Patterns of Survival Looking at it another way, demographers want to know the pattern of survival for a given population. This can best be determined by identifying a cohort, which is defined as a group of individuals that are born at about the same time. That cohort is then followed over time, and the number of survivors is counted at set time intervals. The census stops after the last member of the cohort dies. The pattern of survival exhibited by the whole cohort is called its survivorship. Ecologists have examined the survivorship patterns of a wide array of species, including vertebrate animals, invertebrates, plants, fungi, algae, and even microscopic organisms. They have also investigated organisms from a variety of habitats, including oceans, deserts, rain forests, mountain peaks, meadows, and ponds. Survivorship patterns vary tremendously. Some species have a survivorship pattern in which the young and middle-aged individuals have a high rate of survival, but old individuals die in large numbers. Several species of organisms that live in nature, such as mountain sheep and rotifers (tiny aquatic invertebrates), exhibit this survivorship pattern. At the other extreme, many species exhibit a survivorship pattern in which mortality is heaviest among the young. Those few individuals that are fortunate enough to survive the period of heavy mortality then enjoy a high probability of surviving until the end of their natural life span. Examples of species that have this pattern include marine invertebrates such as sponges and clams, most species of fish, and parasitic worms. An intermediate pattern is also observed, in which the probability of dying stays relatively constant as the cohort gets older. American robins, gray squirrels, and hydras all display this pattern. These survivorship patterns are usually depicted on a graph that has the age of individuals in the cohort on the x axis and the number of survivors on the y axis. Each of the three survivorship patterns gives a different curve when the number of survivors is plotted as a function of age. In the first pattern (high survival among juveniles), the curve is horizontal at first but then swings downward at the right of the graph. In the second pattern (low survival among juveniles), the curve drops at the left of the graph but then levels out to form a horizontal line. That curve resembles a backward letter J. The third survivorship pattern (constant mortality throughout the life of the cohort) gives a straight line that runs from the upper-left corner of the graph to the lower right (this is best seen when the y axis is expressed as the logarithm of the number of survivors). In the first half of the twentieth century, demographers Raymond Pearl and Edward S. Deevey labeled each survivorship pattern: Type I is high survival among juveniles, type II is constant mortality through the life of the cohort, and type III is low survival among juveniles. That terminology became well entrenched in the biological literature by the 1950's. Few species exhibit a pure type I, II, or III pattern, however; instead, survivorship varies so that the pattern may be one type at one part of the cohort's existence and another type later on. Perhaps the most common survivorship pattern, especially among vertebrates, is composed of a type III pattern for juveniles and young adults followed by a type I pattern for older adults. This pattern can be explained biologically. Most species tend to suffer heavy juvenile mortality because of predation, starvation, cannibalism, or the inability to cope with a stressful environment. Juveniles that survive this hazardous period then become strong adults that enjoy relatively low mortality. As time passes, the adults reach old age and ultimately fall victim to disease, predation, and organ-system failure, thus causing a second downward plunge in the survivorship curve.
The Physiology of Vision In simple terms, the visual system takes a signal in the form of light and translates it into a chemical change and later a nervous impulse in the brain; this nervous impulse is what the animal perceives as sight. The two main characteristics of eyes, no matter how complex or simple, are light-sensitive receptors (photoreceptors), and a mechanism to control light. In simple eyes, the light-sensitive receptors make up a layer known as the retina. The nature of a photoreceptor is dictated by opsin, the photosensitive proteinaceous pigment present within the membranes in the photoreceptor. The photoreceptors are variable in size, shape, and content. There are two types in vertebrates: rods and cones. Rods are especially sensitive to light and as such makeupthe majority of the retina of nocturnal species, who require as sensitive a system as possible. The photopigment in rods is called rhodopsin. Cones are less sensitive to light, but there are different types which are sensitive to light of different wavelengths (or colors), and can give animals color vision. There are many different types of cones, and hence many types of color vision; possession of these cone types and their specific positioning within the retina is an evolutionary adaptation particular to animals who benefit from color vision. There are two main types of eye design in the animal kingdom: simple eyes and compound eyes. Simple eyes have a single layer of photoreceptors, which, in the least complicated case, form a cup of photosensitive material. The human eye, with its complex light-focusing apparatus, is still a simple eye. Compound eyes, which are present in most insects, have many separate optical units, called ommatidia. Each ommatidium has a rhabdom, containing a group of up to nine tubular rhabdomeres, with ciliary or microvillar (finger-shaped) photoreceptors. The orientation of groups of photosensitive cilia is structured, often with pairs of rhabdomeres organized at right angles to each other. This is especially key in analysis of polarized light. Many visual systems have mechanisms to control light. Restriction of the amount of light entering the eye is useful; the opening through which light enters is referred to as an aperture, as in a camera. Many animals have a contractible iris which constricts and dilates to control light entry through the pupil. For example, in dim light conditions the iris can dilate and let in as much light as possible. Some eyes have lenses which enable light to be focused on the retina, allowing for better resolution of objects in the visual field. Many are able to change the shape of the lens in order to bring an image to focus on the retina, a process called accommodation. Other animals use a cornea to bend light onto the retina, although the cornea is rigid and cannot change shape.Onthe other hand, there are many species with much simpler eyes, which do not possess any kind of light control apparatus. Upon reception of light by a photoreceptor, a biochemical cascade of events occurs within the photoreceptor itself, which amplifies the original signal received. The result of this cascade is a nervous signal which proceeds through many neural layers to the brain. Throughout most of the retina in the simple eyes of vertebrates, several photoreceptors connect to one neuron (convergence), but there is often an area of the retina where one photoreceptor connects to one neuron. This area is called the fovea and is the part of the retina which has best acuity. The area within the retina which comprises the fovea is variable. Fish possess what is known as a "visual streak" fovea, which gives excellent vision along a horizontal slice of the visual field. This is an ideal adaptation for fish given their particular habitat. Information is passed through the nervous system in layers of neurons. The retina is, in fact, an extension of the brain, and contains many nerve cells. The exact arrangement and mechanism of action of neural cell types, and the precise pathway to and within the brain, differ greatly from species to species. Phototransduction in vertebrates is different from that of invertebrates, from the arrangement of the retina to the biochemical cascade and the types of neurons involved.
