Summary
1 The invasion of habitats by non-native plant and animal species is a global phenomenon with potentially grave consequences for ecological, economic, and social systems. Unfortunately, to date, the study of invasions has been primarily anecdotal and resistant to generalization.

2 Here, we use insights from experiments and from long-term monitoring studies of vegetation to propose a new theory in which fluctuation in resource availability is identified as the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions.

3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1046/j.1365-2745.2000.00473.x

Fluctuating resources in plant communities: a general theory of invasibility

Meta-analysis provides formal statistical techniques for summarizing the results of independent experiments and is increasingly being used in ecology. The response ratio (the ratio of mean outcome in the experimental group to that in the control group) and closely related measures of proportionate change are often used as measures of effect magnitude in ecology. Using these metrics for meta-analysis requires knowledge of their statistical properties, but these have not been previously derived. We give the approximate sampling distribution of the log response ratio, discuss why it is a particularly useful metric for many applications in ecology, and demonstrate how to use it in meta-analysis. The meta-analysis of response-ratio data is illustrated using experimental data on the effects of increased atmospheric CO2 on plant biomass responses.

The meta-analysis of response ratios in experimental ecology

Summary 1 Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized It is timely to consider the progress made by research in this field 2 We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant‐plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory We then consider future directions for facilitation research 3 With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies 4 There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity 5 Certain ecological modelling approaches (eg individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes 6 Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms

/pdf/facilitation-in-plant-communities-the-past-the-present-and-572accn7oh.pdf

Facilitation in plant communities: the past, the present, and the future

Soils are the most heterogeneous parts of the biosphere, with an extremely high differentiation of properties and processes within nano- to macroscales. The spatial and temporal heterogeneity of input of labile organics by plants creates microbial hotspots over short periods of time – the hot moments. We define microbial hotspots as small soil volumes with much faster process rates and much more intensive interactions compared to the average soil conditions. Such hotspots are found in the rhizosphere, detritusphere, biopores (including drilosphere) and on aggregate surfaces, but hotspots are frequently of mixed origin. Hot moments are short-term events or sequences of events inducing accelerated process rates as compared to the average rates. Thus, hotspots and hot moments are defined by dynamic characteristics, i.e. by process rates. For this hotspot concept we extensively reviewed and examined the localization and size of hotspots, spatial distribution and visualization approaches, transport of labile C to and from hotspots, lifetime and process intensities, with a special focus on process rates and microbial activities. The fraction of active microorganisms in hotspots is 2–20 times higher than in the bulk soil, and their specific activities (i.e. respiration, microbial growth, mineralization potential, enzyme activities, RNA/DNA ratio) may also be much higher. The duration of hot moments in the rhizosphere is limited and is controlled by the length of the input of labile organics. It can last a few hours up to a few days. In the detritusphere, however, the duration of hot moments is regulated by the output – by decomposition rates of litter – and lasts for weeks and months. Hot moments induce succession in microbial communities and intense intra- and interspecific competition affecting C use efficiency, microbial growth and turnover. The faster turnover and lower C use efficiency in hotspots counterbalances the high C inputs, leading to the absence of strong increases in C stocks. Consequently, the intensification of fluxes is much stronger than the increase of pools. Maintenance of stoichiometric ratios by accelerated microbial growth in hotspots requires additional nutrients (e.g. N and P), causing their microbial mining from soil organic matter, i.e. priming effects. Consequently, priming effects are localized in microbial hotspots and are consequences of hot moments. We estimated the contribution of the hotspots to the whole soil profile and suggested that, irrespective of their volume, the hotspots are mainly responsible for the ecologically relevant processes in soil. By this review, we raised the importance of concepts and ecological theory of distribution and functioning of microorganisms in soil.

Microbial hotspots and hot moments in soil: Concept & review

Human activities have increased the availability of nutrients in terrestrial and aquatic ecosystems. In grasslands, this eutrophication causes loss of plant species diversity, but the mechanism of this loss has been difficult to determine. Using experimental grassland plant communities, we found that addition of light to the grassland understory prevented the loss of biodiversity caused by eutrophication. There was no detectable role for competition for soil resources in diversity loss. Thus, competition for light is a major mechanism of plant diversity loss after eutrophication and explains the particular threat of eutrophication to plant diversity. Our conclusions have implications for grassland management and conservation policy and underscore the need to control nutrient enrichment if plant diversity is to be preserved.

/pdf/competition-for-light-causes-plant-biodiversity-loss-after-4d1fqaeyvb.pdf

Competition for light causes plant biodiversity loss after eutrophication

Quantitative synthesis across studies requires consistent measures of effect size among studies. In community ecology, these measures of effect size will often be some measure of the strength of interactions between taxa. However, indices of interaction strength vary greatly among both theoretical and empirical studies, and the connection between hypotheses about interaction strength and the metrics that are used to test these hypotheses are often not explicit. We describe criteria for choosing appropriate metrics and methods for comparing them among studies at three stages of designing a meta-analysis to test hypotheses about variation in interaction intensity: (1) the choice of response variable; (2) how effect size is calculated using the response in two treatments; and (3) whether there is a consistent quantitative effect across all taxa and systems studied or only qualitatively similar effects within each taxon–system combination. The consequences of different choices at each of these stages are illu...

/pdf/empirical-approaches-to-quantifying-interaction-intensity-4jpw8ztdc9.pdf

Empirical approaches to quantifying interaction intensity: competition and facilitation along productivity gradients

Summary 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 aboveground 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.

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

Relationships between productivity and diversity in plant communities have been widely documented. Unimodal productivity-diversity relationships are most common along natural productivity gradients, and fertilization generally reduces diversity. Five distinct hypotheses invoke changes in competition to explain why diversity should decline from intermediate to high productivity. Because experiments measuring the effects of competition on diversity are rare, four of the five hypotheses have not been directly tested, but each hypothesis makes unique predictions that allow for indirect tests. The indirect evidence is often conflicting, and while none of the hypotheses can be rejected, only the dynamic equilibrium hypothesis is consistently supported. A new hypothesis, however, is supported by indirect evidence and may help to explain the variation in the shape of productivity-diversity relationships, as well as the most common patterns. Diversity may be high in environments that promote size symmetric competition, where soil resources limit growth and are homogeneously distributed within the soil volume explored by individual plants. Conversely, diversity may be low in environments that promote size asymmetric competition, where light is limiting, or where soil resources are limiting and are patchily distributed within rooting zones.

Explaining productivity‐diversity relationships in plants

Summary


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.

/pdf/root-competition-can-cause-a-decline-in-diversity-with-24gwoox7iw.pdf

Root competition can cause a decline in diversity with increased productivity

The root foraging strategy of a plant species can be characterized by measuring foraging scale, precision, and rate. Trade-offs among these traits have been predicted to contribute to coexistence of competitors. We tested for trade-offs among root foraging scale (total root mass and length of structural roots), precision (ln-ratio of root lengths in resource-rich and resource-poor patches), and rate (days required for roots to reach a resource-rich patch, or growth rate of roots within a resource-rich patch) in eight co-occurring species. We found that root foraging scale and precision were positively correlated, as were foraging scale and the rate of reaching patches. High relative growth rate of a species did not contribute to greater scale, precision, or rate of root foraging. Introduced species had greater foraging scale, precision, and rate than native species. The positive correlations between foraging scale and foraging precision and rate may give larger species a disproportionate advantage in competition for patchy soil resources, leading to size asymmetric competition below ground.

Tara K. Rajaniemi

Papers

Empirical approaches to quantifying interaction intensity: competition and facilitation along productivity gradients

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

Explaining productivity‐diversity relationships in plants

Root competition can cause a decline in diversity with increased productivity

Root foraging for patchy resources in eight herbaceous plant species