18-L-Distributions

Biology 20C - Fall 1998

ECOLOGY AND EVOLUTIONARY BIOLOGY

Lecture 18 - Distributions

Problem: An important goal of ecological work has always been to provide explanations about why a particular species is present in some places but absent from others.

Old Approach: Beginning in mid-19th century, the main question that ecologists asked was:

"Why is the species present in the areas that it occupies?"

This question is usually answered by describing adaptations (physiological, anatomical, morphological, behavioral, life history) that permit individuals to tolerate the environmental extremes and to exploit the resources in the area.This approach was a major influence on the development of "Physiological Ecology" as the dominant sub-discipline of ecology during the first half of the 20th century, and it flowed naturally from the physiological emphasis on Zones of Tolerance and Response Curves. It assumed that organisms live up to the limits of their physiological tolerances; and therefore, that studies of adaptations to extreme conditions will predict (and explain) observed distributions. In other words, distributions are expected to approximate the physiological "Fundamental Niche."

New Approach: About 1955, ecologists began to reverse the question and ask:

"Why is the species absent beyond the area it occupies?"

This question can only be answered experimentally. Today, the basic tool used to study limits of distributions is the transplant experiment. Groups of individuals are transplanted beyond their observed range, then monitored to determine whether they survive, grow and reproduce--and if not, why not. Transplant experiments may use five kinds of treatments:

1.	Unconfined organisms	Free to do whatever they like, without altering the environment in any way.
2.	Confined organisms	(e.g. in cages) that prevent them from leaving the experimental area, but without altering any other aspect of the environment.
3.	Exclusion or inclusion of other species	(e.g. potential predators, competitors, food, etc.) either by transplanting the other species, or by using cages to keep them in or out.
4.	Modification of physical or chemical factors	Experimentally alter the abiotic surroundings (e.g. water, nutrients) of the transplants.
5.	Controls	To detect and measure possible artifacts of the experimental methods that may alter or obscure treatment effects.

In any particular experiment, any or all of these treatments may be used singly, or in various combinations. They are often applied sequentially to eliminate whole classes of factors by following the experimental protocol outlined below.

Lecture 18 - Protocol for Distribution Studies

Experiment: Transplant species beyond its range, with and without habitat modifications

Results: Transplants establish permanent populations…		Conclusions: Species absent because of…			Mechanisms: Area is not occupied because of …

1. …when unconfined and without habitat manipulations	Þ YES	è DISPERSAL è
				- Barriers to dispersal
					- Historical events
ß					- Not enough time
NO
2. …only when confined	Þ YES è BEHAVIOR è				- Habitat selection
					- Habitat recognition
NO
ß

3. … only if another species is manipulated by either…	Þ YES	è OTHER SPECIES è
			Sp. Excluded			Sp. Included
			- Predation			- Symbiosis
ß			- Parasitism			- Mutualism
NO			- Competition			- Food
ß			- Disease

4. …only if abiotic conditions are modified.	Þ YES	è PHYSICAL/CHEMICAL CONDITIONS è
			Physical			Chemical
			- Temperature			- Water
ß			- Light			- Oxygen
NO			- Moisture			- Salinity
ß			- Fire			- pH
			- Soil structure			- Soil nutrients
ß			- etc.			- etc.
[Problem of Logic]

Details of each example differ, but general conclusions from many transplant experiments are:

1. that observed distributions are usually smaller than predicted from physiological studies;

2. that many populations are not limited by physiological tolerances.

The order of the protocol (above) also reflects the common rankings of probable answers. A species is likely to be absent from an area because:

1st	It can’t get there, hasn’t had time or was recently disturbed	(Dispersal; History)
2nd	It doesn’t recognize a suitable habitat.	(Behavior)
3rd	It is excluded by predators, competitors, parasites; or by lack of food organisms.	(Other species)
4th	It is excluded by abiotic conditions or by lack of resources.	(Physical/Chemical)

While all these factors may interact in complex ways at a site, and different factors may act at different sites, the usual consequences are:

1. The observed (realized) range is smaller than the potential (physiological) distribution.

2. Observed distributions reflect the "Realized Niche," rather than the "Fundamental Niche".