Local Communities Competition

Interspecific competition generally leads to a reduction of the contribution of poor competitors to the community, whereas superior competitors are able to gain dominance. Thus, competition is a process strongly increasing dominance. The degree of competitive dominance depends on two factors: (1) on the asymmetry of the competition and (2) the time for superior species to develop their dominance (see further below). The former aspect depends on the distribution of traits in an assemblage of competing species. If traits are similar, and thus competitive advantage of a certain trait low, dominance will be low. On the other hand, strong dominance can be expected if traits are skewed and competitive advantage of a certain trait is large (Figure 3). Competition of terrestrial plants for light is a lucid example for such an asymmetric competition for a unidirectional resource, as competitive success mainly depends on one single trait (plant height), and species growing higher are able to strongly dominate communities. Similarly, filamentous species highly dominate many undisturbed assemblages of benthic algae (periphyton) as they are able to compete successfully for light and water column nutrients.

The skewed distribution of successful traits is also the reason why anthropogenic input of fertilizers often leads to enhance dominance (Figure 2a). Fertilization of terrestrial or aquatic environments (eutrophication) generally enhances the availability of one or few resources, but not of others. Thereby, the array of traits leading to competitive success narrows. A typical example is represented by the addition of phosphorus (P) to lakes, which often leads to the dominance of cyanobacteria species which need a lot of P but are able to fix atmospheric nitrogen (N).

More generally speaking, dominance increases when the number of potentially limiting nutrients decreases (Figure 3 ). It has been shown that the evenness of lake phytoplankton increases with increasing number of limiting nutrients. Similarly, the evenness of plant assemblages peaks at intermediate ratios of available N:P, but is low at low or high N:P ratios, respectively. Thus, a strong imbalance of N and P, which leads to limitation by either only

Ecosystem functioning

Figure 3 Conceptual depiction of processes affecting dominance. Gray ovals comprise biotic processes, white ovals abiotic constraints. Direct effects are represented by bold arrows, indirect effects by thin arrows. '+' and '-' characterize processes or traits increasing or decreasing dominance, respectively.

Ecosystem functioning

Figure 3 Conceptual depiction of processes affecting dominance. Gray ovals comprise biotic processes, white ovals abiotic constraints. Direct effects are represented by bold arrows, indirect effects by thin arrows. '+' and '-' characterize processes or traits increasing or decreasing dominance, respectively.

N or only P, leads to greater dominance of single species. Similarly, gradients in productivity implicitly are also gradients in the number of limiting resources. Very unproductive terrestrial ecosystems often are systems with strong water shortage and dominated by species most successful in accessing water. Very productive terrestrial ecosystems tend to be limited only by light and again single species with certain traits will dominate. Therefore, dominance is least pronounced in mid-productive ecosystems.

Environmental harshness has often been supposed to reduce competition, which however has been falsified in recent years. Instead harsh conditions often require certain adaptations and the array of traits leading to survival and competitive success are also highly narrowed for organisms in extraordinarily harsh conditions. Thus dominance of single species is the general case in these environments.

The second aspect, time of development, describes whether the competitive advantage of a species can fully develop (Figure 3). The gain of competitive dominance takes time, while the extent of time strongly depends on the generation time of the organisms and the already-mentioned asymmetry in competitive advantage. A highly superior fast-growing species will gain dominance more rapidly than a slow-growing species which has only a marginal competitive advantage compared to the co-occurring species. Competitive dominance in competing microalgae can arise within days or weeks, whereas it may last years in long-lived trees or in ecological similar moss species in a bog.

Because the gain of competitive dominance takes time, spatial as well as temporal heterogeneity may stop or even reverse this process. Spatial heterogeneity can be provided by patchiness in resource supply or by biotic or abiotic architecture affecting competitive success. Patchy resource supply creates a mosaic of resource ratios and competitively advantageous strategies. Architecture often opens up refuges or represents the basis for the success of other life-forms (such as epiphytes). Temporal heterogeneity is often investigated in the form of fluctuations in resource availability, which also reduces competitive dominance or reverses competitive ranks of the interacting species.

Disturbances, defined as an irregular mortality-inducing event, has been proposed as a univariate explanation of dominance and diversity patterns, most prominently by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at disturbance regimes of intermediate frequency and intensity. Similarly, it predicts that low-disturbance ecosystems are dominated by competitively superior species, whereas high disturbance creates dominance by species able to cope with the extreme mortality in these systems. Dominance is thus expected to be low in mid-disturbance regimes, where dominance opens niche opportunities for species by creating patchiness of environmental conditions allowing species to differentially express life-history tradeoffs. Strong evidence has accumulated that how disturbance (and also consumption as a biotic mortality agent) affects community structure is coupled to the productivity of the system, which affects the rate of biomass accrual and dispersal and thus the speed of competitive displacement in a community (see below).

Consumption

Consumers can act as a mortality agent keeping competitively superior species from gaining dominance. Consumption in this meaning comprises all kinds of consumer-prey interactions including herbivory, predation, and pathogens. Dominance can be enhanced or reduced by consumer presence, and the sign of the effect often is mediated by changing competitive interactions between prey species. Consumers can enhance dominance when they favor competitive success of well-defended or less-preferred prey species. More often, however, consumption reduces dominance if the superior competitor is especially prone to consumption.

The generality or selectivity of the consumer is highly important for their effect on dominance in prey communities. Selective consumers increase dominance if they favor species with a certain trait but they can reduce dominance ifthey are specialized on competitively superior species. The latter interaction is known in the ecological literature as keystone predation, which describes the preferential consumption of a competitively dominant species and thus the fostering of subdominant species. Keystone predation generally decreases dominance ratios and has been observed in many ecosystems strongly structured by competition.

Generalist consumers are also able to reduce dominance, mainly in situations where dominance is fostered by strong competition and fast competitive displacement. In highly productive ecosystems, dominance by single species is generally high, but these dominant species are often highly prone to consumption. Especially plants allocate less resources to defense (structural or chemical) with increasing ecosystem productivity (aka the growth rate hypothesis of defense). Therefore, plants adapted to dominate at high resource supply tend to be highly vulnerable to grazing and grazer presence reduces dominance. Thus, productivity is a key factor to understand the degree of consumer control on diversity and dominance. Several meta-analyses concurred on a dominance-reducing effect of consumers in highly productive ecosystems characterized by strong dominance of fast-growing species, which is strongly reduced by the presence of consumers. At low productivity, consumer effects on diversity are mainly negative, which translates to an increase in dominance.

Many trophic interactions are able to propagate through the food web and may change dominance indirectly on the next trophic level (trophic cascades). Predators can alter the dominance ratio and the identity of dominance species in plant assemblages via the reduction in herbivore density or activity. Other indirect effects in food webs have also been identified as altering and regulating the dominance in community structure. Apparent competition and associational resistance are two pathways mediating the contribution of different prey species to the entire prey assemblage. Also nutrient regeneration may change competitive advantages in communities where recycled nutrients play a major role in primary production. The presence of a P-rich consumer may enhance N-regeneration compared to P-regenera-tion and thus shift the available N:P elemental ratio and therewith competitive success of different prey taxa.

Facilitation

The role of positive interactions for community structure and dominance has only recently become a focus of ecological studies. The importance of mutualistic interactions is especially seen in the dominance of life-forms with long-evolved symbioses in certain ecosystems. The dependence of trees dominating boreal forests on mycor-rhiza and the dependence of corals on symbiotic zooxanthellae are vivid examples highlighting the effects of positive interactions in community structure. Also nitrogen fixation via rhizobia can foster the dominance of certain plant groups such as the Fabaceae in terrestrial ecosystems with high light but poor N-availability.

Additionally, facilitation clearly plays a role in temporal gradients of dominance, as the presence of a certain pioneer species often is necessary in order to allow the dominance of late successional species. Facilitation can thus create dominance by the favored species, but it also can reduce dominance by allowing coexistence of species which would not be able to thrive without the presence of a facilitative species.

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