Quantifying individual- and community-level effects of competition using experimentally-determined null species pools

The effects of competition on individual fitness and species diversity were investigated in a first-year old field by comparing the natural community to an experimentally-determined null community. The species pool for the null community was estimated from low-density plots, and hypothetical sample plots in the null community were constructed by random sampling from the species pool. Individual plants were larger in low-density plots than control plots, indicating that competition reduced individual fitness. Competition appeared to reduce diversity in half the plots (i.e. species richness and diversity were lower than in hypothetical null community plots with the same number of individuals), but did not affect diversity in the other plots. However, the reduction in diversity could be explained as an artifact caused by spatial aggregation in control plots. The magnitude of the effects of competition on diversity did not change with plot density, and no species consistently increased or decreased in relative abundance as plot density increased. We conclude that competition had no effect on diversity in this community, despite the strong effect on individual growth.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73205/1/3236636.pd

Root competition can cause a decline in diversity with increased productivity

1  Plant community theory often invokes competition to explain why species diversity declines as productivity increases. Competition for all resources might become more intense and lead to greater competitive exclusion or, alternatively, competition for light only could become more intense and exclude poor light competitors. 2  To test these hypotheses, we constructed communities of seven old-field species using combined monocultures. Constructs experienced no interspecific competition, only shoot competition or only root competition, with and without fertilizer. Diversity in these limited interaction communities was compared to diversity in unfertilized and fertilized mixtures of the seven species. 3  Diversity decreased with fertilization in mixtures and in communities with only root competition. Shoot competition had small effects on the community and did not contribute to changes in diversity with fertilization. 4  Root competition may strongly impact plant community structure in unproductive communities where light never becomes limiting, or under non-equilibrium conditions following human disturbances.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75541/1/j.1365-2745.2003.00768.x.pd

Local Filters Limit Species Diversity, but Species Pools Determine Composition

In a series of three experiments, we tested for effects of species pools, resource stress, and species interactions on four aspects of community structure: species richness, evenness, species composition, and functional group composition. We also examined whether the impacts of species interactions on the community varied with resource availability or species pool. Communities of sand dune annuals grew from seed bank samples collected from two sites in three different years, so that the species pool differed at two levels: the source site and the year of seed bank collection. Communities experienced one of three irrigation treatments and a range of sowing densities, which varied resource supply (stress) and the potential for species interactions, respectively. Species richness and evenness were most affected by local factors: higher densities and lower water availability decreased species diversity. In contrast, species composition was influenced most by the species pool. Functional group composition had an intermediate response, and was affected by both species pools and local filters. Resource stress and species interactions strongly filtered species from the community, but the identity of species remaining was variable. Furthermore, the magnitude of species interaction effects on richness and evenness varied with species pools. Thus, the outcome of species sorting among biotic and abiotic environments was dependent on the pool of available species. Contrary to predictions from theory, the effects of species interactions on the community did not vary consistently with resource levels

Quantitative partitioning of regional and local processes shaping regional diversity patterns

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73256/1/j.1461-0248.2005.00855.x.pd

Why does fertilization reduce plant species diversity? Testing three competition-based hypotheses

1 Plant species diversity drops when fertilizer is added or productivity increases. To explain this, the total competition hypothesis predicts that competition above ground and below ground both become more important, leading to more competitive exclusion, whereas the light competition hypothesis predicts that a shift from below-ground to above-ground competition has a similar effect. The density hypothesis predicts that more above-ground competition leads to mortality of small individuals of all species, and thus a random loss of species from plots. 2 Fertilizer was added to old field plots to manipulate both below-ground and above-ground resources, while shadecloth was used to manipulate above-ground resources alone in tests of these hypotheses. 3 Fertilizer decreased both ramet density and species diversity, and the effect remained significant when density was added as a covariate. Density effects explained only a small part of the drop in diversity with fertilizer. 4 Shadecloth and fertilizer reduced light by the same amount, but only fertilizer reduced diversity. Light alone did not control diversity, as the light competition hypothesis would have predicted, but the combination of above-ground and below-ground competition caused competitive exclusion, consistent with the total competition hypothesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75695/1/j.1365-2745.2001.00662.x.pd

Explaining the productivity -diversity relationship in plants: Changing effects of competition on the community.

Plant species diversity frequently declines from intermediate to high productivity. I review three competition-based hypotheses to explain the productivity-diversity relationship, and test them in an old-field perennial community. The total competition hypothesis predicts that competition for all resources becomes more intense and increases competitive exclusion. The light competition hypothesis predicts that competition for light becomes more intense while root competition becomes less intense, and light competition increases competitive exclusion. The density hypothesis predicts that increased competition for light kills small individuals of all species, and species are lost randomly from small plots. Many previous tests of these hypotheses measured the effects of competition on individuals, not on the community. In a first-year old field, I used low-density plots to characterize the community with no competition. Although competition had strong effects on individuals, it had no effect on species diversity. Clearly, community-level experiments are necessary to test these hypotheses. Two community-level tests support none of the three hypotheses. Artificial shading and fertilization both reduced light, but shading did not decrease diversity while fertilization did. Thus, increased competition for light did not decrease diversity. Fertilized plots lost more species than predicted from random mortality, so the density hypothesis was not supported. When competition was directly manipulated with a combined-monocultures design, diversity dropped with fertilizer in communities experiencing full competition or only root competition, but not in communities with no interactions or only shoot interactions. Therefore, belowground competition alone explained reduced diversity. Neither the total competition hypothesis or the light competition hypothesis predicted this pattern. Two further experiments explore the mechanisms of root competition in this community. Species that increase in abundance with fertilizer have high root allocation and can further increase root allocation to exploit added nutrients. Size asymmetry of root competition may also contribute to competitive success. At least one species, Bromus inermis, had size asymmetric effects on neighbors in root competition. Thus, the previous hypotheses to explain the productivity-diversity relationship were rejected, and a new hypothesis was proposed. Diversity may decline with productivity only when competition becomes more size asymmetric, either because light becomes limiting or because small-scale heterogeneity of soil resources increases.Ph.D.Biological SciencesBotanyEcologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126311/2/3016940.pd

Community-level consequences of species interactions in an annual plant community

Question : How does the intensity of species interactions affect species and functional group composition of an annual plant community? Location : Sede Boqer, Negev Desert, Israel. Methods : The potential for competitive interactions in two annual plant communities (desert and coastal) from semi-stabilized sand dunes was manipulated by varying seed bank density and therefore the number of potentially interacting individuals. Communities were exposed to three different irrigation regimes, mimicking precipitation at the desert site, the coastal site, and an intermediate precipitation level. Plots were maintained for 3 years, and percentage cover of each species in the plots was recorded at the end of each growing season. We used redundancy analysis to test for effects of initial density, irrigation, and year on the species and functional group composition of the communities. Results : Initial density had significant effects on species composition, and these effects remained significant over 3 years, even as total community percentage cover became more similar among treatments over time. Density effects did not depend on resource availability (irrigation level). Functional group identity or individual plant size did not predict which species would be good competitors, and a species' competitive ability did not predict its abundance in the field. Conclusions : Species interactions strongly affect community composition, and those effects carry over into subsequent years such that competition does not lead to convergence in community structure over time. However, the particular changes in composition observed were not predictable by some of the traits that have been found important in individual-level experiments. We speculate that the outcome of competition in diverse communities will depend on multiple traits, in contrast to the outcome of individual-level pairwise experiments. We also speculate that the shift in composition with density could mean that local variation in density may contribute to maintenance of diversity in this system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74125/1/j.1654-1103.2009.01086.x.pd