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Xenobiology 101: Part 4 - Ecology, Environment, and Evolution

Introduction

Ecology is that science that explores how organisms interact with each other and their environment.

For surface life, factors that determine climate will influence the variety of environment types present.

A key determinant of climate is the amount of energy available from the primary and the ability of the world to retain some of this input. The properties of the atmospheric gas mix are important in this regard. For example, carbon dioxide and methane are well known "greenhouse" gases, as they limit the amount of infrared radiation reflected into space.

The presence of life is critical to maintaining atmospheric composition. The present levels of oxygen in Earth's atmosphere are due to photosynthesis. Life with alternate biochemistries using gaseous reactants will doubtless affect the atmospheres of their worlds in a similar fashion.

Local differences in the albedo or surface reflectivity of a planet causes heat gradients to develop in the atmosphere. For example, water (oceans) is less reflective than land which is usually less reflective than clouds.

These heat gradients, in combination with planetary rotation (the Coriolis effect), form the basis of weather.

Solvent (and some nutrient) cycles will have an atmospheric phase. Vapour will be taken up into warm atmospheric gases, and precipitate out as liquid or solid from cold ones - e.g. rain. In the case of some nutrients, some living things actively incorporate compounds into their bodies.

Three Earthly examples:

  1. The water cycle. Almost all (over 97%) of water is in the oceans. Polar ice contains about 2%. The balance is in relatively rapid circulation (weeks), in the form of lakes, rivers, water vapour and rain.
  2. The nitrogen cycle. Nitrogen is a key constituent of amino acids and nucleotides, the building blocks of proteins and DNA respectively. The most important reservoir of nitrogen is the atmosphere. Converting gaseous N2 to usable compounds such as nitrates, ammonium salts and urea is called 'nitrogen fixation' and is performed predominantly by bacteria.

    Most animals and plants maintain a fairly even nitrogen balance except during periods of growth (positive nitrogen balance as protein is deposited) or illness (negative nitrogen balance with protein breakdown).

    Urea and purines (one of the nitrogen containing bases that make up the core of nucleotides) are excreted by animals, and recycled by bacteria.

  3. The carbon cycle. Atmospheric carbon dioxide is the 'fast compartment'. The CO2 living things 'breathe out' is incorporated into glucose by photosynthesis. Significant amounts of carbon dioxide is dissolved in the oceans (n.b.: gas solubility varies inversely with temperature). Carbonate based rocks e.g. limestone make up the 'slow compartment'.

Terrain features influence rainfall. As air masses move over a mountain range, they cool and water vapour precipitates out. The formation of deserts may be assisted by the effects of equatorial heating - warm, moist air masses move to cooler regions, and dumps its water. The now dry air masses circulate back towards the equator, creating dry regions in the mid-latitudes.

The density of plant life also influences rainfall. Plants produce water vapour from photosynthesis and vent it into the atmosphere (except those plants native to very dry climates): transpiration. Increased humidity, in combination with effects on albedo, alter the likelihood of rain.

Soils

The interaction of geology and weather determines what elements and compounds are available for plant life to use. Soils are the substrates on which plants grow. They are formed by partly by erosion, but mainly by the action of living things. Their key function is to act as a reservoir of solid nutrients (and solvent) for plants.

Essentially, they vary in moisture content, aeration and nutrient content. Areas that enjoy high rainfall or are in the catchment of run-off from hills, mountains, etc. will obviously have more moist soils.

Aeration is a function of both animal and plant activity - making the soil increasingly fine grained enhances diffusion of oxygen, nitrogen and water into Earthly soils, optimising conditions for animal and plant growth.

Nutrients may be either intrinsic to the soil or washed in by solvent flows. Most soils are complex mixtures of organic and inorganic compounds. Their fertility or carrying capacity varies with their ability to retain nutrients - or toxic materials e.g. clay soils and aluminium salts.

On other worlds with more acidic solvents, erosion may be greatly accelerated and soils relatively better aerated. However the latter factor may be offset by the decreased stability of nutrients or their increased solubility (the only large fertile areas may be where rivers empty into oceans).

Differences in local gravity, the presence or absence of plate tectonics, and atmospheric pressure and constitution will also alter the rate and extent of erosion and other geological processes.

Energy flows and food webs

Organisms can be loosely divided on how they obtain the energy they need for metabolism.

Food webs are maps that describe how each group of living things interacts with each other from a nutritional standpoint. There are two broad types:

At each level, energy is consumed in metabolic activity. Each level of a food pyramid is 10% the size of the level below i.e. 100kg of producers (plants, etc.) supports 10kg of primary consumer (herbivore) supports 1kg of secondary consumer (carnivore), etc.

This 'biomass' relation is universal. Pyramids based on number of organisms may be partly inverted e.g. in a forest ecosystem, there are a few large producers (trees) with many small primary consumers (insects, and other herbivores) and few  secondary consumers (predators).

Ecosystems and the Koppen classification system.

(examples kindly provided by Leonard Erickson) An ecosystem can be defined as a 'spatially explicit unit that includes all of the organisms, along with all components of the abiotic environment within its boundaries'. A synonymous term for ecosystem is biome.

Variation in temperature (available energy) and precipitation ranges (nutrient flows) are the most important determinants of what types of biome will appear. One descriptive system used in geography is the Koppen classification based on temperature and rainfall.

Koppen classification and example ecosystems

  1. hot, mean monthly temperature > 18 C
     Tropical/ dry  Thorn scrub, Savanna
     Tropical/ Moderate  Savanna, thorn forest, tropical seasonal
     Tropical/ humid  Tropical seasonal or rain forest
     Tropical/ wet  Tropical rain forest
     Tropical/ very wet  Tropical Rain forest
     Tropical/ wetsoil  Tropical swamp forest, mangrove swamp
     SubTropical/ Dry  Temperate grassland, Thorn scrub,Temperate woodland, Savanna, Thorn forest
     SubTropical/ Moderate  Temperate Grassland, Woodland, Forest
    Thorn forest, Tropical seasonal forest
     SubTropical/ humid  Temperate forest, temperate rain forest
    Tropical seasonal or rain forest
     SubTropical/ Wet  Tropical rain forest
     SubTropical/ Wet Soil  Temperate Swamp forest,
    Tropical swamp forest, Mangrove swamp
  2. dry (average rainfall less than 300mm/yr)
    Variants : h (average temp > 18 C) or k (average temp < 18 C)
     Tropical/ Arid  Barrens, Tropical desert
     SubTropical/ Arid  Barrens, Warm temperate desert,tropical desert
     Warm Temperate/Arid  Barrens, Warm Temperate desert, semidesert
     Cold Temperate/ Arid  Barrens, Semidesert scrub,
    Temperate shrub, Taiga
  3. temperate, mean temperature of coldest month -3 to 18 C
     Warm Temperate/dry  Temperate shrub, temperate grass,
    Temperate woodland
     Warm Temperate/Moderate  Temperate Forest (Deciduous or evergreen)
     Warm Temperate/Humid  Temperate Rain forest
     Warm Temperate/Wet soil  Marsh, Temperate Swamp forest
    Temperate shrub, Thorn scrub
  4. cool, mean temperature of coldest month < -3 C
     Cold Temperate/ Dry  Taiga
     Cold Temperate/moderate  Taiga, Elfin woodland
     Cold Temperate/Wet  Soil Bog.
  5. cold climates, mean temp of hottest month < 10 C
    Variant : H > 1500m above sea level
     Arctic-Alpine/ Arid  Barrens, Arctic-alpine desert, Tundra
     Arctic-alpine/ Dry  Tundra, Arctic-alpine desert
     Arctic-alpine/ wetsoil  Tundra, Bog
     Polar/ Arid  Barrens

To this list we can add the following aquatic biomes :-

Two other biomes that have been recently discovered are geothermal (deep ocean vents, hot springs) and deep rock (e.g. bacteria found in oil fields and even deeper porous rocks). As noted in the first post, subsurface microscopic life exploiting chemical and thermal gradients for energy may constitute much of a world's biomass.

Another less obvious biome is the environment occupied by the "aerial plankton" - bacteria, etc. that waft about in the air around us, massing 0.1 grams or less.

On other worlds with lower gravity or chemistry and temperature favouring aerostatic flight, the atmosphere might look more like the sea, with jellyfish-like 'floaters' competing with bird-like creatures for prey... a common motif for life in gas giant atmospheres in science fiction.

So in general terms, ecosystems vary in the energy and solvent available to them, which influences the amount of life they can support. Life's presence helps maintain conditions that the ecosystem continues to florish. In time, however, the effect of life on an area may cause conditions to change, favouring different organisms. This process is called succession and will be discussed in a later section.

Niches, habitats and species interactions

(with assistance from Leonard Erickson and Ian Ferguson) A 'niche' can be defined as 'the functional role of an organism within its community'.

Niches are well described in the various Traveller rules (e.g. scavenger, hijacker, omnivore, etc.)

In general terms, habitats can be divided into the following areas: aquatic, land and air. Most organisms span across two or more of these. Examples:

Adaptations to a given habitat are driven by remorseless evolutionary pressure, based on the relevant physical laws that governit.

The fish that can swim faster will escape a predator; its progressively more streamlined descendants will eventually dominate the watery niche in which it lives.

So traits that assist survival are passed on (though sometimes 'bad' genes hitch-hike on the 'good' ones).

Species interactions result from the interaction of niches and the food web.

Types include:

Evolution and succession

Charles Darwin's 'The Origin of Species' is one of the most important works in all science. In that book, the theory of evolution was first brought to the attention of a wider audience.

Drawing from observations made over many years as a naturalist, Darwin proposed that organisms were in a continual struggle to secure their place in a given environment. 'Natural selection' is the term he used to describe this process and its consequences. As mentioned above, natural selection is based upon the gradual accumulation of improvements.

Consider flight as an example. There are many theories as to how flight began. Given the long history of life on Earth, it was probably discovered independently several times throughout the animal kingdom.

Wings may have begun as radiators for insects; as the insects grew larger over evolutionary time, their radiators grew too - eventually attaining a critical wingspan. Halteres or wing stubs still persist on flies, but act as control surfaces during flight.

Vertebrates variously sport wings that are modified arms or forepaws, gliding membranes between fore and hind limbs and even modified ribs. Leaping from tree to tree, or making running jumps from the ground to flee or catch prey are logical starting points for the development of flight.

Lastly there are the cephalopods - octopi and squid can forcibly empty their swim bladders to effect 'jet propulsion'. There is at least one species of flying squid.

Vision has a similar history of many independent starts (at least forty times). Across the animal kingdom a vast range of visual systems, from clumps of light-sensitive skin cells to sophisticated camera and compound eyes can be seen. Interestingly, the genes that code for overall eye development are tightly conserved across species; there is not very much difference between those of a fruit fly, mouse or man.

Succession can be seen as a form of ecosystem evolution. Changes in the environment are brought about by a progression of plant communities and the formation of increasingly nutrient-rich soils. E.g. on a sand dune, the bare sand of the beach gives way to dune grass, then perennial herbs, shrubs, light-loving and drought-tolerant trees, and finally shade-tolerant trees that require moderate to high soil moisture.

There are two types of succession. Primary succession begins with bare rock, sand, or mineral soil. Examples are sand dunes or a volcanic blast zone. In secondary succession, nutrient rich soil is already present; because of this, changes in plant communities are more rapid and are driven by the availability of light and water. An example would be an old field becoming forest.

Sucesssion can be divided into three stages :- pioneer, intermediate and climax. In the pioneer stage, diversity of species is low as the supporting plants are small and sparse. Over the course of years, species diversity expands greatly as more plants arise; this intermediate stage is the 'richest' one in terms of diversity and plant density. In the climax stage, the environment has matured. Trees dominate and support a wide range of animal life.

Next Part: Case Study 1 - The Viji