Are red imported fire ants facilitators of native seed dispersal?

Invasive ants threaten native communities, in part, through their potential to disrupt mutualisms, yet invasive species may also facilitate native species. The red imported fire ant (Solenopsis invicta) is one of the most conspicuous invasive ants in North America and its high densities, combined with its potential to displace native ants, have led to concerns that it may disrupt ant-plant seed dispersal mutualisms. We examined the potential of fire ants to disperse seeds in the longleaf pine ecosystem by comparing the removal of elaiosome-bearing seeds by fire ants versus native ants. A total of 14 ant species were observed removing seeds, with fire ants responsible for more than half of all removals. While fire ants were the dominant seed remover in this system, they did not remove significantly more seeds than would be expected based on their population density (46% of ground-dwelling ants). Moreover, red imported fire ants were similar to native ants with respect to distance of seed movement and frequency of moving seeds back to the nest. Areas of higher fire ant densities were found to have greater rates of seed removal by ants without a subsequent drop in seed dispersal by native ants, suggesting that fire ant-invaded areas may experience overall higher levels of seed dispersal. Thus, fire ants may actually facilitate dispersal of elaiosome-bearing plant species in the longleaf pine ecosystem.     

Bet‐hedging and germination in the Australian arid zone shrub Acacia ligulata

The diaspore of the Australian arid zone shrub Acacia ligulata is dispersed by birds and ants. To investigate the benefits of providing a dispersal structure attractive to both groups, we compared the germination response and viability of seeds eaten by birds, handled by ants or collected from trees to simulated precursors of germination: scarification, fire and rainfall were simulated. Seed germination and viability were related to the degree of preheating disturbance to the seed coat. Heating increased the germinability of seeds not scarified or eaten by birds. In the absence of heating, ingestion by birds increased germinability. Heating increased the mortality of seeds. Our results suggest that ingestion of seeds by birds may break seed dormancy and hence enable some seeds to germinate soon after dispersal. Alternatively, seeds not eaten by birds are likely to remain dormant until sufficiently scarified by soil or stimulated by fire. Consequently, in areas such as the Simpson Desert, A. ligulata may be able to use a range of seedling establishment ‘windows’ provided by monsoon rains, post‐fire environments and unseasonal winter rains, and also spread the risk of unsuccessful seedling establishment by retaining dormant seeds in the seedbank.     

Foraging behaviour of harvesting ants determines seed removal and dispersal

Harvesting ants can affect the regeneration of plants through at least two different processes: seed removal and seed dispersal. We analyse the role of different foraging strategies of ants on patterns of seed removal and dispersal by three Messor species with considerable differences in their foraging systems. Messor capitatus workers rarely leave the nest in well-formed columns, while the other two species form foraging trails, with M. bouvieri forming temporary trails and M. barbarus foraging on a stable system of permanent foraging trails. Overall seed intake of M. capitatus colonies is considerably less than that of the two group-foraging species. There are also differences in the size of seeds collected: M. barbarus and M. capitatus harvest similar amounts of large and small seeds, while M. bouvieri harvests small seeds more intensely than large ones, due to the smaller size of the worker caste. The three Messor species differ in the percent of seed dropping of the different seed type and in the seed dispersal distance. Moreover, M. bouvieri and M. capitatus redistributed dropped seeds preferentially in bare soil and low sparse vegetation habitats, while M. barbarus redistributed seeds mainly in the high vegetation habitat. These results show that the foraging systems of these harvesting ants determine different patterns of seed removal and dispersal and, thus, affect the abundance and redistribution of seeds in the area.     

ANT DISPERSAL OF CALATHEA (MARANTACEAE) SEEDS BY CARNIVOROUS PONERINES (FORMICIDAE) IN A TROPICAL RAIN FOREST

Two tropical herbaceous plant species, Calathea microcephala and Calathea ovandensis, exhibit morphological adaptations for seed dispersal by ants. In the field, 21 ant species are attracted to the arils of the seeds. Previously known as predatory carnivores, the ponerine ants Odontomachus laticeps, O. minutus, Pachycondyla harpax, and P. apicalis carry the seeds toward their nests, behaving as though they were carrying prey. These ants remove the aril in the nest, and seeds without arils germinate more readily than seeds with arils. Ant foraging distances can account for seedling distributions. Myrmecochory may be far more common in tropical rain forests than has been previously suspected.     

Dispersal distance as a benefit of myrmecochory

Summary Nutrient-enrichment and predator avoidance are generally considered the major benefits of myrmecochory, but this is apparently not so in Australia where some of the greatest known concentrations of myrmecochorus plants occur. Here I demonstrate that distance dispersal is a potential benefit of myrmecochory in the Australian environment. Although mean dispersal distance at a site in southeastern Australia was only 2.1 m, the dispersal curve was characterised by a narrow peak and long tail. A dispersal curve of this shape has been shown by Green (1983) to be optimal when safe sites for seedling establishment are rare, as is typically the case for Australian myrmecochores in the absence of fire. Both mean disperal distance and shape of the dispersal curve are influenced strongly by nest density and dispersion, population size, and territoriality of seed-dispersing ants. I argue that distance dispersal is likely to be a benefit of myrmecochory throughout Australia, independent of any targeting of seeds to ant nests.     

Ant-mediated expansion of an obligate seeder species during the first years after fire

Most obligate seeder species build up a soil seed bank that is associated with massive seed germination in the year immediately after a fire. These species are also shade‐intolerant and disappear when vegetation cover closes, creating unsuitable conditions for seedling recruitment. The only way for these plants to expand their populations is when habitats suitable for seedling recruitment arise (i.e. in years immediately after a fire). However, short primary seed dispersal of obligate seeders does not allow these plants to colonise the suitable habitats, and these habitats can only be colonised by secondary seed dispersion. We hypothesised that Fumana ericoides, an obligate‐seeding small shrub, not only establishes abundantly in the first year after fire, but also expands its local range in the following years due to secondary dispersal by ants while suitable habitats are still available. We tested this hypothesis using experimental studies and a simulation model of potential population expansion in a recently burned area. Results showed that F. ericoides not only established prolifically in the year immediately after fire, but was also able to recruit new individuals and expand its population in the years following the fire, despite a low germination rate and short primary seed dispersal. Ant‐mediated seed dispersal and availability of suitable habitats were key factors in this phenomenon: ants redistributed seeds in suitable habitats while they were available, which accelerated the expansion of F. ericoides because new plants established far away from the core population.     

Forest Succession and Distance from Forest Edge in an Afro‐Tropical Grassland1

Forest succession on degraded tropical lands often is slowed by impoverished seed banks and low rates of seed dispersal. Within degraded landscapes, remnant forests are potential seed sources that could enhance nearby forest succession. The spatial extent that forest can influence succession, however, remains largely unstudied. In abandoned agricultural lands in Kibale National Park, Uganda, recurrent fires have helped perpetuate the dominance of tall (2–3 m) grasses. We examined the effects of distance from forest and grassland vegetation structure on succession in a grassland having several years of fire exclusion. At 10 and 25 m from forest edge, we quantified vegetation patterns, seed predation, and survival of planted tree seedlings. Natural vegetation was similar at both distances, as was seed (eight species) and seedling (six species) survival; however, distance may be important at spatial or temporal scales not examined in this study. Our results offer insight into forest succession on degraded tropical grasslands following fire exclusion. Naturally recruited trees and tree seedlings were scarce, and seed survival was low (20% after 7 mo). While seedling survival was high (95% after 6 to 8 mo), seedling shoot growth was very slow (x?= 0.5 cm/100 d), suggesting that survivorship eventually may decline. Recurrent fires often impede forest succession in degraded tropical grasslands; however, even with fire exclusion, our study suggests that forest succession can be very slow, even in close proximity to forest.     

An update of the world survey of myrmecochorous dispersal distances

We update the global assessment of seed dispersal by ants and test the hypothesis that the body size of seed‐dispersing ant species varies with latitude in the same way as dispersal distance. We compiled all published data about seed dispersal distance by myrmecochory through March, 2011. We then broke the data down by vegetation type, geography and taxonomy. We also compiled data on body size (body length) of the seed‐dispersing ant species from the studies consulted. Based on 7889 observations, the mean dispersal distance was 1.99 m, although the curve has a long tail extending to 180 m. Considering the mean dispersal distance by ant species and study as independent data, the mean dispersal distance was 2.24 ± 7.19 m (n = 183). Shorter distances are associated with smaller ant species, while the tail of the dispersal curve is due to larger ant species. The mean dispersal distance of myrmecochorous seeds dispersed by ants decreased with increasing latitude, but there was no significant relationship between the body size of dispersing ant species and latitude (i.e. myrmecochorous seed‐dispersing ant species do not follow Bergmann's rule). In 1998 we made three predictions: 1) the dispersal distances of the Southern Hemisphere will decrease with as more data from mesophyllous vegetation are obtained; 2) assuming that ant nest density is higher at lower latitudes, the differences in distances between hemispheres would probably decrease with more data; and 3) numerical differences between dispersal distances in mesophyllous and sclerophyllous vegetation zones would increase with more data. The results obtained since 1998 support the only the third prediction. The dispersal distances in mesophyllous vegetation zones are shorter than in the sclerophyllous vegetation zones, and the difference between 1998 have increased. The differences in dispersal distances between hemispheres are consistent with the avoidance of parent–offspring competition (escape hypothesis).     

Establishment of chasmogamous and cleistogamous seedlings of an ant-dispersed UNDERSTORY HERB,CALATHEA MICANS (Marantaceae)

This study compares the establishment success of cleistogamous (CL, obligately selfed) and chasmogamous (CH, potentially outcrossed) seeds of the neotropical understory herb, Calathea micans (Marantaceae). In this species, CH and CL seeds are morphologically identical and are both dispersed by ants. I measured dispersal distances and analyzed seedling recruitment and seedling size for ant-planted CH and CL seeds, a protocol made possible by the fact that the seed coat of C. micans can be indelibly marked and remains attached to the base of the seedling for a few months. Seeds were taken by ants from experimental depots placed in natural conditions. In a second experiment, I planted CH and CL seeds in gap and understory sites to determine the effects of seed type and light on germination percentage, date of emergence, and seed and seedling survivorship. Ant-planted CH and CL seedlings did not differ in dispersal distance. CH and CL seeds did not differ significantly in recruitment probability and seedling size in either experiment. However, for the ant-planted seedlings, CH recruitment was higher in the understory than in the gap, while CL recruitment was uniform across light environments. In conclusion, I found a difference in the establishment success of CH and CL seeds in this understory herb, only after dispersal by ants in field conditions.     

Ecological benefits of myrmecochory for the endangered chaparral shrub Fremontodendron decumbens (Sterculiaceae)

Fremontodendron decumbens grows in a single county in central California, USA. Prior research showed that its elaiosome-bearing seeds are dispersed by the harvester ant Messor andrei. I tested several hypotheses regarding the positive role of ant-mediated dispersal to F. decumbens: (1) Does ant-mediated seed dispersal facilitate seed escape from rodent predation?; (2) Does ant processing of seeds stimulate germination?; (3) Are ant middens more suitable microsites for seed or seedling survival in unburned chaparral areas?; and (4) Do survival benefits of dispersal occur post-fire in the form of differences in seedling survival probabilities and, if so, why? Results of tests of each hypothesis were: (1) similar percentages of seeds placed on ant middens and under F. decumbens shrub canopies were destroyed by rodents, but seeds from which elaiosomes had been removed were more likely to escape rodent predation; (2) seeds processed by ants did not germinate more readily than seeds removed directly from shrub branches; (3) seedling predation was a major cause of mortality in unburned chaparral on both ant middens and under shrubs, and overall seedling survival did not differ between the two microsites; (4) post-burn seedling survival was significantly greater for seedlings dispersed away from F. decumbens shrub canopies, because dispersed seedlings were both less likely to be killed by predators and more likely to be growing in a gap created by the fire-caused death of an established shrub. I concluded that the major ecological benefit to F. decumbens of ant-mediated seed dispersal was elevated post-fire seedling survival resulting from enhanced escape by dispersed seedlings from both predation and competition.     

Combining distances of ballistic and myrmecochorous seed dispersal in Adriana quadripartita (Euphorbiaceae)

The separate contributions of different vectors to net seed dispersal curves of diplochorous systems have rarely been characterised. In Australia, myrmecochory is a common seed dispersal syndrome and in the majority of such systems, seeds are initially dispersed ballistically. We measured ballistic and myrmecochorous seed dispersal distances in relation to canopies of Adriana quadripartita (Euphorbiaceae) and used a simulation model to estimate the net dispersal curve. We also compared seed removal rates and ant abundances under, and outside, plant canopies to examine how foraging patterns by ants may affect net dispersal.Overall ant abundance did not show a significant numerical response to seedfall; however, the abundance of the main seed dispersing ant, Rhytidoponera ‘metallica’ did. Despite this, seed removal rates did not differ significantly between canopy and open locations. Rhytidoponera ‘metallica’ account for 93% of observed seed dispersal events. On average, the ants dispersed seeds 1.54 m and in doing so, moved seed a mean radial distance of 0.76 m away from canopy edges. This contribution to net dispersal distance by ants is considerable since ballistic dispersal moved seeds a median distance of 7.5 cm. Our simulation model indicated that the combination of ballistic and ant seed dispersal is expected to result in seeds being transported a median net radial dispersal distance of 1.05 m from the canopy edge.Thus in this system, an important function of diplochory may simply be to move a higher proportion of seeds from under the canopy of parent plants than is possible by ballistic dispersal alone. This ‘dispersal-for-distance’ may result in reduced parent–offspring competition or may increase the probability that seeds reach rare safe sites for germination and recruitment.     

Interactions between ants and seeds of two myrmecochorous plant species in recently burnt and long‐unburnt forest sites

Myrmecochory (seed dispersal by ants) is a common seed dispersal strategy of plants in fire‐prone sclerophyll vegetation of Australia, yet there is little understanding of how fire history may influence this seed dispersal mutualism. We investigated the initial fate of seeds of two myrmecochorous plant species, the small‐seeded Pultenaea daphnoides J.C. Wendl. and the large‐seeded Acacia pycnantha Benth., in replicated burnt (3.25 years since fire) and unburnt (53 years since fire) forest plots in the Mount Lofty Ranges, South Australia. Specifically we measured (i) seed removal rates; (ii) the frequency of three ant–seed interactions (seed removal, elaiosome robbery and seed ignoring); (iii) the relative contribution of different ant species to ant–seed interactions; and (iv) the abundance of common interacting ant species. Rates of seed removal from depots and the proportion of seeds removed were higher in recently burnt vegetation and the magnitude of these effects was greater for the smaller‐seeded P. daphnoides. The overall proportion of elaiosomes robbed was higher in unburnt vegetation; however, the decrease in elaiosome robbery in burnt vegetation was greater for P. daphnoides than for A. pycnantha. Ants ignored seeds more frequently in burnt vegetation and at similar rates for both seed species. In total, 20 ant species were observed interacting with seeds; however, three common ant species accounted for 66.3% of ant–seed interactions. Monomorium sydneyense almost exclusively robbed elaiosomes, Rhytidoponera metallica typically removed seeds and Anonychomyrma nr. nitidiceps showed a mix of the three behaviours towards seeds. Differences in the proportions of seeds removed, elaiosomes robbed and seeds ignored appeared to be largely driven by an increase in abundance of A. nr. nitidiceps and a decrease in abundance of M. sydneyense in burnt vegetation. Understanding how these fire‐driven changes in the initial fate of myrmecochorous seeds affect plant fitness requires further investigation.     

蚂蚁觅食与搬运行为对阜平黄堇和小花黄堇种子散布的影响

蚂蚁是无脊椎动物中重要的种子传播者,蚂蚁散布影响植物种子的传播和扩散,进而会影响种苗的空间分布格局。在野外研究了蚂蚁觅食及搬运行为对阜平黄堇(Corydalis wilfordii Regel)和小花黄堇(C.racemosa(Thunb.)Pers.)种子散布的影响。结果显示,双针棱胸蚁和束胸平结蚁是两种植物种子的共同搬运者,前者行使群体募集,后者行使简单协作募集。在搬运阜平黄堇种子时,双针棱胸蚁在原地或搬运途中取食油质体后抛弃的种子约占种子总数的56%,而拖至蚁巢的种子约占种子总数的44%,平均搬运距离为(1.85±0.24)m,搬运效率为(43.8±7.5)粒/h;而束胸平结蚁将完整种子全部直接搬运至蚁巢,平均搬运距离为0.45 m,搬运效率为(7.3±2.2)粒/h。在搬运小花黄堇种子时,双针棱胸蚁和束胸平结蚁均将完整种子全部直接搬运至蚁巢,平均搬运距离分别为(6.27±4.40)m和(6.65±1.64)m,搬运效率分别为(34.2±6.5)粒/h和(10.6±3.2)粒/h。这说明行使群体募集的蚂蚁比行使简单协作募集的蚂蚁有较高的搬运效率,蚂蚁散布导致阜平黄堇和小花黄堇种子到达蚁巢的数量和搬运距离不同,而这种不同与相应搬运蚂蚁的觅食对策、搬运行为和种子特征有关。阜平黄堇种子比小花黄堇种子大,但阜平黄堇的油质体质量比小于小花黄堇的油质体质量比,讨论了种子特征对蚂蚁散布的影响。     

Seed dispersal by ants in the semi-arid Caatinga of North-East Brazil

BACKGROUND AND AIMS: Myrmecochory is a conspicuous feature of several sclerophyll ecosystems around the world but it has received little attention in the semi-arid areas of South America. This study addresses the importance of seed dispersal by ants in a 2500-km(2) area of the Caatinga ecosystem (north-east Brazil) and investigates ant-derived benefits to the plant through myrmecochory. METHODS: Seed manipulation and dispersal by ants was investigated during a 3-year period in the Xingó region. Both plant and ant assemblages involved in seed dispersal were described and ant behaviour was characterized. True myrmecochorous seeds of seven Euphorbiaceae species (i.e. elaiosome-bearing seeds) were used in experiments designed to: (1) quantify the rates of seed cleaning/removal and the influence of both seed size and elaiosome presence on seed removal; (2) identify the fate of seeds dispersed by ants; and (3) document the benefits of seed dispersal by ants in terms of seed germination and seedling growth. KEY RESULTS: Seed dispersal by ants involved one-quarter of the woody flora inhabiting the Xingó region, but true myrmecochory was restricted to 12.8 % of the woody plant species. Myrmecochorous seeds manipulated by ants faced high levels of seed removal (38-84 %) and 83 % of removed seeds were discarded on ant nests. Moreover, seed removal positively correlated with the presence of elaiosome, and elaiosome removal increased germination success by at least 30 %. Finally, some Euphorbiaceae species presented both increased germination and seedling growth on ant-nest soils. CONCLUSIONS: Myrmecochory is a relevant seed dispersal mode in the Caatinga ecosystem, and is particularly frequent among Euphorbiaceae trees and shrubs. The fact that seeds reach micro-sites suitable for establishment (ant nests) supports the directed dispersal hypothesis as a possible force favouring myrmecochory in this ecosystem. Ecosystems with a high frequency of myrmecochorous plants appear not to be restricted to regions of nutrient-impoverished soil or to fire-prone regions.     

The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines

Ants have been traditionally considered either as predators or dispersers of seeds, but not both. That is, ant dispersal is restricted to myrmecochorous seeds, while almost all seeds removed by seed‐harvesting ants are eaten. However, harvesting ants might be simultaneously antagonistic and mutualistic towards seeds. This study analyzes the predation–dispersal relationship between seed‐harvesting ants and seeds of Lobularia maritima, a non‐myrmechorous perennial herb, in order to disentangle the dual role of ants as dispersers and predators of L. maritima seeds. The results obtained confirm the role of harvesting ants as both predators and dispersers of the non‐myrmechorous seeds of L. maritima. The removal activity of Messor bouvieri on L. maritima seeds is very important, particularly in autumn, which is the flowering and fruiting peak of this plant. It can be estimated that harvesting ants collect more than 85% of seeds, and almost 70% of them are effectively lost to predation. However, these granivorous ants also have drawbacks as seed dispersers. There is a relatively small percent of seeds collected by ants that escape predation, either because they are dropped on the way to the nest (16.4% of seeds harvested), or because they are mistakenly rejected on the refuse pile (0.9%). Abiotic dispersal of L. maritima seeds in the absence of ants occurs over very short distances from the plant stem. As seeds dispersed by ants reach a considerably greater distance than that obtained by gravity, this might represent a real advantage for the species, because it reduces intraspecific adult competition for seedlings, which directly influences seedling survivorship. These results challenge the generalization that seed removal by ants generally leads to successful seed dispersal if done by legitimate seed dispersers, or seed loss if done by seed consumers that eat them, and confirm that harvesting ants might have a dual role as both predators and dispersers of nonmyrmechorous seeds.     

Myrmecochorous dispersal distances: a world survey

Abstract. Myrmecochorous dispersal distances are reviewed; the seed dispersal curve generated by ants shows a characteristic peak at short distances and a long tail, a shape suited to small densities of safe sites. Mean global distance is of 0.96 m (n= 2524) with a range of 0.01–77 m. Data have been broken down by geography (Northern hemisphere v. Southern hemisphere), taxonomy (ant subfamilies) and ecology (vegetation: sclerophyllous v. mesophyllous). Although a statistical difference exists between dispersal curves from the Northern hemisphere and the Southern hemisphere, this may be an artefact of lack of data from mesophyllous myrmecochores from this hemisphere. The four ant subfamilies do show also numerical differences but could not be subjected to statistical analysis. A difference between the shape of dispersal curve for sclerophyllous myrmecochores and mesophyllous myrmecochores has also been detected. We hypothesize that this difference is related to the myrmecological communities from both types of vegetation: dispersing ants from sclerophyllous vegetation would have smaller nest densities and/or bigger foraging areas than dispersing ants from mesic environments.     

Does the morphology of animal foraging pits influence secondary seed dispersal by ants?

Secondary seed dispersal by ants (myrmecochory) is an important process in semi‐arid environments where seeds are transported from the soil surface to an ant nest. Microsites from which ants often remove seeds are the small pits and depressions made by native and exotic animals that forage in the soil. Previous studies have demonstrated greater seed retention in the pits of native than exotic animals, but little is known about how biotic factors such as secondary seed dispersal by ants affect seed removal and therefore retention in these foraging pits. We used an experimental approach to examine how the morphology of burrowing bettong (Bettongia lesueur), greater bilby (Macrotis lagotis), short‐beaked echidna (Tachyglossus aculeatus) and European rabbit (Oryctolagus cuniculus) foraging pits and ant body size influenced ant locomotion and seed removal from pits along an aridity gradient. Ants took 3.7‐times longer to emerge from echidna pits (19.6 s) and six‐times longer to emerge from bettong pits (30.5 s) than from rabbit pits (5.2 s), resulting in lower seed removal from bettong pits than other pit types. Fewer seeds were removed from pits when cages were used to exclude large body‐sized (>2 mm) ants. Few seeds were removed from the pits or surface up to aridity values of 0.5 (humid and dry sub‐humid), but removal increased rapidly in semi‐arid and arid zones. Our study demonstrates that mammal foraging pit morphology significantly affects ant locomotion, the ability of ants to retrieve seeds, and therefore the likelihood that seeds will be retained within foraging pits.     

Seed removal in a tropical North American desert: an evaluation of pre‐ and post‐dispersal seed removal in Stenocereus stellatus

• To determine seed removal influence on seed populations, we need to quantify pre‐ and post‐dispersal seed removal. Several studies have quantified seed removal in temperate American deserts, but few studies have been performed in tropical deserts. These studies have only quantified pre‐ or post‐dispersal seed removal, thus underestimating the influence of seed removal. We evaluated pre‐ and post‐dispersal seed removal in the columnar cactus Stenocereus stellatus in a Mexican tropical desert.
• We performed selective exclosure experiments to estimate percentage of seeds removed by ants, birds and rodents during the pre‐ and post‐dispersal phases. We also conducted field samplings to estimate abundance of the most common seed removers.
• Birds (10–28%) removed a higher percentage of seeds than ants (2%) and rodents (1–4%) during pre‐dispersal seed removal. Melanerpes hypopolius was probably the main bird removing seeds from fruits. Ants (62–64%) removed a higher percentage of seeds than birds (34–38%) and rodents (16–30%) during post‐dispersal seed removal. Pogonomyrmex barbatus was probably the main ant removing seeds from soil.
• Birds and ants are the main pre‐ and post‐dispersal seed removers in S. stellatus, respectively. Further studies in other S. stellatus populations and plants with different life forms and fruit types will contribute to evaluate seed removal in tropical American deserts.

     

What are the consequences of ant–seed interactions on the abundance of two dry-fruited shrubs in a Mediterranean scrub?

Strong interactions between dry-fruited shrubs and seed-harvesting ants are expected in early successional scrubs, where both groups have a major presence. We have analysed the implications of the seed characteristics of two dry-fruited shrub species (Coronilla minima and Dorycnium pentaphyllum) on seed predation and dispersal mediated by harvester ants and the consequences of these processes on spatio-temporal patterns of plant abundance in a heterogeneous environment. We found that large C. minima seeds were collected much more (39%) than small D. pentaphyllum seeds (2%). However, not all of the removed seeds of these plant species were consumed, and 12.8% of the seeds were lost along the trails, which increased dispersal distances compared with abiotic dispersal alone. Seed dropping occurred among all microhabitats of the two plant species, but especially in open microhabitats, which are the most suitable ones for plant establishment. The two plant species increased their presence in the study area during the study period: C. minima in open microhabitats and D. pentaphyllum in high vegetation. The large size of C. minima seeds probably limited the primary seed dispersal of this species, but may have allowed strong interaction with ants. Thus, seed dispersal by ants resulted in C. minima seeds reaching more suitable microhabitats by means of increasing dispersal distance and redistribution among microhabitats. In contrast, the smaller size of D. pentaphyllum seeds arguably allows abiotic seed dispersal over longer distances and colonization of all types of microhabitats, although it probably also limits their interaction with ants and, consequently, their redistribution in suitable microhabitats. We suggest that dyszoochory could contribute to the success of plant species with different seed characteristics in scrub habitats where seeds are abundantly collected by seed-harvesting ants.     

Functional extinction of a desert rodent: implications for seed fate and vegetation dynamics

Population declines of once‐abundant species have often preceded understanding of their roles within ecosystems. Consequently, important drivers of environmental change may remain undiagnosed because we simply do not know how species that are now rare or extinct shaped ecosystems in the past. Australia's desert rodents are thought to have little numerical impact on seed fate and vegetation recruitment when compared with ants or with desert rodents on other continents. However most research on granivory by Australian desert rodents has occurred in areas where rodents were rare or functionally extinct. Here we ask if the paradigm that rodents are relatively un‐important granivores in Australian deserts is an artefact of their historical decline. In the Strzelecki Desert, the endangered rodent, Notomys fuscus is rare where introduced mesopredators are abundant but common where dingoes (an apex predator) suppress mesopredator populations. We used foraging trays to compare rates of seed removal for a common shrub (Dodonaea viscosa angustissima hopbush) between areas where N. fuscus, hopbush shrubs and their seedlings were rare and common and found that seed removal was consistently higher where rodents were common and hopbush rare. By excluding ants and rodents from foraging trays we show that ants removed more seeds than rodents where rodents were rare but rodents removed far more seeds than ants where rodents were common. By manipulating rodents’ access to the soil seed‐bank we show that hopbush seeds persisted in greater numbers where rodents were excluded than where they had access. Our results support the hypothesis that granivory by rodents may once have been a far more important process influencing the fate of seeds and shaping plant communities in arid Australia and suggest that dingo extirpation has cascading effects on shrub seeds. Our study highlights that functional extinction of rodents may be an under‐appreciated driver of vegetation change.