Competition and Coexistence. Competition for Resources

Without impediments, even the slowest growing, poorest reproducing plant species would be able to cover the inhabitable surface of the planet within only a few dozen generations. This Malthusian law of exponential growth led Darwin (1859) to understand competition as one of the main selective forces behind natural selection. Of all the seeds that fall onto a given area, only a few can become established, and only a species that is able to win this struggle for space is part of the next generation (i.e. has fitness >0). Thus, conceptually, the link between competition and evolution is a very close one. As a consequence, competition has existed as a research topic in plant ecology for at least a century, with a wide variety of approaches, models and, regrettably, misunderstandings. Competition not only affects individual populations but also community structure and ecosystem-level processes (Chap. 13).

Concepts and Definitions of Competition.Competition, in the widest possible definition of the term, represents a situation where the winner (read: competitively superior, dominant species or individual) is picked from a set of candidates (contest competition). Superiority is not sufficient for survival, however: many individuals may compete without any of them “winning” enough to make a living (scramble competition). In plant competition, the establishing individual may be picked independently of the species (see the lottery model below) or its success may be correlated with some trait such as shade tolerance or seed mass. Competition, ubiquitous as we assume it to be in plant communities, is difficult to detect “in action”, requiring tracer experiments and manipulation of resources. And even failure to detect competition may well be due to generations of intense competition having removed unsuccessful species from the species pool (“the ghost of competition past”: Connell 1980).

Often competition changes qualitatively during the lifetime of a plant. It starts with competition for a germination site (called the “regeneration niche” by Grubb 1977), and it may then turn into competition for water or nitrogen, to possibly end in a competition for light (as in forest trees). Competition for light, nutrients and water is called resource competition; it has attracted the bulk of scientific interest. Allelopathy, discussed earlier, is sometimes referred to as “interference competition” to highlight the active initiative of a plant in preventing other plants exploiting the resources in its immediate vicinity. While the production of allelochemicals is common, our understanding is that scramble or diffuse competition is the more common mode of interaction. Scrambling refers to the disorderly uptake of resources by all plant individuals (of possibly several species).

Since any resource unit used by one plant is no longer available to another plant (which is why it is sometimes referred to as “pre-emptive competition”), and since plants acquiring more resources can grow more and hence continue foraging for resources, scrambling often leads to a run-away process, resulting in the dominance of a small number of individual plants. This process becomes particularly prominent for light, where competition is strongly asymmetric: taller trees shade smaller ones (preempting the struggle for light), but not the other way around. The taller trees can thus grow even more and will hence shade out the smaller trees in forests even more. In the case of Fagus syl- vatica (European beech, Fagaceae, but also for Nothofagus spp., southern beeches), this can lead to a complete suppression of all other tree species in its understorey (although differential sensitivity to deer browsing may contribute to this outcome; Sect. 19.2). Competition for nutrients, for example, nitrogen and water, is much less asymmetric, allowing in particular small individuals to snatch up some resources.

Competition for Resources.The most obvious resource plants compete for are nutrients (N, P, cations), water and light. The reason these resources are the target of competition is that they become limiting under some circumstances (see Liebig’s law of minimum, Chap. 11). Only when plants deplete the local resource pool does competition come into play. If nitrogen is still available in the soil but shading limits plant growth (say in forest understorey), then competition for nitrogen is ecologically irrelevant.

Across all biomes, nitrogen is the de facto most commonly limiting resource (Vitousek and Howarth 1991). This can be explained by its virtual absence in bedrock, its erratic non-biotic production through volcanic eruptions and lightning, and its high abundance in plants and animals. Before the invention of chemical nitrogen fertilisers in the early twentieth century (winning Haber and Bosch the 1918 Nobel Prize in Chemistry), all nitrogen in plants and animals had been fixed by bacteria (free-living marine and terrestrial cyanobacteria, endo-symbiontic protobacteria (Rhizobia in legumes) or actinobacteria (e.g. Frankia in alder; see also Sect. 7.4 in Chap. 7 and Sect. 16.3 in Chap. 16 for more details on nitrogen fixation). Today, industrial N fixation is about twice that of all natural N fixation (both biogenic and lightning). It is unsurprising that this huge increase in nitrogen availability had substantial effects on plant communities (Sect. 19.4).

The obvious agricultural implication of nitrogen limitation and the “green revolution” of fertilisation have blinded plant ecologists to the probably far more ubiquitous and observable limiting resource for plants: light. Trees are the result of the struggle for adapting to taller competitors, leading to a run-away process of investing a huge proportion of assimilates into structural support (stem, roots and branches), which delays reproduction and increases susceptibility to damage and pests. But forests are not the only light- competition systems; even grassland systems, less than 1 m tall, can be light-limited, as has been shown experimentally (Sect. 20.3 in Chap. 20). Surprisingly, even for aquatic systems, where algal blooms reduce light penetration further down the water column, only recently has a theory of light competition been properly developed (Huisman and Weissing 1995). The key feature of light competition, and its biggest difference to competition for nutrients or water, is the asymmetry mentioned earlier. The consequence is a strong evolutionary pressure towards tallness in light- limiting systems or an adaptation to low-light conditions (exhibited by many light-sensitive forest understorey plants such as Paris quadrifolia: herb-paris, or the seedling stages of dominant tree species such as European beech or sycamore maple, or indeed many tropical tree species).

The other common limiting resources (such as phosphorus or water) are covered in Chaps. 10 and 11. They are also common subjects of competition but provide no known qualitative difference to nitrogen in terms of the way competition is carried out. Nitrogen (because of the tremendously increased availability) and light (because of its asymmetry) cover most situations of competition.

Much less investigated is the role of “negative resources”, that is, substances or settings that impair plant growth and that plants may thus compete for to avoid. While soil water, which transports soluble nutrients and allows their tran- spirational transport up into plant leaves (Chap. 10), is imperative for growth, too much or too little soil water is detrimental to many plant species. Avoiding water-logged soils (on which only a minority of plants are actually able to grow, while many more can briefly tolerate it) or soil desiccation (which stimulates root abscission and impairs photosynthesis by inducing stomata closure) actually means that plants are now competing for positions along gradients where environmental conditions are favourable (“mesic”) in all aspects. On dry, wet, polluted, nutrient-poor, cold, dark sites, any adaptation to adversity will immediately also give competitive advantage with respect to all other resources since only a few species will be able to survive. For example, light competition on lead-/arsenic-rich soils is virtually absent, since few species have adapted to such conditions, none of them a shrub or tree able to outshade others. Competition for herbivore- or pathogen-free space will be discussed in what follows (Sect. 19.4).