Chapter 10: Population Dynamics
static life table
A second way to estimate patterns of survival in wild populations is to record the age at death of a large number of individuals. This method differs from the cohort approach because the individuals in your sample are born at different times. This method produces a static life table. The table is called static because the method involves a snapshot of survival within a population during a short interval of time. To produce a static life table, the biologist often needs to estimate the age at which individuals die. This can be done by tagging individuals when they are born and then recovering the tags after death and recording the age at death. An alternative procedure is to somehow estimate the age of dead individuals. The age of many species can be determined reasonably well. (ex: growth rings on horns of mountain sheep)
survivorship curve
A survivorship curve summarizes the pattern of survival in a population. Patterns of survival vary a great deal from one species to another and, depending on environmental circumstances, can vary substantially even within a single species.
age distribution -what are the underlying assumptions?
A third way of determining patterns of survival is from the age distribution. An age distribution consists of the proportion of individuals of different ages within a population. You can use an age distribution to estimate survival by calculating the difference in proportion of individuals in succeeding age classes. This method, which also produces a static life table, assumes that the difference in numbers of individuals in one age class and the next is the result of mortality. What are some other major assumptions underlying the use of age distributions to estimate patterns of survival? This method requires that a population is neither growing nor declining and that it is not receiving new members from the outside or losing members because they migrate away. Since most of these assumptions are often violated in natural populations, a life table constructed from this type of data tends to be less accurate than a cohort life table. Static life tables are often useful, however, since they may be the only information available.
numerical responses to increase prey availability
C. S. Holling (1959a) also observed numerical responses to increased prey availability. Numerical responses are changes in the density of predator populations in response to increased prey density. Holling studied populations of mice and shrews preying on insect cocoons and attributed the numerical responses he observed to increased reproductive rates. He commented that, "because the reproductive rate of small mammals is so high, there was an almost immediate increase in density with increase in food." However, some other predators, with much lower reproductive rates, also show strong numerical responses. These numerical responses to prey density are almost entirely due to dispersal.
To calculate the total number of seeds produced by this population during the year of study, multiply
To calculate the total number of seeds produced by this population during the year of study, multiply 2.4177 by 996, which was the initial number of plants in this population. The result, 2,408, is the number of seeds that this population of P. drummondii will begin with the next year.
to track birthrates in a sexually reproducing species, what do population biologists need to know? -what is this data called?
Tracking birthrates in a population is similar to tracking survival rates. In a sexually reproducing population, the population biologist needs to know the average number of births per female for each age class and the number of females in each age class. In practice, the ecologist counts the number of eggs produced by birds or reptiles, the number of fawns produced by deer, or the number of seeds or sprouts produced by plants. The numbers of offspring produced by parents of different ages are then tabulated. The tabulation of birthrates for females of different ages in a population is called a fecundity schedule.
mullers colonization cycle and aquatic insect larvae
Many studies support Müller's hypothesized colonization cycle, especially among aquatic insects. Aquatic insect larvae disperse upstream as well as downstream by swimming, crawling, and drifting, while most adult aquatic insects disperse by flying. Because of continuous dispersal, which reshuffles stream populations, new substrates put into streams are quickly colonized by a wide variety of stream invertebrates, algae, and bacteria. Most of these dynamics are difficult to observe because they occur too quickly, within the substratum, or at night, or they involve microorganisms impossible to observe directly without the aid of a microscope.
maple vs hemlock northward movement towards its present range following ice age
Though the distributions of maple and hemlock overlap today, they did not during the height of the last ice age. In addition, maple colonized the northern part of its present range from the lower Mississippi Valley region, while hemlock colonized its present range from a refuge along the Atlantic coast. The two trees dispersed at very different rates. Of the two species, maple dispersed faster, arriving at the northern limits of its present-day range about 6,000 years ago. In contrast, hemlock didn't reach the northwestern limit of its present distribution until 2,000 years ago.
3 main ways of estimating patterns of survival within a population
1) cohort life table 2) static life table 3) age distribution
A life table combined with a fecundity schedule can be used to estimate (4 things)
A life table combined with a fecundity schedule can be used to estimate: net reproductive rate (R0), geometric rate of increase (λ), generation time (T), and per capita rate of increase (r). In addition to survival rates, population ecologists are concerned about another major influence on local population density—birthrates.
summary of the three types of survivorship curves
A relatively high rate of survival among young and middle-aged individuals followed by a high rate of mortality among the aged is known as a type I survivorship curve. This is the pattern of survival we saw in populations of Dall sheep, P. drummondii, and rotifers (see figs. 10.14 and 10.15). Constant rates of survival throughout life produce the straight-line pattern of survival known as a type II survivorship curve. American robins, white-crowned sparrows, and common mud turtles show this pattern of survival (see fig. 10.16). A type III survivorship curve is one in which a period of extremely high rates of mortality among the young is followed by a relatively high rate of survival. The desert plant Cleome provides an excellent example of a type III survivorship curve (see fig. 10.17).
Why doesn't the flowing water of streams eventually wash all stream organisms, including fish, insects, snails, bacteria, algae, and fungi, out to sea?
All stream dwellers have a variety of characteristics that help them maintain their position in streams. Some fish, such as trout, are streamlined and can easily swim against swift currents, while other fish like sculpins and loaches are well designed for avoiding the full strength of currents by living on the bottom and seeking shelter among or under stones. Microorganisms resist being washed away by adhering to the surfaces of stones, wood, and other substrates. Many stream insects are flattened and so stay out of the main force of the current, while others are streamlined and fast-swimming.
Now let's turn from elevation responses to changes in the latitudinal ranges of species over the period of recent warming.
As with changes in elevation, documenting latitudinal changes depends on the availability of baseline surveys of earlier distributions. One of the few places with extensive long-term records of latitudinal distributions over a large geographic area is the UK. Researchers have taken advantage of the extensive national inventory of species available there to study whether the range limits of species have moved northward within the British Isles. In this study, Rachael Hickling of the University of York and colleagues estimated the latitudinal shifts in the distributions of 16 groups of organisms ranging from birds, mammals, and butterflies to spiders, dragonflies, and ground beetles over a roughly 25-year period. results: Of the 329 species included in their analysis, 275, or roughly 84%, had shifted their ranges northward, 52 had moved southward, while the distributions of 2 species remained unchanged. The results of the studies reviewed here and hundreds of others have demonstrated changes in population distributions in response to well-documented increases in global temperatures over the past century. The distribution of earth's biota is changing as we watch.
Eurasian collared doves
Birds provide some of the best examples of rapid population expansion. The expansion of Eurasian collared doves into Europe was notable in a number of ways. First, the spread began suddenly and, once begun, was relentless. By the 1980s, the doves were found in every country of western and eastern Europe (fig. 10.3). Eurasian collared doves have also rapidly colonized North America. A population of the species was established in the Bahamas in the 1970s by birds released during a burglary of a pet shop in Nassau. By the early 1980s, Eurasian collared doves colonized nearby Florida, subsequently spreading rapidly across North America and reaching California by the mid-1990s. The expansion took place in small jumps. Adult Eurasian collared doves are highly sedentary, and dispersal is limited to young doves. Most dispersing young stay within a few kilometers of their parent's nest, but some disperse hundreds of kilometers (fig. 10.4). Once they have chosen a mate, the young birds nest and become sedentary like their parents. These pulses of dispersal by young birds spread the Eurasian collared dove population across Europe at a rate of about 45 km per year.
How does this rate of expansion by Eurasian collared doves compare to rates of expansion by other populations?
Compared with the dispersal rate of Africanized honeybees across the Americas, 45 km per year is a modest rate. However, compared with dispersal rates for most other animals that have been studied, 45 km per year is rapid. Figure 10.5, which summarizes rates of dispersal by several species of mammals and birds, shows that rates of dispersal differ by three orders of magnitude.
Whatever its cause, stream organisms drift downstream in large numbers. Why doesn't drift eventually eliminate organisms from the upstream sections of streams? -Karl Müller and the *colonization cycle* of stream invertebrates
Karl Müller (1954, 1974) hypothesized that drift would eventually wash entire populations out of streams unless organisms actively moved upstream to compensate for drift. He proposed that stream populations are maintained through a dynamic interplay between downstream and upstream dispersal that he called the colonization cycle. The colonization cycle is a dynamic view of stream populations in which upstream and downstream dispersal, as well as reproduction, have major influences on stream populations (fig. 10.8).
per capita rate of increase -equation -what two variables do you need?
Knowing R0 and T allows us to estimate r, the per capita rate of increase for a population: ln R0 is the natural logarithm of R0.) We can interpret r as birthrate minus death rate: r = b−d.
Is there any supporting evidence for high rates of movement by kestrels and owls?
Korpimäki (1988) captured 217 kestrels from 1983 to 1987. Because European kestrel populations have an annual survival rate of 48% to 66%, he predicted a high rate of recapture of marked birds. However, only 3% of the female and 13% of the male kestrels were recaptured. These very low rates of recapture indicated that kestrels were moving into and out of the study area. From their data, Korpimäki and Norrdahl concluded that the kestrels and owls in western Finland are nomadic, moving from place to place in response to changing vole densities.
spates and drift
Despite these means of staying in place, stream organisms do get washed downstream in large numbers, particularly during flash floods, or spates. Stream ecologists refer to this downstream movement of stream organisms as drift. Some drift is due to displacement of organisms during flash floods. However, some is due to the active movement of organisms downstream.
what is a stable age distribution
In a population with a stable age distribution, the proportion of individuals in each of the age classes is constant.
What value of R0 would produce a stable turtle population?
In a stable population, R0 would be 1.0, which means that each female would replace only herself during her lifetime. In a growing population, such as the population of Phlox, R0 would be greater than 1.0.
Africanized honeybees -just how they came to be
In an attempt to improve the adaptability of managed honeybees to their tropical climate, Brazilian scientists imported queens of the African subspecies Apis melifera scutellata in 1956. These queens mated with the European honeybees used by Brazilian beekeepers, producing what we now call Africanized honeybees.
Applications: Arianta arbustorum. and climate change
In chapter 5, we reviewed a study by Bruno Baur and Anette Baur (1993) that documented local extinctions of this species around Basel, Switzerland, where the urban heat island has warmed habitats that once supported populations of the snail. The Baurs' study also provided a potential mechanism to explain those local extinctions: reduced reproduction. In experiments designed to test the influence of temperatures on reproduction, the Baurs observed reduced hatching rates of A. arbustorum eggs at temperatures as low as 22°C and no hatching at temperatures of 25°C and above. How might A. arbustorum respond to large-scale climate warming? The Baurs explored the possibility that A. arbustorum may be shifting its distribution to higher elevations in Switzerland, as the landscape warms (Baur and Baur 2013). Because average air temperature is lower at higher elevations (see figure 2.39), their expectation was that the species would shift its distribution upward. Because species inventories were made soon after the establishment of the Swiss National Park, the Baurs had a basis for determining whether the distribution of A. arbustorum has changed over time. Yet another critical piece of information for such studies is a reliable long-term weather record. Fortunately, weather data have been collected at a weather station on the edge of the Swiss National Park, just 16 km from the study sites, since 1917. The weather records at the station show that during the period since the initial studies of A. arbustorum, the average annual temperature in the Swiss National Park has risen 1.6°C, while precipitation has not changed significantly. results and conclusion: The Baurs' resurvey revealed significant increases in the upper elevation limit of A. arbustorum along slopes in the park. In conclusion, the Baurs' research verified significant increases in the upper elevation limit of A. arbustorum, a species sensitive to warm temperatures, during a century of climate warming.
birthrate definition
In mammals and other live-bearing organisms, from sharks to humans, the term birthrate means the number of young born per female in a period of time. Population biologists also use the term birth more generally to refer to any other processes that produce new individuals in the population. In populations of birds, fish, and reptiles, births are usually counted as the number of eggs laid. In plants, the number of births may be the number of seeds produced or the number of shoots produced during asexual reproduction. In bacteria, the birth, or reproductive, rate is taken as the rate of cell division.
life tables -what do they list
In response to practical challenges of discerning patterns of survival, population biologists have invented bookkeeping devices called life tables that list both the survivorship and the deaths, or mortality, in populations.
dispersal within a metapopulation of Lesser Kestrels (migratory falcons that breed in colonies of monogamous pairs)
Lesser kestrels have suffered a high rate of decline across their range and are listed as a globally threatened species. In contrast to its global circumstance, the lesser kestrel population of the Ebro River valley of northeastern Spain has grown dramatically in 7 years. Serrano and Tella attribute this regional growth to sustained traditional farming practices in the Ebro River valley. However, they warn that plans to modernize farming practices in the Ebro River valley may lead to the population declines seen elsewhere. Serrano and Tella used numbered, color leg bands to mark and track individual kestrels in their study population. Within colonies, the percentage of banded adults of known age ranged from 60% to 90%. The data gathered indicate that a substantial percentage of birds leave the breeding colony where they hatched to join other subpopulations in their first year of breeding. However, females in this species are more likely to move than males. The rate of emigration by first-breeding females is approximately 30% versus 22% for first-breeding males. In contrast, less than 4% of older adults emigrate from a colony on any given year. Though some lesser kestrels in the study population dispersed more than 100 km, Serrano and Tella found a negative correlation between distance between colonies and the frequency of dispersal between them.
Estimating rates when generations overlap (unlike annual plants which do not have overlapping generations) calculating R0, and T for common mud turtle
Let's examine some of the details of this turtle's reproductive patterns in order to calculate the net reproductive rate of this population. About half (0.507) of the turtles nest each year. Of those females that do nest, most nest once during the year. However, some nest twice and a few even nest three times during the year. The average number of nests per year for the nesting turtles is 1.2, which means that 0.2, or one-fifth, of the turtles produce a second nest each year. The average clutch size, the number of eggs produced by a nesting female, is 3.17. So, the average number of eggs produced by nesting females each year is 3.17 eggs per nest × 1.2 nests per year = 3.8 eggs per year. However, remember that only half the females in the population nest each year. Therefore, the number of eggs per female per year is 0.507 × 3.8 = 1.927 eggs per female per year. This is the average total number of eggs per female. On average, half of these eggs will develop into males and half into females. Since the sex ratio in this turtle population is 1 male:1 female, we multiply 1.927 by 0.50 to calculate the number of female eggs per adult female in the population, which equals 0.96 female egg. This is the value listed in column 3 of table 10.2 (which is a *life table*) As in the Phlox population, the sum of lxmx, ∑lxmx provides an estimate of R0, the net reproductive rate of females in this population. In this case, R0=0.601. We can interpret this number as the average number of daughters produced by each female in this population over the course of her lifetime. If this number is correct, the mothers in this population are not producing enough daughters to replace themselves. It appears that this population is declining.
How might other species in the region have responded to climate warming? The work of another research group suggests widespread movement upslope by species in the region. This study examined changes in the upper elevation limits of over 600 species of plants, insects, and birds in the Bavarian Forest National Park in southeastern Germany, located approximately 400 km northeast of the Swiss National Park, where the Baurs studied A. arbustorum (Bässler et al. 2013).
Like the Baurs' study, researchers had access to long-term weather data at their study area. The Bavarian Forest National Park is also protected from most human disturbance and the upper elevation limits of species in the park had been documented in 1902-04, over a century before Bässler and his colleagues conducted their study. results: In contrast to many studies elsewhere in central and northern Europe (e.g., see Parolo and Rossi 2008), plants did not shift their elevation range in the Bavarian Forest National Park. However, the 433 species of insects in the study increased their elevation limits an average of 260 m, while 57 bird species were recorded an average of 165 m higher than previously. These results suggest that large numbers of both vertebrate and invertebrate species are extending their distributions upslope with climate warming.
In 1923, R. B. Miller published data on the age distribution of a population of white oak, Quercus alba, in a mature oak-hickory forest in Illinois. -Miller first determined the relationship between -miller then used this info to estimate ?
Miller first determined the relationship between the age of a white oak and the diameter of its trunk. Miller used diameter to estimate the ages of hundreds of trees.
How well does this classification (the three curves) of survivorship represent natural populations?
Most populations do not conform perfectly to any one of the three basic types of survivorship but show virtually every sort of intermediate form of survivorship between the curves. If survivorship can be so variable within species, what good are these idealized, theoretical survivorship curves? Their most important value, like most theoretical constructs, is that they set boundaries that mark what is possible within populations. Regardless of how closely actual survivorship curves approximate the theoretical curves, they serve excellent summaries of survival patterns within populations.
What was the age distribution of the oaks in Miller's study?
Most white oaks in Miller's study forest were concentrated in the youngest age class of 1 to 50 years, with progressively fewer individuals in the older age classes (fig. 10.19). The oldest white oaks in the forest were over 300 years old. In other words, the age distribution of white oak in this forest was biased toward the young trees.
Adolph Murie (1944) studied patterns of survival among Dall sheep
Murie estimated survival patterns by collecting the skulls of 608 sheep that had died from various causes. He determined the age at which each sheep in his sample died by counting the growth rings on their horns and by studying tooth wear. The major assumption of this study was that the proportion of skulls in each age class represented the typical proportion of individuals dying at that age results: Notice that in this population of Dall sheep, there are two periods when mortality rates are higher: during the first year and during the period between 9 and 13 years. Juvenile mortality and mortality of the aged are higher in this population, while mortality in the middle years is lower. The overall pattern of survival and mortality among Dall sheep is much like that for a variety of other large vertebrates, including red deer, Cervus elaphus, Columbian black-tailed deer, Odocoileus hemionus columbianus, East African buffalo, Syncerus caffer, and humans. The key characteristics of survival among these populations are relatively high rates of survival among the young and middle-aged and high rates of mortality among the older members. This pattern of survival has also been observed in populations of annual plants and small invertebrate animals. pic: summarizes the survival patterns for Dall sheep based on Murie's sample of skulls. The upper portion of the figure shows the static life table that Murie constructed. The first column lists the ages of the sheep, the second column lists the number surviving in each age class, and the third column lists the number dying in each age class. Notice that although Murie studied only 608 skulls, the numbers in the table are expressed as numbers per 1,000 individuals. This adjustment is made to ease comparisons with other populations.
equation for the processes that contribute to population size
Nt=N(t − 1)+ B + I − D − E note that N(t-1) is N sub t-1 where Nt is the number of individuals in a population at some time, t; Nt−1 is the number of individuals in the population at some previous time, t − 1; B is the number of births that have occurred during the interval between t − 1 and t; I is the number of immigrants to the population during that time interval; D is the number of deaths; and E is the number of individuals that have emigrated, or left, the population.
subpopulation vs metapopulation
Ongoing dispersal can join numerous subpopulations to form a metapopulation. Populations of many species occur not as a single continuously distributed population but as spatially separated subpopulations. A subpopulation is a part of a larger population, with which it sustains a limited exchange of individuals through immigration and emigration. A group of subpopulations living on such patches connected by exchange of individuals among patches make up a metapopulation.
The age distribution of a population reflects
The age distribution of a population reflects its history of survival, reproduction, and potential for future growth. Population ecologists can tell a great deal about a population just by studying its age distribution. Age distributions indicate periods of successful reproduction, periods of high and low survival, and whether the older individuals in a population are replacing themselves or if the population is declining. By studying the history of a population, population ecologists can make predictions about its future.
Why have Rio Grande cottonwoods failed to reproduce?
Reproduction by Rio Grande cottonwoods depends on seasonal floods, which play two key roles. First, floods create areas of bare soil without a surface layer of organic matter and without competing vegetation, ideal conditions for germination and establishment of cottonwood seedlings. Floods also keep these nursery areas of bare soil moist until cottonwood seedlings can grow their roots deep enough to tap into the shallow water table. Historically, these conditions were created by spring floods, the timing of which coincided with dispersal of cottonwood seeds by wind. The annual rhythm of seed bed preparation and seeding has been interrupted by the construction of dams on the Rio Grande for flood control and irrigation. The tamed Rio Grande no longer floods like it once did, and though Rio Grande cottonwoods produce seeds each year, their age distribution indicates that these seeds find few suitable places to germinate. The age distributions of tree populations change over the course of many decades or centuries. Meanwhile, other populations can change significantly on much shorter timescales. One of these dynamic populations has been thoroughly studied on the Galápagos Islands.
geometric rate of increase
Since P. drummondii has pulsed reproduction, we can estimate the rate at which its population is growing with a quantity known as the geometric rate of increase λ. The geometric rate of increase is the ratio of the population size at two points in time: (see image) In this equation, Nt+1 is the size of the population at some future time and Nt is the size of the population at some earlier time (fig. 10.22). The time interval t may be years, days, or hours; which time interval you use to calculate the geometric rate of increase for a population depends on the organism and the rate at which its population grows.
calculated geometric rate of increase for P. drummondii
Since P. drummondii is an annual plant, the most meaningful time interval would be 1 year. The initial number, Nt, of P. drummondii in the population was 996. The number of individuals (seeds) in the population at the end of a year of study was 2,408. This is the number in the next generation, which is Nt+1. Therefore, the geometric rate of increase for the population over the period of this study was: λ= 2,408 / 996=2.4177
high mortality among the young
Some organisms produce large numbers of young with very high rates of mortality. The eggs produced by marine fish such as the mackerel, Scomber scombrus, may number in the millions. Out of 1 million eggs laid by a mackerel, more than 999,990 die during the first 70 days of life as eggs, larvae, or juveniles. Survival rates are similar in populations of the prawn Leander squilla off the coast of Sweden. Similar patterns of survival are shown by other marine invertebrates and fish and by plants that produce immense numbers of seeds. One of these plants is Cleome droserifolia, a desert shrub studied by Ahmad Hegazy (1990). Hegazy estimated that a local population of approximately 2,000 plants produce almost 20 million seeds each year. Of these, approximately 12,500 seeds germinate and produce seedlings. Only 800 seedlings survive to become juvenile plants. about 39 survive to the age of 1 year, a survival rate of only 0.0039%.
Rio Grande cottonwoods, Populus deltoides age distibution comparison to that of the white oak
The age distribution of this white oak population contrasts sharply with the age distributions of populations of Rio Grande cottonwoods, Populus deltoides subsp. wislizenii. studies of age distributions indicate that these populations are declining. Older trees, which can live to about 130 years, are not being replaced by younger trees (fig. 10.20). In contrast to the white oak population in Illinois, the Rio Grande cottonwood population is dominated by older individuals. At the study site represented by figure 10.20, there has been no reproduction for over a decade.
Estimating rates for an annual plant -explanation of table (see pic) -interpretation of the table
Table 10.1 combines survivorship with seed production by the annual plant P. drummondii. The first column, x, lists age intervals in days. The second column, nx, lists the number of individuals in the population surviving to each age interval. The third column, lx, lists survivorship, the proportion of the population surviving to each age x. The fourth column, mx, lists the average number of seeds produced by each individual in each age interval. Finally, the fifth column, lxmx, is the product of columns 3 and 4.
how africanized honeybees differ from european honeybees
Temperate and tropical environments have apparently selected for markedly different behavior and population dynamics. Natural selection by a high diversity and abundance of nest predators, including humans, has probably produced the greater aggressiveness shown by Africanized honeybees. The warmer climate and greater stability of nectar sources eliminate the advantages of storing large quantities of honey and maintaining large colonies for survival through the winter. Most important to this discussion of dispersal, Africanized honeybees produce swarms that disperse to form new colonies at a much higher rate than do European honeybees. High rates of colony formation and dispersal have caused a rapid expansion of Africanized honeybees through South and North America. The honeybees stopped spreading southward through South America by about 1983, stopping at about 34° S latitude. However, they continue to spread northward through North America and will continue to do so until stopped by climatic factors.
snails and the colonization cycle -rio claro
The Rio Claro flows approximately 30 km through tropical forest on the Osa Peninsula of Costa Rica before flowing into the Pacific Ocean. One of the most easily observed inhabitants of the Rio Claro is the snail Neritina latissima, which occupies the lower 5 km of the river. The eggs of Neritina hatch to produce free-living planktonic larvae that drift down to the Pacific Ocean. After the larvae metamorphose into small snails, they reenter the Rio Claro and begin moving upstream in huge migratory aggregations of up to 500,000 individuals (fig. 10.9). These aggregations move slowly and may take up to 1 year to reach the upstream limit of the population. Daniel Schneider and John Lyons (1993) discovered that the population of Neritina in the Rio Claro consists of a mixture of migrating and stationary subpopulations, with exchange between them. Individual snails migrate upstream for some distance and then leave the migrating wave and enter a local subpopulation. At the same time, individuals from the local subpopulation enter the migratory wave and move upstream. Thus, individuals move upstream in steps and immigration continuously adds to local subpopulations, while emigration removes individuals. Because an organism that is visible to the naked eye does all this in a clear stream, and does it at a snail's pace, we are provided with a unique opportunity to observe that dispersal can strongly influence local population density.
What might we infer from this age distribution?
The age distribution indicates that reproduction is sufficient to replace the oldest individuals in the population as they die. That is, this population of white oaks appeared to be either stable or growing.
dynamic population in a variable climate Rosemary Grant and Peter Grant (1989) have spent decades studying Darwin's finch populations. One of their most thorough studies has concerned the large cactus finch, Geospiza conirostris, on the island of Genovesa
The age distributions of the large cactus finch during 1983 and 1987 show that the population can be very dynamic (fig. 10.21). The 1983 age distribution shows a fairly regular distribution of individuals among age classes. However, there were no 6-year-old individuals in the population. This gap is due to a drought in 1977, during which no finches reproduced. Now, compare the 1983 and 1987 age distributions. The distributions contrast markedly, though they are for the same population separated by only 4 years! The 1977 gap is still present in the 1987 age distribution, and another has been added for 2- and 3-year-old finches. This second gap is the result of 2 years of reproductive failure during a drought that persisted from 1984 to 1985. Another difference is that the 1987 age distribution is dominated by 4-year-old birds that were fledged during 1983. The 1983 class dominates because wet weather that year resulted in very high production of food that the finches depend on for reproduction. *This long-term study of the large cactus finch population of Genovesa Island demonstrates the responsiveness of population age structure to environmental variation.*
population dynamics
The dynamic population processes underlying distribution and abundance are the subject of chapter 10. We call this area of ecology population dynamics, which is concerned with the factors influencing the expansion, decline, and maintenance of populations.
cohort cohort life table
The first and most reliable way is to identify a large number of individuals that are born at about the same time and keep records on them from birth to death. A group born during the same time period—for example, the same year—is called a cohort. A life table made from data collected in this way is called a cohort life table. The cohort studied might be a group of plant seedlings that germinated at the same time or all the lambs born into a population of mountain sheep in a particular year.
comparison of the lesser ketrels falcon to the rocky mountain parnassian butterfly
The lesser kestrels of the Ebro River valley and the Rocky Mountain Parnassian butterflies of southern Alberta, Canada, interact with their environments on greatly different scales. In addition, the butterfly population appears to be contracting spatially; the lesser kestrel population is expanding. However, these populations also have a number of features in common. First, they both are spatially organized into metapopulations. Another feature that the two populations share is the influence of local population size on the tendency to disperse and the direction of dispersal. Like P. smintheus, lesser kestrels in smaller subpopulations are more likely to emigrate than are individuals in larger subpopulations. Second, lesser kestrels are more likely to disperse from small colonies to larger colonies.
Using this method, the estimated per capita rate of increase for the common mud turtle population of Ellenton Bay is: -what does the negative value of r indicate?
The negative value of r in this case indicates that birthrates are lower than death rates and the population is declining. A value of r greater than 0 would indicate a growing population, and a value equal to 0 would indicate a stable population.
What mechanisms produce the numerical responses by kestrels and owls to changing vole densities?
The peaks in raptor densities in 1977, 1982, and 1986 match the peaks in vole densities almost perfectly. If reproduction were the source of numerical response by kestrels and owls, there would have been more of a delay, or time lag, in kestrel and owl numerical response. From this close match in numbers, Korpimäki and Norrdahl proposed that kestrels and owls must move from place to place in response to local increases in vole populations.
metapopuilation of alipine butterfly P. smintheus
The range of P. smintheus extends from northern New Mexico along the Rocky Mountains to southwestern Alaska. Along this range P. smintheus caterpillars feed mainly on the leaves and flowers of stonecrop, Sedum sp., in areas of open forest and meadows. Because of their tie to a narrow range of host plants, P. smintheus populations are often distributed among the habitat patches occupied by their host plant, appearing to form metapopulations. A combination of fire suppression by forest managers and global warming appears to be decreasing the size of alpine meadows and increasing their isolation from each other by intervening forest. In 1952, the study meadows averaged approximately 36 ha in area. By 1993, the average area of these meadows had declined to approximately 8 ha, a decrease in area of approximately 77%. These changes motivated the research team of Roland, Keyghobadi, and Fownes to study the influences of meadow size and isolation on movements of P. smintheus during the summers of 1995 and 1996.
Roland, Keyghobadi, and Fownes to study the influences of meadow size and isolation on movements of P. smintheus (rocky mountain parnassian butterfly) during the summers of 1995 and 1996.
The research team marked and recaptured butterflies to estimate population size in each meadow and to follow butterfly movements. Upon recapture, dispersal distance of an individual was estimated as the straight line distance from its last point of capture. results:. Over the course of the study, the size of P. smintheus populations in the 20 study meadows ranged from 0 to 230. The average movement distance by males and females in 1995 was approximately 131 m. In 1996, the average movement distances of males and females was 162 m and 118 m, respectively. The maximum dispersal distance for a butterfly in 1995 was 1,729 m and in 1996 the maximum dispersal distance was 1,636 m. Most of the movements determined by recaptures were the result of dispersal within meadows. In 1995 only 5.8% of documented dispersal movements were from one meadow to another, and in 1996 dispersal between meadows accounted for 15.2% of total recaptures. analysis: As shown in figure 10.11, average butterfly population size increased with meadow area. It turned out that butterflies are more likely to leave small populations than large populations. Butterflies leaving small populations generally emigrate to larger populations. The results of this study suggest that as alpine meadows in the Rocky Mountains decline in area, populations of P. smintheus will become progressively more compressed into fewer and fewer small meadows, perhaps disappearing entirely in parts of their range. Some of the patterns of dispersal within this metapopulation of alpine butterflies have been observed in a study of dispersal in a metapopulation of a small falcon.
λ relation to R0
This is the same value we got for R0. But, before you jump to conclusions, you should know that R0, which is the average number of female offspring per female per generation, does not always equal λ. In this case, λ equaled R0 because P. drummondii is an annual plant with pulsed reproduction. If a species has overlapping generations and continuous reproduction, R0 will usually not equal λ.
What are some other major assumptions underlying the use of age distributions to estimate patterns of survival?
This method requires that a population is neither growing nor declining and that it is not receiving new members from the outside or losing members because they migrate away. Since most of these assumptions are often violated in natural populations, a life table constructed from this type of data tends to be less accurate than a cohort life table. Static life tables are often useful, however, since they may be the only information available.
Erkki Korpimäki and Kai Norrdahl (1991) conducted a 10-year study of voles and their predators.
The study began in 1977 during a peak in vole densities of about 1,800 per square kilometer and continued through two more peaks in 1982 (960/km2) and 1985-86 (1,980 and 1,710/km2). The researchers estimated that between these population peaks vole densities per square kilometer fell to as low as 70 in 1980 and 40 in 1984. During this period, the densities of the European kestrel, Falco tinnunculus, short-eared owls, Asio flammeus, and long-eared owls, Asio otus, closely tracked vole densities
constant rates of survival
The survivorship curves of many species are nearly straight lines. In these populations, individuals are equally likely to die at any age. This pattern of survival has been commonly observed in birds, such as the American robin, Turdus migratorius, and the white-crowned sparrow, Zonotrichia leucophrys nuttalli (fig. 10.16). Also, the common mud turtle, Kinosternon subrubrum, has a relatively constant rate of survival. Though the mud turtle has a high rate of mortality during the first year of life, thereafter, survival follows a straight line.
calculating the net reproductive rate in the annual plant P. drummondii -*DEFINITION of net reproductive rate* -what are the assumptions about reproductive rates in this table? -equation
We've already used the data in column 3, lx, to construct the survivorship curve for this species (see fig. 10.15). Now, let's combine those survivorship data with the seed production for P. drummondii, mx, to calculate the net reproductive rate, R0. The calculations of reproductive rates in this section assume that lx and mx for each age class in a population are constant and, as a result, the population has a stable age distribution. In general, the net reproductive rate is the average number of *female offspring* produced by an individual female in a population during her lifetime. In the case of the annual plant P. drummondii, the net reproductive rate is the average number of seeds left by an individual. We don't have to consider the sex of the Phlox seeds, since all germinating and surviving to maturity will produce flowers with fully functional female and male reproductive organs. You can calculate the net reproductive rate from table 10.1 by adding the values in the final column. The result is: (see image)
generation time (T) -definition -equation
the average age of reproduction Equation: x is age in years
What effect does current have on the lives of stream organisms?
the effects of current are substantial and influence everything from the amount of oxygen in the water to the size, shape, and behavior of stream organisms.
