Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain.

/pdf/effects-of-biodiversity-on-ecosystem-functioning-a-consensus-13j8vabcus.pdf

Effects of biodiversity on ecosystem functioning: a consensus of current knowledge

Publisher Summary This chapter discusses a search for physiological causes and ecological consequence in reference with the variation in growth rate between higher plants. When grown under optimum conditions, plant species from fertile, productive habitats tend to have inherently higher relative growth rates (RGR) than species from less favorable environments. Under these conditions, fast-growing species produce relatively more leaf area and less root mass, which greatly contributes to their larger carbon gain per unit plant weight. Fast-growing species also have higher respiration rates per unit organ weight, due to demands of a higher RGR and higher rate of nutrient uptake. Fast-growing species have a greater capacity to acquire nutrients, which is likely to be a consequence, rather than the cause, of their higher RGR. This chapter analyses variations in morphological, physiological, chemical, and allocation characteristics underlying variation in RGR, to arrive at an appraisal of its ecological significance. The lower specific leaf area (SLA) of slow-growing species is because of the relatively high concentration of cell-wall material and quantitative secondary compounds, which may protect against detrimental abiotic and biotic factors. This chapter concludes that it is likely that there are trade-offs between growth potential and performance under adverse conditions, however, the current ecophysiological information explaining variation in RGR is too limited to support this contention quantitatively.

/pdf/inherent-variation-in-growth-rate-between-higher-plants-a-26yywnhi7b.pdf

Inherent Variation in Growth Rate Between Higher Plants: A Search for Physiological Causes and Ecological Consequences

Variation in plant functional traits results from evolutionary and environmental drivers that operate at a variety of different scales, which makes it a challenge to differentiate among them. In this article we describe patterns of functional trait variation and trait correlations within and among habitats in relation to several environmental and trade‐off axes. We then ask whether such patterns reflect natural selection and can be considered plant strategies. In so doing we highlight evidence that demonstrates that (1) patterns of trait variation across resource and environmental gradients (light, water, nutrients, and temperature) probably reflect adaptation, (2) plant trait variation typically involves multiple‐correlated traits that arise because of inevitable trade‐offs among traits and across levels of whole‐plant integration and that must be understood from a whole‐plant perspective, and (3) such adaptation may be globally generalizable for like conditions; i.e., the set of traits (collections of t...

/pdf/the-evolution-of-plant-functional-variation-traits-spectra-1753e9n900.pdf

The evolution of plant functional variation: traits, spectra, and strategies

1 Possible patterns in nutrient resorption efficiency (% of the leaf nutrient pool resorbed) from senescing leaves of perennials were examined at both the intra- and the interspecific level. Most of the data used originated from studies with evergreen and deciduous shrubs and trees. 2 Combining all data, mean nutrient resorption efficiency was 50% for N (n = 287) and 52% for P (n = 226). N resorption efficiency of evergreen shrubs and trees (47%) was significantly lower than in deciduous shrubs and trees (54%), whereas P resorption efficiency did not differ significantly between these growth-forms (51 and 50%, respectively). Although nutrient resorption is an important nutrient conservation mechanism at the species level, it does not differ strongly between growth-forms. 3 Mean N and P concentrations in leaves of deciduous shrubs and trees were about 60% higher than in evergreen species. There were only small differences in mean resorption efficiency and nutrient concentrations in leaf litter of deciduous species were therefore much higher than in evergreens. This implies that, in comparison with deciduous species, the low nutrient concentrations in mature leaves of evergreens contribute far more to nutrient conservation than does nutrient resorption. 4 Relations between leaf nutrient status and leaf nutrient resorption were absent or very weak. Assuming that leaf nutrient status reflects nutrient availability, this implies that nutrient resorption is only weakly controlled by nutrient availability. 5 At the intraspecific level, nutrient resorption was not very responsive to increased nutrient availability. There was no response in 63% of the experiments analysed (covering 60 spp.), whereas in 32% there was a decrease in N resorption in response to increased nutrient availability. For P (37 species analysed) there was no response in 57% of the cases and in 35% of the cases P resorption decreased upon enhanced nutrient supply. Evergreen shrubs and trees showed especially low responsiveness. 6 This review shows that there are no clear nutritional controls on nutrient resorption efficiency. Future research should focus on the biochemical basis of variation in nutrient resorption efficiency and on the factors, other than nutrient availability, that control nutrient resorption efficiency.

https://www.jstor.org/stable/pdfplus/2261481.pdf

Nutrient resorption from senescing leaves of perennials: are there general patterns?

A general model is presented describing ecosystem degradation to help decide when restoration, rehabilitation, or reallocation should be the preferred response. The latter two pathways are suggested when one or more “thresholds of irreversibility” have been crossed in the course of ecosystem degradation, and when “passive” restoration to a presumed predisturbance condition is deemed impossible. The young but burgeoning field of ecological restoration, and the older field of rehabilitation and sustainable range management of arid and semiarid lands (ASAL), are found to have much in common, especially compared with the reallocation of lands, which is often carried out without reference to pre-existing ecosystems. After clarifying some basic terminology, we present 18 vital ecosystem attributes for evaluating stages of degradation and planning experiments in the restoration or rehabilitation of degraded ecosystems. Finally, we offer 10 hypotheses concerning ecological restoration and rehabilitation as they apply to ASAL and perhaps to all terrestrial ecosystems.

Restoration and Rehabilitation of Degraded Ecosystems in Arid and Semi‐Arid Lands. I. A View from the South

The term relative nutrient requirement is introduced in order to measure and to compare the nutrient losses from different perennial plant populations and the amount of nutrient that they need to absorb to compensate these losses. The relative nutrient requirement (L) is defined as the amount of the growth-limiting nutrient that must be taken up to maintain or replace each unit of biomass during a given time interval (e.g., mgN g-1 biomass year-1). It is derived that in a plant community with two competing perennial plant populations, species1 will become dominant if the relative competition coefficient k 12 (sensu De Wit 1960) exceeds the ratio between the relative nutrient requirements of the two species (L 1/L 2), whereas species 2 will become dominant, if k 12 is below this critical ratio. The above-ground litter production was measured inwet heathland communities dominated by Erica tetralix or by Molinia caeruleain order to estimate N and P losses from theaboveground biomass and to calculate the relative N and P requirements of these species. Molinia lost during one year 63% and 34%, respectively, of the amount of N and P present in the above-ground biomass at the end of the growing season. These losses were in Erica 27% and 31%, respectively. The relative N requirements of the two species show the same difference: 7.5 and 2.6 mg N g-1 yr-1, respectively, in Molinia and in Erica. Also the relative P requirement of Molinia is higher as well as that of Erica (0.18 versus 0.08 mg P g-1 yr-1). The relative competition coefficient of Molinia with respect to Erica (k me ) is equal to unity under unfertilized conditions but increases with increasing nutrient supply. Under nutrient-poor conditions k me is below the critical ratio of the relative nutrient requirements of the two species (L m /L e =2.9 or 2.3), so that Erica will be the dominant species. After an increase in nutrient availability k me increases and exceeds this critical limit which results in Molinia replacing Erica. During the last 20 years this replacement of Erica-dominant communities by monocultures of Molinia has been observed in almost all wet heathlands in The Netherlands along with a strong increase in nitrogen availability.

/pdf/a-comparative-study-on-nutrient-cycling-in-wet-heathland-4syc952niw.pdf

A comparative study on nutrient cycling in wet heathland ecosystems : I. Litter production and nutrient losses from the plant.

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A comparative study on nutrient cycling in wet heathland ecosystems : I. Litter production and nutrient losses from the plant.