Supplementing Seed Banks to Rehabilitate Disturbed Mojave Desert Shrublands: Where Do All the Seeds Go?

Revegetation of degraded arid lands often involves supplementing impoverished seed banks and improving the seedbed, yet these approaches frequently fail. To understand these failures, we tracked the fates of seeds for six shrub species that were broadcast across two contrasting surface disturbances common to the Mojave Desert—sites compacted by concentrated vehicle use and trenched sites where topsoil and subsurface soils were mixed. We evaluated seedbed treatments that enhance soil‐seed contact (tackifier) and create surface roughness while reducing soil bulk density (harrowing). We also explored whether seed harvesting by granivores and seedling suppression by non‐native annuals influence the success of broadcast seeding in revegetating degraded shrublands. Ten weeks after treatments, seeds readily moved off of experimental plots in untreated compacted sites, but seed movements were reduced 32% by tackifier and 55% through harrowing. Harrowing promoted seedling emergence in compacted sites, particularly for the early‐colonizing species Encelia farinosa, but tackifier was largely ineffective. The inherent surface roughness of trenched sites retained three times the number of seeds than compacted sites, but soil mixing during trench development likely altered the suitability of the seedbed thus resulting in poor seedling emergence. Non‐native annuals had little influence on seed fates during our study. In contrast, the prevalence of harvester ants increased seed removal on compacted sites, whereas rodent activity influenced removal on trenched sites. Future success of broadcast seeding in arid lands depends on evaluating disturbance characteristics prior to seeding and selecting appropriate species and seasons for application.     

The growth and form of tunnelling networks in ants

Many biological networks grow under strong spatial constraints, where the large-scale structure emerges from the extension, the branching and intersection of growing parts of the network. One example is provided by ant tunnelling networks, which represent the most common nest architecture in ants. Our goal was to understand how these network structures emerge from the tunnel growth dynamics. We used a standardized two-dimensional set-up shaped as a disk and studied the characteristics of tunnel growth in terms of initiation, propagation and termination of new digging sites and found that they can be described with simple probabilistic laws. We show that a model based on these simple laws and for which parameters were measured from the sand disks experiments can account for the emergence of several topological properties that were observed in experimental networks. In particular, the model accurately reproduced an allometric relation between the number of edges and the number of nodes, as well as an invariance of the node degree distribution. The model was then used to make predictions about the resulting networks' topology when the geometry of the sand substrate was shaped as a square. Experiments aimed at testing the model's predictions showed that the predictions were indeed validated. Both in the model and in the experiments, there was a similar trend for the node degree distribution tail to be steeper in the square sand patch than in the disk sand patch, while other characteristics such as the meshedness (i.e. how densely the network is internally connected) remained constant. Because network growth based on branching/fusion events is widespread in biological systems, this general model might provide useful insights for the study of other systems and, more generally, the evolution of spatial networks in biological systems.     

Two new myrmecophilous species of the genus Scutacarus (Acari: Prostigmata: Scutacaridae) with world keys to related species groups

In a survey on heterostigmatic mites (Acari: Prostigmata: Heterostigmata) associated with insects including ants in northwestern Iran, two new species of myrmecophilous mites of the genus Scutacarus (Acari: Scutacaridae) were found. They are S. shajariani sp. nov. phoretic on Tetramorium sp. (Hymenoptera: Formicidae) and S. ebermanni sp. nov. phoretic on Messor sp. The mentioned new species belong to groups of species with reduced setae e and h₂, and no setae 4b, respectively. These two new species are described and illustrated, and world keys are provided for respective species groups.     

Effects of harvester ants on plant species distribution and abundance in a serpentine grassland

Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messor andrei, the black seed-harvesting ant, on a serpentine grassland plant community in northern California. First, to determine if selective seed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundances of all species on these plots and on control plots were measured before and after exclosure. Second, to determine if M. andrei nest mounds affect plant community composition, we compared plant species abundances on and off nest mounds. M. andrei deposit large amounts of organic matter on their nest mounds over a foraging season, so mounds may alter the edaphic environment. The exclusion of seed-harvesting activity did not cause changes in the plant community. Nest mounds had a strong effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, produced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentine grassland. Received: 31 March 1997 / Accepted: 6 May 1997     

Is foraging in the desert ant,Messor pergandei (Hymenoptera: Formiceidae), limited by water?

Abstract.

• 1 The seed-harvesting ant Messor (Veromessor) prrgandei (Mayr) is a common inhabitant of southwestern deserts of the U.S.A. Foragers vary in size from less than 1 mg to more than 10 mg in body mass and may travel over 80 m on a single foraging trip. Their small size, long foraging range, and hot, arid habitat suggest that water stress may limit foraging activity. We examined intercolony and interindividual variation in water loss of M.pergandei foragers under several different situations in the field.
• 2 Colonies differed significantly in minimum critical water content (W_c) of individual foragers (water content below which foragers are incapable of normal locomotion). In one colony small workers had disproportionately higher W_c than larger workers; in the other colony W_c was isometric with body size.
• 3 Groups of workers confined in the field approached W_c only after normal foraging stopped and substrate temperatures exceeded 45°C, while water content of individual foragers did not approach the W_c during normal foraging periods. Moreover, seed load and distance travelled did not negatively affect forager water content, as measured on return to the nest: indeed, our results suggest that forager hydration level may influence load selection and/or foraging distance. We conclude that, under normal circumstances, foraging in M.pergandei is not water-limited.

     

Do seed harvesting ants threaten the viability of a critically endangered non-myrmecochorous perennial plant population? A complex interaction

This study analyzes the fate of seeds removed by ants, as well as the seed supply, seedling survival, and the ability to form persistent seed banks in the soil, in a critically endangered population of the non-myrmecochorous perennial halophyte Helianthemum polygonoides, in order to determine the extent to which seed removal by ants represents a real bottleneck for recruitment and thus a threat for long-term population viability. Apparently, the impact of seed-harvester ants was dramatic: the primary seed shadow was reduced by up to 93% by the activity of Messor bouvieri and Aphaenogaster dulcineae. However, M. bouvieri lost 7% of fruits collected by dropping them on trunk trails, mostly under plant cover, the most propitious microenvironment for recruitment. In addition, some seeds were mistakenly rejected in refuse piles, although here recruitment was extremely low. A significant fraction of seeds (≈40%) remained viable in the soil for over 2 years. In addition, H. polygonoides had the ability to form short-term persistent seed banks, in spite of the intense ant seed removal. In general, seedling recruitment was very low, denoting the scarcity of safe sites in the habitat. Overall, the data corroborate that seed removal by ants, although highly intense, does not compromise the viability of perennial-plant populations, because seed supply and seed reserve in the soil are enough to exploit current and future safe sites. A population viability analysis demonstrated that other threats affecting the survival of reproductive H. polygonoides plants actually constitute the real risk for the conservation of this critically endangered species.     

Effects of harvester ants on seed availability and dispersal of Lavandula stoechas subsp. pedunculata in a Mediterranean grassland-scrubland mosaic

The role of harvester ants in Mediterranean grassland and scrubland has mostly focused on seed consumption. However, recent studies have reported their role as accidental dispersal agents of some of the collected seeds via refuse piles. The objective of this study is to examine the effect of the ant Messor barbarus on seed availability and dispersal of one of its major diet components, Lavandula stoechas subsp. pedunculata, in scrubland, grassland and the ecotones between them. After confirming and quantifying the Lavandula contribution to M. barbarus diet, we described the spatial and temporal patterns of pre- and post-dispersal seed predation, seed content and seedling occurrence in the refuse piles. Our results show that: (1) Lavandula propagules constitute a high proportion of the prey items collected by M. barbarus, with particularly intense collection activity in mid-summer, spring and autumn, in decreasing order. (2) Pre-dispersal predation rate was significantly higher in the ecotone than in the scrubland (76% and 13.5% of total seed production lost respectively). (3) Season and propagule type (seed vs. fruit) were the most significant variables explaining the post-dispersal predation probability, which approached 100% of seeds after 48 h in mid-summer. (4) Viable Lavandula seeds were found in refuse piles at densities of 0.06–0.2 per g of refuse pile material, or 58.8–207.2 per refuse pile. On the one hand, these results indicate that the ecotones are most affected by M. barbarus pre-dispersal consumption, which may locally limit Lavandula colonisation. On the other hand, the small proportion of consumed seeds that is dispersed to refuse piles may be relevant at the population level, as this dispersal implies arrival at potentially favourable sites for establishment.     

西北荒漠草原针毛收获蚁的筑巢行为

针毛收获蚁Messoraciculatus(F .Smith)是我国北方特别是西北荒漠草原的优势种蚂蚁 ,亦是重要的种子收获性蚁类 ,收获、贮藏和取食 1 0余种荒漠植物种子。自然巢穴深 80cm～ 1 3 7cm ,单一巢口 ,沿主隧道不同深度有水平延伸的各层巢室 ,巢室向四周的延伸范围随巢深减小 ,层间距拉大。自然巢穴的空间结构与社群越冬与种子的深层贮藏有着密切的关系。饲养社群筑巢行为由蚁后主导 ,起始营巢位点数与蚁后头数有关 ,每一蚁后带领 5 0～ 70头工蚁 ,营巢时程 0 5～ 0 67d 60工蚁 蚁后 ,蚁巢有多个巢口。自然与饲养社群的巢穴结构差异很大 ,但两者单一蚁后与工蚁所占的有效巢穴空间是基本一致的 ,自然社群 1 81 3 7cm3 蚁后 ,2 99cm3 工蚁 ,饲养社群 1 5 6 2 6cm3 蚁后 ,2 60cm3 工蚁     

针毛收获蚁社群密度与分布及社群各品级在自然穴巢中的季节迁移行为

针毛收获蚁Messoraciculatus(F .Smith)是我国北方特别是西北荒漠草原的优势种蚂蚁 ,亦是重要的种子收获性蚁类 ,收获、贮藏和取食 1 0余种荒漠植物种子。本文以沙坡头自然保护区红卫固定沙丘地段为样地 ,对该蚁的社群密度与分布及社群各品级在自然穴巢中的季节迁移行为作了研究。最近邻体法测定表明 ,社群在新生生境中起初随机定居 ,随社群密度的不断定居 ,空间分布型逐渐向均匀六边形过渡 ;社群密度为成熟社群 34 .98巢 hm2 ,发展中社群 2 6 .99巢 hm2 ,新建社群 4 4.98巢 hm2 ,种群密度 2 .6 8工蚁 m2 。社群结构的季节动态总蚁量和幼蚁为双峰型 ,蚁后和雄蚁为单峰型。室内人工蚁巢饲养表明 ,社群结构约 1月后趋于稳定 ,蚁量 5 0～ 70头 ,单蚁后以 4个品级混合越冬。自然社群的挖掘表明 ,各品级在蚁巢中的季节迁移行为明显呈现上迁、前层持续活动、下迁和深层越冬四期。此外 ,讨论了该蚁对荒漠草原环境的适应性。