Why so many flowers? A preliminary assessment of mixed pollination strategy enhancing sexual reproduction of the invasive <i>Acacia longifolia</i> in Portugal

Abstract

Abstract. Acacia longifolia, a native legume from Australia, has been introduced in many European countries and elsewhere, thus becoming one of the most important global invasive species. In Europe, its flowering occurs in a period unsuitable for insect activity: nonetheless it is considered entomophilous. Floral traits of this species are puzzling: brightly coloured and scented as liked by insects, but with abundant staminate small-sized flowers and relatively small pollen grains, as it is common in anemophilous species. Invasion processes are especially favoured when reshaping local ecological networks, thus the interest in understanding pollination syndromes associated with invasive plant species that may facilitate invasiveness. Moreover, a striking difference exists between its massive flowering and relatively poor seed set. We introduced a novel approach: first, we consider the possibility that a part of the pollination success is carried on by wind and, second, we weighted the ethological perspective of the main pollinator. During the flowering season of A. longifolia (February-April 2016), we carried on exclusion experiments to detect the relative contribution of insects and wind. While the exclusion experiments corroborated the need for pollen vectors, we actually recorded a low abundance of insects. The honeybee, known pollinator of acacias, was relatively rare and not always productive in terms of successful visits. While wind contributed to seed set, focal observations confirmed that honeybees transfer pollen when visiting both the inflorescences to collect pollen and the extrafloral nectaries to collect nectar. The mixed pollination strategy of A. longifolia may then be the basis of its success in invading Portugal's windy coasts.

Contrasting effects of exotic plant invasions and managed honeybees on plant–flower visitor interactions

“…Although many exotic plants have become naturalized without imposing strong impacts on native plant communities (Timóteo et al, 2018), invasive species with attractive flowers rich in nectar and pollen can compete with native plants for pollinators, ultimately altering the structure of mutualistic networks (Benadi, Hovestadt, Poethke, & Blüthgen, 2014; Grass, Berens, Peter, & Farwig, 2013; Hansen et al, 2018; Maruyama et al, 2016; Russo, Nichol, & Shea, 2016; Stout & Tiedeken, 2017). At the same time, invasions of mass‐flowering plants can have positive, long‐term effects on pollinator population dynamics by complementing nectar and pollen resources provided by native plants (Albrecht, Ramis, & Traveset, 2016; Davis, Kelly, Maggs, & Stout, 2018; Giovanetti, Ramos, & Máguas, 2018; Russo et al, 2016; Stout & Tiedeken, 2017). Despite this variability, most of the available research has focused on analysing changes in plant–pollinator interactions in invaded vs. uninvaded plant communities without considering potential interactions between exotic plant invasion and other abiotic and biotic drivers of environmental change (González‐Varo et al, 2013, but see Grass et al, 2013).…”

Are floral traits good predictors of effective pollinators? A test of pollination syndromes

“…We hypothesize that, as has been found in diverse studies (Danieli-Silva et al, 2012;Hingston & McQuillan, 2000;L azaro et al, 2008;Strakosh & Ferguson, 2005), the insect groups inferred as pollinators will be associated with the floral traits that define the floral syndrome of each plant species. Moreover, we expect that those insect groups carry a high proportion of pollen grains of the plant species on which they were collected, confirming their effectiveness as pollinators (Benning, 2015;Danieli-Silva et al, 2012;Giovanetti et al, 2018;Hargreaves et al, 2004;Pérez et al, 2006). Finally, we hypothesize that pollination syndromes will be met in plant species with long life cycles (i.e., perennial and herbaceous-perennial [plants with underground perennation structures, with production of aerial biomass only during short periods of time every year; Crawley, 1997]), but not in species with annual life cycles as it has been suggested previously based on the restrictions experienced by each plant species to establish specialized relationships with pollinators (Bond, 1994;Johnson & Steiner, 2000;Waser et al, 1996).…”

Invasiveness, ecological impacts and control of acacias in southwestern Europe – a review

“…Schortemeyer et al (2002) found that when Australian acacias are not limited in water and nutrient resources, an increase in CO 2 concentration could enhance the growth of most of the studied species (including A. dealbata and A. melanoxylon) and increase N 2 fixation (in A. melanoxylon among others but not in A. dealbata). However, water deficiency due to the variation in climate parameters could favour native species over Acacia invasive species, as predicted for A. dealbata (González-Muñoz et al, 2014). In NW Iberia, regional cli-mate change models for the second half of the 21st century predict a temperature increase of 2-3 • C, especially in summer and interior areas, and a decrease in precipitation, especially in spring and summer (Álvarez et al, 2011).…”

“…Acacias' high colonization capacity is related to their vegetative reproduction (Lorenzo et al, 2010a; and their high investment in flower production, especially for A. dealbata (Correia et al, 2014). Mixed pollination (through honeybees and wind) in A. longifolia has been proposed as a strategy the species uses to succeed in Portugal coastal ecosystems (Giovanetti et al, 2018). Moreover, invasive Acacia species are able to rapidly build a massive and persistent seed bank (Gibson et al, 2011;Vazquez-de-la-Cueva, 2014) owing to a short generation time, long fruiting period, large seed number, small seed size, and prolonged and high seed viability (Alpert et al, 2000; Table 5).…”