappreciated property of phenological change.

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RESUMEN

Climate change can cause changes in expression of organismal traits that influence fitness. In flowering plants, floral traits can respond to drought, and that phenotypic plasticity has the potential to affect pollination and plant reproductive success. Global climate change is leading to earlier snow melt in snow-dominated ecosystems as well as affecting precipitation during the growing season, but the effects of snow melt timing on floral morphology and rewards remain unknown. We conducted crossed manipulations of spring snow melt timing (early vs. control) and summer monsoon precipitation (addition, control, and reduction) that mimicked recent natural variation, and examined plastic responses in floral traits of Ipomopsis aggregata over 3 years in the Rocky Mountains. We tested whether increased summer precipitation compensated for earlier snow melt, and if plasticity was associated with changes in soil moisture and/or leaf gas exchange. Lower summer precipitation decreased corolla length, style length, corolla width, sepal width, and nectar production, and increased nectar concentration. Earlier snow melt (taking into account natural and experimental variation) had the same effects on those traits and decreased inflorescence height. The effect of reduced summer precipitation was stronger in earlier snow melt years for corolla length and sepal width. Trait reductions were explained by drier soil during the flowering period, but this effect was only partially explained by how drier soils affected plant water stress, as measured by leaf gas exchange. We predicted the effects of plastic trait changes on pollinator visitation rates, pollination success, and seed production using prior studies on I. aggregata. The largest predicted effect of drier soil on relative fitness components via plasticity was a decrease in male fitness caused by reduced pollinator rewards (nectar production). Early snow melt and reduced precipitation are strong drivers of phenotypic plasticity, and both should be considered when predicting effects of climate change on plant traits in snow-dominated ecosystems.

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While feeding, foragers can alter their environment. Such alteration constitutes ecological niche construction (ENC) if it enables future benefits for the constructor and conspecific individuals. The environmental modification may also affect non-constructing, bystander species, especially if they share resources with constructor species. If so, ENC could confer the constructor species a competitive advantage by both enhancing its foraging returns and reducing those of bystander species. Expectations - (E1) ENC frequency should vary positively with the recent and current density of the constructor species, and (E2) constructors should use modifications disproportionately. In contrast, bystanders should (E3) experience intensified competition for the affected resource, and (E4) exhibit diverse, possibly mitigating, responses to ENC, depending on opportunity and relative benefits. We investigated these expectations in Argentina for competition for Fuchsia magellanica nectar between an invasive bumble bee Bombus terrestris (terr: putative constructor), which often bites holes at the bases of floral tubes to rob nectar, and native B. dahlbomii (dahl: bystander), which normally accesses Fuchsia nectar through the flower mouth (front visits). Robbing holes constitute ENC, as they persist until the 7-day flowers wilt. The dynamics of the incidence of robbed flowers, abundance of both bees and the number and types of their flower visits (front or robbing) were characterised by alternate-day surveys of plants during 2.5 months. After initially accessing Fuchsia nectar via front visits, terr switched to robbing and its abundance on Fuchsia increased 20-fold within 10 days (E2). Correspondingly, the incidence of robbed flowers varied positively with recent and past terr abundance (E1). In contrast, dahl abundance remained low and varied negatively with the incidence of robbed flowers (E3). When terr ceased visiting Fuchsia, dahl abundance increased sixfold within 10 days (E3), possibly because many dahl previously had avoided competition with terr by feeding on other plant species (E4). While terr was present, dahl on Fuchsia used front visits (tolerance) or used existing robbing holes (adoption: E4). The diverse dahl responses suggest partial compensation for competition with terr. ENC alters competitive asymmetry, favouring constructor species. However, bystander responses can partially offset this advantage, perhaps facilitating coexistence.

Los animales pueden alterar su ambiente como consecuencia del forrajeo. Esta alteración constituye una forma de construcción de nicho ecológico (CNE) cuando la misma promueve beneficios para el constructor y otros individuos de la misma especie. Esta modificación ambiental puede también afectar otras especies no involucradas directamente en la CNE, especialmente cuando comparten recursos con la especie constructora. En este caso, la CNE puede conferir una ventaja competitiva a la especie constructora, incrementado sus beneficios alimenticios en detrimento de los de las especies no constructoras. Expectativas - (E1) La frecuencia de eventos de CNE debería variar positivamente con la densidad presente y pasada de la especie constructora, y (E2) los individuos de la especie constructora deberían usar las modificaciones asociadas a la CNE en forma diferencial. Por el contrario, los individuos de las otras especies que usen el mismo recurso deberían (E3) experimentar una intensificación de la competencia, y (E4) exhibir una diversidad de respuestas, algunas que atenúen las consecuencias negativas de la CNE, lo que va a depender de la oportunidad y de los beneficios relativos. En Argentina, investigamos el cumplimiento de estas expectativas en la competencia por el néctar de Fuchsia magellanica entre el abejorro invasor, Bombus terrestris (terr: la especie constructora), quien frecuentemente agujerea la base de tubos florales para robar néctar, y el abejorro nativo B. dahlbomii (dahl: la especie afectada), quien normalmente accede al néctar de Fuchsia en forma legítima a través de la apertura de la flor (visitas frontales). Los agujeros realizados por terr constituyen un caso de CNE, ya que persisten a lo largo de la vida de la vida de la flor (aprox. 7-day). En este trabajo relevamos la incidencia de flores robadas, la abundancia de las dos especies de abejas, y el número y tipo de sus visitas florales (frontales o de robo) día por medio durante 2.5 meses. Inicialmente terr accedió las flores de Fuchsia frontalmente, pero más tarde cambió su comportamiento convirtiéndose en un robador casi exclusivo a lo largo de un período de 10 días en que su abundancia se incrementó 20 veces (E2). En consecuencia, la incidencia de flores robadas varió positivamente con la abundancia de terr presente y pasada (E1). Por el contrario, la abundancia de dahl fue baja y varió negativamente con la incidencia de flores robadas (E3). En particular, la abundancia de dahl se incrementó seis veces en un período de 10 días una vez que terr cesó de visitar las flores de Fuchsia (E3), posiblemente porque dahl evitó la competencia con terr forrajeando en otras especies de plantas (E4). Cuando terr estuvo presente, dahl visitó las flores de Fuchsia frontalmente (tolerancia) o usó los agujeros existentes (adopción de comportamiento robador: E4). Estas respuestas diversas de dahl sugieren una compensación parcial de los efectos negativos de la competencia con terr. La CNE incrementa la asimetría de la competencia, favoreciendo la especie constructora. Sin embargo, las diferentes respuestas de las especies afectadas pueden compensar parcialmente esta desventaja competitiva, tal vez facilitando la coexistencia.

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