Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

The 141-year period for Dr. Beal's seed viability experiment: A hybrid surprise

PREMISE

In 1879, Dr. William Beal buried 20 glass bottles filled with seeds and sand at a single site at Michigan State University. The goal of the experiment was to understand seed longevity in the soil, a topic of general importance in ecology, restoration, conservation, and agriculture, by periodically assaying germinability of these seeds over 100 years. The interval between germination assays has been extended and the experiment will now end after 221 years, in 2100.

METHODS

We dug up the 16th bottle in April 2021 and attempted to germinate the 141-year-old seeds it contained. We grew germinants to maturity and identified these to species by vegetative and reproductive phenotypes. For the first time in the history of this experiment, genomic DNA was sequenced to confirm species identities.

RESULTS

Twenty seeds germinated over the 244-day assay. Eight germinated in the first 11 days. All 20 belonged to the Verbascum genus: Nineteen were V. blattaria according to phenotype and ITS2 genotype; and one had a hybrid V. blattaria × V. thapsus phenotype and ITS2 genotype. In total, 20/50 (40%) of the original Verbascum seeds in the bottle germinated in year 141.

CONCLUSIONS

While most species in the Beal experiment lost all seed viability in the first 60 years, a high percentage of Verbascum seeds can still germinate after 141 years in the soil. Long-term experiments such as this one are rare and invaluable for studying seed viability in natural soil conditions.

Ecotypic differentiation reveals seed colour-related alkaloid content in a crop wild relative

Crop wild relatives can be a useful source of genotypes that maximise crop survival and yield in specific habitats. Lupinus angustifolius is an annual forb with crop varieties derived from a narrow genetic basis but that are cultivated worldwide. Its seeds have high nutritional value, but they naturally contain alkaloids with anti-nutritive factors. The study of its wild populations can be useful to find genotypes that contribute to higher climate resilience and greater yield under stressing environmental conditions. Using a common garden, we evaluated ecotypic differentiation in four natural populations from two contrasting latitudes in terms of plant biomass, seed mass and number, alkaloid content in seeds for the three main alkaloids present in the plant and seed colour, including its possible influence on post-dispersal predation. Correlations among traits were also assessed. We found differences among populations for all traits except final biomass. Northern populations had lighter seeds and a tendency to yield more seeds when they produced white seeds, compared to southern populations and variegated seeds. Regardless of latitude, populations showed differences in alkaloid concentration, with all three alkaloids found generally in high or low concentrations in each population. Proportion of white seed morphs varied in each population. Seed colour did not influence predator preference. In addition, white seed colour was related to a low alkaloid content. Our results evidence the existence of natural ecotypic differentiation in L. angustifolius not only due to latitudinal range, but also to local environmental factors. White seed coat colour could be used as a visual clue for identification of low-alkaloid genotypes, a priority trait in L. angustifolius breeding programmes.

Tracking Seed Fates of Tropical Tree Species: Evidence for Seed Caching in a Tropical Forest in North-East India

Rodents affect the post-dispersal fate of seeds by acting either as on-site seed predators or as secondary dispersers when they scatter-hoard seeds. The tropical forests of north-east India harbour a high diversity of little-studied terrestrial murid and hystricid rodents. We examined the role played by these rodents in determining the seed fates of tropical evergreen tree species in a forest site in north-east India. We selected ten tree species (3 mammal-dispersed and 7 bird-dispersed) that varied in seed size and followed the fates of 10,777 tagged seeds. We used camera traps to determine the identity of rodent visitors, visitation rates and their seed-handling behavior. Seeds of all tree species were handled by at least one rodent taxon. Overall rates of seed removal (44.5%) were much higher than direct on-site seed predation (9.9%), but seed-handling behavior differed between the terrestrial rodent groups: two species of murid rodents removed and cached seeds, and two species of porcupines were on-site seed predators. In addition, a true cricket, Brachytrupes sp., cached seeds of three species underground. We found 309 caches formed by the rodents and the cricket; most were single-seeded (79%) and seeds were moved up to 19 m. Over 40% of seeds were re-cached from primary cache locations, while about 12% germinated in the primary caches. Seed removal rates varied widely amongst tree species, from 3% in Beilschmiedia assamica to 97% in Actinodaphne obovata. Seed predation was observed in nine species. Chisocheton cumingianus (57%) and Prunus ceylanica (25%) had moderate levels of seed predation while the remaining species had less than 10% seed predation. We hypothesized that seed traits that provide information on resource quantity would influence rodent choice of a seed, while traits that determine resource accessibility would influence whether seeds are removed or eaten. Removal rates significantly decreased (p < 0.001) while predation rates increased (p = 0.06) with seed size. Removal rates were significantly lower for soft seeds (p = 0.002), whereas predation rates were significantly higher on soft seeds (p = 0.01). Our results show that murid rodents play a very important role in affecting the seed fates of tropical trees in the Eastern Himalayas. We also found that the different rodent groups differed in their seed handling behavior and responses to changes in seed characteristics.

Effect of an invasive plant and moonlight on rodent foraging behavior in a coastal dune ecosystem

Understanding how invasive plants may alter predator avoidance behaviors is important for granivorous rodents because their foraging can trigger ripple effects in trophic webs. Previous research has shown that European beach grass Ammophila arenaria, an invasive species in coastal California, affects the predation of other seeds by the rodents Microtus californicus, Peromyscus maniculatus, and Reithrodontomys megalotis. This may be due to lower perceived predation risk by rodents foraging in close proximity to the cover provided by Ammophila, but this mechanism has not yet been tested. We examined the perceived predation risk of rodents by measuring the ‘giving up density’ of food left behind in experimental patches of food in areas with and without abundant cover from Ammophila and under varying amount of moonlight. We found strong evidence that giving up density was lower in the thick uniform vegetation on Ammophila-dominated habitat than it was in the more sparsely and diversely vegetated restored habitat. There was also evidence that moonlight affected giving up density and that it mediated the effects of habitat, although with our design we were unable to distinguish the effects of lunar illumination and moon phase. Our findings illustrate that foraging rodents, well known to be risk-averse during moonlit nights, are also affected by the presence of an invasive plant. This result has implications for granivory and perhaps plant demography in invaded and restored coastal habitats. Future research in this system should work to unravel the complex trophic links formed by a non-native invasive plant (i.e., Ammophila) providing cover favored by native rodents, which likely forage on and potentially limit the recruitment of native and non-native plants, some of which have ecosystem consequences of their own.

Adaptive divergence in seed color camouflage in contrasting soil environments

Although adaptive plant population divergence across contrasting soil conditions is often driven by abiotic soil factors, natural enemies may also contribute. Cryptic matching to the native soil color is a form of defensive camouflage that seeds can use to avoid detection by seed predators. The legume Acmispon wrangelianus occurs across a variety of gray-green serpentine soils and brown nonserpentine soils. Quantitative digital image analysis of seed and soil colors was used to test whether genetically based seed color is a closer match to the color of the native soil than to the color of other nearby soils. Lineages bear seeds that more closely match the color of their native serpentine or nonserpentine soil type than the opposing soil type. Further, even within a soil type, lineages bear seeds with a closer color match to the soil at their native site than to other sites. The striking concordance between seed and native soil color suggests that natural selection for locally camouflaged seed color morphs, probably driven by seed predators, may maintain adaptive divergence in pigmentation, despite the opportunity for migration between soil environments.

Rodent seed predation: effects on seed survival, recruitment, abundance, and dispersion of bird-dispersed tropical trees

Tropical tree species vary widely in their pattern of spatial dispersion. We focus on how seed predation may modify seed deposition patterns and affect the abundance and dispersion of adult trees in a tropical forest in India. Using plots across a range of seed densities, we examined whether seed predation levels by terrestrial rodents varied across six large-seeded, bird-dispersed tree species. Since inter-specific variation in density-dependent seed mortality may have downstream effects on recruitment and adult tree stages, we determined recruitment patterns close to and away from parent trees, along with adult tree abundance and dispersion patterns. Four species (Canarium resiniferum, Dysoxylum binectariferum, Horsfieldia kingii, and Prunus ceylanica) showed high predation levels (78.5-98.7%) and increased mortality with increasing seed density, while two species, Chisocheton cumingianus and Polyalthia simiarum, showed significantly lower seed predation levels and weak density-dependent mortality. The latter two species also had the highest recruitment near parent trees, with most abundant and aggregated adults. The four species that had high seed mortality had low recruitment under parent trees, were rare, and had more spaced adult tree dispersion. Biotic dispersal may be vital for species that suffer density-dependent mortality factors under parent trees. In tropical forests where large vertebrate seed dispersers but not seed predators are hunted, differences in seed vulnerability to rodent seed predation and density-dependent mortality can affect forest structure and composition.

Can spatial isolation help predict dispersal-limited sites for native species restoration?

When the distribution of species is limited by propagule supply, new populations may be initiated by seed addition, but identifying suitable sites for efficiently targeted seed addition remains a major challenge for restoration. In addition to the biotic or abiotic variables typically used in species distribution models, spatial isolation from conspecifics could help predict the suitability of unoccupied sites. Site suitability might be expected to increase with spatial isolation after other factors are accounted for, since isolation increases the chance that a site is unoccupied only because of propagule limitation. For two native annual forbs in Californian grasslands, we combined experimental seeding and niche modeling to ask whether suitability of unoccupied sites could be predicted by spatial variables (either distances from, or densities of, conspecific populations), either by themselves or in combination with niche models. We also asked whether experimental tests of these predictions held up not only in the short term (one year), but also in the longer term (three years). For Lasthenia californica, seed additions were only successful relatively near existing populations. For Lupinus nanus, seeding success was low and was positively related to the number of conspecifics within 1 km. For both species, a few previously unoccupied sites remained occupied three years after seeding, but this subset was not predictable based on either spatial or niche variables. Seed addition alone may be a limited means of native forb restoration if suitable unoccupied sites are either rare or unpredictable, or if they tend to be close to where the species already occurs.

Multiple mechanisms enable invasive species to suppress native species

PREMISE OF THE STUDY

Invasive plants represent a significant threat to ecosystem biodiversity. To decrease the impacts of invasive species, a major scientific undertaking of the last few decades has been aimed at understanding the mechanisms that drive invasive plant success. Most studies and theories have focused on a single mechanism for predicting the success of invasive plants and therefore cannot provide insight as to the relative importance of multiple interactions in predicting invasive species' success.

METHODS

We examine four mechanisms that potentially contribute to the success of invasive velvetgrass Holcus lanatus: direct competition, indirect competition mediated by mammalian herbivores, interference competition via allelopathy, and indirect competition mediated by changes in the soil community. Using a combination of field and greenhouse approaches, we focus on the effects of H. lanatus on a common species in California coastal prairies, Erigeron glaucus, where the invasion is most intense.

KEY RESULTS

We found that H. lanatus had the strongest effects on E. glaucus via direct competition, but it also influenced the soil community in ways that feed back to negatively influence E. glaucus and other native species after H. lanatus removal.

CONCLUSIONS

This approach provided evidence for multiple mechanisms contributing to negative effects of invasive species, and it identified when particular strategies were most likely to be important. These mechanisms can be applied to eradication of H. lanatus and conservation of California coastal prairie systems, and they illustrate the utility of an integrated set of experiments for determining the potential mechanisms of invasive species' success.

Apparent competition with an invasive plant hastens the extinction of an endangered lupine

Invasive plants may compete with native plants by increasing the pressure of native consumers, a mechanism known as "apparent competition." Apparent competition can be as strong as or stronger than direct competition, but the role of apparent competition has rarely been examined in biological invasions. We used four years of demographic data and seed-removal experiments to determine if introduced grasses caused elevated levels of seed consumption on native plant species in a coastal dune system in California, USA. We show that the endangered, coastal dune plant Lupinus tidestromii experiences high levels of pre-dispersal seed consumption by the native rodent Peromyscus maniculatus due to its proximity to the invasive grass, Ammophila arenaria. We use stage-structured, stochastic population models to project that two of three study populations will decline toward extinction under ambient levels of consumption. For one of these declining populations, a relatively small decrease in consumption pressure should allow for persistence. We show that apparent competition with an invasive species significantly decreases the population growth rate and persistence of a native species. We expect that apparent competition is an important mechanism in other ecosystems because invasive plants often change habitat structure and plant-consumer interactions. Possible implications of the apparent-competition mechanism include selective extinction of species preferred by seed consumers in the presence of an invasive species and biological homogenization of communities toward non-preferred native plant species.

Fire and mice: seed predation moderates fire's influence on conifer recruitment

In fire-adapted ecosystems, fire is presumed to be the dominant ecological force, and little is known about how consumer interactions influence forest regeneration. Here, we investigated seed predation by deer mice (Peromyscus maniculatus) and its effects on recruitment of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) seedlings in unburned and recently burned fire-adapted montane forests in west-central Montana, USA. Deer mice were almost twice as abundant in burned than unburned stands. Deer mouse removal of seeds from petri dishes was two times higher in burned than in unburned stands, and seed removal levels were 8% higher for ponderosa pine than for the smaller Douglas-fir seeds. In seed-addition experiments, emergence of seedlings in deer mouse-exclusion cages was almost six times higher in burned compared to unburned forest. In both burned and unburned forest, emergence was lower for ponderosa pine than for Douglas-fir. Seedling survival to establishment did not differ between conifer species but was considerably higher in burned than in unburned forest. However, effects of seed predation on recruitment prevailed over fire effects: in cages allowing access by deer mice, emergence and establishment were extremely rare for both conifer species in both burned and unburned forest. This research suggests that consumer interactions can substantially influence recruitment even in fire-adapted forest ecosystems.

Small-mammal seed predation limits the recruitment and abundance of two perennial grassland forbs

Although post-dispersal seed predators are common and often reduce seed density, their influence on plant population abundance remains unclear. On the one hand, increasing evidence suggests that many plant populations are seed limited, implying that seed predators could reduce plant abundance. On the other hand,.it is generally uncertain whether the magnitude of seed limitation imposed by granivores is strong enough to overcome density-dependent processes that could compensate for seed loss at later stages. We examined the impact of seed predation by small mammals, primarily deer mice (Peromyscus maniculatus), on seedling recruitment and subsequent plant establishment of two perennial grassland forbs in western Montana, USA: Lupinus sericeus (Fabaceae) and Lithospermum ruderale (Boraginaceae). The experiment combined graded densities of seed addition for each species with a small-mammal exclusion treatment. Seedling recruitment and plant establishment were monitored in the experimental plots for up to three years. For both species, small-mammal exclusion increased the total number of seedlings that emerged, and these effects were still significant three years after seed addition, resulting in greater numbers of established plants inside exclosures than in control plots. We also found evidence of seed limitation, with increasing density of seeds added leading to increased numbers of seedlings. Results from seed addition and small-mammal exclusion experiments in later years also revealed significant impacts of small mammals on seedling emergence. These results suggest that granivores can have potentially important impacts in limiting forb abundance in grasslands communities.

Seed longevity and germination characteristics of six fen plant species

Fens are among the most threatened habitats in Europe as their area has decreased considerably in the last centuries. For successful management and restoration conservationists need detailed knowledge about seed bank formation and seed longevity of plants, as these features are closely related to successional and vegetation dynamical processes. I analysed seed longevity and the germination characteristics of six fen plant species by seed burial experiments. Based on seed weight, seed bank was expected for long-term persistent for the light-seeded Schoenus nigricans, Carex appropinquata, C. pseudocyperus, C. davalliana and Peucedanum palustre and also that for the medium-seeded Cicuta virosa. It was proved that, the latter two species have short-term persistent seed banks, while Carex pseudocyperus has a transient seed bank, therefore these species may only have a limited role in restoration from seed banks. It was found that Schoenus nigricans, Carex appropinquata and C. davalliana have persistent seed banks, because some of their four-year-old seeds have emerged. Fresh seeds had low germination rate in all studied species and majority of seeds emerged after winter, except for Carex pseudocyperus. After the germination peak in spring, the majority of the ungerminated seeds of Schoenus nigricans, Peucedanum palustre, Carex appropinquata, C. davalliana and Cicuta virosa entered a secondary dormancy phase that was broken in autumn. I found the seasonal emergence of the latter three species highly similar.

Weed-biocontrol insects reduce native-plant recruitment through second-order apparent competition

Small-mammal seed predation is an important force structuring native-plant communities that may also influence exotic-plant invasions. In the intermountain West, deer mice (Peromyscus maniculatus) are prominent predators of native-plant seeds, but they avoid consuming seeds of certain widespread invasives like spotted knapweed (Centaurea maculosa). These mice also consume the biological-control insects Urophora spp. introduced to control C. maculosa, and this food resource substantially increases deer mouse populations. Thus, mice may play an important role in the invasion and management of C. maculosa through food-web interactions. We examined deer mouse seed predation and its effects on seedling emergence and establishment of a dominant native grass, Pseudoroegneria spicata, and forb, Balsamorhiza sagittata, in C. maculosa-invaded grasslands that were treated with herbicide to suppress C. maculosa or left untreated as controls. Deer mice readily took seeds of both native plants but removed 2-20 times more of the larger B. sagittata seeds than the smaller P. spicata seeds. Seed predation reduced emergence and establishment of both species but had greater impacts on B. sagittata. The intensity of seed predation corresponded with annual and seasonal changes in deer mouse abundance, suggesting that abundance largely determined mouse impacts on native-plant seeds. Accordingly, herbicide treatments that reduced mouse abundance by suppressing C. maculosa and its associated biocontrol food subsidies to mice also reduced seed predation and decreased the impact of deer mice on B. sagittata establishment. These results provide evidence that Urophora biocontrol agents may exacerbate the negative effects of C. maculosa on native plants through a form of second-order apparent competition-a biocontrol indirect effect that has not been previously documented. Herbicide suppressed C. maculosa and Urophora, reducing mouse populations and moderating seed predation on native plants, but the herbicide's direct negative effects on native forb seedlings overwhelmed the indirect positive effect of reducing deer mouse seed predation. By manipulating this four-level food chain, we illustrate that host-specific biological control agents may impact nontarget plant species through food-web interactions, and herbicides may influence management outcomes through indirect trophic interactions in addition to their direct effects on plants.

Consumers Limit the Abundance and Dynamics of a Perennial Shrub with a Seed Bank

For nearly 30 years, ecologists have argued that predators of seeds and seedlings seldom have population-level effects on plants with persistent seed banks and density-dependent seedling survival. We parameterized stage-based population models that incorporated density dependence and seed dormancy with data from a 5.5-year experiment that quantified how granivorous mice and herbivorous voles influence bush lupine (Lupinus arboreus) demography. We asked how seed dormancy and density-dependent seedling survival mediate the impacts of these consumers in dune and grassland habitats. In dune habitat, mice reduced analytical lambda (the intrinsic rate of population growth) by 39%, the equilibrium number of aboveground plants by 90%, and the seed bank by 98%; voles had minimal effects. In adjacent grasslands, mice had minimal effects, but seedling herbivory by voles reduced analytical lambda by 15% and reduced both the equilibrium number of aboveground plants and dormant seeds by 63%. A bootstrap analysis demonstrated that these consumer effects were robust to parameter uncertainty. Our results demonstrate that the quantitative strengths of seed dormancy and density-dependent seedling survival--not their mere existence--critically mediate consumer effects. This study suggests that plant population dynamics and distribution may be more strongly influenced by consumers of seeds and seedlings than is currently recognized.

Rodent seed predation promotes differential recruitment among bird-dispersed trees in temperate secondary forests

We investigated the role of seed predation by rodents in the recruitment of the fleshy-fruited trees Taxus baccata, Ilex aquifolium and Crataegus monogyna in temperate secondary forests in NW Spain. We measured the densities of dispersed seeds, early emerged seedlings, established recruits and adults, at four sites over a period of 2 years. Seed predation among species was compared by seed removal experiments and analysis of rodent larder-hoards. The three species differed markedly in local regeneration patterns. The rank order in the seed rain following decreasing seed density was Ilex, Taxus and Crataegus. However, Crataegus established 3.3 times more seedlings than Taxus. For all species, there was a positive linear relationship between the density of emerged seedlings and seed density, suggesting that recruitment was seed- rather than microsite-limited. A consistent pattern of seed selection among species was exerted by rodents, which preferred Taxus and, secondarily, Ilex seeds to Crataegus seeds. Predation ranking was the inverse of that of seed protection against predators, measured as the mass of woody coat per mass unit of the edible fraction. Recruitment potential, evaluated as the ratio of seedlings to seeds, was negatively related to seed predation, with the rank order Crataegus > Ilex > Taxus. The selective early recruitment limitation exerted by predation may have a demographic effect in the long term, as judged by the positive relationship between early seedling emergence and the density of established recruits. By modulating the pre-emptive competition for seed safe sites, rodents may preclude the progressive exclusion of species that produce low numbers of seeds (i.e. Crataegus) by those dominant in seed number (i.e. Ilex, Taxus), or at least foster the evenness for site occupation among seedlings of different species.

Influence of fire, topography, and consumer abundance on seed predation in tallgrass prairie

We assessed seed predation by vertebrates and invertebrates in three fire-frequency treatments (<1 year, 1–4 years, and >4 years since fire) and in three topographic positions (upland, limestone breaks, and lowland) in tallgrass prairie. Two types of seed trays, one for vertebrates and one for invertebrates, were placed in each treatment during each nocturnal and diurnal period. Vertebrates removed significantly more seeds than did invertebrates. Fire frequency and topographic position affected seed removal by both vertebrates and invertebrates. Seed removal by invertebrates was influenced negatively by fire; the greatest seed removal occurred in uplands and lowlands in unburned prairie. Vertebrates removed the most seeds in burned prairie and in lowlands and limestone breaks. Time of day also influenced seed removal by vertebrates, as nocturnal vertebrates (assumed to be rodents) removed more seeds than diurnal vertebrates. Abundance of rodents, however, did not predict accurately seed removal in fire treatments or topographic positions, as rodents removed fewer seeds than expected in prairie that had not been burned in >4 years and in lowlands. This pattern likely was due to the presence of a well-developed plant litter layer in both unburned and lowland habitats, which reduces the likelihood of a rodent locating seeds.

Temporal patterns in seedling establishment on pocket gopher disturbances

Disturbances often facilitate seedling establishment, and can change the species composition of a community by increasing recruitment of disturbance-adapted species. To understand the effects of pocket gopher disturbances on alpine seedling dynamics, we examined the gopher disturbances' effects on seedling emergence and survival on gopher disturbances 0 to 5 years old. In contrast to results from most other ecosystems, these recently created gopher mounds had lower seedling emergence and survival rates than undisturbed areas. A lack of correlation between species' abundances on gopher mounds and undisturbed sites in one of the two communities studied suggested that a suite of disturbance-adapted species recruited onto the mounds. To explain low seedling emergence on recent gopher mounds, we quantified gopher mound seed banks and studied recruitment in a site with mounds that ranged from 0 to >20 years old. Seed numbers in first-year gopher mound soils were extremely low relative to undisturbed soils, and this pattern was mirrored in seedling establishment patterns over the long term. Gopher disturbance depressed seedling emergence density for the first 5 years. Subsequently, emergence density increased until at least 20 years following the disturbance. Emergence on disturbances more than 20 years old was higher than on undisturbed sites. Therefore, gopher disturbances probably facilitate seedling establishment in alpine dry and moist meadow; however, this process takes place over decades.