Plant communities of the world
Plants define the biological communities of the world. Within one biome, communities vary greatly and different communities are found in swamps, on mountains and on particular soils. Within one biome, species richness and composition differs greatly and oceanic islands are frequently poor in species. People have modified many habitats, particularly through removal of trees.
When colonizing bare ground there is a primary succession of plants from pioneers through one or more transitional stages to a climax community. Secondary succession of similar nature happens within more mature communities after disturbance. Succession is often not orderly, can be deflected and end points may differ. Within all communities plants differ in their ability to colonize and pioneer plants are a feature of all plant communities.
Dominance and diversity
Some communities are dominated by one or a few plant species, something expected on the principle of competitive exclusion, particularly for plants of similar life form, but many are not. Dominance occurs in high nutrient conditions where competition may be intense and on certain soils, but many communities have numerous coexisting species.
Diversity can be divided into a diversity, species living in the same habitat, βdiversity, species occupying separate habitats and g diversity,species occupying separate geographical areas. β diversity can be explained easily although habitat distinctions may be small and subtle; g diversity varies from region to region depending on the topography and history of the land. a diversity is much harder to explain within one life form. The critical stage of a plant’s life cycle is the stage from germination to establishment, often not seen in a mature community, but the conditions at this stage may affect which species establish. Pioneer species may persist for decades. Herbivores and diseases can kill mainly the young stages, preventing dominance. The climate fluctuates and plants are migrating continuously, leaving many communities in a state of permanent transition.
In some places, particularly where modified by people, communities are well defined and have been classified. In others there is continuous variation. Classification is mainly useful within a small well-defined region.
Plants define the world’s natural communities, providing the habitat for all its other inhabitants as well as providing the great majority of the biomass. A general picture of the distribution of biomes is given but there are many different plant communities within each biome. For instance, across the northern hemisphere in the boreal and temperate zones there is a patchwork of deciduous broad-leaved and coniferous forest, often with a single species dominant in any one place, and with more broad-leaved forest as one moves south. In the Mediterranean climate regions of the world (in both hemispheres), there is a rich mixture of, mainly evergreen, tree and shrub communities extending to grasslands in the center of the main continents where the climate is drier at similar latitudes. In the seasonal subtropics and tropics there is a range of savannah communities dominated by grasses with increasing numbers of trees as the climate gets wetter.
Within all these places there are patches of other communities and specialist communities in swamps, on mountains, near the sea and in response to particular soil conditions. Different continents differ in their plant communities, and species richness can vary greatly between communities within one biome. Oceanic islands are usually much poorer in species than continental areas. People have modified many communities and, in particular, removed trees to replace them with grassland or other open communities for pasture and agricultural communities and dwellings.
One of the best known ecological theories is that of succession which is seen to occur in places such as sand dunes and marshes, on abandoned fields and by retreating glaciers. Certain fast-growing pioneer plant species with good seed dispersal colonize when the new habitat is first available. These species modify the conditions both in the soil and in the biotic environment that allows other species to colonize. There may be a period of open herbaceous vegetation followed by shrubs or pioneer trees, eventually in many places leading to a community dominated by trees as the climaxvegetation. The pioneer species are usually outcompeted by later colonizers, although some communities are halted at one point in this succession, e.g. by grazing animals. This description applies to a primary succession but, in a gap in a mature community, there is secondary succession involving new colonization by pioneer species. This is followed by changes in the plant community, though with less change in the soil conditions. This classic view of succession is simplistic, in that very often the climax community species are present early on, particularly in secondary succession. There is a large role of chance in how the succession progresses, and the end point may not always be the same or there may not be an end point as such. It does provide a clear illustration of some of the differences between plants in their ecology and an insight into the formation of plant communities.
Plants differ in their abilities to colonize and in their speed of growth and response to light and nutrients. There are plants which are specialist colonizers and do not persist in any one place but move around in a community. Secondary succession is happening continuously as trees or other dominant plants die or are blown over, or herbivorous mammals graze or uproot certain plants, creating gaps. In any gap some form of succession can take place and this will take a different form depending on the size and nature of the gap and the presence of any animals. There are pioneer species in all plant communities colonizing these gaps and any ‘climax’ plant community is in a dynamic state.
Some plant communities are dominated by a single species, some by a small number and some appear to have no dominant species at all. The principle of competitive exclusion, developed in relation to microorganisms, states that if two species occupy the same ecological niche, defined as the total of all a species’ requirements, one will outcompete the other; if they coexist some aspect of their lives, i.e. their niches, must differ. The definition of a plant’s niche is not well worked out, but the life forms of plants will, clearly, allow many species to coexist by occupying different parts of a community and, within each life form, there are many different requirements, e.g. for lightdemanding and shade-tolerant trees. Despite this, by applying the competitive exclusion principle, one would expect most plant communities to have a dominant species within each major life form present, but this is only true under certain limited conditions. It is true for some communities on soils rich in nutrients, particularly in temperate zones. Although many plants thrive best under high nutrient conditions, competition is intense and one species may become dominant through faster growth or stronger competitive ability. Dominant plant species also occur in places subjected to an environmental stress, such as waterlogging or the presence of a normally toxic element. In most places, however, there is a diversity of species within each life form, this diversity increasing towards the tropics until, in many tropical rainforests, there are numerous species all apparently with similar requirements growing in the same plant community.
Diversity can be divided into three categories: a diversity, the diversity within one habitat or microhabitat; β diversity, that between habitats; and gdiversity, that between different geographical areas within one overall area, e.g. two mountains within one range. β diversity may include differences in subtle features such as particular nutrient rich patches in a poor soil, e.g. in an animal dung heap, or a small change in gradient, but it is easily comprehensible and, given sensitive equipment, straightforward to explain. g diversity varies greatly, with some areas such as the Cape region of South Africa having a particularly high g diversity and many species with a narrowly restricted distribution, probably owing to a combination of its varied topography and history. Other places, such as northern Europe, have low g diversity.
a diversity is much harder to explain, beyond the differences in life form already mentioned. It probably arises from a combination of many factors. One of the most significant details is that, in many species, we rarely see the most vulnerable stage in a plant’s life cycle, the period between germination and establishment. Plants can live for many years, some many centuries, so the existence of a group of individuals of one species may reflect conditions that applied several decades or centuries earlier. Communities are periodically disturbed, e.g. by freak weather conditions or by large animals, and germination conditions for any one species may only occur infrequently. Gap formation in any plant community is significant for the germination of most plants and which species establishes will depend on the size of the gap and how frequently gaps are formed as well as precise soil conditions. There must also be seeds of that species in position when favorable conditions appear, so chance must play a part. Plants are vulnerable to attack from herbivores, particularly insects, and diseases and pathogens such as fungi (Topics M3 and M4). This is well known in crop monocultures and will affect mostly abundant or dominant plants, and mostly at the young stages. Attack is likely to be periodic and may not be seenin a limited study period, but it will reduce the dominance by any one species.
A plant community will always be subject to change. The climate has changed markedly over the last million years with periodic glaciations of greater or lesser extent and more minor climatic fluctuations in any one area over a shorter time scale. These have given rise not just to cool and warm periods but, in the tropics, drier periods during glacial times and the rising and falling of sea level and, over a time scale of centuries, dry or wet periods and cool or warm ones. Plants have migrated north and south and across tropical land masses in response to the large changes and with smaller changes in community boundaries and local invasions and extinctions with the smaller changes. If we consider this with the long generation time of many of the dominant plants in a community, it suggests that many communities may be in a state of continuous succession, with the persistence of pioneer species for decades or centuries and the whole community not reaching any equilibrium.
All of the above factors probably affect the structure of a plant community to a greater or lesser extent and contribute to an explanation of the adiversity. In the most diverse environments in the world such as some tropical rainforests, there is high a, β and gdiversity.
Certain plants tend to grow together and form a community and these communities can be well defined, with clear boundaries in places. This is mainly true where people have modified the vegetation or where there are abrupt changes in soil conditions, e.g. at the edge of a woodland and pasture, or where limestone intrudes among acidic rocks. This has led to a classification of plant communities, particularly in Europe where these transitions are most clear. In many places there are gradual transitions from one community to another. These are best regarded as collections of species that happen to be able to exist under certain conditions but have different limits; the community as a whole is not a precise entity and its composition is changing continuously across its extent. All communities vary like this and classification is most useful within one limited geographical region. In most parts of the world the communities have not been classified except in broad terms.