Plant Ecology

Learn about Plant Communities

There are two main types of plant communities. Open plant communities consist of open canopies with sparse vegetation and usually one or two canopy layers. Closed communities include more dense vegetation with a closed canopy. Closed communities may consist of rain forest plants and include larger leafed species.

In contrast with closed communities, open communities develop where there is shortage of resources, like African savannas, North American prairies and Asian steppes, all suffering from shortage of moisture for at least several months a year, or open forests on rocky grounds lacking enough soil to grow on.

AIMS

• Define the term ecosystem
• Explain the importance of plants as energy producers within ecosystems
• Explain basic ecological principles
• Define the terms open and closed plant communities, semi-natural vegetation, dominant species, climax association.
• Describe the effects of plant association and competition on the succession of plants
• Describe how plant communities respond to environmental stresses.
• Explain how the development, structure and function of an organism depends on the interaction of that organism with its environment
• Describe the effects of a range of abiotic environmental factors on plant growth and development
• Explain the importance of monitoring abiotic environmental factors
• Describe plant modifications to withstand extreme environmental conditions
• Describe the weather and climate in a particular region.
• Relate plant distribution, growth and natural selection to soil, geography, weather and climate.
• State how soil, geography, weather and climate affect the horticulturist’s selection of plants for any specific growing location.
• Evaluate the use of meteorological records in relation to plant growth and development
• Define the terms xerophyte, hydrophyte and halophyte
• Describe the structure and function of xerophytes, hydrophytes and halophytes
• Describe how xerophytes, hydrophytes and halophytes can be utilised in garden or landscape situations
• Describe the significance of xeromorphy in temperate zone plants and its importance in the garden or landscape situation.
• Evaluate the methods by which environmental conditions can be manipulated to improve the growth and development of plants
• State the factors affecting the choice of plants for garden or landscape sites with extreme conditions
• Assess the value of using protective structures to grow plants
• Describe the sources and nature of pollutants and possible effects on plants
• Describe how the environment may be affected by a range of horticultural practices
• Explain how planning, environmental assessment and impact analysis may contribute to the conservation process
• State the major sources of grant aide available to support environmental conservation on horticultural sites
• Review the role of national and international organisations in the conservation of plants and gardens.

Lesson Structure

There are 8 lessons in this course:

1. Introductory Ecology
   • Definitions for ecology, ecosystems
   • Constituents of an ecosystem
   • Ecological concepts
   • Interrelationships between climate, soil and living things (consumers, decomposers)
   • The food web
   • Habitat and niche
   • Biomes
   • Terminology
2. Plant Communities
   • Open and closed plant communities
   • Habitat types
   • Location and characteristics of biomes
   • Semi natural vegetation
   • Competition
   • Succession of plant communities
   • Community stability and equilibrium
   • Environmental stress
   • Edge effects
   • Terminology
3. Plants and their Environment
   • Development, structure and function
   • Plant modifications: functional adaptions
   • Environmental factors: light, temperature, fires, wind
   • Monitoring abiotic factors
   • Introduction to Environmental assessment
   • Pre purchase inspection of a site
   • Background data
   • Flora and fauna surveys
   • Open space management plans
   • Compliance with licencing conditions
   • Detection of pollutants
   • Use of plants
   • Remediation of a polluted site
4. Plants, Soils and Climate
   • Natural conditions and plants distribution
   • Climate classification
   • Examples: climate in the UK, climate in Australia
   • Meteorological data
   • Plant distribution
   • Geographic location
   • Rainfall
   • Evaporation
   • Effective rainfall
   • Circulation features
   • The walker circulation
   • Southern oscillation
   • El nino
   • La Nina
   • GAIA theory
   • Carbon dioxide cycle
   • Wind descriptions
   • Soil problems
   • Erosion
   • Salinity
   • Soil structure decline and soil compaction
   • Soil acidification
   • Build up of dangerous chemicals
5. Plant Adaptations to Extreme Environments
   • Ecological groups of plants: hydrophyte, xerophyte, mesophyte, halophyte
   • Xeromorphy
   • Common environmental problems when growing plants: foliage burn, pollution, lack of water, frost, shade, humidity, temperature, wind, etc
   • Desert landscapes
   • Xeriscapes
   • Coastal gardens
   • Water plant environments
   • Greenhouse plants
6. Manipulating Plant Environments
   • Controlling environmental conditions
   • Tolerance levels for different plants
   • Matching plants with their environment
   • Managing light
   • Managing water
   • Protective structures
   • Windbreaks
   • Tree guards
7. Environmental Conservation
   • Water pollution
   • Soil pollution
   • Atmospheric pollution
   • Effects of horticulture
   • Pesticides
   • Fertilisers
   • Deforestation problems
   • Loss of agricultural land
   • Loss of biodiversity
   • Environmental weeds
   • The greenhouse effect
   • Other environmental problems affecting plant communities
   • Greenhouse gases
   • Ozone depletion
   • Introduction to recycling
8. Environmental Organisations, Assessment and Funding
   • Plant conservation
   • Conservation of individual species
   • Conservation organisations
   • Conservation funding

Why Study Plant Ecology?

Competition
Competition occurs both within a species and among species.  When a shared resource is scarce, then organisms will compete for these resources.  In competition, one species interferes with another species enough to keep the second species from gaining access to the resource. Therefore, those that are more successful will survive.  It is possible that within some plant populations, the resources will be shared among individuals so that none of them obtain sufficient quantities to survive as adults, or to be able to reproduce.

Unlike animals, which ingest food, green plants are dependent on photosynthesis to obtain their energy. Competition in plants is therefore observable in terms of a struggle for light. Those plants with C4 and CAM photosynthesis are strong competitors in a limiting environment. It enables them to dominate climatic areas where other plants would have perished or been competitively excluded. Plants are considered to have CAM if their photosynthetic cells have the ability to fix CO2 in the dark via the activity of PEP carboxylase. C4 plants evolved primarily in the tropics and they especially adapted to high light intensities, high temperatures, and dryness. The optimal temperature range for C4 photosynthesis is high, and C4 plants flourish at temperatures that would be lethal to many other species. Also, they use CO2 efficiently with smaller stomatal openings and less water loss. There are many advantages.

Individual plants claim and hold on to a site until they lose vigour or die.  These organisms prevent other individuals from surviving by controlling light, moisture and nutrients in their immediate vicinities. A perfect example is that of an emergent in the rainforest canopy. It has successfully competed for its place in the rainforest, which was offered by a falling tree some time before.

Competition between members of different species results in the division of resources in a community.  Certain plants have roots that grow to different depths in the soil.  Some plants have shallow roots that permit them to use moisture and nutrients near the ground surface.  Other plants growing in the same place have deep roots enabling them to exploit deeper moisture and nutrients unavailable to the short rooted plants.

Parasitism
Closely related to predation is parasitism.  This is where two organisms live together, one drawing its nourishment at the expense of the other.  Parasites are much smaller than their hosts, and they include many viruses and bacteria.

Parasite is taken from the Greek word "parasitos" which, loosely translated, means one who eats at another's table. Parasitism is a dependency relationship and as a result, killing their hosts in the manner of predators is not normal for parasites.  Because of this the hosts and the parasites coexist with a mutual tolerance.  However, parasites may regulate some host populations, by modifying their behaviour and lowering their reproductive success.

There are many examples of parasites, but just a few are mentioned below:

• Mosquitoes, taking blood from animal bodies
  
• Fungus such as lichen, which feed from trees
  
• Bacteria that lives in an animal’s digestive track

Receiving their food in an assimilated form from the tissues of their hosts, they do not have to digest it in any complete manner, and consequently the alimentary canal tends towards degeneration or total disappearance.  Other things, such as organs of locomotion can also degenerate.  For instance, fleas once had wings, but no longer needing them, they have degenerated.

Coevolution
This is the joint evolution of two unrelated species that have a close ecological relationship.  By that it is meant that the evolution of one species depends partly on the evolution of the other. Coevolution is also involved in predator-prey relationships.

As predators evolve more efficient methods of capturing or consuming prey, the prey evolves more ways to escape predation.  Plants have acquired such defensive mechanisms as:

• Thorns
• Spines
  
• Hard Seed Coatings
• Poisonous ill-tasting Sap

However, some herbivores can breach these defences and attack the plant.
Some organisms may have a colour, pattern or shape that imitates an unpalatable species.  For instance, stick insects are fawny or green in colour and resemble tree twigs.  This allows them protection from their predators, by being difficult to discern from surrounding foliage.  Obnoxious odours or emitting poisons is also a defence mechanism.  This is a common trait in many snakes and spiders in Australia.

Mutualism is another coevolutionary relationship, in which two or more species depend upon one another and cannot live outside such an association.  For instance, fig trees in eastern Australia are pollinated by fig wasps.  The tree cannot survive without cross-pollination, so it provides a source of food and protection for the wasp, to guarantee that the wasp will reside there, and pollinate the trees.

Mutualism
This is a biological interaction in which the growth, survival, and/or reproduction of both interacting species are enhanced. In most situations neither partner can survive without the other. The relationship between legumes and the nitrogen-fixing bacteria that live in nodules on the legume roots is a good example of mutualism. In the symbiotic association between rhizobia bacteria and legumes, the bacteria provide the plant with a form of nitrogen that it can use to make protein, while the plant provides the bacteria with both an energy source and a highly regulated oxygen environment. This nitrogen fixation is carried out by only certain bacteria, which reduces dinitrogen (N2) to ammonium (NH4+). All living organisms are dependent on this process.

An outstanding example are lichens. Lichens are mutualistic symbiotic associations between fungi (ascomycetes) and certain genera of green algae. The photosynthetic members of these associations (the algae) provide the carbon compounds for both partners, and they are protected from environmental extremes by their fungal partners, which pass on to them mineral nutrients that reach the fungi from the external environment. Because of this relationship lichens are able to exist in some of the harshest habitats on earth.

Another type of mutualism and one of the most interesting is that between fungi and plants (mycorrhizae). The fungi mycorrhizae, which are at the roots of vascular plants, play a vital role in the absorption of phosphorus and other essential nutrients. Without this fungi the normal growth of plants would be impossible.

Plant-herbivore and pathogen interactions
The importance of plant-herbivore and plant-pathogen interactions in determining the structure of natural communities is not always obvious. The effects of herbivores on plants are profound, in the short and the long-term. Herbivores control such things as the reproductive potential of plants by destroying the photosynthetic surfaces, their food-storage organs, or their reproductive structures. These interactions have led to the evolution by plants of a wide variety of chemical defences. This ability to produce toxic chemicals gives plants a huge competitive advantage. This advantage is similar to the thorns or tough, leathery leaves, which protect plants from grazing. Some protective chemicals that plants produce may display other features, other than begin distasteful, that deter herbivores. Pyrethrum is a natural insecticide produced from a species of Chrysanthemum. Chromenes can interfere with insect juvenile hormone and act as true insecticides.

The secondary plant products ingested by herbivores may, in turn, play a role in the animals’ ecological relationships with other animals. Some insects store these poisons within their tissues and are protected from their predators. Some sex attractants in insects are derived from the plants on which they feed.

Relationships within a community are incredibly complex. Organisms that coexist within a community often have evolved together and affect one another in a huge variety of ways.