Environmental Problems, Their Causes, and Sustainability 1

The size of a population that includes a large proportionof young organisms in their reproductivestage or that will soon enter this stage is likely to increase.In contrast, the size of a population dominatedby individuals past their reproductive stage is likely todecrease. The size of a population with a fairly evendistribution between these three stages will likely remainstable because the reproduction by younger individualswill be roughly balanced by the deaths ofolder individuals.What Limits Population Growth? Resourcesand CompetitorsNo population can grow indefinitely becauseresources such as light, water, and nutrients arelimited and because of the presence of competitorsor predators.Populations vary in their capacity for growth, alsoknown as the biotic potential of a population. The intrinsicrate of increase (r) is the rate at which a popula-0Growth factors(biotic potential)AbioticFavorable lightFavorable temperatureFavorable chemical environment(optimal level of critical nutrients)POPULATIONSIZEDecrease factors(environmental resistance)AbioticToo much or too little lightTemperature too high or too lowUnfavorable chemical environment(too much or too little of critical nutrients)tion would grow if it had unlimited resources. Mostpopulations grow at a rate slower than this maximum.Individuals in populations with a high rate ofgrowth typically reproduce early in life, have short generationtimes (the time between successive generations),can reproduce many times (have a long reproductivelife), and have many offspring each time they reproduce.Some species have an astounding biotic potential.Without any controls on population growth, the descendantsof a single female housefly could totalabout 5.6 trillion houseflies within about 13 months. Ifthis exponential growth kept up, within a few yearsthere would be enough houseflies to cover the earth’sentire surface!Fortunately, this is not realistic because no populationcan grow indefinitely. In the real world, a rapidlygrowing population reaches some size limit imposedby a shortage of one or more limiting factors, such aslight, water, space, or nutrients, or by too many competitorsor predators. In nature there are always limits topopulation growth. This important lesson from nature isthe main message of this chapter.Environmental resistance consists of all factorsthat act to limit the growth of a population. The sizeof the population of a particular species in a givenplace and time is determined by the interplay betweenits biotic potential and environmental resistance(Figure 9-3).Together biotic potential and environmentalresistance determine the carryingcapacity (K). This is the maximum numberof individuals of a given species thatcan be sustained indefinitely in a givenspace (area or volume). Thegrowth rate of a populationdecreases as its size nears thecarrying capacity of its environmentbecause resourcessuch as food and water beginto dwindle.BioticHigh reproductive rateGeneralized nicheAdequate food supplySuitable habitatAbility to compete for resourcesAbility to hide from or defendagainst predatorsAbility to resist diseases and parasitesAbility to migrate and live in otherhabitatsAbility to adapt to environmental changeBioticLow reproductive rateSpecialized nicheInadequate food supplyUnsuitable or destroyed habitatToo many competitorsInsufficient ability to hide from or defendagainst predatorsInability to resist diseases and parasitesInability to migrate and live in otherhabitatsInability to adapt to environmental changeFigure 9-3 Ecological trade-offs:factors that tend to increase or decreasethe size of a population.Whether the size of a populationgrows, remains stable, or decreasesdepends on interactions between itsgrowth factors (biotic potential) andits decrease factors (environmentalresistance).http://biology.brookscole.com/miller14165