Disturbances can be defined as natural or land-use-related events that remove biomass or individuals. The environment can be considered to act as a filter in the prevention of the establishment of species lacking a specific combination of traits. The species that ultimately comprise the community are those that survive these filter effects (Keddy 1992; Duckworth and al. 2000; Lavorel and al. 2007). This arises simply as a result of the non-random demographic processes of dispersal, growth, survival, and reproduction of individuals within and between species, as determined by the functional traits they possess, where the interspecies variations of these functional traits are greater than their intraspecies variations. Species sorting by the environment is a process of selection over short (ecological) time scales, which results in changes in species composition over an environmental gradient (Keddy 1992; Diaz and al. 1998; Shipley and al 2006b; Lavorel and al. 2007).
The rules of community assembly provide a means to predict future states of communities. Functional classification often has two relatively distinct goals, one of which is to investigate the effects of species on ecosystem properties (functional effect groups), and another of which is to investigate the responses of species to changes in the environment, such as disturbance, resource availability and climate (functional response groups) (Figure 2). Most studies have focused on functional effect groups, rather than using groupings based on species responses. However, a merging of these two perspectives is needed to better understand the effects of biodiversity on the properties of an ecosystem (Keddy 1992; Hooper and al. 2002).
Early syntheses of changes in species traits along nutrient gradients recognized that species from nutrient-rich habitats tend to be inherently fast growing. Rapid resource capture and fast turn-over of organs leads to the poor internal conservation of resources, while the reverse is true for nutrient-poor habitats. Soil disturbance, therefore, favours plants with a suite of traits that goes beyond the ruderal syndrome, including a prostrate stature with either the stoloniferous architecture in perennial grasses or the flat rosettes in forbs, and a high fecundity and a small seed pool. Nutrient and/or water limitation tends to select for a conservative competitor strategy, with leaf traits promoting resource conservation, such as low specific leaf area, high tissue density, long life-span, and low nutritive value. As a consequence of these primary traits, the predominant plants in these environments will compete with their neighbours by sequestering nutrients, possibly with the help of mycorrhizal associations, and by accumulating poorly degradable litter, and sometimes through allelopathy or nutrient immobilization (Westoby 1998; Westoby and al. 2002; Lavorel and al. 2007).
Under intense anthropogenic pressure, patterns of biodiversity become increasingly variable. There are three main factors that limit species richness: harshness of the environment, competitive exclusion, and species-pool limitation. Species loss from ecosystems is usually caused by increasing effects of these three factors. The current experimental evidence indicates that the least productive species are those that have recently been excluded from temperate European grasslands. This also means a reduction in the available diaspore pool on a landscape scale, and could result in increased species-pool limitation in other communities (Leps 2004). As productivity is often a non-linear, concave function of the number of species or functional groups, an increase in spatial variability of biodiversity can cause dramatic decreases in the mean productivity of the ecosystems. Thus the impact of the loss of biodiversity on productivity may be larger than current estimates indicate (Benedetti-Cecchi 2005). In addition, the strengths of interactions among plants are expected to co-vary with other key traits along environmental gradients (Lavorel and al. 2007), supporting an expected increase in the importance of mycorrhizal interactions as a result of the increased importance of the trade-offs between costs and benefits between both partners in extremely disturbed sites. Current results support the hypothesis of mycorrhizal associations as a plant strategy for exploitation of nutrient resources and resistance to stress, with its most intense development in periods of increased plant demands (Read 1991; Regvar and al. 2006; Vogel-Mikus and Regvar 2006; Pongrac and al. 2007).
Was this article helpful?