4Abiotische Faktoren und die ökologische Nische

Definition and Scope of Abiotic Factors

• General Concept: Abiotic factors are all environmental influences that do not originate from other living organisms (non-biological) but are essential or impactful for a specific organism's survival, growth, and reproduction.

• Examples of Abiotic Factors:

  • Temperature

  • Light intensity

  • Salinity (salt content)

  • $\text{pH}$ value

  • Nutrient availability

  • Soil characteristics (e.g., grain size)

  • Environmental pollution (e.g., chemical contamination)

Influence of Abiotic Factors on Biological Processes

There are three primary patterns describing how abiotic factor intensity affects ecological and biological processes:

• 1. Normal Distribution (The Optimum Model):

  • Typically applies to factors like $\text{pH}$ and temperature.

  • The Optimum: A specific range where the organism functions best.

  • Biological Thresholds:

  • Reproduction: Possible only within a very narrow, central range of intensity.

  • Growth: Possible within a slightly wider range than reproduction.

  • Survival (Long-term): Possible in an even wider range; however, outside this, the organism may survive briefly but will eventually perish.

  • Death (Extreme): Occurs rapidly if intensity is too high or too low.

• 2. Deleterious Factors (Non-essential/Harmful):

  • Applies to factors like radioactivity or chemical pollution.

  • Zero Intensity: The ideal state for the organism.

  • Compensation: Organisms can tolerate small amounts without noticeable effects or significant metabolic costs.

  • Critical Threshold: As intensity increases, metabolic costs rise, eventually making reproduction, growth, and survival impossible.

• 3. Essential Trace Elements (The Plateau Model):

  • Applies to substances like metals ($Cu$, $Zn$) and salts that organisms require in trace amounts.

  • Deficiency: Without a minimal amount, the organism cannot survive.

  • Tolerance Plateau: Once the minimum requirement is met, the organism is healthy across a broad range of intensities.

  • Toxicity: Beyond a certain threshold, the substance becomes toxic, and the negative effects mirror those of harmful factors (reduced survival and growth).

Temperature as a Dominant Abiotic Factor

Temperature is considered the most important abiotic factor because it directly influences almost all metabolic processes, particularly enzyme function.

• Metabolic Rate: Metabolism increases with temperature, roughly doubling for every increase of $10\,^{\circ}C$.

• Upper Limit: Above a specific threshold, enzymes undergo denaturation (they break down), leading to the death of the organism.

Classification of Organisms by Temperature Regulation

• Poikilothermic (Ectothermic/Variable-temperature):

  • These organisms, such as the earthworm, adapt their body temperature to the surrounding environment.

  • They use behavioral mechanisms (e.g., seeking microclimates) to regulate their temperature.

• Homeothermic (Endothermic/Constant-temperature):

  • These organisms, such as mammals, maintain a stable internal body temperature through metabolic heat production (e.g., respiration).

  • Core vs. Extremities: Note that only the core body temperature remains stable; extremities like the face, hands, or feet are often cooler.

  • Exceptions: Some homeotherms undergo hibernation (Winterschlaf), allowing their body temperature to drop significantly lower than normal (though still above ambient) to conserve energy.

Metabolic Costs and the Thermal Neutral Zone

• The Thermal Neutral Zone: A specific temperature range where an endotherm's heat loss equals its internal heat production. In this zone, the organism expends minimum energy to maintain its core temperature.

• Energy Expenditure: Maintaining a stable body temperature is extremely "expensive" in terms of energy. Outside the neutral zone, the organism must burn more energy to generate heat.

• Lethal Limits: At extreme temperatures, the organism can no longer produce enough heat (or cool down sufficiently), and the body temperature deviates from the optimum, leading to death.

• Efficiency comparison: Neither strategy (Poikilothermic vs. Homeothermic) is inherently "better." For example, insects (Poikilotherms) are far more species-rich and numerous than mammals.

Structural and Behavioral Adaptations to Temperature

Structural Adaptations in Plants and Animals

• Plants: Unlike animals, plants cannot move to avoid heat. They utilize:

  • Reflective Surfaces: White or "mirrored" surfaces to deflect solar energy.

  • Spines and Fuzz: These provide shading for the plant surface to keep it cooler.

• Animals: Use "heat windows" (areas with less fur, like on a Wallaby's belly) or rest on cool surfaces (conductive cooling).

Behavioral and Evaporative Adaptations

• Evaporative Cooling: Utilizing the energy-sucking effect of water evaporation to lower temperature.

  • Plants: Use stomata (pores on leaves) for transpiration cooling (this costs water).

  • Animals: Dogs pant (hecheln), humans sweat, and Wallabies lick their forepaws to cool down.

• Microclimates: Small-scale regional conditions that differ from the overall area.

  • Ants ($Lasius \, niger$): Utilize the thermal properties of stones. They move their larvae up under warm stones in the morning and evening to accelerate growth, but move them deeper when it becomes too hot at midday.

  • Survival Strategy: Animals often survive in regions far outside their theoretical climate tolerance by consistently moving between different microclimates.

Ecological Laws of Tolerance

Liebig's Law of the Minimum (attributed to Karl Sprengel)

• Developed originally for agriculture (yields).

• Definition: The growth or yield of an organism is limited by the single resource that is in the shortest supply relative to the organism's needs.

• Example: If wheat requires $5\,\text{units}$ of water, $2\,\text{units}$ of sun, and $1\,\text{unit}$ of minerals per gram, but the environment provides $6\,\text{water}$, $5\,\text{sun}$, and $3\,\text{minerals}$, the plant can only grow $1\,\text{gram}$ because water is the limiting factor.

• Note: Factors are often interdependent (synergistic). For example, a plant may need less Zinc in the shade than in the sun.

Shelford's Law of Tolerance

• Definition: An organism's success is limited not only by a minimum amount of a resource but also by a maximum threshold.

• Rare/Extreme Events: Even a single brief event outside the tolerance range can limit the distribution of a species.

• Case Study: The Saguaro Cactus ($Carnegiea \, gigantea$). These cacti contain high water content and cannot tolerate frost. If the temperature remains below $0\,^{\circ}C$ for more than $36\,\text{hours}$ without thawing, they freeze and die. This specific threshold defines the geographic distribution limit of the species.

Ecological Terminology

• Eury-: Wide range of tolerance (e.g., Eurypotent).

• Steno-: Narrow range of tolerance (e.g., Stenopotent).

• Oligo-: Preference for low intensity.

• Meso-: Preference for medium intensity.

• Poly-: Preference for high intensity.

• Examples:

  • Artemia: Polyuruhalin (tolerates high and wide ranges of salt).

  • $Lasius \, niger$: Mesoeurypotent regarding soil moisture (prefers medium moisture but has wide tolerance).

The Ecological Niche Concept

There are three historically significant ways to define a niche:

• 1. The Grinnellian Niche (The "Address"): Focused on the place or habitat. It is the physical space where all necessary resources and conditions exist for a species to survive and reproduce.

  • Example: The Yellow-eared Bat (Zeltfledermaus) requires tropical climates and specifically banana leaves to construct its "tents."

• 2. The Eltonian Niche (The "Profession"): Focused on the functional role of an organism within its ecosystem (what it eats, what eats it, and its impact on the environment).

  • Example: An owl's niche is defined as a nocturnal, flying predator of small mammals.

• 3. The Hutchinsonian Niche (n-dimensional Hypervolume): The currently dominant scientific concept.

  • Definition: The niche is an n-dimensional volume where each axis represents a different environmental condition or resource (e.g., temperature, $pH$, moisture).

  • The area within this hypervolume represents all the combined conditions necessary for reproduction.

Fundamental vs. Realized Niche

• Fundamental Niche: The theoretical volume of conditions where a species can survive and reproduce in the absence of biotic interactions (competition, predation, parasites).

• Realized Niche: The actual, smaller volume that the species occupies once biotic interactions like competition and predation are accounted for.

• Niche Partitioning: When niches overlap, competition often leads to a reduction of the realized niche, forcing species to specialize in different parts of the gradient. This promotes biodiversity.

Ecological Zonation and Gradients

Organisms are distributed along gradients of abiotic factors, leading to zonation (regular changes in ecosystems).

Higher Altitude Gradients

• Factors changing with altitude include temperature, solar radiation (stronger higher up), wind (increases evaporation), and precipitation (highest in middle altitudes; lower at high peaks where air is dry).

• Zones:

  • Nivale Zone: Above the limit of closed vegetation; often snow-covered.

  • Alpin Zone: Above the tree line, characterized by mats, dwarf shrubs, and specialized flora.

  • Montana (Montane) Zone: The middle mountain range, below the tree line.

Coastal/Marine Zonation

• Based on exposure to salt water/fresh water and wave energy.

• Supralittoral (Spray Zone): Never submerged, but affected by salt spray.

• Upper Eulittoral: Affected by surf; occasionally submerged.

• Lower Eulittoral: Frequently submerged; must tolerate periods out of water.

• Infralittoral: Always submerged.

Dynamics of Distribution and Invasive Species

• Glacial Legacy: Species distribution is not a stable state. For example, some trees are still migrating to fill areas made habitable $10,000$ years ago after the last ice age.

• Climate Change Impacts: Abiotic factors are shifting faster than many organisms can migrate (e.g., trees move slowly). Conversely, mobile organisms like disease-carrying mosquitoes (e.g., Malaria carriers) may expand their range into new areas like Germany.

• Invasive Species: These organisms often have a larger realized niche in their new environment because they are released from the "pressure factors" (predators/competitors) found in their native range. This allows them to expand massively and take over new territories.

Questions & Discussion

Q: How would a power plant that warms river water to $30\text{-}35\,^{\circ}C$ impact Salmon spawning grounds?A: While Gedeihkurven (tolerance curves) might suggest adults can survive, we must look at the most sensitive life stage (spawning). Even if temperatures are not immediately lethal, being further from the optimum range reduces the overall success of the species. Thus, while not necessarily killing all salmon instantly, it would negatively impact all spawning grounds by moving conditions away from the optimum.

Note on Upcoming Event: Professor Urban Lewis from the University of Oxford will give a lecture in approximately 3 weeks (Monday, 22nd at 12:05 PM) on "Insect Food Webs and the Extraordinary Diversity of Tropical Rainforests" in the Large Zoology Lecture Hall. Attendance is highly encouraged for practical insights into scientific research.