Studies in ecology deal with a variety of questions. These are some of them:
- Why do populations fluctuate?
- What is controlling population density?
- How do organisms allocate energy to competing life processes?
- Why do predators consume one prey and not another?
- Where do new organisms come from and how are they dispersed?
- How can a sustainable population be achieved?
- What is the effect of removing or adding species to a food web?
- Why do biodiversity hotspots occur? What processes enable it? Why do they occur where they occur?
- What triggers abrupt shifts in ecosystems and do they result in alternate states?
- How are human impacts damaging ecological systems?
Within an ecosystem, there are subtle, and sometimes indirect, interactions that are numerous because of the many different species and the wide variety of factors that play a role. It should also be said that ecological interactions are not static and are constantly evolving.
When talking about evolution, we should mention genetic change, natural selection and fitness. Genetic change consists of mutations and other processes that produce new and variant forms of genes. Natural selection is a process that operates on individuals with different combinations of these genes to endow the most fit a survival advantage. It occurs by differential reproduction and survival of genetically distinct individuals in the population, and involves death and limits to reproductions. Natural selection also ensures that existing species are suited to their environment. Fitness is the proportionate contribution of individuals to future generations. The fittest individual equals 1.0, while all the rest are <1.0. The individuals leaving the greatest number of descendants relative to others are the fittest of the population. There are ways to measure the correlates of fitness, such as reproduction and fertility, survival and mortality, and growth.
Endler (1986) offers ~140 demonstrations of natural selection in the wild. It is one of the best evidences demonstrating a) variability in a trait, b) that trait variation is heritable, and c) that a trait can confer a fitness advantage. These traits can also be observed in Grant and Grant (1993).
Rapid evolution is defined as the evolutionary changes occurring in a population in less than a hundred years. Four categories of rapid evolution affecting species' interaction are:
- Evolution of trophic links via specialization
- During the 1982-1983 El Niño there was an especially high amount of rainfall which caused a change in seeds. This put selective pressure on finches, leading to a rapid evolution of beak sizes. This event favored granivorous finches. (Grant and Grant 1993)
- Evolution of defense
- Cryptic coloration in moths responded to changes in color of habitat. By 1900, tree bark was darkened by pollution and the percentage of dark moths increased significantly. (Kettlewell 1995)
- Rapid loss of traits in absence of interaction
- Guppies exhibited loss of defensive antipredator traits when predation diminished. These traits included cryptic coloration, body size, and specific behaviors. (Endler 1995)
- Change in outcome of interaction
- The Myxoma virus was introduced in Australia to control the rabbit population, but soon rabbits evolved resistance while the virus evolved decreased virulence. This is an example of coevolution and reciprocal genetic change.
--------------------------------------------------------------
Niche Theory
A niche is an organism's place in the environment. It is defined by both physiological tolerances and resource requirements and availability. A fundamental niche is a multidimensional space that is quantifiable. Also referred to as a 3D space called hypervolume. It is where a species could live. A realized niche is the space constrained by biological interactions among species. It is the "post-interactive" niche and it is where the species actually lives. The key question now is what determines the width of the realized niche?
No comments:
Post a Comment