Diminishing importance of elaiosomes for acacia seed removal in non-native ranges

Abstract

Myrmecochorous plants produce seeds with lipid-rich appendages (elaiosomes) which act as a reward for seed-dispersing ants. Seed dispersal is important for exotic species, which often need to establish new mutualistic interactions in order to colonize new non-native habitats. However, little is known about the importance of elaiosomes for seed removal in many of their non-native ranges. We studied ant–seed interactions of elaiosome-bearing and elaiosome-removed seeds of the Australian trees Acacia dealbata and Acacia longifolia in order to assess the relative importance of elaiosomes for seed removal between their native (Australia) and non-native (Portugal) ranges. In Portugal, we also studied the co-occurring native plant species with myrmecochorous seeds, Pterospartum tridentatum and Ulex europaeus, across three contiguous levels of acacia invasion: control (i.e. no acacia), low, and high acacia tree density. Acacia seeds were successfully removed by ants in their non-native region by a diversified assemblage of ant species, even in sites where native plants interacted with only one specialized ant species. In the invaded range, diminishing relative importance of elaiosomes was associated with changes in the ant community due to acacia invasion, and for A. dealbata, ant species richness decreased with increasing acacia tree density. Although seed removal was high for both acacia species, the importance of elaiosomes was proportionally lower for A. dealbata in the non-native region. Native plant species experienced significant reductions in seed removal in areas highly invaded by acacia, identifying another mechanism of displacement of native plants by acacias.

Appendices

Appendix 1: Rarefaction curves

See Fig. 3.

Fig. 3

Rarefaction curves for ant–seed interactions observed at each of the study sites

Appendix 2: Interaction networks

See Fig. 4 and Tables 3, 4 and 5.

Fig. 4

Ant–seed mutualistic networks based on the observed interactions rates (seed removal) for sites with A. dealbata (top), or A. longifolia (below), on either Portugal (left), or Australia (right)

To visualize the relative strength of interactions on each site, we calculated ant–seed interaction networks for each of the two Portuguese sites with the statistical package “bipartite” on R 3.1.2 (Dormann et al. 2009). For each network, we calculated the following indexes: connectance, indicative of the realized proportion of possible links, obtained by the total sum of links divided by the number of cells in the interaction matrix (Dunne et al. 2002); nestedness (weighted NODF) indicating how the system is organized, with values closest to zero indicating high nestedness and values nearing 100 low nestedness (Rodriguez-Girones and Santamaria 2006); and network specialization index H2′, describing the level of specialization of the network, and ranging from zero (no specialization) to 1 (complete specialization) (Blüthgen et al. 2006); animal robustness, measuring the sensitivity of the system to the loss of plant species; and plant robustness, measuring the sensitivity of the system to the loss of animal species (Memmott et al. 2004; Burgos et al. 2007). For detailed descriptions of the different indexes used, see Dormann et al. (2009) and references therein. The study of interaction networks requires at least two species at each interaction level. Since we only had one plant species in Australia, we could only calculate interaction indexes for the Portuguese populations, for which two plant species were considered. However, we plotted interaction graphs for the Australian observations to quantitatively compare their interaction strength to those of Portugal.