2. Limiting factors: critical limiting factor and tolerance limits
Abundance and Distribution of Species
Liebig's law of the minimum:
The single factor closest to these survival limits is the critical limiting factor that determines where a particular organism can live. The major factor is one in the minimum.
or: single environmental factor in shortest supply relative to demand is the critical determinant in species distribution
Shelford added to Liebig's work:
Single environmental factor closest to tolerance limits determines where a particular organism can live
Shelford's principle of tolerance limits:
Each environmental factor has both minimum and maximum levels called Tolerance limits, beyond, which a particular species cannot survive or is unable to reproduce.
For terrestrial organisms: temperature, moisture, and nutrient availability.
For aquatic organisms: salinity, gradient (water current).
Sometimes the requirements and tolerances of species are useful indicators of specific environmental characteristics - environmental indicators (trout, lichens etc.).
Limiting factors include ABIOTIC (physical) factors and BIOTIC factors
Abiotic Factors: Light availability/day length, seasonality; moisture/water availability; temperature, diurnal temperature range; height or depth, pressure; salinity; wind; pH, CO2, O2, availability of N, P, K, Ca, etc.
Biotic Factors: Competition; predation; parasitism; disease; pollinators, dispersal agents; food availability; species density /distance to nearest neighbour).
Who Lives Where, and Why?
Generalists vs. Specialists
2 basic strategies for surviving
- in a broad range of environmental conditions: "generalists“.
- in a narrow set of environmental conditions: "specialists”.
TOLERANCE RANGE
All species have a tolerance range within which they can cope with the conditions they experience.
The tolerance range is within the organism, i.e., it is the ability of the organism to withstand extremes of different limiting factors.
Different ecological groups:
Eyrobionts – organisms with large range tolerance to an ecological factor.
Stenobionts – organisms with narrow range tolerance to an ecological factor.
Xerophytes – organisms that are adapted to drought and environmental water shortage.
Homeothermics – organisms with constant temperature not depending on environmental temperature.
Poikilotehermics – organisms with body temperature depending on environmental temperature.
Endemics – species that are localized and may have just one population that inhabits a small area.
Eurytopic Species
Some species have wide tolerance ranges (eurytopic/ecologically tolerant species) and are found over a wide range of habitats and locations (i.e., have wider geographical ranges)
Stenotopic Species
Some species have very narrow tolerance ranges (stenotopic/ecologically intolerant species) and are not widespread and very limited in their ranges. E.g., the amphipod crustacean species (Gammarus spp) and their varying tolerance to salinity).
Tolerance Limits
Each environmental factor (temperature, humidity, food supply, etc.) has both minimum and maximum levels beyond which a species cannot survive or is unable to reproduce: lower & upper limits of tolerance.
Tolerance range is the range between these 2 limits.
An environmental gradient is a stepwise increase or decrease in an environmental factor – for example, pH, rainfall, temperature, and so on. Species can tolerate different environmental gradients. For instance, humans have a relatively narrow range of temperatures at which they can survive without clothing and shelter. Similarly, humans can live on relatively few types of unprepared food. Becoming very efficient at living in a narrow environmental range is a type of specialization. Generally, the more highly organisms are adapted to a narrow environmental range, the less adapted they are to changes in the environment. Why would this be true?