Ant preference for seeds without awns increases removal of exotic relative to native grass seeds

Abstract

1. The removal of seeds by granivorous ants can affect plant recruitment through either seed loss from predation or the dispersal and recruitment of seeds that are removed but not consumed. Consequently, variation in ant selection preferences can influence patterns of seed removal and affect plant community composition, including the spread of exotic plant species. 2. We conducted a seed removal experiment to determine whether: 1) rates of removal by ants differed between three native and three exotic grass species in an Australian temperate grassland; and 2) differences in removal rates were associated with the presence or absence of awns. 3. We found that seeds of the three exotic species, none of which had awns, were removed by ants at a higher rate than those of the three native species, all of which had awns. Removal rates of native species increased when awns were manually removed, suggesting the awns of native species acted as a removal barrier. 4. While we do not know the fate of seeds removed from our experiment, differences among species in removal rates mirrored differences in their spatial spread in a separate seed addition experiment. Exotic species removed by ants at a higher rate in the removal experiment had more widely dispersed seedlings than native species in the seed addition experiment, potentially indicating a role for granivorous ants in dispersing exotic seeds. Identifying ant selection preferences and directly linking removal to seed fate could help explain how exotic grass species move around the landscape.

Niche partitioning overrides interspecific competition to determine plant species distributions along a nutrient gradient

“…We found no evidence that plot contamination affected recruitment outcomes (Supporting information), possibly due to strong niche differentiation among our species. We did observe ants removing seeds during our experiment, and a separate experiment in the same grassland confirmed the preferential removal of exotic grass seeds by ant seed predators (Wandrag et al 2021). High rates of seed predation will strongly limit plant recruitment (Hulme 1994) and are common in grasslands globally (Turnbull et al 2000).…”

Seasonal effects of granivory on broadcast seeding restoration in a degraded semiarid grassland

“…Morphology is another common driver of seed preference, and the ACHY and HEAN seeds preferred by P. occidentalis are morphologically similar, with smooth, awnless seed coats. This result supports recent studies that find lower ant granivory for seeds with a structure or coating (Crist & MacMahon 1992;Paolini et al 2020;Wandrag et al 2021) or a higher seed to seed plus structure ratio (Martyn et al 2022), possibly because large awns make seeds cumbersome for transport. In contrast, an older study by Pulliam and Brand (1975) found that Pogonomyrmex spp.…”

Granivorous ants prefer small and unprotected seeds—implications for restoration in arid ecosystems

“…Seed movement by granivores is likely an underestimated challenge for restoration success. Granivores can remove (predate) native seed spread on the landscape (Linabury et al 2019), assist native seed movement (Brewer & Rejmanek 1999), and even increase exotic seed movement (Ortiz et al 2021; Wandrag et al 2021). For grasslands, understanding and reducing ant‐seed predation could be important components of restoration success.…”

“…Our results compliment those of previous studies that showed a seed structure or coating can reduce seed predation by ants. For example, Wandrag et al (2021) conducted a comparative study with three exotic seed species naturally with no awns, three native seed species with awns intact, and the same three native species with the awns removed in an Australian grassland. They found that seed removal rate by ants was highest for the exotic species but that removing the awns increased these rates almost fivefold.…”