Chapter 12: Life Histories
4 types of growth forms of plants created by Westoby, Leishman, and Lord
Westoby, Leishman, and Lord recognized four plant growth forms. Grasses and grasslike plants, such as sedges and rushes, were classified as graminoids. Herbaceous plants other than graminoids were assigned to a forb category. Species with woody thickening of their tissues were considered as woody plants. Finally, climbing plants and vines were classified as climbers.
Westoby and his coauthors recognized six dispersal strategies.
1) They classified seeds with no specialized structures for dispersal as *unassisted dispersers*. 2) If seeds had hooks, spines, or barbs, they were classified as *adhesion-adapted*. 3) Meanwhile, seeds with wings, hairs, or other structures that provide air resistance were assigned to a *wind-dispersed category*. 4-6) Animal-dispersed seeds in the study included ant-dispersed, vertebrate-dispersed, and scatterhoarded. 4) Westoby, Leishman, and Lord classified seeds with an *elaiosome*, a structure on the surface of some seeds generally containing oils attractive to ants, as ant-dispersed. 5) Seeds with an *aril*, a fleshy covering of some seeds that attracts birds and other vertebrates, or with flesh were classified as vertebrate-dispersed. 6) Finally, they classified as *scatterhoarded* those seeds known to be gathered by mammals and stored in scattered caches or hoards.
Applications: Climate Change and timing of reproduction and migration -phenology
Although the duration of this effort is impressive, the record of cherry tree blooming dates at Kyoto, Japan, the occasion for a traditional festival, spans an incredible 1,200 years. When modern researchers (Arakawa 1956; Aono and Saito 2010) began connecting the timing of that blooming to climate, they entered the realm of phenology. Phenology is the study of the timing of ecological events, especially in relation to climate and weather, for example, the arrival of migratory birds on their breeding grounds or the date of flowering by a plant species, such as the cherry trees at Kyoto.
animal phenology
Animals are also adjusting the timing of life histories as the climate warms. For example, salmon are migrating from the ocean to freshwater earlier in the spring, and many species of butterflies and moths are increasing the number of generations they produce in a year. However, migratory birds likely offer the best-documented changes in phenology. One of the most commonly recorded migratory events has been the first arrival date of migratory birds on their breeding grounds, an event long noted by ornithologists and amateur birders. Like first flowering date in plant populations in temperate regions, the first arrival date by migratory birds is now occurring earlier in the spring compared to historical records. In general, birds migrating from greater distances have shifted their first arrival dates less than shorter-distance migrants. Birds wintering in North America have shifted their first arrival dates an average of 21 days per century, while those wintering in South America or on Caribbean islands have shifted their arrivals an average of 12 and 10 days per century, respectively
Jakobsson and Eriksson also investigated the relationship between seed size and recruitment among 50 plant species living in the meadows of their study region, using a field experiment. -background, not results
At their field sites, Jakobsson and Eriksson planted the seeds of each species in 14 small 10×10 cm plots. Each plot was sown with 50 to 100 seeds of the study species. They left half of the study plots undisturbed, while the other plots were disturbed before planting by scratching the soil surface and removing any accumulated litter. Of the 50 species of seeds planted, the seeds of 48 species germinated and those of 45 species established recruits. Jakobsson and Eriksson observed no recruitment of any of the study species on the control plots. Therefore they could be confident that new plants recruited into their experimental plots came from seeds that they had planted. Though plants recruited to both undisturbed and disturbed plots, the number of recruits was generally higher in disturbed plots. Further, eight species of plants recruited only on disturbed plots.
bertschy and fox variation in life history characteristics of pumpkinseed fish across different lakes (summary)
Bertschy and Fox found significant variation in most life history characteristics across their study lakes. Pumpkinseed sunfish matured at ages ranging from 2.4 to 3.4 years in the different study lakes, and they showed reproductive investments (gonadosomatic indexes, or GSI) ranging from 6.9% to 9.3%. The relationship between survival rate and age at maturity found by Bertschy and Fox suggests that populations with higher adult survival mature at a greater age (fig. 12.17). The correlation between survival rate and age at maturity was not high enough to be statistically significant; however, the relationship between adult survival and reproductive effort was very clear and highly significant The patterns of life history variation across the pumpkinseed populations studied by Bertschy and Fox support the theory that, where adult survival is lower relative to juvenile survival, natural selection will favor allocating greater resources to reproduction.
Charnov's approach -I/M -product of C and E
Charnov's approach was to take a few key life history features and convert them to dimensionless numbers. One of his variables was relative size of offspring. He created this dimensionless variable by dividing the mass of offspring at independence from the parent, I, by the adult mass at first reproduction, m. The result, I/m, is the size of offspring expressed as a proportion of adult body mass. Charnov's approach allows us to determine whether one is relatively larger than the other. The second measure was proportion of adult body mass allocated to reproduction per unit time, C, multiplied by the adult life span, E, which gives us an estimate of the fraction of adult body mass allocated to reproduction over a life span. As we have seen, higher reproductive effort is associated with shorter adult life span (see fig. 12.13), so Charnov reasoned their product might be similar for closely related taxa.
Donald Gunderson
Donald Gunderson (1997) explored patterns in adult survival and reproductive effort among several populations of fish. Gunderson suggested that there should be a strong relationship between adult mortality in populations and reproductive effort because some combinations of mortality and reproductive effort have a higher probability of persisting than others. For instance, a population showing a combination of high mortality and high reproductive effort would have a higher chance of persisting than one experiencing high mortality but allocating low reproductive effort. The life history information Gunderson summarized in his analysis included mortality rate, estimated maximum length, age at reproductive maturity, and reproductive effort.
The great diversity of life histories can be classified on the basis of a few population characteristics. Examples include (4)
Examples include fecundity or number of offspring, survival, relative offspring size, and age at reproductive maturity.
fecundity
Fecundity is simply the number of eggs or seeds produced by an organism.
Charnov chose these two dimensionless numbers (I/m and C•E) for two particular reasons.
First, R0, the net reproductive rate (see chapter 10, section 10.5) is a measure of an individual's fitness in populations that are not growing. In addition, R0 can be rewritten solely in terms of these two numbers and the chance of surviving to adulthood.
-Grime and the four extreme environmental types
Four environmental extremes envisioned by Grime were (1) low disturbance-low stress, (2) low disturbance-high stress, (3) high disturbance-low stress, and (4) high disturbance-high stress. Drawing on his extensive knowledge of plant biology, Grime suggested that plants occupy three of his theoretical environments but that there is no viable strategy among plants for the fourth environmental combination, high disturbance-high stress.
stress-tolerant category of plant life-history -definition of stress -sources of stress
Grime (1977) began his discussion of the second type of plant life history, stress-tolerant, with a definition of stress as "external constraints which limit the rate of dry matter production of all or part of the vegetation." In other words, stress is induced by environmental conditions that limit the growth of all or part of the vegetation. Stress is the result of extreme temperatures, high or low, extreme hydrologic conditions, too little or too much water, or too much or too little light or nutrients. The important point that Grime made, however, was that in every biome, some species are more tolerant to the environmental extremes that occur. These are the species that he referred to as "stress-tolerant." Stress-tolerant plants are those that live under conditions of high stress but low disturbance. Grime proposed that, in general, stress-tolerant plants grow slowly; are evergreen; conserve fixed carbon, nutrients, and water; and are adept at exploiting temporary favorable conditions. In addition, stress-tolerant plants are often unpalatable to most herbivores. Because stress-tolerant species endure some of the most difficult conditions a particular environment has to offer, they are there to take advantage of infrequent favorable periods for growth and reproduction.
Grime next described plant strategies, or life histories, that match the requirements of the remaining three environments (name the three)
Grime next described plant strategies, or life histories, that match the requirements of the remaining three environments. His strategies were ruderal, stress-tolerant, and competitive (fig. 12.20).
How does Grime's system of classification compare with the r and K selection contrast proposed by MacArthur and Wilson and Pianka?
Grime proposed that r selection corresponds to his ruderal strategy or life history, while K selection corresponds to the stress-tolerant end of his classification. Meanwhile, he placed the competitive life history category in a position intermediate between the extremes represented by r selection and K selection. However, while attempting this reconciliation of the two classifications, Grime suggested that a linear arrangement of life histories, with r selection and K selection occupying the extremes, fails to capture the full variation shown by organisms. He suggested that more dimensions are needed and, of course, Grime's triangular arrangement (see fig. 12.20) adds another dimension. The factors varying along the edges of Grime's triangle are intensity of disturbance, stress, and competition. Other ecologists have also recognized the need for more dimensions in representing life history diversity.
Gunderon -gonadosomatic index (GSI)
Gunderson estimated reproductive effort as each population's gonadosomatic index, or GSI. GSI was taken as the ovary weight of each species divided by the species body weight and adjusted for the number of batches of offspring produced by each species per year. For example, because the northern anchovy spawns three times per year, the weight of its ovary was multiplied by 3 for calculating its GSI. Meanwhile, the ovary weight for dogfish sharks, which reproduce only every other year, was divided by 2.
Gunderson analysis of mortality and reproductive effort
Gunderson's analysis also gives information on variation in reproductive effort among species. His calculations of a gonadosomatic index, or GSI, for each of the 28 species included in the analysis spanned more than a 30-fold difference from a value of 0.02 for the rougheye rockfish to 0.65 for the northern anchovy. When Gunderson plotted GSI against mortality rates (fig. 12.13), the results supported the prediction from life history theory that species with higher mortality would show higher relative reproductive effort.
Eric Charnov, with Robin Warne and Melanie Moses (2002, 2007), developed a new approach to life history classification. Charnov has focused his attention on mammals, altricial birds, and lizards Why remove the influences of size and time?
His goal has been to develop a classification free of the influences of size and time that would facilitate the exploration of life history variation within and among groups of closely related taxa. Our discussion of r and K selection underscored the relationship between size of organisms and timing of life history features (see table 12.1). The influences of size and timing are responsible for many of the obvious life history differences among species of closely related taxa, for instance, the differences among large and small mammal species, such as between a deer mouse and an African elephant (see fig. 12.19). By removing size and time effects, we may be able to more clearly detect life history differences among evolutionary lineages.
Description of ruderals strategy
His strategies were ruderal, stress-tolerant, and competitive (fig. 12.20). Ruderals are plants that live in highly disturbed habitats and that may depend on disturbance to persist in the face of potential competition from other plants. Grime summarized several characteristics of ruderals that allow them to persist in habitats experiencing frequent and intense disturbance, which he defined as any mechanisms or processes that limit plants by destroying plant biomass One of the characteristics of ruderals is their capacity to grow rapidly and produce seeds during relatively short periods between successive disturbances. This capacity alone would favor persistence of ruderals in the face of frequent disturbance. In addition, however, ruderals also invest a large proportion of their biomass in reproduction, producing large numbers of seeds that are capable of dispersing to new habitats made available by disturbance.
application of Winemiller and Roses' theory
However, it may be that the analysis by Winemiller and Rose has laid the foundation for a more general theory of life histories. By basing their classification system on some of the most basic aspects of population ecology, lx, mx, and α, Winemiller and Rose (1992) established a common currency for representing and analyzing life history information for any organism. As a model for how such a translation might be done, Winemiller (1992) plotted the distributions of life history parameters of representative animal groups on their life history classification axes (fig. 12.22). By plotting life history variation among vertebrate groups on the same axes using the same variables, figure 12.22 demonstrates differences in the amount of life history variation between the groups. Notice that fish show the greatest variation and mammals the least, while birds and reptiles and amphibians include intermediate levels of variation.
miller-rushing and primack
In all, there were 25 years of blooming records in the data set. Of the hundreds of species of plants in the record, Miller-Rushing and Primack chose 43 for which there were the most observations across the years of record. They used these data to test the hypothesis that changes in climate since Thoreau made his observations have altered the flowering phenology of plants in Concord. Miller-Rushing and Primack found that, since 1852 when Thoreau began his observations, temperatures in Concord have increased and plants are blooming earlier in spring. Concord show that average annual temperatures have increased 2.5°C since 1852 and the average date of first flowering by the 43 species in the study is now 7 days earlier than in 1852
Gunderson comparison to Shine and Charnov study
In contrast to Shine and Charnov, Gunderson provides estimates of mortality rates rather than survival rates. In addition, his estimates are of "instantaneous" mortality rates instead of annual rates. However, like Shine and Charnov, his results show a clear relationship between adult mortality and age of reproductive maturity (fig. 12.12b). These results support the idea that natural selection has acted to adjust age at reproductive maturity to rates of mortality experienced by populations.
Comparison of Winemiller and Rose's classification of life history strategies to either the r-K continuum of MacArthur and Wilson and Pianka or the triangular classification of plant life histories developed by Grime
It is difficult to compare them. For instance, opportunistic species share characteristics with r selected and ruderal species. However, opportunistic species differ from the typical r selected species because they tend to produce small clutches of offspring. The equilibrium strategy, which combines production of high juvenile survival, low numbers of offspring, and late reproductive maturity, approaches the characteristics of typical K selected species. Winemiller and Rose point out, however, that many fish classified as "equilibrium" are small, while typically K selected species tend toward large body size (see table 12.1). Periodic species are not captured by the linear r to K selection gradient. Meanwhile, the periodic and equilibrium species in Winemiller and Rose's classification share some characteristics with Grime's stress-tolerant and competitive species but differ in other characteristics.
How do differences in egg size and number translate into differences in gene flow among populations? (in other words, why does a larger amount of small eggs correspond to greater gene flow?)
It turns out that the larvae of darters that hatch from larger eggs are larger when they hatch. These larger larvae begin feeding on prey that live on the streambed at an earlier age, and spend less time drifting with the water current. Consequently, larvae hatching from larger eggs disperse shorter distances and therefore carry their genes shorter distances. As a result, populations of species producing fewer larger eggs will be more isolated genetically from other populations. Because of their greater isolation, such populations will differentiate genetically more rapidly compared to populations of species that produce many smaller larvae that disperse longer distances.
What role did differences in seed size play in the rate of recruitment by different species?
Jakobsson and Eriksson calculated recruitment success in various ways. One of the most basic ways was by dividing the total number of recruits by the total number of seeds of a species that they planted, giving the proportion of seeds sown that produced recruits. While 45 of 50 species established new recruits in the experimental plots, the rate at which they established varied widely among species from approximately 5% to nearly 90%. Jakobsson and Eriksson found that differences in seed size explained much of the observed differences in recruitment success among species (fig. 12.10). On average, larger seeds, which produce larger seedlings, were associated with a higher rate of recruitment. Therefore it appears that by investing more energy into a seed, the maternal plant increases the probability that the seed will successfully establish itself as a new plant. This advantage associated with large seed size is probably very important in environments such as the grasslands studied by Jakobsson and Eriksson, where competition with established plants is likely to be high.
results of bertschy and fox study on pumpkinseed fish (2/2) -estimations of juvenile survival -relative ratio of adult to juvenile survival
Juvenile survival to adulthood in the study lakes ranged from 0.004, or about 4 out of 1,000 larvae, to 0.016, or about 16 out of 1,000 larvae. Because they were interested in the relative rates of adult and juvenile survival, Bertschy and Fox represented survival in their study lakes as the ratio of adult to juvenile survival probabilities. Figure 12.16 shows that this ratio ranged widely among study lakes from a low of 10.6 to 116.8, a tenfold difference among lakes.
How might patterns in seed and seedling size vary among woody plants? Kenji Seiwa and Kihachiro Kikuzawa (1991)
Kenji Seiwa and Kihachiro Kikuzawa (1991) studied the relationship between seed size and seedling size among tree species native to Hokkaido, the northernmost large island of Japan. Seiwa and Kikuzawa were especially focused on the influences of shade on seedling establishment. The trees studied by Seiwa and Kikuzawa were all broad-leaved deciduous trees that grow in the temperate deciduous forests of Hokkaido In the laboratory the research team removed any fruit pulp from the seeds, washed them, and then allowed them to air dry for 24 hours. Seiwa and Kikuzawa then estimated average seed mass by weighing one to five groups of 100 to 1,000 randomly chosen seeds. A week later they planted seeds at depths of 1 to 2 cm in a clay loam soil and watered, until the soil was saturated, three times a week. results: Seiwa and Kikuzawa's results showed clearly that larger seeds produced taller seedlings (fig. 12.11). They explained this pattern as the result of the larger seeds providing greater energy reserves to boost initial seedling growth. Seiwa and Kikuzawa observed that seedlings from large-seeded species unfolded all of their leaves rapidly in the spring and shed all of their leaves synchronously in the autumn. They concluded that this timing allows the seedlings from large-seeded species to emerge early in the spring before the trees forming the canopy of the forest have expanded their leaves and have shaded the forest floor. Seiwa and Kikuzawa also pointed out that rapid growth would help seedlings penetrate the thick litter layer on the floor of deciduous forests and help them establish themselves as part of the forest understory.
life history
Life history consists of the adaptations of an organism that influence aspects of its biology, such as the number of offspring it produces, its survival, and its size and age at reproductive maturity. Because all organisms have access to limited energy and other resources, there is a trade-off between the number and size of offspring; those that produce larger offspring are constrained to produce fewer, whereas those that produce smaller offspring may produce larger numbers. This tension between competing demands for resources leads inevitably to trade-offs between functions. One of those is the trade-off between number and size of offspring. Organisms that produce many offspring are constrained, because of energy limitation, to produce smaller offspring (seeds, eggs, or live young). Viewed from the opposite perspective, organisms that produce large, well-cared-for offspring are constrained to produce fewer.
growth form and comparison of sizes of seeds
Many characteristics of plants correlate with their growth form, or life-form, which itself constitutes an aspect of the plant life history. Therefore, comparing seed production of orchids and coconut palms, which mixes data from a species having the growth form of an epiphyte (the orchid) and another with the growth form of a tree (the palm), may not be a valid comparison. Such a comparison may not be valid, since growth form may itself influence the number and size of seeds produced by plants.
r and K selection and body size according to Pianka
Meanwhile, early reproduction and smaller body size will be favored by r selection, while K selection favors later reproduction and larger body size.
Kirk Bertschy and Michael Fox (1999) studied the influence of adult survival on pumpkinseed sunfish life histories.
One of the major objectives of their study was to test the prediction by life history theory that increased adult survival, relative to juvenile mortality, favors delayed maturity and reduced reproductive effort. Bertschy and Fox estimated life history characteristics from annual samples of approximately 100 pumpkinseed sunfish taken from each of the five study lakes. They made several measurements on each individual in their samples, including its age (by counting annual rings in scales), weight (to the nearest 0.1 g), length (in mm), sex, and reproductive status. Bertschy and Fox studied reproductive traits in females only. A female was considered mature if her ovaries contained eggs with yolk. Bertschy and Fox represented female reproductive effort using the gonadosomatic index, GSI, which they calculated as 100 × (ovary mass) ÷ (body mass), which yields GSI values expressed as percentages rather than as proportions. Ages of fish were estimated from their length using the relationship between length and age of individuals of known age from each population. Juvenile survival was estimated by counting the number of pumpkinseed nests and then collecting all the larval fish in a sample of nests.
Charnovs actual study (classification of life histories using mammals, lizards, and altricial birds- which are birds that are born entirely dependent on their parents) -results
One of the striking results of using Charnov's dimensionless analysis is that while there is little variation within mammals, lizards, or birds, there are substantial differences among these groups of animals. Figure 12.23 shows that the birds have the highest I/m (essentially 1, since they raise their young to adult size) and C•E values. In contrast, lizards and mammals share the same C•E value but differ a lot in I/m (0.1 vs 0.3). Previous classifications of life histories have revealed substantial variation within taxa, such as mammals and fish (see fig. 12.22). In contrast, Charnov's classification, by removing the influences of time and size, allows us to see the great similarities within these groups and reveals the substantial differences among them.
Eric Pianka (1970, 1972) developed the concept of r and K selection further in two important papers. -extremes -link of r and K selection to survivorship curves -r and K selection and development rates
Pianka pointed out that r selection and K selection are the endpoints on a continuous distribution and that most organisms are subject to forms of selection somewhere in between these extremes. In addition, he correlated r and K selection with attributes of the environment and of populations. He also listed the population characteristics that each form of selection favors. Following MacArthur and Wilson, Pianka predicted that while r selection should be characteristic of variable or unpredictable environments, fairly constant or predictable environments should create conditions for K selection. In such conditions survivorship among r selected species will approximate type III, while K selected species should show type I or II survivorship In addition, according to Pianka, development should be rapid under r selection and relatively slow under K selection.
Pianka on r and K selection and amount of times an organism reproduces -iteroparity -semelparity
Pianka predicted that reproduction under r selection will tend toward a single reproductive event in which many small offspring are produced. This type of reproduction, which is called semelparity, occurs in organisms such as annual weeds and salmon. In contrast, K selection should favor repeated reproduction, or iteroparity, of fewer larger offspring. Iteroparity, which spaces out reproduction over several reproductive periods during an organism's lifetime, is the type of reproduction seen in most perennial plants and most vertebrate animals. Pianka's contrast may be restated as "small and fast," analogous to r selected species, with ones that are "large and slow," analogous to K selected species (fig. 12.19).
Westoby, Leishman, and Lord: also found relationship between the way that plants that disperse their seeds in different ways and the sizes of the seeds
Plants that they had classified as unassisted dispersers produced the smallest seeds, while wind-dispersed seeds were slightly larger. Adhesion-adapted seeds were of intermediate size, while animal-dispersed seeds were largest. Ant-dispersed seeds were the next largest, vertebrate-dispersed seeds were somewhat larger, and scatterhoarded were the largest by far. Westoby and his team point out that between 21% and 47% of the variation in seed size in the five floras included in their study is accounted for by a combination of growth form and mode of dispersal.
Richard Shine and Eric Charnov (1992) explored life history variation among snakes and lizards to determine whether generalizations developed through studies of fish and marine invertebrates could be extended to another group of animals living in very different environments. -use of energy before and after reaching sexual maturity
Shine and Charnov began their presentation with a reminder that, in contrast to most terrestrial arthropods, birds, and mammals, including humans, many animals continue growing after they reach sexual maturity. In addition, most vertebrate species begin reproducing before they reach their maximum body size. Shine and Charnov pointed out that the energy budgets of these other vertebrate species, such as fish and reptiles, are different before and after sexual maturity. Before these organisms reach sexual maturity, energy acquired by an individual is allocated to one of two competing demands: maintenance and growth. However, after reaching sexual maturity, limited energy supplies are allocated to three functions: maintenance, growth, and reproduction.
Shine and Charnov study methods and results
Shine and Charnov gathered information from published summaries on annual adult survival and age at which females mature for several species of snakes and lizards. Regardless of these cautions, the results of Shine and Charnov's study showed clearly that as survival of adult lizards and snakes increases, their age at maturity also increases
The most fundamental contrasts between the two selective extremes represented by r and K selection are between what two variables?
The most fundamental contrasts are, of course, between intrinsic rate of increase, rmax, which should be highest in r selected species, and competitive ability, which should be highest among K selected species.
relationship between seed characteristics and mode of dispersal -Westoby, Leishman, and Lord
The results showed a clear association between seed size and plant growth form (fig. 12.8a). In most of the floras analyzed by Westoby and his colleagues, the smallest seeds were produced by graminoid plants, followed by the seeds produced by forbs. In all five study regions, woody plants produce seeds that are far larger than those produced by either graminoids or forbs. However, the largest seeds in all regions are produced by vines. The researchers found that the seeds produced by woody plants and vines in the five floras were on average, approximately 10 times the mass of seeds produced by either graminoid plants or forbs.
K selection -occurs when?
The term K selection refers to the carrying capacity of the logistic growth equation summarized in figure 11.13. MacArthur and Wilson proposed that K selection favors more efficient utilization of resources such as food and nutrients. They envisioned that K selection would be most prominent in those situations where species populations are near carrying capacity much of the time.
r selection -who invented the term -strongest when?
The term r selection, which refers to the per capita rate of increase, r, which we calculated in chapter 10, was defined by Robert MacArthur and E. O. Wilson as selection favoring a higher population growth rate. MacArthur and Wilson suggested that r selection would be strongest in species often colonizing new or disturbed habitats. Therefore, high levels of disturbance would lead to ongoing r selection. MacArthur and Wilson contrasted r selected species with those subject mainly to K selection.
competitive plants
The third plant strategy proposed by Grime, the competitive strategy, is in many respects intermediate between the ruderal strategy and the stress-tolerant strategy. In Grime's classification, competitive plants occupy environments where disturbance intensity is low and the intensity of stress is also low. Under conditions of low stress and low disturbance, plants have the potential to grow well. As they do so, however, they eventually compete with each other for resources, such as light, water, nutrients, and space. Grime's model predicts that the plants living under such circumstances will be selected for strong competitive abilities.
the two variables selected by Grime that he considered as most important in exerting selective pressure on plants were
The two variables that he selected as most important in exerting selective pressure on plants were the intensity of disturbance and the intensity of stress. Grime contrasted four extreme environmental types, which he characterized by combinations of disturbance intensity and stress intensity.
Winemiller and Rose
Their trade-offs are among fecundity, survivorship, and age at reproductive maturity. Using variation in fish life histories as a model, Winemiller and Rose proposed that life histories should lie on a semi-triangular surface as shown in figure 12.21. They called the three endpoints on their surface "opportunistic," "equilibrium," and "periodic" life histories.
seed size variation in plants
While some orchids are known to produce billions of seeds, coconut palms produce small numbers of huge seeds. At this scale it is clear that there is a trade-off between seed size and seed number, and although there are complexities that must be accounted for. Botanists long ago described a negative relationship between seed size and seed number.
Anna Jakobsson and Ove Eriksson (2000) of Stockholm University studied the relationships between seed size, seedling size, and seedling recruitment among herbs and grasses living in seminatural grasslands in southeastern Sweden.
To estimate the influence of seed size and seedling size, Jakobsson and Eriksson germinated seeds in pots containing a standardized soil mix. The pots were maintained in a greenhouse under standardized conditions, and seedlings were harvested and weighed 3 weeks after germination. The results of this portion of the study showed clearly that larger seeds produced larger seedlings (fig. 12.9).
What are the factors that maintain variation in seed size?
To maintain such variation, there must be advantages and disadvantages of producing either large or small seeds. What are those advantages and disadvantages? Plants that produce small seeds can produce greater numbers of seeds. Such plants seem to have an advantage where disturbance rates are high and where plants with the capacity to colonize newly opened space appear to thrive. Though plants that produce large seeds are constrained to produce fewer, large seeds produce seedlings that survive at a higher rate in the face of environmental hazards. Those hazards include competition from established plants, shade, defoliation, nutrient shortage, deep burial in soil or litter, and drought
In a study of gene flow among populations of darters, small freshwater fish in the perch family, or Percidae, Tom Turner and Joel Trexler tried to determine the extent to which life history differences among species might influence gene flow between populations.
Turner and Trexler (1998) pointed out that in such a study, it is best to focus on a group of closely related organisms with a shared evolutionary history. They were particularly interested in determining the relationship between egg size and egg number, or fecundity, and the extent of gene flow among populations. hypothesis: Turner and Trexler proposed that gene flow would be higher among populations producing more numerous smaller eggs—that is, among populations with higher fecundity. basis: Turner and Trexler chose the darters for their studies because they are an ideal study group. Darters are small, streamlined benthic fishes that live in rivers and streams throughout eastern and central North America. The darters consist of 174 species in three genera within the family Percidae, which makes them one of the most species-rich groups of vertebrates in North America. Male darters are usually strikingly colored during the breeding season Turner and Trexler sampled 64 locations on streams and rivers of Ohio, Arkansas, and Missouri, the heart of freshwater fish diversity in North America, which supports one of the most diverse temperate freshwater fish faunas on earth. Of the darters they collected at these locations, they chose 15 species, 5 in the genus Percina and 10 Etheostoma species, for detailed study. Turner and Trexler chose darter species that included a wide range of variation in life history traits, especially variation in body size, number of eggs laid, and egg size.
How can the number and kinds of allozymes synthesized by a series of populations be used to determine the extent of gene flow among populations?
Turner and Trexler assumed that the populations differing in allelic frequencies have lower gene flow between them than populations that have similar allelic frequencies. In other words, they assumed that genetic similarity between populations is maintained by gene flow, while genetic differences arise in the absence or restriction of gene flow.
How turner and trexler characterized the genetic structure of darter populations (in other words, how the measured gene flow) -allozymes -polymorphic locus
Turner and Trexler characterized the genetic structure of darter populations using electrophoresis of allozymes, different forms of an enzyme, which are gene products, produced by 21 different genes, or loci. They chose 21 loci out of 40 that they examined because they were polymorphic. A polymorphic locus is one that occurs as more than one allele. In this case each allele synthesizes a different allozyme. Turner and Trexler assessed genetic structure using allelic frequencies. Allelic frequencies were measured as the frequencies of allozymes across the 21 different study loci. Populations with similar allelic frequencies were taken as genetically similar, while those that differed in allelic frequencies were concluded to be different genetically. Gene flow was estimated by the degree of similarity in allelic frequencies between populations.
What relationship is there between egg size and number and gene flow between populations?
Turner and Trexler found a negative relationship between egg size and gene flow but a strong positive relationship of gene flow with the number of eggs produced by females (fig. 12.5). That is, populations of darter species that produce many small eggs showed less difference in allelic frequencies across the study region than did populations that produce fewer larger eggs.
What are the sources of these differences in perspective?
What are the sources of these differences in perspective? One of the sources is that different ecologists have worked with different groups of organisms. While MacArthur and Wilson's system was built after years of work on birds and insects, respectively, Pianka had worked mainly with lizards. Grime's classification was built on and intended for plants. Finally, the perspective of Winemiller and Rose was influenced substantially by their work with fish. Because these ecologists worked with such different groups of organisms, it is not surprising that their classifications of life histories do not overlay precisely.
adult survival and age of reproduction
Where adult survival is lower, organisms begin reproducing at an earlier age and invest a greater proportion of their energy budget into reproduction; where adult survival is higher, organisms defer reproduction to a later age and allocate a smaller proportion of their resources to reproduction.
Reproductive effort
is the allocation of energy, time, and other resources to the production and care of offspring
germination
is the process by which seeds begin to grow or develop, producing the small plant called a seedling in the process.
opportunistic, equilibrium, and periodic and life histories
opportunistic: The opportunistic strategy, by combining low juvenile survival, low numbers of offspring, and early reproductive maturity, maximizes colonizing ability across environments that vary unpredictably in time or space. While the absolute reproductive output of opportunistic species may be low, the percentage of their energy budget allocated to reproduction is high. equilibrium: Winemiller and Rose's equilibrium strategy combines high juvenile survival, low numbers of offspring, and late reproductive maturity. Periodic: The periodic strategy combines low juvenile survival, high numbers of offspring, and late maturity. Among fish, periodic species tend to be large and produce numerous small offspring. By producing large numbers of offspring over a long life span, periodic species can take advantage of infrequent periods when conditions are favorable for reproduction.
J.P. Grime
proposed that variation in environmental conditions has led to the development of distinctive strategies or life histories among plants.
results of tuner and trexler study
results: As they expected, Turner and Trexler found that larger darter species produce larger numbers of eggs (fig. 12.3). Their results also support the generalization that there is a trade-off between offspring size and number. On average, darters that produce larger eggs produce fewer eggs (fig. 12.4).
Kirk Winemiller (1995) pointed out that fish
show more variation in many life history traits than any other group of animals. For instance, the number of offspring they produce per brood—that is, their clutch size—ranges from the one or two large live young produced by mako sharks to the 600 million eggs per clutch laid by the ocean sunfish.
results of bertschy and fox study on pumpkinseed fish (1/2) -proportion of adults surviving from one year to the next and its graph
the proportion of adults surviving from one year to the next ranged from approximately one adult out of five (0.19) to about two adults out of three (0.65). This variation among lakes produced striking differences in the form of survivorship curves (see pic).
