Population Growth and Factors Affecting Growth

2. 32.1 Population Growth • A population is a group of individuals of the same species living together • Critical properties of a population include • Population size • The number of individuals in a population • Population density • Population size per unit area • Population dispersion • Scatter of individuals within a population’s range • Population growth • How populations grow and the factors affecting growth

3. The Exponential Growth Model • Assumes a population is growing without limits at its maximal rate • Rate is symbolized r and called the biotic potential Change over time Intrinsic rate of increase • Growth rate = dN/dt = riN No. of individuals in a population The actual rate of population increase is Birthrate Deathrate Net immigration • r = (b – d) + (i – e) Net emigration

4. Carrying Capacity • No matter how fast populations grow, they eventually reach a limit • This is imposed by shortages of important environmental factors • Nutrients, water, space, light • The carrying capacity is the maximum number of individuals that an area can support • It is symbolized by k

5. K – N ( ) • dN/dt = rN K Fig. 32.2 The Logistics Growth Model • As a population approaches its carrying capacity, the growth rate slows because of limiting resources • The logistic growth equation accounts for this • Growth rate begins to slow as N approaches K • It reaches 0 when N = K

6. Fig. 32.3 The Logistics Growth Model • A graphical plot of N versus t (time) gives an S-shaped sigmoid growth curve • History of a fur seal population on St. Paul Island, Alaska

7. 32.2 The Influence ofPopulation Density • Density-independent effects • Effects that are independent of population size but still regulate growth • Most are aspects of the external environment • Weather • Droughts, storms, floods • Physical disruptions • Fire, road construction

8. Song sparrow Fig. 32.4 32.2 The Influence ofPopulation Density Density-dependent effects • Effects that are dependent on population size and act to regulate growth Reproductive success decreases as population size increases • These effects have an increasing effect as population size increases

9. Fig. 32.5 32.2 The Influence ofPopulation Density Maximizing population productivity • The goal of harvesting organisms for commercial purposes is to maximize net productivity • The point of maximal sustainable yield lies partly up the sigmoid curve

10. 32.3 Life History Adaptations • Life history = The complete life cycle of an animal • Life histories are diverse, with different organisms having different adaptations to their environments • r-selected adaptations • Populations favor the exponential growth model • Have a high rate of increase • K-selected adaptations • Populations experience competitive logistic growth • Favor reproduction near carrying capacity

11. Most natural populations exhibit a combination of the r/k adaptations

12. Greek demos, “people” Greek graphos, “measurement” 32.4 Population Demography Demography is the statistical study of populations • It helps predict how population sizes will change in the future • Growth rate sensitive to • Age structure • Sex ratio

13. Age structure • Cohort = A group of individuals of the same age • Has a characteristic • Birth rate or fecundity • Number of offspring born in a standard time • Death rate or mortality • Number of individuals that die in that period • The relative number of individuals in each cohort defines a population’s age structure • Sex ratio • The proportion of males and females in a population • The number of births is usually directly related to the number of females

14. Fig. 32.7 • Provide a way to express the age distribution characteristics of populations • Survivorship is the percentage of an original population that survives to a given age • Survivorship curves • Type I • Mortality rises in postreproductive years • Type II • Mortality constant throughout life • Type III • Mortality low after establishment

15. 32.5 Communities • All organisms that live together in an area are called a community • The different species compete and cooperate with each other to make the community stable • A community is often identified by the presence of its dominant species • The distribution of the other organisms may differ a good deal • However, the ranges of all organisms overlap

16. 32.6 The Niche and Competition • A niche is the particular biological role of an organism in a community • It is a pattern of living • Competition is the struggle of two organisms to use the same resource • Interspecific competition occurs between individuals of different species • Intraspecific competition occurs between individuals of a single species

17. Fig. 32.9 Competition among two species of barnacles limits niche use • Because of competition, organisms may not be able to occupy their fundamental (theoretical) niche • Instead, they occupy their realized (actual) niche

18. Fig. 32.10 Competitive Exclusion • In the 1930s, G.F. Gause studied interspecific competition among three species of Paramecium • P. aurelia; P. caudatum; P. bursaria • All three grew well alone in culture tubes

19. Fig. 32.10 • However, P. caudatum declined to extinction when grown with P. aurelia • The two shared the same realized niche and the latter was better! • Gause formulated the principle of competitive exclusion • No two species with the same niche can coexist • But is one competitor always eliminated? • No, as we shall soon see!

20. Fig. 32.10 • P. caudatum and P. bursaria were able to coexist • The two have different realized niches and thus avoid competition • Gause’s principle of competitive exclusion can be restated • No two species can occupy the same niche indefinitely • When niches overlap, two outcomes are possible • Competitive exclusion or resource partitioning

21. Fig. 32.11 Resource Partitioning • Persistent competition is rare in natural communities • Either one species drives the other to extinction • Or natural selection reduces the competition between them Five species of warblers subdivided a niche to avoid direct competition with one another

22. Resource Partitioning • Sympatric species occupy same geographical area • Avoid competition by partitioning resources • Allopatric species do not live in the same geographical area and thus are not in competition • Sympatric speciestend to exhibit greater differences than allopatric species do • Character displacement facilitates habitat partitioning and thus reduces competition

23. Fig. 32.12 Character displacement in stickleback fish Resource Partitioning Feeds on both resources Feeds on plankton Feeds on larger prey

24. 32.7 Coevolution and Symbiosis • Coevolution is a term that describes the long-term evolutionary adjustments of species to one another • Symbiosis is the condition in which two (or more) kinds of organisms live together in close associations • Major kinds include • Mutualism – Both participating species benefit • Parasitism – One species benefits while the other is harmed • Commensalism – One species benefits and the other neither benefits nor is harmed

25. Fig. 32.14 Mutualism • Symbiotic relationship in which both species benefit Ants and Aphids • Aphids provide the ants with food in the form of continuously excreted “honeydew” • Ants transport the aphids and protect them from predators

26. Fig. 32.15 Mutualism • Symbiotic relationship in which both species benefit Beltian body Ants and Acacias • Acacias provide the ants with food in the form of Beltian bodies • Ants provide the acacias with organic nutrients and protect it from herbivores and shading from other plants

27. Fig. 32.16a Parasitism • Symbiotic relationship that is a form of predation • The predator (parasite) is much smaller than the prey • The prey does not necessarily die • External parasites • Ectoparasites feed on the exterior surface of an organism • Parasitoids are insects that lay eggs on living hosts • Wasps Dodder is a chlorophyll-less parasitic plant

28. Sarcocystis Fig. 32.16 Meadow pipit Cuckoo Internal parasites • Endoparasites live within the bodies of vertebrates and invertebrates • Marked by much more extreme specialization than external parasites • Brood parasitism • Birds lay their eggs in the nests of other species Foster parent Brood parasite • Brood parasites reduce the reproductive success of the foster parent hosts

29. Fig. 32.17 Commensalism • Symbiotic relationship that benefits one species and neither harms nor benefits the other Clownfishes and Sea anemones • Clownfishes gain protection by remaining among the anemone’s tentacles • They also glean scraps from the anemone’s food

30. Fig. 32.18 Cattle egrets and African cape buffalo • Egrets eat insects off of the buffalo Note: • No clear distinction between commensalism and mutualism • Difficult to determine if second partner benefits at all • Indeed, the relationship maybe even parasitic

31. Fig. 32.20 32.8 Predator-Prey Interactions Predation is the consuming of one organism by another, usually of a similar or larger size • Under simple laboratory conditions, the predator often exterminates its prey • It then becomes extinct itself having run out of food!

32. Fig. 32.21a 32.8 Predator-Prey Interactions • In nature, predator and prey populations often exhibit cyclic oscillations • The North American snowshoe hare (Lepus americanus) follows a “10-year cycle” • Two factors involved • 1. Food plants • Willow and birch twigs • 2. Predators • Canada lynx (Lynx canadensis)

33. Fig. 32.21b 32.8 Predator-Prey Interactions

34. 32.8 Predator-Prey Interactions • Predator-prey interactions are essential in the maintenance of species-diverse communities • Predators greatly reduce competitive exclusion by reducing the individuals of competing species • For example, sea stars prevent bivalves from dominating intertidal habitats • Other organisms can share their habitat • Keystone species are species that play key roles in their communities

35. 32.9 Plant and Animal Defenses • Plants have evolved many mechanisms to defend themselves from herbivores • Morphological (structural) defenses • Thorns, spines and prickles • Chemical defenses • Secondary chemical compounds • Found in most algae as well • Mustard oils • Found in the mustard family (Brassicaceae)

36. Adult Green caterpillar Fig. 32.23 The Evolutionary Response of Herbivores • Mustard oils protected plants from herbivores at first • At some point, however, certain insects evolved the ability to break down mustard oil • These insects were able to use a new resource without competing with other herbivores for it • Cabbage butterfly caterpillars

37. Blue jay Fig. 32.24 Animal Defenses • Some animals receive an added benefit from eating plants rich in secondary chemical compounds • Caterpillars of monarch butterflies concentrate and store these compounds • They then pass them to the adult and even to eggs of next generation • Birds that eat the butterflies regurgitate them I’m not eating this again!

38. Inchworm caterpillar Fig. 32.25 Dendrobatid frog Fig. 32.26 • Cryptic coloration • Color that blends with surrounding • Aposematic coloration • Showy color advertising poisonous nature • Defensive coloration Camouflage! Warning! • Chemical defenses • Stings – Bees and wasps • Toxic alkaloids – Dendrobatid frogs

39. 32.10 Mimicry • Many non-poisonous species have evolved to resemble poisonous ones with aposematic coloration • Two types of mimicry have been identified • Batesian mimicry • After Henry Bates, a 19th century British naturalist • Müllerian mimicry • After Fritz Müller, a 19th century German biologist

40. Monarch butterfly Fig. 32.27 Viceroy butterfly Batesian Mimicry • A harmless unprotected species (mimic) resembles a poisonous model that exhibits aposematic coloration • If the mimics are relatively scarce, they will be avoided by predators

41. Yellow jacket Masaridwasp Fig. 32.28 Sand wasp Anthidiine bee Müllerian Mimicry • Two or more unrelated but protected (toxic) species come to resemble one another • Thus a group defense is achieved

42. Self Mimicry • Involves adaptations where one animal body part comes to resemble another • This type of mimicry is used by both predator and prey • Example • “Eye-spots” found in many butterflies, moths and fish

43. 32.11 Ecological Succession • Succession is the orderly progression of changes in community composition that occur over time • Secondary succession • Occurs in areas where an existing community has been disturbed • Primary succession • Occurs on bare lifeless substrates, like rocks • The first plants to appear from a pioneering community • The climax community comes at the end

44. Why Succession Happens • Three dynamic critical concepts • 1. Tolerance • First to come are weedy r-selected species that are tolerant of the harsh abiotic conditions • 2. Facilitation • Habitat changes are introduced that favor other, less weedy species • 3. Inhibition • Habitat changes may inhibit the growth of the species that caused them

45. Why Succession Happens • As ecosystems mature, more K-selected species replace r-selected ones • Species richness and total biomass increase • However, net productivity decreases • Thus, agricultural systems are maintained in early successional stages to keep net productivity high

46. 32.12 Biodiversity • Biologically diverse ecosystems are in general more stable than simple ones • Species richness refers to the number of species in an ecosystem • It is the quantity usually measured by biologists to characterize an ecosystem’s biodiversity • Two factors are important in promoting biodiversity • Ecosystem size • Latitude

47. Ecosystem Size • Larger ecosystems contain more diverse habitats and therefore have greater number of species • A reduction in an ecosystem size, will reduce the number of species it can support • Faunal collapse (extinction) may occur in extreme cases

48. Fig. 32.32 Latitude • The number of species in the tropics is far more than that in the arctic region • Two principal reasons • 1. Length of growing season • 2. Climatic stability

49. Island Biodiversity • In 1967, Robert MacArthur and Edward O. Wilson proposed the equilibrium model • The species richness on islands is a dynamic equilibrium between colonization and extinction • Two important factors • Island size • Larger islands have more species than smaller ones • Distance from mainland • Distant islands have less species than those near the mainland

50. Small distant islands have fewer bird species Fig. 32.33 The equilibrium model of island biogeography Equilibrium Shifting equilibrium