The role of specialist parasites in structuring host communities

Natural enemies can be a powerful force when structuring natural communities, and in facilitating or preventing species coexistence depending on the nature of the trophic interaction. In particular, “keystone” predators can promote species coexistence, provided they preferentially attack the competitively dominant species. However, it is not clear whether parasites can play a similar structuring role; parasites typically form chronic associations with their victims, reducing their fitness (i.e., fecundity) rather than survival, and allowing infected hosts to remain viable competitors within the community. Therefore the density-dependent suppression of the host is likely to be more subtle than that due to predation. Using a series of simple population-dynamic models we show that specialist parasites can facilitate species coexistence, although possibly less so than predators. These results contrast with those typically found with models of generalist parasites, which can reduce the likelihood of species coexistence through apparent competition. In addition, we show that the likelihood of parasite-facilitated species coexistence depends greatly on the specific type of parasite. In particular, macroparasites (e.g., parasitic helminths) may be less likely to facilitate species coexistence than microparasites (e.g., viruses or bacteria) due to their typically highly aggregated distribution amongst their hosts. Furthermore, species coexistence is more likely if the parasite is relatively benign to its host. Parasitism by apparently “harmless” specialist parasites may provide an important but overlooked factor in the maintenance of species diversity, facilitating species invasions into new communities and the emergence of novel infectious diseases.     

Competitive ability and the potential for lotteries among territorial reef fishes

Stegastes diencaeus and S. dorsopunicans are mutually territorial Caribbean damselfishes. S. diencaeus is larger, grows faster and lives longer than S. dorsopunicans. S. diencaeus is a habitat specialist that shares its primary habitat mainly with S. dorsopunicans. Field manipulations show that both S. diencaeus and S. dorsopunicans readily take over living space from smaller, but not larger, heterospecific neighbors. Natural changes in the use of living space by both species occur frequently and adult S. diencaeus often aggressively usurp the living areas of smaller S. dorsopunicans. Lunar and seasonal patterns of juvenile recruitment by S. diencaeus and S. dorsopunicans are similar. Large size bestows competitive superiority on S. diencaeus by giving its adults a superior ability to aggressively acquire living space, and by enabling its juveniles to quickly escape the period when they lack a size advantage. Hence they spend much of their lives as competitive dominants. There is no evidence that competitive advantages arising from large size are offset either by other adult attributes or by differences in temporal patterns of recruitment that affect priority of access to space. The lottery hypothesis for species coexistence relies on patterns of abundance being determined by patterns of recruitment to vacant space because different species have equal space-holding abilities. These data show that the existence of such a mechanism is doubtful.     

Dispersal network heterogeneity promotes species coexistence in hierarchical competitive communities

Understanding the mechanisms of biodiversity maintenance is a fundamental issue in ecology. The possibility that species disperse within the landscape along differing paths presents a relatively unexplored mechanism by which diversity could emerge. By embedding a classical metapopulation model within a network framework, we explore how access to different dispersal networks can promote species coexistence. While it is clear that species with the same demography cannot coexist stably on shared dispersal networks, we find that coexistence is possible on unshared networks, as species can surprisingly form self‐organised clusters of occupied patches with the most connected patches at the core. Furthermore, a unimodal biodiversity response to an increase in species colonisation rates or average patch connectivity emerges in unshared networks. Increasing network size also increases species richness monotonically, producing characteristic species–area curves. This suggests that, in contrast to previous predictions, many more species can co‐occur than the number of limiting resources.     

Spatial heterogeneity of mortality and temporal fluctuation in fertility promote coexistence but not vice versa: a random-community approach

Both spatial heterogeneity and temporal fluctuation of the environment are important mechanisms promoting species coexistence, but they work in different manners. We consider many pairs of species with randomly generated survivorship and fertility in the lottery model, and examine how the variability in demographic processes affects the outcome of competition. The results are: [1] Coexistence is easier if habitat difference in mortality is greater, or if year-to-year variation in reproductive rate is larger. But neither habitat-difference in fertility nor temporal variation in mortality promotes coexistence. [2] Mean fertility does not affect the outcome if CV remains constant. In contrast, enhanced mean mortality decreases the fraction of coexisting pairs if the environment fluctuates temporally. [3] We also investigate the effect of limited dispersal of propagules between habitats. Compared with the complete mixing case, the fraction of coexisting pairs is clearly enhanced if the spatial heterogeneity is the major source of environmental variation, but shows slight increase if the temporal fluctuation is dominant. We conclude that spatial heterogeneity is likely to work more effectively in promoting species coexistence than temporal fluctuation, especially when the species suffer relatively high mortality, and disperse their propagules in a limited spatial scale.     

Functional characters combined with null models reveal inconsistency in mechanisms of species turnover in lacustrine fish communities

Functional characters have the potential to act as indicators of species turnover between local communities. Null models provide a powerful statistical approach to test for patterns using functional character information. A combined null model/functional character approach provides the ability to distinguish between the effect of competition and environmental filtering on species turnover. We measured 13 functional characters relating directly to resource use for the fish species found in French lakes. We combined this functional character data with a null model approach to test whether co-occurring species overlapped more or less than expected at random for four primary niche axes. We used an environmentally constrained null model approach to determine if the same mechanisms were responsible for species turnover at different sections of the altitudinal gradient. Functional diversity indices were used to examine the variation in functional character diversity with altitude, as a test of the hypothesis that competitive intensity decreases with increasing environmental adversity. The unconstrained null model showed that environmental filtering was the dominant influence on species turnover between lakes. In the constrained null model, there was much less evidence for environmental filtering, emphasising the strong effect of altitude on turnover in functional character values between local communities. Different results were obtained for low-altitude and high-altitude lake subsets, with more evidence for the effect of environmental filtering being found in the high-altitude lakes. This demonstrates that different processes may influence species turnover throughout an environmental gradient. Functional diversity values showed a slight decrease with altitude, indicating that there was only weak evidence that competitive intensity decreased with increasing altitude. Variation resource availability and environmental stress probably cause the observed turnover in functional characters along the altitudinal gradient, though the effects of dispersal limitation and species introductions in high-altitude lakes cannot be ruled out.     

Reduced competitive ability in an invasive plant

One explanation for successful plant invaders is that they evolved to be more competitive. An intuitive prediction of this Evolution of Increased Competitive Ability (EICA) hypothesis never previously tested is that invasive populations should outcompete their native ‘ancestors’ in a common environment. We tested this idea in a diallel competition experiment with Alliaria petiolata where offspring from native and invasive populations were grown alone or in all pairwise combinations. While without competition, there were no differences between native and invasive populations, native populations outperformed invasive ones when competing against each other. Our results contradict the EICA hypothesis and we conclude that it does not not hold for Alliaria petiolata. Instead, we formulate a new ERCA (Evolutionary Reduced Competitive Ability) hypothesis: if there is less competition in the invasive range and competitive ability involves traits that have a fitness cost, then selection might act against it, thereby reducing intraspecific interactions too.     

Root foraging traits and competitive ability in heterogeneous soils

The responses of plant roots to nutrient patches in soil may be an important component of competitive ability. In particular, the scale, precision, and rate of foraging for patchy soil resources may influence competitive ability in heterogeneous soils. In a target–neighbor experiment in the field, per-individual and per-gram competitive effects were measured for six old-field species with known root foraging scale, precision, and rate. The presence and number of nutrient patches were also manipulated in a full factorial design. Number and presence of patches did not influence the outcome of competition. Competitive ability was not related to total plant size, growth rate, or root:shoot allocation, or to root foraging precision. Per-individual competitive effects were marginally correlated with root foraging scale (biomass of roots) and root foraging rate (time required to reach a patch). Therefore, competitive ability was more closely related to ability to quickly fill a soil volume with roots than to ability to preempt resource-rich patches.     

The risk of competitive exclusion during evolutionary branching: Effects of resource variability, correlation and autocorrelation

Evolutionary branching has been suggested as a mechanism to explain ecological speciation processes. Recent studies indicate however that demographic stochasticity and environmental fluctuations may prevent branching through stochastic competitive exclusion. Here we extend previous theory in several ways; we use a more mechanistic ecological model, we incorporate environmental fluctuations in a more realistic way and we include environmental autocorrelation in the analysis. We present a single, comprehensible analytical result which summarizes most effects of environmental fluctuations on evolutionary branching driven by resource competition. Corroborating earlier findings, we show that branching may be delayed or impeded if the underlying resources have uncorrelated or negatively correlated responses to environmental fluctuations. There is also a strong impeding effect of positive environmental autocorrelation, which can be related to results from recent experiments on adaptive radiation in bacterial microcosms. In addition, we find that environmental fluctuations can lead to cycles of repeated branching and extinction.     

When the exception becomes the rule: The disappearance of limiting similarity in the Lotka-Volterra model

We investigate the transition between limiting similarity and coexistence of a continuum in the competitive Lotka-Volterra model. It is known that there exist exceptional cases in which, contrary to the limiting similarity expectation, all phenotypes coexist along a trait axis. Earlier studies established that the distance between surviving phenotypes is in the magnitude of the niche width 2σ provided that the carrying capacity curve differs from the exceptional one significantly enough. In this paper we studied the outcome of competition for small perturbations of the exceptional (Gaussian) carrying capacity. We found that the average distance between the surviving phenotypes goes to zero when the perturbation vanishes. The number of coexisting species in equilibrium is proportional to the negative logarithm of the perturbation. Nevertheless, the niche width provides a good order of magnitude for the distance between survivors if the perturbations are larger than 10%. Therefore, we conclude that limiting similarity is a good framework of biological thinking despite the lack of an absolute lower bound of similarity.     

Co-occurrence of tree species at fine spatial scale in a woodland cerrado,southeastern Brazil

Species co-occurrence at fine spatial scales is expected to be nonrandom in relation to species phylogenetic relatedness and functional similarity. On the one hand, closely related species that occur together and experience similar environmental conditions are likely to share phenotypic traits due to the process of environmental filtering. On the other hand, species that are too similar are unlikely to co-occur due to competitive exclusion. We surveyed a woodland cerrado, southeastern Brazil, to test whether co-occurrence in tree species shows functional or phylogenetic structuring at fine spatial scale. Searching for correlations between an index of species co-occurrence and both functional trait differences and phylogenetic distances, we provided evidence for a predominant role of environment filters in determining the co-occurrence of functionally similar tree species in cerrado. However, we did not find any effect of phylogenetic relatedness on tree species co-occurrence. We suggest that the phylogenetic relatedness of co-occurring cerrado tree species did not present a pattern, because the species functional traits were randomly distributed on the phylogeny. Thus, phylogenetic relatedness and functional similarity do not seem to limit the co-occurrence at fine spatial scale of cerrado tree species.     

Effects of river level fluctuation on plant species richness, diversity, and distribution in a floodplain forest in Central Amazonia

River levels in Central Amazonia fluctuate up to 14 m annually, with the flooding period ranging from 50 to 270 days between the rising and falling phases. Vast areas of forest along the rivers contain plant species that are well adapted to annual flooding. We studied the effect of flooding level on tree species richness, diversity, density, and composition in lake, river, and stream habitats in Jaú National Park, Brazil. 3051 trees >10 cm diameter (at 1.3 m diameter at breast height, dbh) were measured and identified in 25 10 m × 40 m randomly selected plots in each habitat. Ordination methods and analysis of variance results showed that forested areas near lakes had significantly lower species richness of trees than riverine and streamside habitats. Plot species richness and diversity were strongly negatively correlated with the water level and duration of flooding. The drier (stream) habitat had more total species (54 species of trees) and more unique species of trees (6 tree species) than the riverine (52 tree species; 3 unique species) and lake (33 tree species; 3 unique species) habitats. Species composition overlap among habitats was surprisingly high (42.6–60.6% overlap), almost one-third of the species were found in all three habitat types, and few species were unique to each habitat. We conclude that: (1) duration of flooding has a strong impact on species richness, diversity and plant distribution patterns; (2) most species are adapted to a wide range of habitats and flood durations; and (3) while flood duration may decrease local diversity, it also creates and maintains high landscape-scale diversity by increasing landscape heterogeneity. Received: 20 April 1997 / Accepted: 14 January 1999     

Relative herbivory tolerance and competitive ability in two dominant : subordinate pairs of perennial grasses in a native grassland

We evaluated herbivory tolerance and competitive ability within twodominant : subordinate pairs of C₄, perennial grasses at each of twosites to determine the contribution of these processes to herbivore-inducedspecies replacement. Herbivory tolerance was assessed by cumulative regrowthfrom defoliated plants of each species and competitive ability was evaluated byrelative uptake of a ¹⁵N isotope placed into the soil between pairedspecies in the field. Herbivory tolerance was similar for the dominant andsubordinate species in both plant pairs and defoliation intensity had a greaterinfluence on herbivory tolerance than did defoliation pattern. Both specieswithin the Sorghastrum nutans : Schizachyriumscoparium pairs exhibited comparable nitrogen acquisition from a¹⁵N enriched pulse with or without defoliation. In contrast,S. scoparium acquired more ¹⁵N than did itssubordinate neighbor, Bothriochloa laguroides when thisspecies pair was undefoliated. Uniform defoliation of this species pair at adefoliation intensity removing 70% of the shoot mass accentuated this responsefurther demonstrating the greater competitive ability of the dominant comparedto the subordinate species. Although the 90% defoliation intensity reducednitrogen acquisition by the dominant relative to the subordinate species,B. laguroides, it did not reduce nitrogen acquisition bythe dominant below that of the subordinate neighbor. The occurrence of similarherbivory tolerance among dominant and subordinate species indicates thatselective herbivory suppressed the greater competitive ability, rather than thegreater herbivory tolerance, of the dominant grasses in this experimentaldesign. These data suggest that interspecific competitive ability may be ofequal or greater importance than herbivory tolerance in mediatingherbivore-induced species replacement in mesic grasslands and savannas.     

Joint control by rodent herbivory and nutrient availability of plant diversity in a salt marsh–salty steppe transition zone

Questions: Do current models that predict shifting effects of herbivores on plant diversity with varying nutrient conditions apply to stressful systems like salt marshes? Do herbivores affect different components of the diversity as nutrient availability varies? Location: Salt marsh–salty steppe transition zone at the SW Atlantic Mar Chiquita coastal lagoon (37°44′52″S, 57°26′6″W), Argentina. Methods: We experimentally evaluated the separate and interactive effect of nutrients and rodent (Cavia aperea) herbivory, using exclosures and applying fertilizer (mostly nitrogen), following a factorial design in 50 cm × 50 cm plots. Results: We found a negative effect of herbivory on diversity in the resource‐poor scenario (due to a reduction in species richness), but a positive effect when nutrients were added, by reducing the abundance of the dominant plant (and hence increasing evenness). Conclusions: Our experimental results contribute to the limited factorial evidence evaluating the role of nutrients and herbivory on the diversity of terrestrial plant communities, even in highly stressful environments like salt marsh–salty steppe transition zones. Our results also support the model that predicts negative effects of herbivores on plant diversity in low‐nutrient conditions and positive effects in nutrient‐enriched scenarios, and also support the mechanism assumed to act in these situations.     

Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie

In tallgrass prairie, plant species interactions regulated by their associated mycorrhizal fungi may be important forces that influence species coexistence and community structure; however, the mechanisms and magnitude of these interactions remain unknown. The objective of this study was to determine how interspecific competition, mycorrhizal symbiosis, and their interactions influence plant community structure. We conducted a factorial experiment, which incorporated manipulations of abundance of dominant competitors, Andropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal symbiosis using the fungicide benomyl under two fire regimes (annual and 4-year burn intervals). Removal of the two dominant C₄ grass species altered the community structure, increased plant species richness, diversity, and evenness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community structure, although plant species richness and diversity increased. Responses of individual plant species were associated with their degree of mycorrhizal responsiveness: highly mycorrhizal responsive species decreased in abundance and less mycorrhizal responsive species increased in abundance. The combination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, indicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that mycorrhizal fungi affect plant communities indirectly by influencing the pattern and strength of plant competitive interactions. Burning strongly influenced the outcome of these interactions, which suggests that plant species diversity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis. Received: 7 April 1999 / Accepted: 30 July 1999     

Coexistence through spatio-temporal heterogeneity and species sorting in grassland plant communities

The effect of spatial heterogeneity on species coexistence relies on the degree of niche heterogeneity in the habitat and the ability of species to exploit the available niche opportunities. We studied species coexistence in a perennial grassland, and tested whether small-scale disturbances create environmental heterogeneity that affects coexistence and whether the functional diversity of species in the species pool affects the ability of community composition to reflect heterogeneity through species sorting. We manipulated the spatio-temporal heterogeneity of disturbance and the functional diversity of species added as seed and measured their impact on the spatial turnover of species composition. Disturbance increased environmental heterogeneity and spatial turnover, and the effect of heterogeneity on turnover was greatest in the presence of a functionally diverse species pool, showing the importance of trait variation among species for exploiting environmental heterogeneity, and suggesting that coexistence occurred due to species sorting among heterogeneous niches.     

Phenotypic variability promotes diversity and stability in competitive communities

Intraspecific variation is at the core of evolutionary theory, and yet, from an ecological perspective, we have few robust expectations for how this variation should affect the dynamics of large communities. Here, by adapting an approach from evolutionary game theory, we show that the incorporation of phenotypic variability into competitive networks dramatically alters the dynamics across ecological timescales, stabilising the systems and buffering the communities against demographic perturbations. The beneficial effects of phenotypic variability are strongest when there are substantial differences among phenotypes and when phenotypes are inherited with moderately high fidelity; yet even low levels of variation lead to significant increases in diversity, stability, and robustness. By identifying a simple and ubiquitous stabilising force in competitive communities, this work contributes to our core understanding of how biological diversity is maintained in natural systems.     

Factors associated with plant species richness in a coastal tall‐grass prairie

Abstract. In this study we examine the factors associated with variations in species richness within a remnant tall‐grass prairie in order to gain insight into the relative importance of controlling variables. The study area was a small, isolated prairie surrounded by wetlands and located within the coastal prairie region, which occurs along the northwestern Gulf of Mexico coastal plain. Samples were taken along three transects that spanned the prairie. Parameters measured included micro‐elevation, soil characteristics, indications of recent disturbance, above‐ground biomass (including litter), light penetration through the plant canopy, and species richness. Species richness was found to correlate with micro‐elevation, certain soil parameters, and light penetration through the canopy, but not with above‐ground biomass. Structural equation analysis was used to assess the direct and indirect effects of micro‐elevation, soil properties, disturbance, and indicators of plant abundance on species richness. The results of this analysis showed that observed variations in species richness were primarily associated with variations in environmental effects (from soil and microtopography) and were largely unrelated to variations in measures of plant abundance (biomass and light penetration). These findings suggest that observed variations in species richness in this system primarily resulted from environmental effects on the species pool. These results fit with a growing body of information that suggests that environmental effects on species richness are of widespread importance.