How do weeds fit into the big picture of ecosystem development?

How do weeds fit into the big picture of ecosystem development?
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    How do weeds fit into the big picture of ecosystem development? David Cooke Biosecurity SA, GPO Box 1671, Adelaide, SA 5001 Ecosystem development means succession - an ordered, directional process of vegetation development. Succession is driven by changes in the physical environment - development of the soil profile, changing hydrology and land forms. But these changes in turn are caused or modified by the action of plants and other living organisms Succession leads to a stabilised ecosystem, a community of plants and their associated organisms in equilibrium with its physical environment. And this climax ecosystem maximises biomass and information content (negentropy) per unit of available energy flow. Plant species occupy many different niches or roles in succession. They become established at various stages of the succession and may disappear at later stages as the structure of the plant community changes. There are various models for classifying the niches that species fill and the types of selection pressure that adapt them to these niches. Plant synecology began a century ago in North America with Frederic Clements who compared the development of a plant community to the sequence of stages in development of an individual organism - both are subject to a cycle of growth, maturity, and decay. He introduced the idea of a single climax community, the final state towards which vegetation succession in a given habitat converges. After a complete or partial disturbance, vegetation tends to grow back toward the mature climax state, which is the vegetation best suited to utilise these conditions. Odum (1969) made some generalisations about the trends that occur as a system develops from pioneer to climax stages and these have stood up pretty well ever since. Take his example of a mixed forest developing on srcinally bare ground. Nett production of organic material by photosynthesis peaks fairly early in succession, in this case while the forest still has several layers of foliage and before the tree canopy has severely reduced the light reaching the ground. It slowly declines almost to zero at the climax stage as respiration of the whole forest ecosystem increases until it almost balances the gross production. The accumulated biomass of the system l evels off more slowly but doesn’t decline because it includes all the organic material stored in trunks, roots and leaf litter. Succession leads to a climax state where maximum biomass is maintained per unit of available energy flow. Theory predicts that stability is maximised in the climax vegetation. The community then has a maximum inertia against gradual changes such as leaching of nutrients and climate cycles.  The ability of the vegetation to capture solar energy increases in the early stages of succession as it deploys more photosynthetic tissue. But the energy input from the incident sunlight is a fixed limit for a given habitat. Later as the volume of living material per unit area increases, the biomass supported by the limited unit energy flow (power) increases. In other words, the energy is being captured and used more fully. Food chains become longer as the number and diversity of species in the system increases. Succession leads to a climax state where maximum biomass and information content (or negentropy) are maintained per unit of available energy flow. Natural selection acts to maximise useful energy flow in a system: energy is taken in as sunlight and converted to chemical energy that is passed along the food chain and used to build structure. This useful energy flow can be quantified as Energy Rate Density, the efficiency of energy flow and use (Chaisson, 2010) This measures how much energy flows through a given mass in a given time, or how efficient is an ecosystem in harnessing energy inputs. Natural selection favours the systems best at obtaining the available energy, or tapping additional energy not already being captured, so enlarging the total energy flow. And these changes produce a more buffered and integrated system that is consequently more stable. The more complex a system becomes with more negative feedback loops between its component species, the more it resists external perturbations like fire, sudden increases in herbivore pressure or climatic variations. With increasing diversity a plant community includes species that can increase in response to grazing, and others that take over after a fire. Nutrient cycles become more closed with limiting nutrients such as phosphorus in heathlands being stored in individual plants and recycled more efficiently when these plants die and decay. In other words a developing plant community takes increasing control of its physical environment, stabilising it against random changes. The  r/K selection theory  was a simple classification or ordination on a single axis of traits that promote success in particular environments. The theory srcinates from MacArthur & Wilson (1967) who had animals in mind, primarily, but the same principle can be applied to plants or any other organisms. . limit K  slope r   % of carrying capacity  The terms r and K   come from the logistic curve of population growth - exponential at first in the log phase, then slowed down by density-dependent factors as the population approaches the carrying capacity of the habitat.  r   is the intrinsic rate of increase, the fastest the population can grow. K   is the carrying capacity or the limit that the environment places on population size. Of course this is a theoretical curve, a mathematical model; in the real world populations always overshoot, fall short, or oscillate around K  . r  -selection is the dominant selection pressure in unstable or unpredictable environments, as the ability to reproduce quickly is crucial, and there isn’t much advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Traits characteristic of r  -selection include: small size; short generation time, implying a simple life cycle and early maturity onset; high fecundity; ability to disperse progeny widely since a habitat may not stay suitable for long. In effect, it’s selection for rapid growth rates of individuals or populations. Organisms whose life history is subject to r-selection are often referred to as r-  strategists e.g. micro-organisms, through insects and annual weeds of cultivation. In stable or predictable environments  K  -selection predominates, as the ability to compete successfully for limited resources is crucial, and populations of K  -selected organisms typically are very constant and close to the maximum that the environment can bear. Characteristic traits favoured by K  -selection include: large size, long generation time and complex life cycles, slow population increase with heavy investment in each of fewer progeny; less adaptation for dispersal, needing to stay in one place where the habitat remains suitable. K  -selection might be called adaptation for more precise feedback control between a population and its environment. Dividing introduced or invasive species into r  -strategists and K  -strategists separates the obvious fast invaders from other introduced plants with slower lifecycles that may have greater impacts in the long run. It also gives some hints about how ecosystems develop through succession. From pioneer communities that are all r-selected, toward climax communities of K  -strategists. As the community becomes more complex, K-selected specialists dominate over r-selected generalists, and niches become narrower. Grime (2001) published a theoretical model of vegetation that went a long way further. This was a triangular ordination of a plant's ’ adaptation to three factors - disturbance, competition, and stress. On this model, succession progresses from species with a ruderal strategy to exploit disturbed sites, through stages characterised by competition in denser vegetation, to a climax community in equilibrium with the limiting stresses of the environment.  This can be useful in providing a more detailed analysis of the roles that various weeds can take in wild or cultivated vegetation systems. Taking a simplified view, plant species may be shaped mainly by just one of the three selection pressures: 1 Ruderals  are plants that occupy empty space, ground bared by disturbance, which is defined as any process that limits biomass by causing its destruction. Apart from soil movements like ploughing, fire, flood, mowing and grazing are also disturbances. Ruderals have a short life cycle; as pioneers with a precarious hold on their habitat, they have to start producing seeds early in their life cycle and continue while they live. They respond to stress by shortened life cycles - think of tiny annuals flowering in droughted paddocks. So they rapidly accumulate biomass, which they invest in seeds or other propagules rather than storing in underground tubers or rhizomes, and form seed banks in soil They have little investment in defence from herbivores (poisons, spines) or competition (perennation) because they have the space to themselves. They are often free-seeding, largish annual herbs like capeweed (Arctotheca calendula) , rampion mignonette ( Reseda phyteuma  ), turnip weeds ( Brassica   spp.), oats ( Avena   spp.). They evolved in natural habitats that were frequently disturbed like river floodplains and beaches, and they became the classic weeds of cultivation and bare ground - our annual field crops and their associated annual weeds both evolved from ruderals after humans invented agriculture. They have high invasiveness but not necessarily high impact as they may not persist for long. Onion weed ( Asphodelus fistulosus  ) and African daisy ( Senecio pterophorus  ) are spectacular colonists of cleared or denuded land but give way to more competitive perennials when these get established. 2 Competitors are adapted to hold their own in denser vegetation, by competing in growth to maximise their share of the energy that the system receives as sunlight. Competition is the tendency for neighbouring plants to compete for the same resources - quanta of light, ion of nutrient, molecule of water, volume of space. This becomes important in habitats where stress and disturbance are both low. Competitors grow in the denser vegetation that comes after the pioneer phase, so they must have high rates of growth, but are not necessarily short-lived. David Ashton, my ecology lecturer at Melbourne Uni, used to say that plant synecology might at a pinch be reduced to competition for the number one resource of incident light. That’s surely true at least for the wet sclerophyll and temperate rainforests of Victoria where he worked. Competitors have high rates of biomass accumulation and turnover, and incidentally, a high response to added fertilizer. They invest in producing a dense canopy of foliage rather than in high seed production.  Typical life forms include robust perennial herbs like, herbaceous climbers like bridal creeper ( Asparagus asparagoides  ), and fast-growing, shade-intolerant trees and shrubs like box elder ( Acer negundo  ), desert ash ( Fraxinus angustifolia  ) and tree-of-heaven ( Ailanthus altissima  ). Many problem weeds belong here, the ones that may take longer to colonise an area but are difficult to control once they’re there. Weeds with lower invasiveness but higher impacts, such as the deep-rooted perennials. 3 Stress-tolerators  Stress is defined as any external constraint limiting the rate of biomass production, and stress-tolerators have strategies to occupy niches defined by special limitations - low light intensity, low moisture, low nutrient levels. They are typically plants with a long life cycle and low investment in seed production which may only occur in favourable years. They invest heavily in defence against herbivores (by poisons, spines, or unpalatability) and environmental stresses (by sclerophylly and succulence, adaptations) with slow biomass accumulation and growth rates. Typical forms are large succulents like cacti; sclerophyll shrubs as dominate the heathlands; shade-dwelling perennials and trees as found in the understorey of rainforests; and large geophytes like Amaryllis belladonna  . Not many extreme stress-tolerators are recognised as weeds yet, but they include some slow invaders that may be problems in the future. Grime’s model is robust because it is an application to plant ecology of a much more general principle. The triangle is a useful model because it is n’t a static classification, but a diagram of a process. He showed that as succession occurs on a low nutrient site, the vegetation developing from a pioneer phase to the climax state, ruderals are replaced by stress tolerators. On sites with higher nutrient levels, there is a higher trajectory across the diagram with more plants showing competitor traits in the mid part of the succession.
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