The urban fire ant paradox: native fire ants persist in an urban refuge while invasive fire ants dominate natural habitats

In contrast to the widespread extirpation of native fire ants (Solenopsis geminata) across southern US following the invasion by imported red fire ants (S. invicta), some residential areas of Austin form unexpected refuges for native fire ants. Ironically, these urban environments provide refuges for the native fire ants while adjacent natural habitats have been overrun by invasive fire ants. Resistance to invasive fire ants in these urban areas occurs mainly in older residential properties constructed prior to the S. invicta invasion, while more recent construction has allowed establishment by S. invicta. The invasive ability of S. invicta is often attributed to escape from parasitoids and efficient dispersal of polygyne multiple queen colonies. Here we also show the importance of landscape parameters in the invasion process, where low levels of disturbance and continuous plant cover in older residential areas form possible barriers to colonization. Dense leaf cover (high NDVI) was also found to be associated with native ant refuges. Long term residential land ownership may have resulted in lower recent disturbance levels and increased plant cover that support refuges of native fire ants.     

The role of ants and mammals in dispersal and post-dispersal seed predation of the shrubs Grevillea (Proteaceae)

The role seed predators play in influencing the dynamics of plant populations has been little studied in Australia. The interaction of ant dispersal and seed predation on the soil seedbank in six shrubby species of Grevillea from the Sydney region of southeastern Australia was examined in selective exclusion experiments, seed array trials and placement of single seeds on the ground.Two distinct seed types in Grevillea were examined and different seed dispersal and post-dispersal seed predation patterns were associated with each: (a) seeds lacking an elaiosome were not attractive to ants and annual seed losses of between 82 and 95% were found in vegetation unburnt for greater than 8 years. Native rodents, Rattus fuscipes, and macropods, Wallabia bicolor, were responsible for these seed losses; (b) seeds with an elaiosome were rapidly handled by ants. Two functional types of ants were recognised. Most encounters were by ants that were small (Local) relative to seed size and these ants simply removed the elaiosome in situ or moved seeds only small distances (<20 cm). Some 0–24% of ant/seed encounters were by large (Removalist) ant species that were capable of moving seeds back to nests. In addition, Rattus fuscipes and Wallabia bicolor consumed at least 32–68% of seeds of Grevillea species with an elaiosome.Ants may reduce the overall levels of seed predation where seeds moved by Removalist ant species escape predation and are deposited in safe sites, hence allowing more seeds to reach the persistent soil seedbank. Mammals do not consume all seeds when ants are excluded, allowing for the potential for some seed escape from predation after seeds are discarded by Local ant species.     

Spatial variation in the fatty acid composition of elaiosomes in an ant-dispersed plant: Differences within and between individuals and populations

Euphorbia characias is a common myrmecochorous plant of the western Mediterranean Basin whose seeds are dispersed by ants following fruit explosion. The variation in elaiosomes’ fatty acid composition of this species was studied at three hierarchical levels (sub-individual, individual and population) in four populations from the Iberian Peninsula. We found that differences in fruit location on the inflorescence do not seem to influence the fatty acid composition of elaiosomes, providing to each propagule an equal chance of being dispersed. However, significant differences in elaiosome fatty acid composition between individuals and populations were found for most of the compounds identified. The content of oleic acid, a key mediator in the ant–seed interaction, differed widely between populations, probably reflecting geographic variations in co-adaptation between plants and their dispersers. The finding that the fatty acid composition of E. characias elaiosomes is distinct from that of the seed itself, but very similar to that of elaiosomes from unrelated species, reinforces the idea of convergent evolution in the chemical composition of these structures.     

Contrasting effects of resource availability and plant mortality on plant community invasion by Bromus tectorum L.

The positive effect of disturbance on plant community invasibility is one of the more consistent results in invasion ecology. It is generally attributed to a coincident increase in available resources (due to the disturbance) that allows non-resident plant species to establish (Davis MA, Grime JP Thompson K, J Ecol 88:528–534, 2000). However, most research addressing this issue has been in artificial or highly modified plant communities. Our goal in this study was to investigate the interactive effects of resource availability and plant mortality disturbance on the invasion of natural plant communities. We conducted a series of experiments that examined the response of Bromus tectorum L., a highly invasive annual grass, to experimentally created gradients of resource availability [nitrogen (N) and water] and resident plant species mortality. We found that B. tectorum biomass was co-limited by N and water. Biomass at the end of the growing season was a saturating function (i.e., increased to a maximum) of water, which determined maximum biomass, and N, which determined the rate at which maximum biomass was attained. Despite that fact that plant mortality increased N availability, it had a negative impact on invasion success. Plant mortality also decreased foliar cover, standing dead biomass, and soil cover by litter. In harsh environments, removing foliar and soil cover may increase germination and seedling stress by increasing soil temperatures and water loss. Across all treatments, B. tectorum success decreased with decreasing foliar cover and standing dead biomass. This, in combination with the strong limitation of B. tectorum biomass by water in this experiment, suggests that our plant mortality disturbance removed soil cover that may have otherwise aided B. tectorum invasion into this semi-arid plant community by reducing water stress.     

Landscape structure effects on forest plant diversity at local scale: Exploring the role of spatial extent

Since landscape attributes show different patterns at different spatial extents, it is fundamental to identify how the relation between landscape structure and plant species diversity at local scale varies with scale. Then, it is fundamental to assess the appropriate extent at which landscape factors affect plant species richness at the local scale. To investigate this relation, data on plant species richness of forest communities at plot scale were extracted from a large data set and landscape metrics were calculated around the same plots for a range of extents (250–3000 m). Then, multiple regression models and variance partitioning techniques were applied to assess the amount of variance explained by the landscape metrics on plant species richness for a range of extents. In general, we found that increasing extent of the surrounding landscape analyzed, improved the strength of relationship between the landscape metrics and the properties of plant communities at plot scale. The medium-large extent was most informative as it combined a decent total variance explained with high variance explained by the pure fractions of complexity, fragmentation and disturbance and the minimum of collinearity. In conclusion, we found that it is possible and beneficial to identify a specific extent, where the redundancy in the predictor variables is minimized and the explanatory power of the pure fractions (or single groups) maximized, when examining landscape structure effects on local plant species richness.     

Experimental evidence for a delayed response of the above-ground vegetation and the seed bank to the invasion of an annual exotic plant in deciduous forests

Invasions by alien plants significantly affect native biodiversity and ecosystem functioning. We conducted a 5-year field experiment to investigate potential effects of the annual invasive plant Impatiens glandulifera on both the native above-ground vegetation and the soil seed bank in a deciduous forest in Switzerland. Eight years after the establishment of I. glandulifera, we set up plots in patches invaded by the alien plant, in plots from which the invasive plant had been manually removed and in plots which were not yet colonized by the invasive plant. We examined plant species richness, diversity and plant species composition in the above-ground vegetation and soil seed bank in all plots one year and five years after the initiation of the experiment. The 36 plots (3 plot types × 6 replicates × 2 sites) were equally distributed over two forest sites. Neither the native above-ground vegetation nor the soil seed bank was influenced by the presence of I. glandulifera one year after the start of the field experiment. After five years, however, plant species richness of both the above-ground vegetation and the soil seed bank was reduced by 25% and 30%, respectively, in plots invaded by the alien plant compared to plots from which I. glandulifera had been removed or uninvaded plots. Furthermore, plots invaded by the alien plant had a lower total seedling density (reduction by 60%) and an altered plant species composition in the soil seed bank compared to control plots. Our field experiment indicates that negative effects of the annual invasive plant on the native above-ground vegetation and soil seed bank of deciduous forests become visible with a delay of several years.     

Distribution of alien vs. native plant species in roadside communities along an altitudinal gradient in Tenerife and Gran Canaria (Canary Islands)

Roadside plant communities were studied along two roads following an altitudinal gradient in Gran Canaria and Tenerife (Canary Islands). Our aim was to investigate variation in plant species richness, particularly of the alien flora, along a gradient from coastal shrubland to summit vegetation (1950 m a.s.l. in Gran Canaria, 2300 m in Tenerife) in relation to variation in habitat factors (altitude, habitat structure, roadside disturbance, distance to urban nuclei). We compared different species groups that were classified in terms of their biogeographical status, origin and life form. Altitude was the most important factor determining species richness and composition along both roadside transects. Alien plants showed a unimodal distribution pattern along the altitudinal gradient, with less species and lower abundance at low and high altitudes, and highest abundance at intermediate altitude. Alien plant species were also relatively more frequent near urban centres. The number of native and alien species was significantly positively correlated along the altitudinal gradient. Both alien and native, non-endemic species showed differences in their distribution along the altitudinal gradient according to their biogeographical affinities and climatic tolerances. Despite considerable differences in species pools these patterns were consistent among the two islands. Environmental (abiotic) stress is proposed as a primary, altitude-related factor acting as a filter against most alien plants at coastal and high-mountain altitudes. A higher frequency or intensity of disturbance at intermediate altitudes may be a further causal factor promoting alien plants in this zone. Future management efforts to control alien plants along roads should, therefore, concentrate on intermediate altitudinal zones of the higher Canary Islands.     

Savanna fires increase rates and distances of seed dispersal by ants

Myrmecochory (seed dispersal by ants) is a prominent dispersal mechanism in many environments, and can play a key role in local vegetation dynamics. Here we investigate its interaction with another key process in vegetation dynamics—fire. We examine ant dispersal of seeds immediately before and after experimental burning in an Australian tropical savanna, one of the world’s most fire-prone ecosystems. Specifically, our study addressed the effects of burning on: (1) the composition of ants removing seeds, (2) number of seed removals, and (3) distance of seed dispersal. Fire led to higher rates of seed removal post-fire when compared with unburnt habitat, and markedly altered dispersal distance, with mean dispersal distance increasing more than twofold (from 1.6 to 3.8 m), and many distance dispersal events greater than the pre-fire maximum (7.55 m) being recorded. These changes were due primarily to longer foraging ranges of species of Iridomyrmex, most likely in response to the simplification of their foraging landscape. The significance of enhanced seed-removal rates and distance dispersal for seedling establishment is unclear because the benefits to plants in having their seeds dispersed by ants in northern Australia are poorly known. However, an enhanced removal rate would enhance any benefit of reduced predation by rodents. Similarly, the broader range of dispersal distances would appear to benefit plants in terms of reduced parent–offspring conflict and sibling competition, and the location of favourable seedling microsites. Given the high frequency of fire in Australian tropical savannas, enhanced benefits of seed dispersal by ants would apply for much of the year.     

Assessing the impact of plant invasions on soil seed bank communities: use of univariate and multivariate statistical approaches

Questions: Are soil seed banks affected by invasions of alien plants? How can we rigorously assess alterations in seed bank communities associated with invasive species and account for the high spatial variability of seed bank data? How do multivariate approaches compare with more traditional approaches based on analysis of variance? Location: Three riparian sites, Ireland. Methods: A protocol based on a combination of multivariate techniques was used to characterize soil seed bank communities associated with the herbaceous invasive species Heracleum mantegazzianum in May and October. Permutational multivariate analysis of variance (PERMANOVA) was used to test the effects of the factors “invasion”, “site”, “plot” and “depth” on the soil seed bank, while multivariate analysis of dispersion (PERMDISP) provided a measure of the variability of seed bank data at different spatial scales. Similarity percentages analysis (SIMPER) was used to identify the species that contributed most to the differences between invaded and uninvaded communities. A comparison between the results of PERMANOVA and ANOVA analyses was also made. Results: The composition of seed bank communities invaded by H. mantegazzianum differed significantly from that of uninvaded seed banks. Invaded seed banks were less diverse and had reduced abundance, and were dominated by only a few species, such as Urtica dioica and Juncus effusus. Such patterns were recorded at each of three depth categories, indicating that invasive plants can affect both the transient and the more persistent component of the soil seed bank. Seed bank variability was significantly higher within uninvaded areas, supporting the notion that invasions tend to lead to more homogeneous communities. Conclusion: The analytical protocol used in this study was effective in quantifying the effect of plant invasions, at different spatial scales, providing a statistically robust analysis of alterations in soil seed bank communities. Compared to ANOVA, this protocol provided more biological information and was more appropriate for analysis of the data. This approach is therefore recommended in soil seed bank and invasion ecological studies.     

Legacy effects of afforestation on prairie plant and seed bank communities in a northern Canadian prairie

Afforestation resulting from fire suppression, modified grazing, plantation establishment and climate change poses a threat to northern prairie ecosystems. Trees alter the composition and function of plant and soil fauna communities and can compromise the restoration of afforested prairies. To evaluate the hypothesis that legacies of afforestation persist in restored prairie communities and decrease the potential for restoration, we examined the composition, structure, and diversity of plant and seed bank communities along a 20 year chronosequence of plantation tree removal from a northern fescue prairie in Riding Mountain National Park, Manitoba, Canada. Tree removal increased the abundance of weedy species in the plant and seed bank communities of restored prairies and plant diversity peaked and declined over the 20 year period of passive restoration. As a result, time since tree removal and the encroachment of invasive species were key in explaining the composition of restored prairie communities. Low correlation between the species composition of plant and seed bank communities, including the complete absence of Festuca hallii in restored treatments, demonstrated that legacies of afforestation compromised the potential of seed banks to facilitate prairie restoration. We conclude that tree removal alone is insufficient for the restoration of northern fescue prairies and that, in the absence of active management, the persistence of low-diversity plant and seed bank communities constitutes an important legacy of afforestation and an important barrier to future restoration.     

Butterfly seed predation: effects of landscape characteristics,plant ploidy level and population structure

Polyploidization has been suggested as one of the most common mechanisms for plant diversification. It is often associated with changes in several morphological, phenological and ecological plant traits, and therefore has the potential to alter insect–plant interactions. Nevertheless, studies evaluating the effect of plant polyploidy on interspecific interactions are still few. We investigated pre-dispersal seed predation by the butterfly Anthocharis cardamines in 195 populations of two ploidy levels of the herb Cardamine pratensis (tetraploid ssp. pratensis, 2n = 30 vs. octoploid ssp. paludosa, 2n = 56–64). We asked if differences in incidence and intensity of predation among populations were related to landscape characteristics, plant ploidy level and population structure. The incidence of the seed predator increased with increasing plant population size and decreasing distance to nearest population occupied by A. cardamines. The intensity of predation decreased with increasing plant population size and was not affected by isolation. Probability of attack decreased with increasing shading, and intensity of predation was higher in grazed than in non-grazed habitats. The attack intensity increased with increasing mean flower number of plant population, but was not affected by flowering phenology. Individuals in tetraploid populations suffered on average from higher levels of seed predation, had higher mean flower number, were less shaded and occurred more often in grazed habitats than octoploid populations. When accounting for differences in habitat preferences between ploidy levels there was no longer a difference in intensity of predation, suggesting that the observed differences in attack rates among populations of the two ploidy levels are mediated by the habitat. Overall, our results suggest that polyploidization is associated with differentiation in habitat preferences and phenotypic traits leading to differences in interspecific interaction among plant populations. This, in turn, may facilitate further divergence of ploidy levels.     

Chemical differences between seeds and elaiosomes indicate an adaptation to nutritional needs of ants

Ant-dispersed plants usually produce seeds with appendages (elaiosomes) as reward for ants. Plants that produce high-quality elaiosomes benefit because ants preferentially disperse their diaspores. We therefore hypothesized that seeds and elaiosomes differ in chemical composition in ways that make elaiosomes of high nutritional quality for ants, capable of providing essential dietary components that explain the increased fitness and higher gyne production documented for colonies with elaiosome consumption. To test the hypothesis we analysed the content and composition of lipids, amino acids, soluble carbohydrates, proteins and starch in seeds and elaiosomes of 15 central European ant-dispersed plants. After separating the different fractions, total lipids were determined gravimetrically, fatty acids and soluble carbohydrates were detected by gas chromatography (GC) and GC–mass spectrometry, free amino acids by an amino acid analyser while starch and protein were analysed photometrically. Seeds accumulated high molecular weight compounds such as proteins and starch, whereas elaiosomes accumulated more easily digestible low molecular weight compounds such as amino acids and monosaccharides. Analysis of similarities and similarity percentages analysis demonstrated that the composition of fatty acids, free amino acids and carbohydrates differed markedly between elaiosomes and seeds. The most important difference was in total amino acid content, which was on average 7.5 times higher in elaiosomes than in seeds. The difference was especially marked for the nitrogen-rich amino acid histidine. The availability of essential nutrients and, in some species, the higher nitrogen content in elaiosomes suggest that their nutritional value for larvae plays a key role in this interaction. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of ants,ground beetles and the seed-fall patterns on myrmecochory of Erythronium japonicum Decne. (Liliaceae)

Erythronium japonicum (Liliaceae) inhabits deciduous mesic forests of Hokkaido, northern Japan. Myrmecochory of this species was investigated, especially the dispersal frequency, the effect of seed predators and the seed fall pattern. In the quadrat census using marked seeds of E. japonicum, the ant Myrmica kotokui frequently transported the seeds. However, the frequency of seed removal was low and most seeds were dispersed as little as 1 m or less. The spatial distribution of E. japonicum individuals was nearly random and most seedlings were established 5–20 cm away from the fertile plants, indicating that even this small scale of seed dispersal contributes to avoiding crowding of seedlings. Some arthropods, e.g. springtails, spiders and ticks, hindered seed dispersal by devouring elaiosomes and seeds. Although ground beetle species also damaged seeds and elaiosomes, a few of them exhibited seed removal behaviour. E. japonicum dropped their seeds not all at once but bit by bit, taking 3–6 days to drop all seeds. This seed-fall pattern was effective in raising the frequency of seed removal by ants and reducing seed predation by some arthropods.     

Inferring relationships between native plant diversity and Lonicera japonica in upland forests in north Mississippi,USA

Question: Do anthropogenic disturbances interact with local environmental factors to increase the abundance and frequency of invasive species, which in turn exerts a negative effect on native biodiversity? Location: Mature Quercus‐Carya and Quercus‐Carya‐Pinus (oak‐hickory‐pine) forests in north Mississippi, USA. Methods: We used partial correlation and factor analysis to investigate relationships between native ground cover plant species richness and composition, percent cover of Lonicera japonica, and local and landscape‐level environmental variables and disturbance patterns in mature upland forests. We directly measured vegetation and environmental variables within 34 sampling subplots and quantified the amount of tree cover surrounding our plots using digital color aerial photography. Results: Simple bivariate correlations revealed that high species richness and a high proportion of herbs were associated with low Lonicera japonica cover, moist and sandy uncompacted soils, low disturbance in the surrounding landscape, and periodic prescribed burning. Partial correlations and factor analysis showed that once we accounted for the environmental factors, L japonica cover was the least important predictor of composition and among the least important predictors of species richness. Hence, much of the negative correlation between native species diversity and this invasive species was explained by soil texture and local and landscape‐level land‐use practices. Conclusions: We conclude that negative correlations between the abundance of invasive species and native plant diversity can occur in landscapes with a gradient of human disturbance, regardless of whether there is any negative effect of invasive species on native species.     

Uncoupling the effects of seed predation and seed dispersal by granivorous ants on plant population dynamics

Secondary seed dispersal is an important plant-animal interaction, which is central to understanding plant population and community dynamics. Very little information is still available on the effects of dispersal on plant demography and, particularly, for ant-seed dispersal interactions. As many other interactions, seed dispersal by animals involves costs (seed predation) and benefits (seed dispersal), the balance of which determines the outcome of the interaction. Separate quantification of each of them is essential in order to understand the effects of this interaction. To address this issue, we have successfully separated and analyzed the costs and benefits of seed dispersal by seed-harvesting ants on the plant population dynamics of three shrub species with different traits. To that aim a stochastic, spatially-explicit individually-based simulation model has been implemented based on actual data sets. The results from our simulation model agree with theoretical models of plant response dependent on seed dispersal, for one plant species, and ant-mediated seed predation, for another one. In these cases, model predictions were close to the observed values at field. Nonetheless, these ecological processes did not affect in anyway a third species, for which the model predictions were far from the observed values. This indicates that the balance between costs and benefits associated to secondary seed dispersal is clearly related to specific traits. This study is one of the first works that analyze tradeoffs of secondary seed dispersal on plant population dynamics, by disentangling the effects of related costs and benefits. We suggest analyzing the effects of interactions on population dynamics as opposed to merely analyzing the partners and their interaction strength.     

其它可利用食物对小花宽瓣黄堇(Corydalis giraldii Fedde)种子散布的影响

作为蚁播植物种子的重要传播者,蚂蚁不但取食种子上附着的油质体,也喜食其它富含蛋白质、脂类、糖和维生素等的食物,因此环境中其它可利用食物的存在可能会影响蚂蚁对种子的搬运进而影响种子散布,但目前对于这种影响是如何发生的仍不清楚。在野外研究了蚂蚁对小花宽瓣黄堇(Corydalis giraldii Fedde)种子、肉、蜂蜜、苹果、馒头等食物的趋性和偏好程度,以及添加食物后蚂蚁对种子的拜访频率和搬运效率,以揭示其它可利用食物如何影响蚂蚁觅食和取食偏好,进而影响小花宽瓣黄堇种子散布。结果显示,在所诱捕的8种蚂蚁中,玉米毛蚁(Lasius alienus(Foerster))和丝光蚁(Formica fusca Linnaeus)是小花宽瓣黄堇种子的主要搬运者,不同食物诱捕的玉米毛蚁数量无显著性差异(P0.05),但蜂蜜和苹果诱捕的丝光蚁数量均显著大于种子(P0.05)。玉米毛蚁和丝光蚁均为杂食性,在觅食中分别行使群体募集和简单协作性募集。在仅有种子的对照处理中,玉米毛蚁和丝光蚁对种子的拜访频率分别为(38.73±4.57)头和(30.8±2.87)头(40min,n=15),两种蚂蚁对种子的拜访频率差异不显著(P0.05);玉米毛蚁和丝光蚁搬运种子的效率分别为(33.87±4.22)粒和(16.27±3.35)粒(40min,n=15),玉米毛蚁的搬运效率显著高于丝光蚁(P0.05)。与对照相比,添加馒头、苹果和蜂蜜后丝光蚁对种子的拜访频率显著降低(P0.05),分别为(15.6±3.61)头、(9.07±1.4)头和(7.67±1.58)头(40min,n=15);添加苹果和蜂蜜后丝光蚁对种子的搬运效率显著降低(P0.05),分别为(3.47±1.17)粒和(2.87±0.9)粒(40min,n=15);添加不同食物后玉米毛蚁对种子的拜访频率和搬运效率均无显著变化(P0.05)。研究结果表明行使群体募集的玉米毛蚁比行使简单协作募集的丝光蚁有更高的种子搬运效率,添加食物后影响丝光蚁对种子的拜访频率和搬运效率,这说明其它可利用食物对小花宽瓣黄堇种子散布的影响与搬运蚂蚁的种类及其觅食的募集方式有关。研究结果可为进一步研究蚂蚁与植物(种子)间的互利共生关系及其影响因素提供资料。