Selective Foraging byPogonomyrmex salinus(Hymenoptera: Formicidae) in Semiarid Grassland: Implications for a Rare Plant

Abstract

Selective foraging by granivores can have important consequences for the structure and composition of plant communities, and potentially severe consequences for rare plant species. To understand how granivore foraging behavior affects common and rare plant species, diet selection should be viewed relative to the availability of alternative seed options, and with consideration of the individual attributes of those seeds (e.g., morphology, nutrient content). We examined the foraging decisions of Owyhee harvester ants, Pogonomyrmex salinus (Olsen), in semiarid grassland dominated by two species of grass, Poa secunda and Bromus tectorum, and two species of mustard, Sisymbrium altissimum and Lepidium papilliferum The latter is a rare plant endemic to southwestern Idaho, and its seeds are readily consumed by P. salinus We examined the diets of P. salinus colonies in June and July over three years and compared these values to the weekly availability of seeds on the ground in a 3-12 -m radius around individual ant colonies. Small-seeded species (P. secunda, S. altissimum, and L. papilliferum) were usually overrepresented in the diet of ants relative to their availability, whereas the large seeds of B. tectorum were largely avoided despite being abundant and nutritious. The reduced travel time associated with carrying small seeds may overshadow differences in nutritional content among seed types, except in times when small seeds are in short supply. Lepidium papilliferum appears particularly vulnerable to seed predation, likely in part because it grows in dense patches that are easily exploited by foragers.

A New Protocol Using Artificial Seeds to Evaluate Dietary Preferences of Harvester Ants in Semi-arid Environments

“…ontogeny) (Mayntz et al, 2005;Whitford & Jackson, 2007;Dussutour & Simpson, 2008). Moreover, it is worth to mention that the content of our mixture represents highly nutritive resources, thus, they can influence the foraging behavior of harvester ants (Crist & MacMahon, 1992;Whitford & Steinberger, 2009;Schmasow & Robertson, 2016). The mixture was poured in a thin layer and allowed to air dry for 24h before the experiments.…”

“…However, most of Pogonomyrmex species exhibit particular Red de Ecoetología, Instituto de Ecología A.C., Xalapa, Veracruz, Mexico RESEARCH ARTICLE -ANTS preferences in their seed intake, predating some plant species more than others (i.e., differential seed predation) (Taber, 1998;MacMahon et al, 2000;Rico-Gray & Oliveira, 2007). On the other hand, in the wide geographical distribution of Pogonomyrmex species, there is not a consensus about their seed preferences and which are the main factors influencing the seed selection by harvester ants (MacMahon et al, 2000;Pirk & De Casenave, 2006;Belchior et al, 2012;Schmasow & Robertson, 2016). In addition, in arid environments, where harvester ants are found, the landscape is highly heterogeneous (in terms of vegetation, climatic conditions, topography and geology) in both spatial and temporal scales (Bestelmeyert & Wiens 2001;Rietkerk et al, 2002;Whitford, 2002), which has led to the implementation of particular methodologies conceived to be used in a given arid environment (since they cannot operate in the same way and need to be readjusted for other localities (e.g.…”

“…In addition, in arid environments, where harvester ants are found, the landscape is highly heterogeneous (in terms of vegetation, climatic conditions, topography and geology) in both spatial and temporal scales (Bestelmeyert & Wiens 2001;Rietkerk et al, 2002;Whitford, 2002), which has led to the implementation of particular methodologies conceived to be used in a given arid environment (since they cannot operate in the same way and need to be readjusted for other localities (e.g. Huenneke et al, 2001) Although there is no agreement about which factors shape the differential predation of seeds, studies have showed that some factors can directly influence the seed selection by Pogonomyrmex spp., such as: spatial abundance and richness of plants (MacMahon et al, 2000;Pirk & Lopez De Casenave, 2006), nutritional quality of seeds (Whitford & Steinberger, 2009;Schmasow & Robertson, 2016), seed size (MacMahon et al, 2000;Pirk & de Casenave, 2010;, seed density (Flanagan et al, 2012), and the seed distance from the nests (Hölldobler, 1976;Holder & Polis, 1987;Schmasow & Robertson, 2016). However, due to the difficulty to test ecological hypothesis in the field that consider the effect of seed traits (e.g., nutrient content, morphology and physiochemical defenses) on the differential predation by Pogonomyrmex species, these questions have been rarely explored.…”

Structural changes over time in individual‐based networks involving a harvester ant, seeds, and invertebrates

“…Although seed size is not the sole factor determining seed selection by ants (cf. resource density; Schmasow & Robertson, 2016;Luna et al, 2018), it represents, compared with seed nutrient content, a more accessible proxy of food quality. Moreover, the foraging of certain Pogonomyrmex species tend to be influenced by seed size, particularly those with smaller colonies and workers (Taylor, 1977;Pirk et al, 2009;Pol et al, 2011).…”

“…Moreover, the foraging of certain Pogonomyrmex species tend to be influenced by seed size, particularly those with smaller colonies and workers (Taylor, 1977;Pirk et al, 2009;Pol et al, 2011). Therefore, according to the optimal foraging theory (Stephens & Krebs, 1986), it is expected that there is a cost-benefit relationship with seed size (related to energy spent and gained) and removal distance of these seeds by ants (Pirk & Lopez de Casenave, 2010;Schmasow & Robertson, 2016).…”