Ecology/Species and Populations

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Chapter 9. Species and Populations

Species and populations are probably the two most commonly used ecology terms, but they are often difficult concepts to grasp. Throughout this chapter, they will be discussed in detail, expanding on the definitions from previous chapters and explaining why there are no universally acceptable definitions. Factors influencing population growth and ways to measure and predict population growth will also be subjects of specific focus. Populations can be affected by so many factors that it is almost impossible to take every variable into account. Demography is the statistical study of the age structure of a population, and it can be used in research to determine what is causing a decline or increase in population size over time. It is worthwhile to understand species concept and population growth even though they are such broad aspects, because they are fundamental to the science of ecology.

Species Concept
You should have familiarity with the species concept from biology and have been presented with this basic tenant of the biological sciences in Chapter 2 of this textbook. If need be,
 * Reread Species ( Follow links as necessary to understand concepts presented )

When initially considering the concept of species you may wonder why the division of organisms into species is necessary at all. According to Ernst W. Mayr, a renowned evolutionary biologist, the existence and reproductive characteristics of distinct species "prevents the production of too great a number of disharmonious incompatible gene combinations." Therefore, the basis for the existence of species is genotypic in nature. However, the existence of species is not traditionally doubted whereas the appropriate definition of a species is continually a topic of debate.

Although a species can be defined in a number of different ways, the most common definition for a living species, suggested by Ernst W. Mayr, is any group of individuals that can breed with themselves but not with any other group. In other words, a species is any group of individuals that is "reproductively isolated." This definition, termed the Biological Species Concept, has limitations and exceptions. Some animals that would not normally be considered part of the same species can breed with limited success. For example, a horse and a donkey can can breed to produce a mule offspring but, the mule is infertile. Accordingly, there is much debate regarding the amount of successful fertility required for two individuals to belong to the same species. There are also organisms that do not interbreed at all, but propagate through direct cloning of an individual, for example many plants and insects. Other definitions must be used to define the species of organisms such as these.

So, why is such a seemingly straight-forward concept so hard to define? One problem is that the definition of a species is used in many different contexts and undoubtedly fails to satisfy them all simultaneously. In particular, there has been a great deal of discontinuity between the application of the same species concept in a taxonomic or systematic sense and in an evolutionary sense. In systematic biology species serve as fundamental units (taxa) for the description and historical progression of diversity. However, evolutionarily, species are considered only as elements of a process called speciation. Even without further knowledge of the differences between evolution and taxonomy, it is apparent that the concept of a species is used in different contexts by the two. This is just one example of the many obstacles encompassing the complex debate for a universally applicable definition of a species.

In fact, it seems, after years and years of debate, that it is impossible to attain a universally suitable definition for "species". As Pigliucci (2006, p. 50) points out:
 * ...emotions have run high for decades on the "crucial" question of what a biological species is, while at the same time plenty of researchers have conducted brilliant studies on individual species and on the process of speciation, apparently unencumbered by the lack of a universally-agreed-upon definition of what species are...

The species concept is basic to biology and therefore ecology, but it is really the species population that ecologists study.



Population Biology
A species population is a discrete group of potentially interbreeding organisms in the same species in a given locality. Here, potential interbreeding has nothing to do with measures of fertility; instead it is a statement that all members of a population are potentially a part of a single gene pool. In a species population, there are many factors that interfere with an individual’s fitness (its ability to survive and reproduce). Booth et al.(1995) describes that an individual's fitness is increased with group living, also known as the Allee Effect, and that an individual’s ability to survive will be increased by their size and rank. In this study, Booth shows that within the group of damselfish there was a greater chance for survival if they stayed in large groups. Within these groups the smaller individuals would benefit the most from the group living, but consequently the growth of these individuals was reduced, thus taking a longer time to reach a higher social status in the group. Reed et al. 2005 support Booth's findings and also show that there is a linear relationship between log population size and population fitness. Reed suggests that “conservation efforts should ultimately aim at maintaining populations of several thousand individuals to ensure long-term persistence."


 * Read Population The common use of the term in human demographics utilizes concepts and terms important to Population Biology

Diversity as a goal is split between species diversity and diversity within a population. An area that is filled with finches and owls and many species has species diversity. But if all the finches have very similar genes, or if the population has an inbreeding problem, there is not enough diversity within that population. When there is no diversity in the gene pool, serious effects can result. This may result from the Founder Effect. A major cause of Founder Effect is when a small part of a population has moved to an area with limited interaction with other populations from the same species. A well known example is the Old Order Amish in Pennsylvania. The Amish rarely interact with people outside of the community and rarely, if ever, marry outside of the population. With the limited genetic variation, genotypic and phenotypic problems are seen more frequently in the population compared with other human populations. The Amish carry a rare allele that causes polydactylism, in which there are extra fingers or toes. This recessive allele is carried by about 1 in every 14 Amish individuals compared to the outside population, where it is carried by about 1 in every 1000.

Another similar type of disturbance in a population is known as a Bottleneck. This disturbance occurs when a species is almost eliminated by human forces (hunting) or natural disasters. The cheetah is a prime example of a species with limited variation from a bottleneck. The cheetah's genetic make-up is so similar that a skin graft will not be rejected from one cheetah to another.

Even though the worldwide population of a species is potentially very large, not all individuals can interbreed due to obvious limitations such as separate continents, mountains or deserts, or great distances. A local population that does actually interbreed is called a neighborhood, or deme. So even though an overall population may not have inbreeding, neighborhoods could have potentially high inbreeding coefficients, which can increase the frequency of diseases or maintain a mutation in that species.

Population Growth
Population growth is measured using time-specific life tables and age-specific life tables. Time-specific life tables are used for long-lived species, such as humans, and age-specific tables are used for short-lived species, such as annual plants and insects. Time-specific life tables assume equal birth rates for all age classes, and are useful for large populations. Age-specific life tables follow a cohort (a group of organisms of the same species and roughly similar ages) and require periodic sampling. There are many types of useful information that studies of population growth can reveal, such as life histories for a population and life expectancies, such as in the map shown at right. Survivorship curves can also be determined by plotting the number of individuals of a cohort against time. Fecundity can be influenced by abiotic factors, such as precipitation (Milanovich et al. 2006).

Reproductive Rates
Closely related to the study of population growth is the reproductive rate of a population. Fecundity, mx is the average number of offspring produced per breeding individual. A fecundity schedule is R0 = ∑ lxmx, where lx is nx/n0 (the proportion surviving), mx is the mean number of offspring per individual, and R0 is the net reproductive rate of the population. If R0=1, the population is stable. If R0<1, the population is decreasing, and if R0>1, the population is increasing.