Coexistence and relative abundance in annual plant assemblages: the roles of competition and colonization

Although an interspecific trade-off between competitive and colonizing ability can permit multispecies coexistence, whether this mechanism controls the structure of natural systems remains unresolved. We used models to evaluate the hypothesized importance of this trade-off for explaining coexistence and relative abundance patterns in annual plant assemblages. In a nonspatial model, empirically derived competition-colonization trade-offs related to seed mass were insufficient to generate coexistence. This was unchanged by spatial structure or interspecific variation in the fraction of seeds dispersing globally. These results differ from those of the more generalized competition-colonization models because the latter assume completely asymmetric competition, an assumption that appears unrealistic considering existing data for annual systems. When, for heuristic purposes, completely asymmetric competition was incorporated into our models, unlimited coexistence was possible. However, in the resulting abundance patterns, the best competitors/poorest colonizers were the most abundant, the opposite of that observed in natural systems. By contrast, these natural patterns were produced by competition-colonization models where environmental heterogeneity permitted species coexistence. Thus, despite the failure of the simple competition-colonization trade-off to explain coexistence in annual plant systems, this trade-off may be essential to explaining relative abundance patterns when other processes permit coexistence.     

Functional Traits in a Species-Rich Grassland and a Short-Term Change in Management: Is There a Competition-Colonization Trade-Off?

The species richness of grasslands generally cannot be fully restored after changes in management. Some species with small statures and basal leaf rosettes can be lost forever. The same species, however, seem to possess the traits necessary for successful re-colonization – they produce small, easily dispersable seeds, numerous seedlings and have lasting seed banks. We tested the hypothesis that plants in species-rich grasslands can be characterized by a negative correlation between their competitive ability and potential for generative regeneration, i.e. by a competition-colonization trade-off. An analysis of the traits of 95 grassland species supported this hypothesis. We then conducted a manipulative experiment in three different meadow communities in the Bílé Karpaty Mts. The experiment involved characterizing species traits during periods of different grassland management regimes in the years 1997–2000 and comparing these with the original management regime, which was restored between 2000 and 2003. We found out that the hypothesis only holds true for the pooled dataset for all three communities. When the individual meadow communities were analyzed separately, plant traits other than those responsible for the competition-colonization trade-off appear to be characteristic of responsive species, e.g. shoot lifespan or phenology. Our results imply that despite the general trade-offs found in large comparative studies, the plant response in a specific community is constrained by the local species pool.     

On testing the competition-colonization trade-off in a multispecies assemblage

The competition-colonization trade-off has long been considered an important mechanism explaining species coexistence in spatially structured environments, yet data supporting it remain ambiguous. Most competition-colonization research examines plants and the dispersal-linked traits of their seeds. However, colonization is more than just dispersal because rapid population growth is also an important component of colonization. We tested for the presence of competition-colonization trade-offs with a commonly used artificial assemblage consisting of protozoan and rotifer species, where colonization was the ability of a species to establish populations in patches. By ranking species according to their colonization abilities and their pairwise competitive interactions, we show that these species exhibit competition-colonization trade-offs. These results reveal that the competition-colonization trade-off exists within nonplant assemblages and that even in a laboratory setting, species are constrained to be either good competitors or colonizers but not both.     

The competition-colonization trade-off is dead; long live the competition-colonization trade-off

When applied at the individual patch level, the classic competition-colonization models of species coexistence assume that propagules of superior competitors can displace adults of inferior competitors (displacement competition). But if adults are invulnerable to displacement by propagules (as trees are to seeds), and propagules compete to replace adults that die for reasons independent of the outcome of juvenile competition (a lottery system), a competition-colonization trade-off alone is not able to produce coexistence. However, we show that coexistence is possible if patch density varies spatially, such that it becomes a niche axis. We also show how a dispersal-fecundity trade-off can partition variation in patch density. We discuss the application of these models to empirical systems. An important implication of communities coexisting via variation in patch density is that the amount of habitat loss necessarily interacts with the pattern of loss in affecting extinctions, invasions, and coexistence, in contrast to displacement competition models, for which the spatial pattern of loss is not important or is less important. Finally, with respect to mechanisms promoting coexistence, we suggest that trade-offs between different stages of colonization could be far more common in nature than a trade-off between competitive ability and colonization ability.     

Temporal variability in detritus resource maintains diversity of bacterial communities

Competition theory generally predicts that diversity is maintained by temporal environmental fluctuations. One of the many suggested mechanisms for maintaining diversity in fluctuating environments is the gleaner-opportunist trade-off, whereby gleaner species have low threshold resource levels and low maximum growth rates in high resource concentration while opportunist species show opposite characteristics. We measured the growth rates of eight heterotrophic aquatic bacteria under different concentrations of chemically complex plant detritus resource. The growth rates revealed gleaner-opportunist trade-offs. The role of environmental variability in maintaining diversity was tested in a 28-day experiment with three different resource fluctuation regimes imposed on two four-species bacterial communities in microcosms. We recorded population densities with serial dilution plating and total biomass as turbidity. Changes in resource availability were measured from filter-sterilised medium by re-introducing the consumer species and recording short-term growth rates. The type of environmental variation had no effect on resource availability, which declined slowly during the experiment and differed in level between the communities. However, the slowly fluctuating environment had the highest Shannon diversity index, biomass, and coefficient of variation of biomass in both communities. We did not find a clear link between the gleaner-opportunist trade-off and diversity in fluctuating environments. Nevertheless, our results do not exclude this explanation and support the general view that temporal environmental variation maintains species diversity also in communities feeding chemically complex resource.     

Herbaceous plant strategies in disturbed habitats

A systematic theoretical evaluation has been made of three important plant life history traits: adult longevity, seed longevity and seed mass, where seed mass is interpreted as being indicative of dispersal distance and seedling vigour. This model study examined the role of these three traits in relation to environmental disturbance. We chose temperate grasslands, widespread in north Western Europe and northern and eastern America, as our reference system for our simulations. Eight plant strategies were defined by allowing two levels in each of the three and combining them in all eight possible ways. A simple, spatially explicit model was developed to simulate competition among individuals with these eight trait combinations at different levels of disturbance.
Simulation results were compared with the actual occurrence over a disturbance gradient of species with similar plant trait combinations in a large database from the Sheffield area (UK). This showed that with increasing disturbance level, non-dormant perennials, dormant perennials, non-dormant annuals and dormant annuals, respectively, became dominant but only if small-seeded, indicating the relative viability of these particular strategies with respect to disturbance.
A new prediction from the model was that stable coexistence occurs between plant strategies with dormant and with non-dormant seeds over a range of levels of disturbance. Plant strategies with large seeds were inferior to small-seeded ones if competitive ability of seedlings is proportional to seed weight. This difference was highest at low seed densities and low germination probabilities, indicating that large-seeded species secure no advantage from being dormant (i.e. having a low germination probability). Finally, the results indicated that dormancy is superior to dispersal as a method of coping with disturbance.     

Seed supply, recruitment, and assembly: quantifying relative seed and establishment limitation in a plant community context

There is a growing consensus that the relative constraints of seed limitation and establishment limitation in recruitment strongly influence abundance patterns in plant communities. Although these constraints have direct relevance to coexistence, most investigations utilize a seed addition approach that offers limited insight into these dynamics. Here we report the results of an assembly experiment with annual plant species from California grasslands to examine how propagule pool characteristics (dominant species abundance, functional diversity) influence establishment and seed limitation (density independence and density dependence across a gradient of seed supply) for each species, as well as how these constraints affect community diversity. Species were predominantly colimited by seed and establishment constraints, exhibiting saturating recruitment functions with increased seed supply. Consistent with competition-colonization trade-off predictions, recruitment constraints often depended on the degree of seed limitation of the competitive dominant, Brassica nigra; diversity was greatest in communities where Brassica was seed limited. Functional similarity within the propagule pool did not affect recruitment across a range of seed supply; likewise, functional diversity of the propagule pool was not related to community diversity. We conclude that seed limitation of the dominant species rather than niche similarity influences interspecific competition for safe sites and scales up to affect community-level diversity.     

Diversity and Coexistence of Sonoran Desert Winter Annuals

Abstract Annual plants make up ca. 50% of local floras in the Sonoran Desert. As with most plant communities, there is no shortage of potential coexistence generating mechanisms, and several mechanisms are likely contributors to coexistence at different spatial scales in the Sonoran Desert, e.g. spatial heterogeneity and the behaviors of predators and grazers. We explore one mechanism of likely importance for desert annuals: temporal environmental variation. It is widely recognized that coexistence is promoted by temporal variation if species such as desert annuals have "temporal niches" in the sense that each has years in which it out-performs the others. It is usually suggested that some resistent life-history stage, such as a seed bank, is also necessary to buffer each species from the negative population dynamic impact of unfavorable years. Using ten years of demographic data, we document the large year-to-year variation in population dynamics of desert annuals and show that ten species respond differently to temporal variation. Competition experiments document reversals in competitive superiority. Also, all species have a between-year seed bank, such that only a proportion of the seed bank germinates in any given year. Thus this system meets our intuitive requirements for variance-based coexistence. Dynamic models of this system demonstrate that subtle aspects of the species biology determine whether coexistence criteria are actually met. Specifically, variable germination fractions are required and coexistence is most readily favored with "predictive" germination. Germination fractions in this system do vary among years in a species specific fashion. Also, for the three years of available data, germination was predictive, in that each species had greater germination fractions in year of greater demographic success. Thus all of the population dynamic elements necessary for temporal variance mediated coexistence seem to be present in this system.     

General relationships between species diversity and stability in competitive systems

Investigating the effect of biodiversity on the stability of ecological communities is complicated by the numerous ways in which models of community interactions can be formulated. This has led to differences in conclusions and interpretations of how the number of species in a community affects its stability. Here, we derive a simple, general relationship between the coefficient of variation (CV) of combined species densities and the environmentally driven variability in species' per capita population growth rates. For a given level of environmentally driven variability in per capita population growth rates, increasing the number of species in a community decreases the CV of combined species densities, provided that species do not respond to environmental fluctuations in a perfectly correlated way. Thus, a community with more species of competitors will be more stable (have lower CV in combined species densities for a given level of environmental variability) than a species-poor community, provided that the species in both communities show equal variability in per capita population growth rates and provided that species within each community do not show strongly correlated responses to environmental fluctuations. This conclusion also applies to "noninteractive" models in which there is no competition between species.     

Correlates of inter-specific variation in germination response to water stress in a semi-arid savannah

Within arid plant communities species vary considerably in the ability to germinate under water stress. Attempts to correlate this variation with environmental gradients have remained largely inconclusive. Germinating only at high water potentials can be seen as a form of predictive germination. Predictive germination provides a fitness variance reducing mechanism and is therefore expected to show negative correlations with other variance reducing life-history attributes such as large seed size or dormancy. We predicted that differences in life-history attributes rather than edaphic gradients could explain the variation in germination responses to water stress found in arid plant communities. To test our hypothesis we determined the germination response of 28 species from the arid Kalahari savannah to a gradient of osmotic stress, expressed as the water potential needed to reduce germination by 50%. In addition, we determined the life-history variables seed mass and germination fraction and the habitat variables soil texture preference and association with acacias. The data were analysed using phylogenetically independent contrasts in a multiple regression model.Contrary to our hypothesis we found no increase in the capacity to germinate under osmotic stress with increasing seed mass and an increase with increasing germination fraction. However, we also found no significant effect of the habitat variables. This result may be explained by variation in seedling drought tolerance. Drought tolerance will also have a variance-reducing effect and can be expected to trade-off with fractional germination. Our results suggest that in arid plant communities most variation in the capacity to germinate under water stress expresses different ways to make a living under similar conditions rather than adaptations to environmental gradients.     

Environmental variability uncovers disruptive effects of species' interactions on population dynamics

How species respond to changes in environmental variability has been shown for single species, but the question remains whether these results are transferable to species when incorporated in ecological communities. Here, we address this issue by analysing the same species exposed to a range of environmental variabilities when (i) isolated or (ii) embedded in a food web. We find that all species in food webs exposed to temporally uncorrelated environments (white noise) show the same type of dynamics as isolated species, whereas species in food webs exposed to positively autocorrelated environments (red noise) can respond completely differently compared with isolated species. This is owing to species following their equilibrium densities in a positively autocorrelated environment that in turn enables species–species interactions to come into play. Our results give new insights into species'' response to environmental variation. They especially highlight the importance of considering both species'' interactions and environmental autocorrelation when studying population dynamics in a fluctuating environment.     

Influence of seed size on performance of non-native annual plant species in a novel community at two planting densities

Climate warming enables plant species to migrate to higher latitudes and altitudes. Within Europe, the Mediterranean harbours many species that might expand their ranges towards Western Europe. Small seed size may facilitate dispersal, however, it may impair establishment of the range-expanding plant species in the novel vegetation. In a greenhouse experiment, we examined effects of average seed size of Mediterranean plant species on their establishment in a mixed community of Western European plant species. Applying two levels of densities of the natives and a herbivory treatment, we tested how seed size is linked to response in plant growth and fitness in novel vegetation. While all non-native plant species showed a negative response to increased planting density, species with small seeds showed a less negative response. This effect persisted under herbivory. Our data suggest that small-seeded non-native plant species may tolerate competitive pressure from novel plant communities better than large-seeded species, so that small seed size may confer a higher probability of establishment of non-native species in novel communities.     

Seed distribution of four co-occurring grasses around Artemisia halodendron shrubs in a sandy habitat

In a natural population of the perennial semi-shrub Artemisia halodendron in a shifting sandy habitat in the Horqin Desert of eastern Inner Mongolia, six isolated adult A. halodendron individuals of similar canopy size were chosen as target plants. The density of seeds in the top 5 cm soil depth around shrubs was measured using transects aligned to the four main wind directions and at different distances from the shrub base on both the windward and leeward sides. The effects of shrub presence on seed distribution of four co-occurring grasses were examined by linking seed distribution to seed traits. Of the four species, Setaris viridis and Eragrostis pilosa had small but similar seed mass, while Chloris virgata and Aristida adscensionis had large but similar seed mass. The species were grouped into two cohorts: small-seeded vs. large-seeded cohorts, and shrub presence effects on seed distribution of both cohorts were examined. We found marked difference in the seed distribution pattern among species, especially between the small-seeded and large-seeded cohorts. The small-seeded cohort had significantly higher seed accumulation on the windward than the leeward sides in the most and least prevailing wind directions and much higher seed accumulation on the leeward than the windward sides in the second and third most prevailing wind directions, while opposite patterns occurred in the large-seeded cohort. Four species also showed marked variation in the seed distribution pattern among transects and between windward and leeward sides of each transect. This study provided further evidence that shrubs embedded in a matrix of herbaceous plants is a key cause of spatial heterogeneity in seed availability of herbaceous species. However, seed distribution responses to the presence of shrubs will vary with species as well as with wind direction, sampling position (windward vs. leeward sides of the shrub) and distance from the shrub.     

Seed size and plant strategy across the whole life cycle

We compiled information from the international literature to quantify the relationships between seed mass and survival through each of the hazards plants face between seed production and maturity. We found that small-seeded species were more abundant in the seed rain than large-seeded species. However, this numerical advantage was lost by seedling emergence. The disadvantage of small-seeded species probably results from size-selective post-dispersal seed predation, or the longer time small-seeded species spend in the soil before germination. Seedlings from large-seeded species have higher survival through a given amount of time as seedlings. However, this advantage seems to be countered by the greater time taken for large-seeded species to reach reproductive maturity: our data suggested no relationship, or perhaps a weak negative relationship, between seed size and survival from seedling emergence through to adulthood. A previous compilation showed that the inverse relationship between seed mass and the number of seeds produced per unit canopy area per year is countered by positive relationships between seed mass, plant size and plant longevity. Taken together, these data show that our old understanding of a species' seed mass as the result of a trade–off between producing a few large offspring, each with high survival probability, versus producing many small offspring each with a lower chance of successfully establishing was incomplete. It seems more likely that seed size evolves as part of a spectrum of life history traits, including plant size, plant longevity, juvenile survival rate and time to reproduction.     

Scale, Experimental Design and the Detection of Ineterspecific Competition within Plant Communities

Abstract We report preliminary results of a series of experiments designed to explore the importance of interspecific competition within arable weed communities at different scales. Competition hierarchies were apparent from a pot experiment with different levels of nutrients and water. Two field experiments looked at Bromus sterilis, Galium aparine and Papaver rhoeas in winter wheat in the field, in a range of combinations and management treatments, and a fourth field experiment included a wider variety of species. There was little effect of fertilizer on population behaviour in the the field. Bromus increased around ten fold per year on minimum-tilled plots, regardless of other treatments. Galium increased on organically-fertilized and minimum-tilled plots, but only in the absence of Bromus. Papaver densities remained low, but again were depressed in the presence of high densities of Bromus . Taken together, the experiments demonstrate the existence of competition between weed species. However, as the design of the experiment increased to include greater levels of environmental variation, so competition became more difficult to detect, and less useful for interpreting the results than knowledge of the biology of the individual species. At the scale of interest to the farmer, the level of competition is not a good predictor for weed population dynamics.     

Temporal fluctuations and experimental effects in desert plant communities

In the Chihuahuan Desert of the southwestern United States we monitored responses of both winter and summer annual plant communities to natural environmental variation and to experimental removal of seed-eating rodents and ants for 13 years. Analyses of data on population densities of the species by principal component analysis (PCA) followed by repeated measures analysis of variance (rmANOVA) on PCA scores showed that: (1) composition of both winter and summer annual communities varied substantially from year to year, presumably in response to interannual climatic variation, and (2) community composition of winter annuals was also significantly affected by the experimental manipulations of seed-eating animals, but the composition of the summer annual community showed no significant response to these experimental treatments. Canonical discriminant analysis (CDA) was then applied to the data for winter annuals to more clearly identify the responses to the different classes of experimental manipulations. This analysis showed that removing rodents or ants or both taxa caused distinctive changes in species composition. There was a tendency for large-seeded species to increase on rodent removal plots and to decrease on ant removal plots, and for small-seeded species to change in the opposite direction. In the winter annual community there was a significant time x treatment interaction: certain combinations of species that responded differently to removal of granivores also showed opposite fluctuations in response to long-term climatic variation. The large year-to-year variation in the summer annual community was closely and positively correlated across all experimental treatments. The use of multivariate analysis in conjunction with long-term monitoring and experimental manipulation shows how biotic interactions interact with variation in abiotic conditions to affect community dynamics.     

Density‐dependent demographic responses of a semelparous plant to natural variation in seed rain

The link between reproductive and vegetative ecology of flowering plants is rarely explored, despite its importance for understanding population processes and fitness. This link can be studied by using experimental or natural variation in seed input to the soil to assess how reproductive success affects vital rates of offspring. We previously reported for Ipomopsis aggregata that per‐seed probability of germinating is insensitive to density of seeds sown into plots, whereas per capita flower production among adults that grow from the seedlings declines in nonlinear fashion with density. Here we describe a parallel non‐experimental study. We related seedling emergence to estimated natural seed input (‘seed rain’) in three populations across ten summers and monitored seedlings that emerged in the first two summers throughout their life histories. Seedling emergence in 1996 was linearly related to seed rain from plants that flowered in 1995. This density independent seed‐to‐seedling transition recurred over the next nine summers, but the slope varied with springtime precipitation. Total numbers of 1996 seedlings that survived to flower and numbers of flowers they produced increased linearly with seed rain in one population, but did not vary detectably in the other two, consistent with negative density dependence. In consequence λ (the dominant eigenvalue of a population projection matrix) decreased from high values at low densities of seed rain to a relatively constant low value with greater seed rain. We also detected density dependence in the 1995 seedling cohort in survival and flower production. The similarity of results from natural and experimental studies supports a conclusion of nonlinear density dependence and shows that characterizing it requires the full life history. For this plant species and others, studies of pollination and fecundity alone may not suffice to draw conclusions about population change or fitness.     

The structure and stability of model ecosystems assembled in a variable environment

To explore how environmental variability may create non‐random community structure, we simulated the assembly of model communities under varying levels of environmental variability. We assembled communities by creating a large pool of randomly constructed species, and then added species from this pool sequentially, allowing extinctions of invading and resident species to occur until the community became saturated. Because much current research on community structure focuses on single trophic levels, we constructed species pools consisting only of competitors. To compare with more realistic communities, we also created species pools with multiple trophic levels. For both types of communities, following assembly we calculated a variety of metrics of community structure, and five measures of community stability. Communities assembled under high environmental variability had fewer species, fewer and weaker interactions among species, and greater evenness in abundance of persisting species. For single trophic‐level communities, community size was dictated primarily by competitive exclusion. In contrast, for multiple trophic‐level communities, community size was increasingly limited by dynamical instabilities as environmental variability increased. Differences in community structure resulting from assembly under high environmental variability led to differences in community stability. According to two measures of stability related to population variability – the characteristic return rate to equilibrium and the coefficient of variation in individual species densities – stability increased for communities assembled under high environmental variability. In contrast, three additional measures of stability that are not directly related to population variability showed a variety of patterns, either increasing, decreasing, or remaining constant. Thus, communities assembled in highly variable environments are not necessarily generically more stable. Our results demonstrate that environmental variability can structure communities and affect their stability properties in non‐trivial ways. Thus, when making predictions about the response of communities to future extinctions or environmental degradation, account should be given to the forces responsible for community structure.     

Intrinsic and extrinsic drivers of recruitment across the distribution range of a seed-dimorphic herb

Recruitment of new individuals, through germination and seedling survival, is a key process for short-lived plants. Here, we analyzed intraspecific variation in recruitment across the latitudinal range of Plantago coronopus, a widespread herb that produces both large basal seeds with a mucilaginous coat and small apical seeds without coat. We experimentally tested the effects of seed traits and water availability on recruitment, by using seeds from a wide environmental stress gradient from N Africa to N Europe. Our experiments were carried out in controlled environmental conditions and in dunes where the species naturally occurs. Water shortage decreased seed germination and seedling survival for all populations, showing the importance of water supply for P. coronopus. Basal seeds showed higher and faster germination rates than apical seeds. Since among-population variation in seed mass was not related to potential germination rate, it is the mucilaginous coat rather than size difference that likely drives the differential success between seed morphs. Seed mass positively affected seedling survival instead, but only in controlled conditions with regular water supply. An experiment in a dune showed indeed that the highest survival corresponded to the local population and not the one with the largest seeds. Our results demonstrate that both intrinsic and extrinsic factors drive inter-population variation in the early life stages of this short-lived plant, allowing it to adapt across the environmentally heterogeneous distribution range. Gathering information on intraspecific variation in recruitment-related traits will help us to understand and predict plant responses in a context of climatic change.     

Microsite and litter cover effects on seed banks vary with seed size and dispersal mechanisms: implications for revegetation of degraded saline land

Seed movements and fates are important for restoration as these determine spatial patterns of recruitment and ultimately shape plant communities. This article examines litter cover and microsite effects on seed availability at a saline site revegetated with Eucalyptus sargentii tree rows interplanted with 5?C6 rows of saltbush (Atriplex spp.). As litter accumulation decreases with increasing distance from tree rows, soil seed banks were compared between paired bare and litter-covered zones within three microsites; tree row, saltbush row closest to tree row and saltbush mid-row (middle row of saltbush between tree rows). Germinable seed banks of the four most abundant species with contrasting seed sizes and dispersal mechanisms were assessed to test the hypotheses that: (i) microsites with litter cover contain higher seed densities than bare areas, but that (ii) microsite and litter effects will vary depending on seed size and dispersal mechanisms. Overall, litter cover increased seed densities, however, litter effects varied with seed size, with no effect on small-seeded species and litter increasing densities of large-seeded species. Seed bank composition also differed between tree and shrub microsites due to differences in seed morphology and dispersal mechanisms. Water-dispersed species were unaffected by microsite but densities of wind-dispersed species, including Atriplex spp., were higher in saltbush microsites. Densities of wind-dispersed species also differed between the two saltbush microsites despite similar litter cover. Future plantings should consider row spacing and orientation, as well as the dimensions of seeding mounds and associated neighbouring depressions, to maximize litter and seed-trapping by microsites.