Changes in numbers and types of organisms are due to evolutionary and environmental factors, with the latter having the greater effect because of overriding human imposition.
Evolution is a reflection of changes in the gene structures of organisms. At the molecular level, nature and positioning of four bases (two purines and two pyrimidines) within deoxyribonucleic acid (DNA) dictate in large part how the blueprint of life is read. Hence, changes in these DNA bases lead to changes in organisms. The four bases are tied together in alternating moieties of a five-carbon sugar (deoxyribose) and phosphate residues to comprise the DNA structures. It is the connection of these components in a polymeric chain and then the twinning of two complementary chains that constitutes the intact DNA double helix. The latter is wrapped with basic proteins to form genes within chromosomes within the nuclei of cells.
In those cells that are essential for reproduction, namely gametes, any changes in types or positions of the four DNA bases may produce a fusion that can lead to either a viable or nonviable zygote. If viable, an altered organism different from either parent will result. If the genetic alteration is favorable and not merely tolerable for survival in the particular environment in which the organism finds itself, it may have progeny that increase in competition with others. It is the operation of "survival of the fittest" over millions of years that was recognized by Darwin and Wallace, who espoused biologic evolution, which is the driving force for natural selection. Unfortunately we still have a public with a large fraction so poorly educated, especially in science, that they believe in more recent and nearly instant creation.
Random changes occur in the type of base inserted within a given position of DNA. Some of these lead to viable but genetically altered organisms. It is this constantly changing nature of the genomes of organisms that leads to the diversity of life. There are also a plethora of environmental insults, typically due to radiation and chemicals that cause damage in the native structure of DNA. Certain of these harmful alterations can be repaired by enzymatic mechanisms intrinsic to the cell; others cannot be corrected and lead to disease and even death. The number and quantity of environmental toxicants, due solely to humans, has increased markedly in relatively recent times. These impact all life forms, including the perpetrating species, man. These detriments to living organisms must be added to the inexorable decrease in organisms, especially complex higher plants and animals, that are displaced by the ever increasing human population which outstrips habitat and sustenance for all. We not only decrease life forms by our treatment of an earth that must support all, but in many instances we add insult to injury in how we deliberately alter organisms from the micro to the macro level. Such practices that include use of antibiotics and other medications, often in inappropriate amounts for treatment of ourselves, our livestock, and our pets, allow the unintended selection of resistant mutants. Similar extensive applications of fungicides, herbicides, diverse pesticides including rodenticides and the like, lead inevitably to fewer life forms among which are to be found undesirable mutants. If no remediations are undertaken, and soon, we may ultimately effect the demise of our current species, which we name arrogantly and perhaps erroneously as Homo sapiens sapiens. Are we, an aggressive primate in the phylogenetic tree with over 98 % DNA homology to chimpanzees, really so sapient? Biologic evolution at the macro level is a relatively slow process and may not drive the production of an improved, more humane and technically competent hominid species in time to offset or at least decrease the more rapid damage the present one is inflicting on our globe.
We need to consider what can and should be done about the major problem, namely the ongoing diminution of life that we have brought about. First, we need to better educate our children in science as includes evolution. The lack of understanding is made clear repetitively by test results from our school children and the preference to place faith over facts by adults brought through our school system. Second, we need to better inform the public and especially politicians who are supposed to represent mutual concerns for us and for our progeny's lives as relate to causes and effects of current practices. There are numerous environmental issues that must be forcefully dealt with, and the current habit of emphasizing treatment rather than preventive approaches should be altered. The Union of Concerned Scientists is one group that endeavors to collate the state of affairs as concerns species and their proper management. This group attempts to inform and to rally people to educate politicians as to how they should vote on environmental issues. Similar activist groups need our support and actual participation. If we are serious in our actions now, perhaps we can alter the outcome that must effect us all.