Ch 8 Features of a Population Population size Total # of organisms in a population Population density # of organisms per unit area Density is a reflection of the carrying capacity of the environment Structure Ratios of M to F, and different ages Distribution
Clumped, Random, Even (Uniform) How to get information about a population? Sampling: directly (collars) or indirectly (droppings) 2 Distribution Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2
Population Regulation Few species show Exponential Growth Pop Size is regulated by factors that limit population growth Density Independent Factors Physical Rainfall, Temperature, Acidity, Salinity Catastrophic Events Flood, Fire, Drought, Volcanic Eruption Density Dependent Factors Influenced by the density of the population (how crowded it is) Organisms that are more crowded: compete for more resources, easily found by predators, spread disease and parasites more readily Food Supply, Disease, Parasites, Competition, Predation
4 Changes in Population Size: Entrances and Exits Population is a group of organisms of the same species living together in one geographic area Populations increase through births and immigration Populations decrease through deaths and emigration Age Structure: Young Populations Can Grow Fast How fast a population grows or declines depends on its age structure.
Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce. Survivorship Curves Survivorship curve depicts age specific mortality. It is obtained by plotting the number of individuals of a particular cohort against time. Standardized to start at 1000 and as the population ages, the number of survivors progressively decline 3 hypothetical patterns Type 1: Late Loss
Type 2: Constant Loss Type 3: Early Loss Late loss population live to an old age. Constant loss population die at all ages. Most members of early loss population, die at young ages. 7 Reproductive Patterns: Large number of smaller offspring with
little parental care (r-selected species). Fewer, larger offspring with higher invested parental care (K-selected species). R selection- organisms with Type 3 survivorship K selection- organisms with Type 1 survivorship Figure 8-9 Reproductive Patterns
r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10 Population Growth Curves/Graphs Populations grow by increasing in number. 2 Types of Growth: Exponential (J) Logistic (S) No population can increase its size indefinitely. The intrinsic rate of increase (r) is the rate at which a population would grow if it had
unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat. Logistic As their ecosystem and community change, populations of organisms undergo different growth patterns. The most common growth pattern is called logistic growth, which takes the shape of an S. Logistic growth starts off fast, but gradually
slows down as the population encounters environmental resistance. Density Factors The density-dependent growth-limiting factors will cause the populations growth to slow and eventually stabilize. The point at which it stabilizes is the carrying capacity, or the maximum population size that can be sustained by the ecosystem. 11
*Currently Humans show this Exponential Exponential growth, which takes the shape of a J, does not experience growth-limiting factors. The population will continue to grow, eventually exceeding the carrying capacity. This is called an overshoot. When a population overshoots the carrying capacity, it experiences a dieback, often in the form of
mass-starvation. Exponential growth is unusual, and does not typically occur under normal conditions. 12 Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Over time species may increase their carrying capacity by developing
adaptations. Some species maintain their carrying capacity by migrating to other areas. So far, technological, social, and other cultural changes have extended the earths carrying capacity for humans. Figure 8-6 Types of Population Change Curves in Nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.
Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change. Types of Population Change Curves in Nature Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.
Figure 8-7 REPRODUCTIVE PATTERNS Some species reproduce without having sex (asexual). Offspring are exact genetic copies (clones). Others reproduce by having sex (sexual). Genetic material is mixture of two individuals. Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly. Major advantages: genetic diversity, offspring protection. Courtship rituals consume time and energy, can transmit disease, and can
inflict injury on males of some species as they compete for sexual partners.
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