Course 3:
The
Terrestrial Ecosystem
Course Summary
- Biomes
-
Drylands
- Introduction to Ecology
- Plant Succession
- Biogeography
- Population Growth and Competition
- Predation
- Nutrient Cycling
- Biodiversity
- Watershed Change
Section 1: Biomes
Earth is divided into different geographic areas called biomes that have similar climate conditions and ecosystems. The geographical distribution (and productivity) of the various biomes is controlled primarily by the climatic variables of precipitation and temperature even though different places on Earth tend to have similar plant communities. There are 8 major biomes on the Earth:
Distribution
of the Earth's Eight Major Terrestrial Biomes.
(Adapted from: H.J. de Blij
and P.O. Miller. 1996. Physical
Geography of the Global Environment. John Wiley,
This section in course 3 discusses in detail the characteristics, climate, flora and fauna of each biome.
Section 2: Drylands
Drylands encompass areas where available soil water is limited because of low precipitation and high evaporation. These areas do not provide enough year-round distribution of precipitation to sustain the amount of water resources that will meet the society's livelihood. These are the geographical regions climatologically described as hyper-arid, arid, semi-arid, or dry sub-humid.
In dryland systems, primary productivity and nutrient cycling are constrained by water limitation. On a long-term basis in these systems, natural moisture inputs (i.e. precipitation) are exceeded by moisture losses through evaporation from surfaces and transpiration by plants (i.e. evapotranspiration). This potential water deficit affects both natural and managed ecosystems where primary productivity of crops, forage and other plants is equally constrained, with repercussions for livestock and humans.
The condition of drylands can be significantly improved by better and integrated management of water and natural resources. Any methods of managing drylands resources must be viewed in the broader socioeconomic context and opportunities must be provided for local communities to explore viable, alternative livelihoods while maintaining their own cultural and societal fabric.
There are many ways to manage scarce water resources and these methods form the underpinnings of overall resource management in drylands. Some approaches include water harvesting techniques, water storage and conservation measures, safe re-use of treated wastewater for irrigation, afforestation for arresting soil erosion, improving ground water recharge, deficit irrigation, intensifying agriculture using novel technologies that do not increase pressure on dryland water, and soil provision services.
Alternative livelihoods introduce management options that do not follow traditional rangeland management or cultivated systems. These livelihoods are still dependent on the condition of drylands services and can include drylands aquaculture, production of novel products, and tourism-related activities.
This section in course 3 elaborates on what drylands are, the problems associated with this biome, anthropogenic impact on the drylands, climate change and desertification issues, and how innovative IWRM techniques can help better manage this biome area.
Section 3: Introduction to
Ecology
This section introduces the origin of life, the concept of biotic and abiotic species, and how everything fits in the ecosystem. An ecosystem can be defined as “a dynamic entity composed of a biological community and its associated abiotic environment.” Often the dynamic interactions that occur within an ecosystem are numerous and complex. Ecosystems are also always undergoing alterations to their biotic and abiotic components. Some of these alterations begin first with a change in the state of one component of the ecosystem and then, because of relationships, cascades and sometimes amplifies into other components. Other important ecosystem components are soil, atmosphere, radiation from the sun, water, and living organisms.
In recent years, the impact of humans has caused a number of dramatic changes to a variety of ecosystems found on the Earth. Humans use and modify natural ecosystems through agriculture, forestry, recreation, urbanization, and industry.
The most obvious impact of humans on ecosystems is the loss of biodiversity. The number of extinctions caused by human domination of ecosystems has been steadily increasing since the start of the Industrial Revolution. The frequency of species extinctions is correlated to the size of human population on the Earth. This size of the human population is directly related to resource consumption, land-use change, and environmental degradation.
After biodiversity, this section discusses food chains and food webs. A food chain can be grazing food chain or detritus food chain. The grazing food chain is a model that describes the general flow of energy in communities. For most ecosystems the model begins with the photosynthetic fixation of light, carbon dioxide, and water by plant autotrophs (primary producers) that produce sugars and other organic molecules. Once produced, these compounds can be used to create the various types of plant tissues. Primary consumers or herbivores form the second link in the grazing food chain. They gain their energy by consuming primary producers. Secondary consumers or primary carnivores, the third link in the chain, gain their energy by consuming herbivores. Tertiary consumers or secondary carnivores are animals that receive their organic energy by consuming primary carnivores. The illustration below models this process:
The detritus food chain differs from the grazing food chain in several significant ways. First, the organisms found in this chain are, in general, physically smaller. Second, the functional roles of the different organisms do not fall as neatly into categories as does the grazing food chain's trophic levels. Finally, detritivores live in environments (soil, sea bed, etc.) that are rich in scattered food particles. As a result, decomposers are less mobile than herbivores or carnivores.
A model describing the organisms found in a food chain is called a food
web. Food webs describe the complex patterns of energy flow in an
ecosystem by modelling who consumes who. The
illustration below describes a portion of the food web for a typical tidal
marsh ecosystem located on the southern coast of
Typical Tidal Marsh Food Web
Section 4: Plant Succession
The first stage of succession is characterized by the pioneering colonization of annual plant species on bare ground and nutrient poor soils. These annual species have short lifespans (one growing season), rapid maturity, and produce numerous small, easily dispersed seeds. In the first year, the annuals are quickly replaced in dominance by biennial plants and grasses. After about 3 to 4 years, the biennial and grass species give way to perennial herbs and shrubs. These plants live for many years and have the ability to reproduce several times over their lifespans.
The different types of succession are:
-
Primary -
establishment of plants on land that has not been previously vegetated.
-
Secondary
- invasion by plants on land that was previously vegetated.
-
Allogenic
- caused by a change in environmental conditions.
-
Autogenic
- caused by the activities of the plants over time.
-
Progressive
- the community becomes complex and contains more species and biomass over
time.
-
Retrogressive
- the community becomes simplistic and contains fewer species and less
biomass over time.
Section 5: Biogeography
Biogeography is the study of the geographical patterns of plant and animal species. In particular, it looks at species’ spatial and temporal distribution over the surface of the Earth, seeking to understand species diversity at large spatial scales. To study the distributions of plant and animal species across the surface of the earth, a fundamental knowledge of ecology and ecosystem dynamics is required.
As discussed earlier, an ecosystem is comprised of habitats, biological communities, and ecotones. A biome is often referred to as a global-scale community of plants and animals and is the largest subdivision of the biosphere. A biome may contain many different kinds of smaller ecosystems.
Plants and animals disburse throughout the earth and occupy habitats favourable for their survival. A habitat is the specific, physical location of an organism. Each species has specific habitat parameters (temperature, moisture and nutrient availability). Within a habitat, organisms "occupy" a niche.
Habitat occupation may be "limited" by the ability of plants and
animals to disperse throughout the environment. Even though a habitat is
available for occupation, barriers to diffusion may prevent organisms from
inhabiting them. Habitat occupation may depend on several factors. First is the
location of centres of evolution called biogeographical realms. Second is
the fragmentation of previously continuous habitat as explained by the
application of the Island Biogeographic Theory in forecasting faunal changes. For instance, in most of the eastern
Section 6: Population Growth and Competition
Odum (1971) defines population as “a collective group of organisms of the same species (or other groups within which individuals may exchange genetic information) occupying a particular space, that has various characteristics, which, although best expressed as statistical functions, are the unique possession of the group and are not characteristic of the individuals in the group.” Some of these properties are density, natality (birth rate), mortality (death rate), age distribution, biotic potential, dispersion and growth form. Populations also possess genetic characteristics directly related to their ecology: adaptiveness, reproductive (Darwinian) fitness and persistence (i.e. probability of leaving descendents over long periods of time).
We know that populations cannot grow indefinitely (including humans). Resources are limited so a leveling off must occur. This section discusses different population growth theories such an S-shaped population curve called Sigmoid growth curve as well as different theories and models for competition among population/species.
Section 7: Predation
The typical predator is free living and catches, kills and devours individuals of another species for food. The typical parasite, in contrast, lives on or in its host and derives nourishment from it, without killing it. To use an investment analogy, the predator is said to live on “capital”, whereas the parasite lives on “interest.”
Although this concept is generally true as far as the individual aggressors and victims are concerned, it does not necessarily apply to the populations as a whole. Parasites may kill a sufficient number of their hosts so that their stock is greatly reduced. In some cases, parasitic infections reduce the host population to very low numbers, thereby cutting off their own food source and causing their own population numbers crash. This can also happen with the predator-prey interaction.
However, where they have coexisted for long periods of time, the predator population devours only an incremental increase in the prey population annually. In such a situation, the predator population may continue indefinitely to take a limited number of the prey without endangering the breeding stock of the species on which it depends.
Some predators are restricted to one prey species or are dependent upon a
small group of food species, whereas others are more general in their
diet. Many predator species have developed special anatomical and
physiological adaptations for securing, devouring and ingesting specific prey species. Prey species have also developed equally
elaborate specializations to avoid detection or to resist capture.
Generally speaking, predators tend to eat the organisms that are most
available – either because of the organism’s abundance or because of its
accessibility. If a predator has a specific food preference or becomes
conditioned to continue feeding on the same species, serious consequences may
ensue for both the predator and the prey. However, usually, when one type
of food becomes scarce, the predator changes its diet. Minnows (Gambusia) are often stocked in reservoirs because of
their known ability to eat mosquito larvae voraciously. Unfortunately, when the
fish have reduced the larvae to low numbers, they turn to other food and thus
allow the mosquito population to recover.
This section also discusses predator-prey interactions, modeling
interactions, predator control and adaptations, as well as prey adaptations and
selection.
Section 8: Nutrient Cycling
Introduction
The patterns of nutrient cycling in the biosphere involve
not only metabolism by living organisms (biotic), but also a series of strictly
abiotic chemical reactions. Understanding the cycle
of a single element requires the knowledge of a process that depends jointly on
the biology of all organisms that utilize the element, its geological
availability, and its organic and inorganic chemistry. Thus, understanding the
cycling of biologically important elements is truly interdisciplinary in
nature. We generally call this process biogeochemical cycling.
Organic and Inorganic
All biogeochemical cycles for all ecologically significant
elements have both an organic and an inorganic component. Both components are
extremely important! How efficiently the nutrients move through the organic component
back to the inorganic reservoirs determines how much is available to organisms
over the short term. The major reservoirs for all metabolically important
elements are found either in the atmosphere, lithosphere (mainly rock, soil and
other weathered sediments) or hydrosphere. Flow in the inorganic phase
generally tends to be slower than in the organic phase.
This section further discusses natural and anthropogenic
nutrient cycles such as carbon, nitrogen and phosphorus. It also discusses the
impact of natural changes as well anthropogenic changes, including the impact
of eutrophication, on both terrestrial and aquatic systems.
Nutrient Management
Unless current practices are changed, non-point pollution of surface waters will increase in the future. Some factors that drive this expectation are the substantial and growing build-up of P and N in agricultural soils; an increasing human population; people's preference for meat-rich diets, which mandates increasing livestock production; growing urban areas, which increases development and erosion; and increased fixation of N by human activities such as fertilizer production and fossil fuel burning. (Ironically, the increasing use of more efficient engines and turbines for burning fossil fuels has had the inadvertent effect of increasing the fixation of N.)
However, this pessimistic forecast could prove to be incorrect, because there are a number of ways that non-point pollution can be reduced, such as by landscape management, agricultural management (P and N), and setting thresholds for nutrients allowed in water.
Section 9: Biodiversity
Species diversity is a central interest in the field of ecology. Even though diversity is seen as a measure of an ecosystem’s health, there remains considerable debate over its measurement and how exactly several factors contribute to a particular status. In addition, diversity remains difficult to define precisely.
Species diversity is made up of two components – the variety of species present and the relative abundance of each. The first is a measure of the community’s richness and the second a measure of the evenness or balance present. It is important to understand how to measure diversity and what the term and the measurement results mean. This concept remains one of the more exciting and debated in both theoretical and applied ecology. Considerable effort has been devoted to explaining why there are systematic and predictable latitudinal patterns in diversity and why it seems so closely associated with area. In addition, the ecological implication or value of diversity in conferring stability remains important from both a theoretical and applied perspective.
Species diversity is not the only form of ecological diversity. Measures of niche width describe the diversity of resources that an organism or species utilizes. Also, habitat diversity is an index that measures the structural complexity of the environment or the number of communities present. Methods for measuring these aspects of ecological diversity are closely related to those used to measure species diversity.
This section further discusses the different ways to
measure diversity, such as the species richness index, evenness or
equitability, Shannon-Wiener Index, or Simpson’s index; general patterns in
species diversity; the factors controlling diversity; the importance of
biodiversity; and the management of biodiversity.
Section 10: Watershed
Change
Change is both unavoidable and essential in the watershed, as plant and animal communities are continually altered in response to changes in physiographic and climatic conditions. Environmental change or disturbance occurs at varying spatiotemporal scales. Disturbance processes are typically characterized according to the frequencies and intensities with which they occur. Changes of concern are defined as disturbances resulting in significant long-term alteration or loss of primary watershed processes or structural components.
In many cases, watershed change occurs as a result of the cumulative and interactive effects of more than one agent of change. These effects result in widespread changes in such watershed processes as hydrology, sediment transport, nutrient cycling, and trophic interactions. All human activities have an impact on watersheds. This section describes the watershed changes that result from timber harvesting, farming, mining, urbanization, building of dams, fishing, hunting, and climate change.
In many cases, watershed change occurs as a result of several agents of change that act together. Changes of concern often result from interactions of natural and human-induced disturbances. Even where individual stresses are relatively insignificant, ecological processes and relationships may be significantly influenced by the cumulative impacts of multiple stresses. Change is not a single-step phenomenon; the original disturbance may first lead to direct effects that then bring about additional indirect effects.
For example, the overall reproductive success of pacific salmon populations may be significantly affected by the cumulative impacts of relatively minor stresses at various life history stages. Under average environmental conditions (e.g. gravel size, flow, temperature), and in the absence of human influences, survival rates may be as low as 10% for incubating pink salmon eggs, even in relatively productive streams. Of those eggs which survive to emergence, as few as 1% may survive oceanic life and upstream migration during the juvenile and smolt stages to spawn successfully themselves (i.e. total survival on the order of 1/1,000).
Given that the average pink female lays approximately 2,000 eggs, some populations may only just replace themselves with each subsequent generation. Accordingly, these populations may decline rapidly or even become extinct as a result of relatively small increases in mortality at different life history stages due to various anthropogenic stresses (e.g. increased peak flows and bed scour during winter egg incubation, increased commercial harvests at sea, or physical barriers to upstream migration and spawning).
In addition to cumulative impacts, significant change may occur as a result of the interactive effects of different agents of change. Two or more agents of change acting together may produce more severe or entirely different impacts than would result from each change in itself. For example, climatic changes typically entail changes in disturbance regime (e.g. fire frequency) that may alter plant species composition.
Another example of interactive change is the way in which metals in lakes change toxicity as acid precipitation changes the pH in lakes. Metals behave differently at different pH levels. At specific critical pH levels (pHzpc) different metals become less soluble and precipitate out of the water column, and become available to the aquatic food web. Aluminum (Al) for example has a pHzpc of approximately 5.3. In areas with high concentrations of Al in underlying substrates, or where Al is a constituent of urban and industrial pollution, fish may be more likely to die of Al toxicity than acid toxicity as pH values decrease and approach a pH of 5.3.