Introduced galliforms as seed predators and dispersers in Hawaiian forests

In altered communities, novel species’ interactions may critically impact ecosystem functioning. One key ecosystem process, seed dispersal, often requires mutualistic interactions between frugivores and fruiting plants, and functional traits, such as seed width, may affect interaction outcomes. Forests of the Hawaiian Islands have experienced high species turnover, and introduced galliforms, the largest of the extant avian frugivores, consume fruit from both native and non-native plants. We investigated the roles of two galliform species as seed dispersers and seed predators in Hawaiian forests. Using captive Kalij Pheasants (Lophura leucomelanos) and Erckel’s Francolins (Pternistis erckelii), we measured the probability of seed survival during gut passage and seed germination following gut passage. We also examined which seeds are being dispersed in forests on the islands of O’ahu and Hawai’i. We found that galliforms are major seed predators for both native and non-native plants, with less than 5% of seeds surviving gut passage for all plants tested and in both bird species. Gut passage by Kalij Pheasants significantly reduced the probability of seeds germinating, especially for the native plants. Further, larger-seeded plants were both less likely to survive gut passage and to germinate. In the wild, galliforms dispersed native and non-native seeds at similar rates. Overall, our results suggest the introduced galliforms are a double-edged sword in conservation efforts; they may help reduce the spread of non-native plants, but they also destroy the seeds of some native plants. Broadly, we show mutualism breakdown may occur following high species turnover, and that functional traits can be useful for predicting outcomes from novel species’ interactions.

     

Seed predation of two alien Opuntia species invading Mediterranean communities

Invasibility depends on the interaction of the introduced species with the abiotic and biotic factors of the recipient community. In particular, the biotic resistance posed by native herbivores has been claimed to be of great importance in controlling plant invasion. We investigated fruit and seed predation of two exotic Opuntia species within and between Mediterranean communities in order to determine how patterns of predation matched patterns of invasion. Predators were small mammals, presumably mice, which could consume more than 50% of the seeds produced. Predators could be equally effective in consuming fruit and single seeds. O. maxima fruits were slightly preferred to O. stricta fruits, but predators did not distinguish between seeds. Seed predation was more intense in invaded than in non-invaded communities. However, there was a high spatial variation in seed predation that did not always match patterns of invasion, suggesting that seed predation alone is not a good predictor of community invasibility to Opuntia. According to these results invasibility to Opuntia is limited in some (but not all) communities by native mice. Seed losses by predation were high for both species. However, we estimated that more than 75% of seeds dispersed by birds to non-invaded areas are not predated.     

Seed dispersal of Diospyros virginiana in the past and the present: Evidence for a generalist evolutionary strategy

Several North American trees are hypothesized to have lost their co‐evolved seed disperser during the late‐Pleistocene extinction and are therefore considered anachronistic. We tested this hypothesis for the American persimmon (Diospyros virginiana) by studying the effects of gut passage of proposed seed dispersers on seedling survival and growth, natural fruiting characteristics, and modern animal consumption patterns. We tested gut passage effects on persimmon seeds using three native living species, the raccoon (Procyon lotor), Virginia opossum (Didelphis virginiana), and coyote (Canis latrans), and two Pleistocene analogs; the Asian elephant (Elephas maximus) and alpaca (Vicugna pacos). Persimmon seeds excreted by raccoons, coyotes, and elephants survived gut transit. Gut passage did not affect sprouting success, but did tend to decrease time to sprout and increase seedling quality. Under field conditions, persimmon fruits were palatable on the parent tree and on the ground for an equal duration, but most fruits were consumed on the ground. Seven vertebrate species fed upon persimmon fruits, with the white‐tailed deer (Odocoileus virginianus)—a species not capable of dispersing persimmon seeds—comprising over 90% of detections. Conversely, potential living seed dispersers were rarely detected. Our results suggest the American persimmon evolved to attract a variety of seed dispersers and thus is not anachronistic. However, human‐induced changes in mammal communities could be affecting successful seed dispersal. We argue that changes in the relative abundance of mammals during the Anthropocene may be modifying seed dispersal patterns, leading to potential changes in forest community composition.     

The role of avian frugivores in germination of seeds of fleshy-fruited invasive alien plants

Many highly invasive plant species have fleshy fruits which are eaten by native frugivorous animals. These frugivores play an important role in long-distance seed dispersal, and may also affect germination success. The aim of this study was to determine whether generalist frugivores enhance or decrease seed germination of invasive alien species through pulp removal or seed coat abrasion, besides serving as dispersal agents. Fruits of four fleshy-fruited invasive alien plant species, namely Solanum mauritianum, Cinnamomum camphora, Lantana camara and Psidium guajava, were fed to three generalist avian frugivorous species, which have been observed feeding on these fruits in the wild. Seed retention time was recorded as this affects dispersal distance and the duration that seeds are exposed to the effects of the gut. Seeds removed from excreta, seeds from manually de-pulped fruit, and whole fruit were planted in soil trays housed in a greenhouse. Daily germination counts were done. Seed retention times differed significantly between bird species for all fruits, except those of C. camphora. However, all frugivores had a similar effect on the germination success of seeds of S. mauritianum, L. camara and P. guajava, showing that gut retention time was not important. Germination of seeds from manually de-pulped fruits did not differ from that of ingested seeds of all plant species, suggesting that seed coat abrasion was also not important. Pulp removal resulted in significantly higher germination rates, both in the two species with larger, multi-seeded fruit (S. mauritianum and P. guajava), and in the two species having single-seeded fruit with waxy exocarps (C. camphora and L. camara). Pulp removal also resulted in significantly earlier germination of L. camara and P. guajava seeds. Therefore, frugivores not only accelerate dispersal, but also greatly enhance seed germination of all fleshy-fruited invasive alien species in this study.     

Non-native mammals are weak candidates to substitute ecological function of native avian seed dispersers in an island ecosystem

Although prominent examples exist of non-native species causing substantial ecological harm, many have neutral or positive effects, including filling surrogate roles once performed by extinct native organisms. We tested the ecological roles of two non-native mammals as seed dispersers or seed predators in Guåhan, which, due to invasive brown tree snakes (Boiga irregularis), is devoid of native seed dispersers–birds and bats. We conducted feeding trials with captive rats (Rattus spp.), which are present but uncommon due to predation by snakes, and pigs (Sus scrofa), which are abundant. We examined if and how they interacted with common forest fruits. We then compared how any gut-passed or animal-handled seeds germinated compared to seeds left in whole fruit or depulped seeds. Rats and pigs interacted with most of the fruits and seeds (>80%) that they were fed. Of those, most seeds were destroyed—78% for rats and 90% for pigs, across both native and non-native plant species. Compared to seeds germinating within whole fruits, rats improved germination of the seeds that they handled without ingesting, while pigs diminished the germination of seeds that they handled. The small percentage of seeds (approximately 1.5% for rats and 5% for pigs) that survived gut passage germinated in higher proportions than those in whole fruits. Percentages of seed survival to germination are lower than found in similar studies with native avian frugivores. Our results indicate that pigs and rats have mixed effects on seeds, but are not suitable surrogates for native seed dispersers.     

The role of avian ‘seed predators’ as seed dispersers

Seed dispersal is a central process in plant ecology with consequences for species composition and habitat structure. Some bird species are known to disperse the seeds they ingest, whereas others, termed ‘seed predators’, digest them and apparently play no part in dispersal, but it is not clear if these are discrete strategies or simply the ends of a continuum. We assessed dispersal effectiveness by combining analysis of faecal samples and bird density. The droppings of seed dispersers contained more entire seeds than those of typical seed predators, but over a quarter of the droppings of seed predators contained whole seeds. This effect was further magnified when bird density was taken into account, and was driven largely by one frequent interaction: the Chaffinch Fringilla coelebs, a typical seed predator and the most abundant bird species in the area and dispersed seeds of Leycesteria formosa, a non‐native plant with berry‐like fruits. These results suggest the existence of a continuum between seed predators and seed dispersers.     

Gut passage effect of the introduced red‐whiskered bulbul (Pycnonotus jocosus) on germination of invasive plant species in Mauritius

In Mauritius, many of the worst invasive plant species have fleshy fruits and rely on animals for dispersal. The introduced red‐whiskered bulbul (Pycnonotus jocosus) feeds on many fleshy‐fruited species, and often moves from invaded and degraded habitats into higher quality native forests, thus potentially acting as a mediator of continued plant invasion into these areas. Furthermore, gut passage may influence seed germination. To investigate this, we fed fleshy fruits of two invasive plant species, Ligustrum robustum and Clidemia hirta, to red‐whiskered bulbuls. Gut passage times of seeds were recorded. Gut‐passed seeds were sown and their germination rate and germination success compared with that of hand‐cleaned seeds, as well as that of seeds in whole fruits. Gut passage and hand‐cleaning had significant positive effects on germination of both species. Gut‐passed seeds of both C. hirta and L. robustum germinated faster than hand‐cleaned seeds. However, for L. robustum, this was only true when compared with hand‐cleaned seeds with intact endocarp; when compared with hand‐cleaned seeds without endocarp, there was no difference. For overall germination success, there was a positive effect of gut passage for C. hirta, but not for L. robustum. For both C. hirta and L. robustum, no seeds in intact fruits geminated, suggesting that removal of pulp is essential for germination. Our results suggest that, first, the initial invasion of native forests in Mauritius may not have happened so rapidly without efficient avian seed dispersers like the red‐whiskered bulbul. Second, the bulbul is likely to be a major factor in the continued re‐invasion of C. hirta and L. robustum into weeded and restored conservation management areas.     

Frugivory and seed dispersal by foxes in relation to mammalian prey abundance in a semiarid thornscrub

Abstract We examine the role of the native fox, Pseudalopex culpaeus, as a frugivore and seed disperser in a semiarid thornscrub of Chile. We quantified the fruit and animal components in its diet versus the availability of fruits and small mammals in the field over a 2‐year period (January 1998 through February 2000). We tested the legitimacy and effectiveness of foxes as dispersers by quantifying the percentages of seed viability and of germination of seeds that passed through fox gut versus those picked from plants. We also studied their efficiency as dispersers, monitoring the fate of seeds in faeces placed in the field. The highest frequencies of fruit consumption by foxes were observed when abundances of small mammal prey were <6 individuals per hectare, regardless of fruit abundance in the field. Thus, foxes consumed fruits as a supplementary food resource. Based on 326 faeces, the total number of fruits consumed was about 34 000 over the 2‐year study period, and fruits from the alien shrub Schinus molle represented 98% of that total, with the native Porlieria chilensis a distant second. Germination and viability of defecated seeds of P. chilensis were reduced by 66% and 48%, respectively, in comparison to controls. In contrast, germination of seeds of S. molle increased by 50% and no effect on viability was observed. With regard to P. chilensis, foxes were legitimate (they defecated viable seeds), but ineffective (seeds in faeces had lower germination than those taken directly from parental plants and there was no seedling establishment in the field) and inefficient dispersers (seeds in faeces were deposited on microhabitats hostile to seed germination and seedling establishment). However, with regard to S. molle, foxes were legitimate, effective (seeds in faeces had higher germination than those taken directly from parental plants; there was germination but no establishment in the field), and efficient dispersers (over 41% of seeds were deposited on safe microsites). Thus, a native fox may be contributing to the spread of an alien shrub, co‐opting existing community processes.     

Asymmetrical Dependence Between a Neotropical Mistletoe and its Avian Seed Disperser

The degree of interdependence among interacting species has major implications for our understanding of the coevolutionary process and biodiversity maintenance. However, the mutualism strength among fruiting plants and their seed dispersers remains poorly understood in tropical ecosystems. We evaluated simultaneously the effectiveness of the avian seed dispersers of the mistletoe Struthanthus flexicaulis (Loranthaceae) and the contribution of its fruits to their diets in a highland rocky savanna in southeastern Brazil. The mistletoe fruits are small lipid‐rich pseudoberries available throughout the year. Four passerine birds fed on fruits, but Elaenia cristata (Tyrannidae) was the most effective disperser, responsible for more than 96 percent of the dispersed seeds. This bird swallowed fruits whole, expelling and depositing undamaged seeds by regurgitation and bill wiping on perches. From 646 dispersed seeds, 56 percent were deposited on safe sites, thin live twigs of 38 susceptible host species. Elaenia cristata were predominantly frugivorous, feeding on typically ornithocoric fruits of at least 12 species, but also on arthropods. Although fruits represented 75 percent of the feeding bouts along the year, S. flexicaulis fruits represented only 34 percent of the E. cristata diet. Our results highlight the asymmetrical nature of this mutualistic interaction, with the mistletoe life cycle locally linked to one highly effective seed disperser that is more weakly dependent on mistletoes fruits as a food source. We suggest that merging the seed dispersal effectiveness framework with diet assessment of seed dispersers is needed to clarify the asymmetries in mutualistic pairwise interactions involving plants and their animal partners.     

Efficiency of endozoochorous seed dispersal in six dry-fruited species (Cistaceae): from seed ingestion to early seedling establishment

We combined laboratory and nursery experiments to analyse the effectiveness of sheep as endozoochorous seed dispersers of six native shrubby Cistaceae species collected in SE Spain (Helianthemum apenninum (L.) Mill., H. violaceum (Cav.) Pers., Fumana ericoides (Cav.) Grand., F. thymifolia (L.) Spach, Cistus monspeliensis L. and C. laurifolius L.), considering the main stages after seed ingestion, i.e. seed recovery, seed germination, seedling emergence and early seedling establishment. Seed recovery after gut passage was high (around 40%) for all the species, except F. thymifolia (12%). Most seeds (ca. 90%) were recovered within 48 h after ingestion for all the species, although seeds were still recovered up to 96 h after ingestion. Gut passage increased germination up to seven-fold compared to non-ingested seeds. Furthermore, seedling emergence from seeds contained in pellets was overall similar (intact pellets) to or higher (crumbled pellets) than emergence from seeds without dung. Survival of emerged seedlings and mass of seedlings after 20 days were not reduced by dung. Sheep act therefore as effective dispersers of these Cistaceae species by scattering seeds and promoting germination, while faeces do not hamper seedling establishment. We conclude that the interaction between herbivorous ungulates and these dry-fruited species may be considered a mutualism qualitatively similar to the mutualism between frugivorous vertebrates and fleshy-fruited plants.     

Role of Brycinus lateralis (Teleostei: Alestidae) in dispersal and germination of Nymphaea nouchali (Angiospermae: Nymphaeaceae) seeds on a seasonal floodplain of the Okavango Delta,Botswana

Seed passage through the gut of vertebrates can be important for seed dispersal, but might influence seed viability. The ability of seeds to germinate after ingestion by seed-eating fish is important for the population dynamics of some plant species, and significant in the evolution of plant–fish interactions. Certain fish in the Okavango Delta, Botswana, are fruit- and seed-eaters and could act as seed dispersers. We sampled 14 fish species in 2013, finding Nymphaea nouchali var. caerulea seeds in the digestive tracts of eight, most commonly in the striped robber Brycinus lateralis. Seeds extracted from the gut of this species had an overall mean germination success of 11.7%. This fish species might well be a legitimate seed disperser, having a positive effect on seed dispersal from parent plants in the Okavango Delta. The current study represents one of the first investigations of the likelihood of seed dispersal by fish on the African continent.     

Seed dispersal effectiveness of the western lowland gorilla (Gorilla gorilla gorilla) in Gabon

The quantitative and qualitative aspects of seed dispersal by the western lowland gorilla (Gorilla gorilla gorilla) were investigated in Gabon. Fresh faeces were collected and washed to identify and count the seeds. Seed germinability after gut passage was estimated with trials in a nursery at the study site. To assess the impact of gut passage on germination success and delay, comparative trials were run with four treatments: (i) gut passed seeds cleaned of faeces, (ii) gut passed seeds within a faecal matrix, (iii) seeds from fresh fruits surrounded by pulp, and (iv) seeds from fresh fruits cleaned of pulp. The analysis of 180 faecal units resulted in the identification of 58 species of seed. Germination trials were realized for 55 species and the mean germination success reached 46%. The impact of gut passage was investigated for Santiria trimera and Chrysophyllum lacourtianum; both species displayed higher germination success after ingestion. This study shows that gorillas effectively disperse seeds of numerous plant species, many of which provide timber or nontimber forest products or are typical of Gabonese forests. Considering the high‐quality of gorilla deposition sites, gorillas is thought to play a unique role in the dynamics of Central African forest.     

Seed Removal Rates Under Isolated Trees and Continuous Vegetation in Semiarid Thornscrub

Clearing native vegetation to increase the amount of land available for agriculture in northeastern Mexico has left remnants ranging in size from fragments of continuous vegetation to isolated individual trees. These provide valuable opportunities for restoring larger areas of native vegetation. We explored whether fragmentation of Tamaulipan thornscrub affects the removal of seeds from 12 woody species that encompass a range of sizes and dispersal mechanisms. We tested whether (1) seed removal rates under isolated trees were higher than under continuous vegetation; (2) dispersal structures, such as fleshy pericarps, made some seeds more attractive to seed removers; and (3) microenvironmental variation affected seed removal rates. Seeds were placed under canopies of Texas ebony (Ebenopsis ebano) and Mesquite trees (Prosopis laevigata). Seed removal trials were conducted three times, each trial lasting 30 days. Most seeds were removed in all trials by the end of one month. Seed removal rate was slower under isolated trees. In general, fleshy fruits were removed faster than other fruits; whole fruits and fleshy tissue were removed faster than depulped seeds. In species with fleshy pericarps, acid washing of seeds, to simulate seeds processed in the digestive tract of dispersers, reduced the seed removal rates, suggesting that it would be a good pre‐treatment for restoration efforts.     

Seed dispersal by lizards on a continental‐shelf island: predicting interspecific variation in seed rain based on plant distribution and lizard movement patterns

Aim We estimated the patterns of seed deposition provided by the eyed lizard, Timon lepidus, and evaluated whether these patterns can be generalized across plant species with different traits (fruit and seed size) and spatial distributions. Location Monteagudo Island, Atlantic Islands National Park (north‐western Spain). Methods We radio‐tracked seven lizards for 14 days and estimated their home ranges using fixed kernels. We also geo‐referenced all fruit‐bearing individuals of four plant species dispersed by eyed lizards in the study area (Corema album, Osyris alba, Rubus ulmifolius and Tamus communis), measured the passage time of their seeds through the lizard gut, and estimated seed predation in four habitats (bare sand, grassland, shrub and gorse). Seed dispersal kernels were estimated using a combination of these data and were combined with seed predation probability maps to incorporate post‐dispersal seed fate (‘seed survival kernels’). Results Median seed gut‐passage times were around 52–98 h, with maximum values up to 250 h. Lizards achieved maximum displacement in their home ranges within 24–48 h. Seed predation was high (80–100% of seeds in 2 months), particularly under Corema shrub and gorse. Seed dispersal kernels showed a common pattern, with two areas of preferential seed deposition, but the importance of these varied among plant species. Interspecific differences among dispersal kernels were strongly reduced by post‐dispersal seed predation; hence, seed survival kernels of the different plant species showed high auto‐ and pairwise‐correlations at small distances (< 50 m). As a result, survival to post‐dispersal seed predation increased with dispersal distance for O. alba and T. communis, but not for C. album. Main conclusions Seed dispersal by lizards was determined primarily by the interaction between the dispersers’ home ranges and the position of the fruit‐bearing plants. As a result, seed rain shared a common template, but showed considerable variation among species, determined by their specific spatial context. Seed predation increased the spatial coherence of the seed rain of the different species, but also resulted in contrasting relationships between seed survival and dispersal distance, which may be of importance for the demographic and evolutionary processes of the plants.     

Ecological implications of seed characteristics of the native Prosopis cineraria and the alien P. juliflora

Seed weight, percentage germination, seedling growth, and nutrient concentrations (Mg, Na, K, Zn, Cu and P) of whole seeds, and of seed coats and embryos separately of two tree species, the native Prosopis cineraria and the invasive alien P. juliflora from semi-arid and arid areas of north and north-west India, were analysed to understand the differences in their ecology. Seeds of P. cineraria were heavier than those of P. juliflora. Percent germination was similar in the two species, but seedling growth was faster in P. juliflora than in P. cineraria. Nutrient concentrations of seeds of the two species were similar (except Cu). Nutrient concentrations in the embryo were higher in P. cineraria, while those in the seed coat were higher in P. juliflora. The relative allocation of nutrients to seed coat was higher in P. juliflora than in P. cineraria. Nutrient-rich embryos and slow growth, along with a staggered seed germination pattern in the native P. cineraria could be linked to delayed establishment as well, in the substratum. Faster growth of the nutrient-poor embryos in P. juliflora along with its simultaneous seed germination pattern, and creation of a favourable microenvironment through leaching of nutrients from a nutrient-rich seed coat can facilitate immediate and successful establishment of this alien species in the invaded habitats.     

Differential passage time of mistletoe fruits through the gut of honeyeaters and flowerpeckers: effects on seedling establishment

McKey's (1975) hypothesis that avian dispersers with a specialized gut provide higher quality seed dispersal than unspecialized frugivores was tested using grey mistletoe (Amyema quandang) fruits, and captive mistletoebirds (Dicaeum hirundinaceum) and spinycheeked honeyeaters (Acanthagenys refogularis) in arid South Australia. Mistletoebirds have a specialized gut, unlike spiny-cheeked honeyeaters. The gut passage time of A. quandang fruits through mistletoebirds was 820±29 s (mean±SE, n=188), compared to 2434±36 s (n=436) for honeyeaters. The seeds defecated by both bird species were deployed on twigs of host trees. Despite the longer retention time of fruit in the gut of honeyeaters, the germination percentage of seeds defecated by mistletoebirds (85% of 485 seeds) and honeyeaters (81% of 485 seeds) did not differ significantly 1 week after deployment. However, after 5 months, a significantly greater proportion of seedlings had established from seeds passed by mistletoebirds (42.7%) than from seeds defecated by honeyeaters (31.1%). The data support the notion that the more gentle treatment of seeds in the gut of specialized dispersers translates into higher seedling establishment.     

Neglected seed dispersers: endozoochory by Javan lutungs (Trachypithecus auratus) in Indonesia

Leaf monkeys are known to be leaf eaters, and thus, their potential role as seed dispersers has been neglected. However, they do also feed on fruits. To examine the role of leaf monkeys as endozoochorous seed dispersers, we studied the Javan lutung (Trachypithecus auratus) in Indonesia. We compared multiple aspects of seed dispersal processes (amount and diversity of seeds ingested, dispersal distance, and germination rate) of lutungs with that of the sympatric long‐tailed macaque (Macaca fascicularis). Over the study period, 54 percent of the lutung feces contained intact seeds, which was equivalent to the macaque feces contained seeds (62%). Seeds of at least six plant species were detected in the lutung feces, which was less than those found in the macaque feces (>19 plant species). The main species of seeds defecated by both lutungs and macaques was Ficus spp. (seed size: 0.7 mm). Seed shadow, estimated from travel distance (range: 1–299 m) and gut passage rate (24–96 h), had a unimodal‐distribution with a peak at 51–100 m, and was shorter than that reported in published accounts of macaques and other similar and smaller sized frugivores. Finally, germination rates of Ficus spp. seeds ingested by both lutungs and macaques were lower than that of the control seeds. These results imply that the dispersal effectiveness of lutungs would be lower than that of the sympatric primate frugivores. However, at a population level, lutungs could play a significant role as seed dispersers for the small‐seeded species, and therefore, more research into their frugivorous habits is warranted.     

The role of frugivorous birds in fruit removal and seed germination of the invasive alien Cotoneaster franchetii in central Argentina

Many invasive plant species have fleshy fruits that are eaten by native frugivorous birds which disperse their seeds and may facilitate their germination, playing an important role in plant invasion success. The fleshy‐fruited shrub Cotoneaster franchetii (Rosaceae) is an important invasive alien in the mountainous regions of central Argentina. To determine the role of avian frugivorous in fruit removal of this species, we conducted a frugivore exclusion experiment including bagged and unbagged branches in 75 plants of C. franchetii. At the end of the dispersal period, we compared the percentage of missing fruits (removed by birds + naturally dropped) in unbagged branches with the percentage of naturally dropped fruits in bagged branches. To assess whether any mechanism acting on seeds during their passage through bird guts (de‐inhibition by pulp removal and/or seed scarification) affects seed germination of this species, we compared percentage and speed of germination among seeds obtained from faeces of the native frugivorous Turdus chiguanco, from manually de‐pulped fruits, and from intact fruits. The percentage of missing fruits per shrub in unbagged branches was significantly higher than the percentage of naturally dropped fruits in bagged branches, suggesting that frugivorous birds play an important role in fruit removal of C. franchetii in the study area. Seeds from bird faeces and from manually de‐pulped fruits germinated in higher percentage and faster than seeds from intact fruits. Germination percentage and speed of seeds from manually de‐pulped fruits were significantly higher than those of gut‐passed seeds. These results indicate that T. chiguanco increases and accelerates seed germination of C. franchetii through pulp removal, but not through seed scarification. Overall, our findings indicate that native frugivorous birds facilitate the dispersal and germination success of C. franchetii, likely playing an important role in its invasion throughout the mountainous region of central Argentina.     

Dormancy as exaptation to protect mimetic seeds against deterioration before dispersal

Background and Aims

Mimetic seeds simulate the appearance of fleshy fruits and arilled seeds without producing nutritive tissues as a reward for seed dispersers. In this strategy of seed dispersal, seeds may remain attached to the mother plant for long periods after maturity, increasing their availability to naïve seed dispersers. The hypothesis that seed coat impermeability in many tropical Fabaceae with mimetic seeds serves as an exaptation to protect the seeds from deterioration and rotting while awaiting dispersal was investigated.

Methods

Seed coat impermeability was evaluated in five mimetic-seeded species of tropical Fabaceae in south-eastern Brazil (Abarema langsdorffii, Abrus precatorius, Adenanthera pavonina, Erythrina velutina and Ormosia arborea) and in Erythrina speciosa, a ‘basal’ species in its genus, which has monochromatic brown seeds and no mimetic displays. Seed hardness was evaluated as a defence against accelerated ageing (humid chamber at 41 °C for 144 h). Seed development and physiological potential of O. arborea was evaluated and the effect of holding mature seeds in pods on the mother plant in the field for a period of 1 year under humid tropical conditions was compared with seeds stored under controlled conditions (15 °C and 40 % relative air humidity).

Key Results

All five mimetic-seeded species, and E. speciosa, showed strong coat impermeability, which protected the seeds against deterioration in accelerated ageing. Most O. arborea seeds only became dormant 2 months after pod dehiscence. Germination of seeds after 1 year on the plant in a humid tropical climate was 56 %, compared with 80 % for seeds stored in controlled conditions (15 °C, 45 % relative humidity). Seedling shoot length after 1 year did not differ between seed sources.

Conclusions

Dormancy acts in mimetic-seeded species as an exaptation to reduce seed deterioration, allowing an increase in their effective dispersal period and mitigating the losses incurred by low removal rates by naïve avian frugivores.     

Potential for ants and vertebrate predators to shape seed-dispersal dynamics of the invasive thistles <Emphasis Type="Italic">Cirsium arvense</Emphasis> and <Emphasis Type="Italic">Carduus nutans</Emphasis> in their introduced range (North America)

The population dynamics of invasive plants are influenced by positive and negative associations formed with members of the fauna present in the introduced range. For example, mutualistic associations formed with pollinators or seed dispersers may facilitate invasion, but reduced fitness from attack by native herbivores can also suppress it. Since population expansion depends on effective seed dispersal, interactions with seed dispersers and predators in a plant species introduced range may be of particular importance. We explored the relative contributions of potential seed dispersers (ants) and vertebrate predators (rodents and birds) to seed removal of two diplochorous (i.e., wind- and ant-dispersed), invasive thistles, Cirsium arvense and Carduus nutans, in Colorado, USA. We also conducted behavior trials to explore the potential of different ant species to disperse seeds, and we quantified which potential ant dispersers were prevalent at our study locations. Both ants and vertebrate predators removed significant amounts of C. arvense and C. nutans seed, with the relative proportion of seed removed by each guild varying by location. The behavior trials revealed clear seed preferences among three ant species as well as differences in the foragers’ abilities to move seeds. In addition, two ant species that acted as potential dispersal agents were dominant at the study locations. Since local conditions in part determined whether dispersers or predators removed more seed, it is possible that some thistle populations benefit from a net dispersal effect, while others suffer proportionally more predation. Additionally, because the effectiveness of potential ant dispersers is taxon-specific, changes in ant community composition could affect the seed-dispersal dynamics of these thistles. Until now, most studies describing dispersal dynamics in C. arvense and C. nutans have focused on primary dispersal by wind or pre-dispersal seed predation by insects. Our findings suggest that animal-mediated dispersal and post-dispersal seed predation deserve further consideration.