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Problems of Small Populations

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1 Problems of Small Populations
Environment 121 Conservation of Biodiversity 28 April 2009 Prof Victoria Sork

2 Outline of lecture topics
Review of population growth models Density independence vs density dependence Age and stage structure populations Demographic issues for small populations Allee effect Demographic stochasticity Environmental stochasticity Genetic issues for small populations Effective population size Genetic drift Inbreeding depression 4/28/09

3 I. Review of Population Growth Model
Density-independent population growth models Without unlimited resources, populations can grow exponentially No competition or other density-dependent factors Birth and death rate are constant No immigration or emigration r = intrinsic rate of increase Examples: Invasive species, insects, pest populations dN/dt = rN 4/28/09

4 Population Growth Models, cont’d
Density Dependent Growth Population growth can be describe be logistic equation K=carrying capacity of environment for that species Determined by resources Modified by landscape change, habitat quality change, Reduced by competition from invasive species, Modify exponential growth model by adding a resistance factor Examples dN/dt = rN [(K— N)/K} = rN{1-(N/K)] 4/28/09

5 Examples of logistic growth

6 Demography Study of vital statistics that affect population structure
birth rate (fecundity) = number of offspring per unit time (in population or per female) this generally varies with age of individual death rate depending on species, environment, highest in juveniles and old individuals, but not always generation time = average time elapsed between birth of individuals and the birth of their offspring. Shorter generation time  fast population growth rate Larger organisms have longer generation time age structure: number of individuals of each age in the population affects population growth rate Survivorship: the likelihood of death at any given age COHORT is all the individuals in a population of the same age by following the death rate of a cohort 4/28/0

7 Age structured populations
First developed by life insurance companies Many organisms have different survival and reproductive rates at different stages Humans are good example The age structure is an indication of future population growth. 4/28/09

8 Age structure indicates future growth

9 Stage structured population
Many organisms show different life history stages a. insect: egg, pupa, n-star b. birds: eggs, fledglings, juveniles, non-reproductive adults, reproductive adults c. Plant: seed stage, seedling stage, sapling stage. juvenile, adult Survivorship and reproduction differs among life stages Different organisms and different circumstances may result in vulnerabilities at different stages Example: Sea Turtles 4/28/09

10 Generalized stage-structure demographic model: Sea turtles
eggs hatchlings Adults Juveniles Return to nesting beach Go to Sea Ocean P f P P P 4/28/09

11 Loggerhead turtles Weight: Adult weigh up to 350 pounds
Diet: Primarily carnivorous and feed mostly on shellfish Threats to Survival: The greatest threat is loss of nesting habitat due to coastal development, predation of nests, and human disturbances (such as coastal lighting and housing developments) that cause disorientations during the emergence of hatchlings. Other major threats include incidental capture in longline fishing, shrimp trawling and pollution. Incidental capture in fisheries is thought to have played a significant role in the recent population declines observed for the loggerhead. Population Estimate: 44,560 nesting females. 4/28/09 Taken from Caribbean Conservation Corporation,

12 Green turtle Weight: pounds Threats to Survival: The greatest threat is from the commercial harvest for eggs and food. Other green turtle parts are used for leather and small turtles are sometimes stuffed for curios. Incidental catch in commercial shrimp trawling is an increasing source of mortality. Diet: Changes significantly during its life. When less than 8 to 10 inches in length eat worms, young crustaceans, aquatic insects, grasses and algae. Once green turtles reach 8 to 10 inches in length, they mostly eat sea grass and algae, the only sea turtle that is strictly herbivorous as an adult. Population Estimate: 88,520 nesting females. 4/28/09 Taken from Caribbean Conservation Corporation,

13 Hawksbills Weight: 100 to 150 pounds Threats to Survival: The greatest threat to hawksbill sea turtle is the harvesting for their prized shell, often referred to as "tortoise shell." In some countries the shell is still used to make hair ornaments, jewelry, and other decorative items. Diet: The hawksbill's narrow head and jaws shaped like a beak allow it to get food from crevices in coral reefs. They eat sponges, anemones, squid and shrimp Population Estimate*: 22,900 nesting females. 4/28/09 Taken from Caribbean Conservation Corporation,

14 Leatherback Turtles Weight: 550-1,545 pounds Diet: feed almost exclusively on jellyfish. Nesting: Nest at intervals of 2 to 3 years, though recent research has indicated they can nest every year. Nests between 6 to 9 times per season, with an average of 10 days between nestings. Lays an average of 80 fertilized eggs, the size of billiard balls, and 30 smaller, unfertilized eggs, in each nest. Eggs incubate for about 65 days. Unlike other species of sea turtles, leatherback females may change nesting beaches, though they tend to stay in the same region. Threats to Survival: Greatest threat to leatherback sea turtles is from incidental take in commercial fisheries and marine pollution (such as balloons and plastic bags floating in the water, which are mistaken for jellyfish). Population Estimate*: 35,860 nesting females. 4/28/09 Taken from Caribbean Conservation Corporation,

15 Kemp's Ridley Sea Turtle
Weight: pounds Diet: Have powerful jaws that help them to crush and grind crabs, clams, mussels, and shrimp. They also like to eat fish, sea urchins, squid and jellyfish. Nesting: nest more often than other species, every 1 1/2 years on average. They also nest in mass synchronized nestings called arribadas. Kemp's ridley nest times each season. They lay an average of 110 eggs in each nest and the eggs incubate for about 55 days. Threats to Survival: The greatest threat to the Kemp's Ridley is from human use activities including collection of eggs and killing adults and juveniles for meat and other products. The significant decline in the number of Kemp's Ridley nests was a result of high levels of incidental take by shrimp trawlers. Population Estimate*: More than 2,500 nesting females. 4/28/09 Taken from Caribbean Conservation Corporation,

16 Olive Ridley Sea Turtle
Weight: pounds Diet: Have powerful jaws that allow for an omnivore diet of crustaceans (such as shrimp & crabs), mollusks, tunicates, fish, crabs, and shrimp. Threats to Survival: Direct harvest of adults and eggs, incidental capture in commercial fisheries and loss of nesting habitat are the main threats to this species. 4/28/09 Taken from Caribbean Conservation Corporation,

17 Flatback Sea Turtle Weight: 198 pounds Diet: Apparently eats sea cucumbers, jellyfish, mollusks, prawns, bryozoans, other invertebrates and seaweed. Threats to Survival: Sea turtles are threatened with capture, harvesting of eggs, destruction of nesting beaches, ocean pollution, oil spills and entanglement in fishing and shrimp nets. Nesting: Nests 4 times per season. Lays an average of 50 eggs at time, but these are comparatively quite large. The eggs incubate for about 55 days. When the hatchlings emerge, they are larger than most species. Population Estimate: 20,285 nesting females. 4/28/09

18 Life stage approach for conservation
Focus on critical phases in life cycle where impact of disturbance is occurring most dramatically What stage is survival interrupted? Is reproduction reduced? Is that a critical phase in the life of a plant? Example: Fruit dispersed by seed predators Gustavia superba, understory tree in Panama. 4/28/09

19 Arcese, Smith, & Hatch 1996 PNAS
Increase in nest predators on edges: Example: Female cowbird as nest predator Brood parasitism by brown-headed cowbirds reduces reproductive success in many passerines that nest in fragmented habitats and ecological edges, where nest predation is also common. --20 yr study of nest failure in song birds --Cowbirds caused nest failure in two ways: Parasitism of song bird nest Predation of other nests --Removal of Cowbirds increase song sparrow survival Year to year variation in population size of song birds can be explained by Cowbird presence. Arcese, Smith, & Hatch 1996 PNAS 4/28/09

20 Special problem for small populations: Allee effect
When population is large, positive relationship between population density and the reproduction and survival of individuals When population is small, increased difficultly of finding mates or reproducing at low population size. A special form of density-dependence 4/28/09

21 African hunting dogs must hunt in groups
Tightly knit social groups Hunt cooperatively;  approach a herd and chase one individual until it's exhausted.  Prey:  grazing animals such as gazelles, springboks, wildebeest and zebras. Mating: cooperative breeding with need for helpers creates Allee effect 4/28/09

22 Allele effect: Self-incompatibility in plants
Aster furcatus , Forked aster: understory plant: found at fewer than 50 sites in six Midwest states, is classified as threatened in Illinois. Gentiana pneumonanthe, European understory shrub. Both species have incompatibility systems: plant cannot mate with self or individuals with same allele at the s-locus. Problematic for isolated, small populations. 4/28/09

23 Demographic uncertainty
Definition  random events in survival and reproduction of individuals. Caused by: skewed sex ratio random sequence of deaths mainly effects small populations Can cause variance in r from  year to year, which reduces population size 4/28/09

24 Northern Right Whale Demography
“Northern right whales (Eubalaena glacialis) were formerly abundant in the northwestern Atlantic, but by 1900 they had been hunted to near extinction. After the end of commercial whaling the population was thought to be recovering slowly; however, evidence indicates that it has been declining since about There are now fewer than 300 individuals, and the species may already be functionally extinct, 3 owing to demographic stochasticity or the difficulty of females locating mates in the vast Atlantic Ocean (Allee effect).” Reference: Demography of the endangered North Atlantic right whale. Masami Fujiwara and Hal Caswell. Nature 414, (29 November 2001) FIGURE 6. The probability distribution of time to extinction assuming demographic stochasticity. 4/28/09

25 Recommendation for Right Whale Survival
Changes due to maternal survival Population growth rate  Fujiwara and Caswell showed that saving females whales is most critical factor. 4/28/09

26 Environmental stochasticity or uncertainty
Defintion: changes in weather, food supply, and the populations of competitors, predators and parasites affects populations of all sizes Can create density-independent conditions Natural catastrophes Extreme cases of environmental uncertainty Infrequent, short in duration, widespread in impact Can cause extinction in small populations 4/28/09

27 Extinction as a function of stochasticity
Brett A. Melbourne1 & Alan Hastings Nature 454, (3 July 2008) Extinction risk depends strongly on factors contributing to stochasticity “Extinction risk in natural populations depends on stochastic factors that affect individuals, and is estimated by incorporating such factors into stochastic models. Stochasticity can be divided into four categories, which include the probabilistic nature of birth and death at the level of individuals (demographic stochasticity), variation in population-level birth and death rates among times or locations (environmental stochasticity), the sex of individuals and variation in vital rates among individuals within a population (demographic heterogeneity). 4/28/09

28 Loss of Genetic diversity in small pops
H=1 - 1/[2 Ne] When effective population size is small, heterozygosity (and genetic diversity) is smaller. See Figure below from 4/28/09

29 What is Effective Population size?
sex ratio --Ne is reduced if number of males and females is not equal Example: If 1 male and 99 females, Ne = 3.96 If 25 males and 75 females, Ne = 75 If 50 males and 50 females, Ne = 10 Variance in Population Size from generation to generation If N1=100,000; N2=20, N3=100,000, What is Ne? ______ How much smaller than arithmetic mean? 4/28/09

30 Effect of family size on Ne
1. For idealized populations size, variance in family size for two 2 offpsring 2. Variation in family size can increase or decrease effective population size. 3. Different species of plants and animals have different mating systems Family ID Even family size Uneven 1 2 3 4 5 9 mean 1.8 Variance 1.7 16.2 4/28/09

31 Effect of variation in family size on Ne
Hypothetical Case: variance Nc Ne Ne/Nc 100 200 2.0 1 133 1.3 2 57 .1.0 5 .57 10 33 .33 Real Example Darwin's Cactus finch variance in family size: 6.74 Ne/Nc = 4/(6.74+4) = .46

32 Inbreeding depression
Fitness is the relative survivorship and fertility of individuals Inbreeding is mating among relatives When populations have a history of outcrossing, inbreeding often results in reduced fitness (known as inbreeding depression) Inbreeding depression: reduction in viablility, birth weight, and fertility due to breeding among relatives Causes: expression of deleterious alleles at one or several genes Potentially major problem for small populations. 4/28/09

33 Inbreeding in zoos Study by Katharine Ralls and Jonathon Ballou, 1983
Surveys of 16 ungulates, 12 small mammals, 16 primates Methods: Compare juvenile mortality of inbred versus non-inbred progeny Results: 41 of 44 species showed significantly higher mortality of inbred progeny 4/28/09

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