Life histories are sensitive to environmental conditions

As we saw in Chapter 4, many traits exhibit flexibility, or plasticity, in response to different environmental conditions, and life history traits are no exception. As a result, researchers have continued to discover a fascinating array of life history traits that can be altered by changing environmental conditions.

Stimuli for Change

Photoperiod The amount of light that occurs each day.

Many events in the life history of an organism are timed to match seasonal changes in the environment. The right timing is essential so behavior and physiology match changing environmental conditions. For instance, flowering plants must bloom when pollinators are present and most birds must breed when there is an abundance of food to feed their chicks. To get their timing right, organisms rely on various indirect cues in the environment.

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Virtually all organisms sense the amount of light that occurs each day, known as the photoperiod. Many can distinguish whether the photoperiod is getting shorter or longer. Within a single species, populations may be exposed to a variety of environmental conditions. Each population develops a particular response to the photoperiod in its environment. For example, consider the grass known as sideoats grama (Bouteloua curtipendula). Southern populations living at 30° N flower in autumn in response to a photoperiod of 13 hours per day. In contrast, northern populations living at 47° N flower in summer in response to a photoperiod that exceeds 16 hours each day.

Another example occurs in water fleas of the genus Daphnia. In Michigan, water fleas enter diapause in mid-September, when the photoperiod declines to fewer than 12 hours of sunlight, but related species in Alaska enter diapause in mid-August, when the photoperiod decreases to fewer than 20 hours of sunlight. Water fleas never see 20-hour days in Michigan, but Alaskan water fleas would perish from the cold if they waited for 12-hour days before entering diapause. From this we see that the critical stimulus for these organisms is the change in environmental conditions associated with a particular photoperiod. The sensitivity of individuals to these cues has been adjusted by natural selection so that the individual’s response to an environmental cue is well matched to the environmental condition.

The Effects of Resources

Many types of organisms undergo dramatic life history changes during the course of their development. One of the most striking changes is the process of metamorphosis in which a larva changes into a juvenile or adult organism. Metamorphosis can be seen in many species of insects and amphibians, as in the transformation from tadpole to frog. Organisms that metamorphose have a wide range of timing options. Environmental conditions that influence timing include the amount of resources available, the temperature, and the presence of enemies.

Let’s consider the different options for timing of metamorphosis by looking at the two growth curves in Figure 8.12. These curves represent the change in the mass of the barking treefrog (Hyla gratiosa) raised under high or low food conditions. On any particular day early in life, the individuals raised under high food conditions have a larger mass then those raised under low food conditions. As time progresses, an individual with access to high food is able to metamorphose at a relatively large mass and young age. An individual with access to low food cannot achieve the same combination of mass and age, but it can follow several alternative strategies. It might wait to mature when it achieves the same mass as individuals raised under high food conditions, though it will take longer to achieve that mass and the delay in reproduction could reduce its fitness. Alternatively, it might metamorphose at the same age as individuals raised under high food conditions, even though it will be significantly smaller. The drawback of this strategy is that a smaller size at metamorphosis makes the organism more vulnerable to predation before reproduction occurs. For most metamorphosing organisms, the optimum solution is usually a compromise between these two strategies. So an organism exposed to low food typically metamorphoses at an older age and smaller mass.

Figure 8.12 Alternative growth curves of a metamorphosing organism. The amount of food available can affect an organism’s mass and age at metamorphosis. In the barking treefrog, an individual living under high food is able to metamorphose at a large mass and young age, represented by point A. An individual living under low food conditions could achieve the same age at metamorphosis if it emerges at a smaller mass, for example at point B. It could achieve the same mass at metamorphosis if it took longer to metamorphose, for example at point D. In reality, the tadpoles reach a compromise and metamorphose at a somewhat smaller mass and a somewhat later age, as indicated by point C. Error bars are standard deviations.
Data from J. Travis, Anuran size at metamorphosis: Experimental test of a model based on intraspecific competition, Ecology 65 (1984): 1155–1160.

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The Effects of Predation

The risk of predation is also an important factor in affecting the life history of organisms. As we saw in Chapter 4, predation can affect a wide range of life history traits including time to and size at hatching, time to and size at metamorphosis, and the time to and size at sexual maturity. One of the more remarkable effects of predators is their impact on the embryos of many species of aquatic organisms. In a variety of groups including fish and amphibians, the embryo that develops inside an egg can detect the presence of an egg predator. Many embryos sense the chemical odors that predators emit while other embryos can detect the vibrations produced by predators. When predators are detected, the embryos can speed up their time of hatching in an attempt to leave the egg before the predator eats it. For example, the red-eyed treefrog (Agalychis callidryas) lives in Central America and the adults lay their eggs on leaves that hang over water. When the embryos have developed sufficiently, they hatch and drop into the water. Should a cat-eyed snake (Leptodeira septentrionalis) appear, however, the frog embryos sense the vibrations of the approaching snake and begin to hatch earlier than usual and drop into the water to avoid the snake (Figure 8.13). However, this response comes at the cost of hatching at a smaller size that can make the hatchling tadpoles more susceptible to predators living in the water. Therefore, when egg predators are not present, the embryo stays in the egg longer and hatches at a larger and safer size.

Figure 8.13 Hatching early in response to predators. As the cat-eyed snake begins to attack the eggs of the red-eyed treefrog, the embryos are stimulated to hatch early. Note the tadpole escaping the snake’s attempt to eat the egg in which the tadpole lived. This photo was taken at the Corcovado National Park in Costa Rica.
Photo by Karen M. Warkentin.

Studies of metamorphosing animals also find that predators commonly play an important role in affecting the size and time at which metamorphosis takes place. For example, many high-elevation streams of western Colorado contain trout, an important predator of mayfly larvae, whereas other streams lack trout. Larval mayflies living with trout metamorphose at smaller sizes and leave the streams earlier than mayflies in comparable streams that lack trout. Growth rates in the two types of streams are similar, so the difference in the time to and size at metamorphosis is entirely the result of predation risk.

Predators can also affect when organisms achieve sexual maturity. Several species of freshwater snails, for example, face higher risks of predation when they are small. As a result, when predators are present, a snail is likely to do better if it delays reproduction and uses its energy to grow. Once it has grown to a safer size, it can reproduce. Although this strategy can improve the snail’s probability of survival in the presence of predators, the cost of delaying sexual maturity can be reduced fecundity. However, once the predator-induced snails begin reproducing, they can produce more eggs in each clutch because they have larger bodies. In these cases, the snails can achieve the same lifetime fecundity as snails raised without predators.

The Effects of Global Warming

We have seen how the life history of organisms responds to different environmental conditions found in nature. However, over the past 100 years human activity has caused a warming trend on Earth. In many regions, the difference in temperature is relatively small—a rise of 1°C or 2°C. However, even small changes in temperature can have substantial impact on an organism’s physiological processes. During the past decade, researchers have begun to discover that the increase in global temperatures has caused changes in the breeding times of many animals and plants.

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Animal Breeding

Researchers interested in the effect of global warming on the life histories of animals have focused on the breeding times of birds and amphibians. Figure 8.14 shows data compiled from 3,450 nesting records of the North American tree swallow (Tachycineta bicolor) collected from 1959 to 1991. As you can see, by the end of the 4-decade period, the mean date of egg laying was 9 days earlier. Moreover, researchers found a negative correlation of the mean egg-laying date to the mean air temperature in May. Across all years, an increase in May temperature was associated with earlier egg-laying dates. From this they concluded that the variation in egg-laying dates was associated with the variation in air temperatures and that the warming temperatures over a 4-decade period might be at least part of the reason the birds are breeding earlier. Similar patterns have been found in many other species of birds across both North America and Europe.

Figure 8.14 Egg laying dates in tree swallows. (a) Egg laying dates collected over a 42-year period demonstrate that tree swallows bred 9 days earlier in 1991 than in 1959. (b) The variation in egg laying date is negatively correlated with the mean air temperature in May.
Data from P. O. Dunn and D. W. Winkler, Climate change has affected the breeding date of tree swallows throughout North America, Proceedings of the Royal Society of London B. 266 (1999): 2487–2490.

A change in breeding dates has also been observed in several species of amphibians. In Britain researchers monitored three species of frogs and three species of salamanders for 17 years. At the end of this period, they found that two of the three frog species monitored were breeding 2 to 3 weeks earlier and all three species of salamanders were breeding 5 to 7 weeks earlier. These changes in breeding times were correlated with the average maximum temperatures that occurred just prior to breeding, which generally increased over the 17 years. A similar study of North American amphibians, however, failed to find a relationship between changes in mean maximum air temperature over time and the initiation of breeding. At the present time, researchers do not know why amphibians in different regions of the world show different responses to global warming.

A major insight into this question came in 2012 when bird researchers reported their results on a captive population of great tits (Parus major). They raised the birds in temperature-controlled chambers over a period of 3 years under different simulated temperature changes. They discovered that the initiation of breeding was not in response to an increase in the mean temperature of the environment, but in response to rapid temperature increases over a period of several days in the spring. This is relevant because global climate change is predicted not only to cause an increase in mean temperatures but also to cause increases in the duration of warm spells and cold spells. The great tit results suggest that increased temperature fluctuations may be the important environmental cue that affects the life histories of many organisms.

Plant Flowering

Plants are also susceptible to change in temperature, which has the potential to alter the initiation of flower production. One of the longest studies began in the nineteenth century with the writer Henry David Thoreau, who is best known for having spent a year in a small cabin at Walden Pond in Concord, Massachusetts, and for his numerous essays about the natural world. Thoreau kept data on more than 500 species of flowering plants in Concord. From 1852 to 1858, he took notes on the dates when each plant species first began flowering in Concord.

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After Thoreau’s death, a local shopkeeper continued his work by observing the first flowering times of more than 700 plant species. More recently, two ecologists realized that the data could help them determine whether long-term changes in global temperatures might be associated with changes in the initial flowering times of plants. Because flowering time is sensitive to temperature as well as to photoperiod, they predicted that warmer global temperatures would cause plants to flower earlier today than in Thoreau’s time. To test their hypothesis, they collected data for plant flowering times in Concord from 2003 to 2006.

In 2008, the researchers reported that over the 154-year period from 1852 to 2006, local temperatures in Concord had increased by 2.4°C. You can see these data in Figure 8.15. They also found that for the 43 most common species of plants, flowering time today is an average of 7 days earlier than in Thoreau’s time. Interestingly, not all plants responded to the temperature change in the same way. In some species, initial flowering time remained unchanged, perhaps because these species use day length as their cue for flowering and day length has not changed. Other species, such as highbush blueberry (Vaccinium corymbosum) and yellow wood sorrel (Oxalis europaea), flower 3 to 4 weeks earlier now than they did in 1852. These unique data collected over a century and a half indicate that a seemingly small change in average annual temperature has been associated with dramatic changes in initial flowering time.

Figure 8.15 First flowering dates for plants in Concord, Massachusetts. (a) The mean flowering time today is 7 days earlier than in the 1850s. Error bars are standard errors. (b) The variation in first flowering time is associated with the mean temperature of the 1 or 2 months preceding each species’ flowering time.
Data from A. J. Miller-Rushing and R. B. Primack, Global warming and flowering times in Thoreau’s Concord: A community perspective, Ecology 89 (2008): 332–341.

Consequences of Altered Breeding Events

The changing breeding seasons of plants and animals in response to global warming does not by itself cause any problems to the species that are responding. Problems can arise, however, when a species depends on the environment to provide the necessary resources with an altered breeding season. The pied flycatcher (Ficedula hypoleuca), for example, is a bird that breeds in Europe each spring. In 1980, researchers in the Netherlands found that the date of egg hatching of the flycatcher began just a few days before the peak abundance of caterpillars, which are a major prey item for the flycatcher chicks. As spring temperatures warmed over the next 2 decades, however, tree leaves appeared 2 weeks earlier and the caterpillars reached their peak abundance 2 weeks earlier. The pied flycatcher, however, retained its normal date of egg hatching, which was 2 weeks later than the new time for peak caterpillar abundance. As a result, the chicks of the flycatcher no longer have a major source of food and the pied flycatcher population has declined by 90 percent.

From our discussion of life history traits, you can see that natural selection has favored a wide variety of strategies for the life history of different species. Different life histories evolve as a result of different selection pressures on traits such as mortality, fecundity, and longevity combined with a considerable number of potential trade-offs among life history traits. As is true for other traits, the genes that code for these traits interact with the environments that organisms experience, ultimately to produce the life history traits of individuals.

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ECOLOGY TODAY CONNECTING THE CONCEPTS

SELECTING ON LIFE HISTORIES WITH COMMERCIAL FISHING

Human selection on the life history of fish. For several decades, commercial fishing vessels, like this boat in the north Atlantic Ocean, have harvested the largest individuals and thereby caused unintentional selection for smaller fish. Because of this selection, some species of fish now achieve sexual maturity earlier.
Photo by Jeff Rotman/Alamy.

Throughout this chapter we have seen how natural selection has shaped the evolution of life histories by favoring individuals that are best suited to their environment. But what if a change in the selection process favored individuals with different suites of traits? This is precisely what happens in many commercial fisheries because they harvest only the largest individuals. For many years this seemed like a wise way to manage the exploitation of wild populations because it protected the small individuals, allowing them a chance to grow. It is also a common practice for state agencies to set a minimum size for fish, such as bass and salmon, that anglers are permitted to keep.

Based on our discussion in this chapter, you should be able to predict what will happen to the life history traits of species that experience a great deal of fishing pressure, particularly from large commercial fishing boats that can collect thousands of fish. When the smallest fish are either not caught or are thrown back into the water, we impose a high mortality rate on large adults and leave the smaller, younger fish behind for breeding. As we have seen in this chapter, high adult mortality favors the evolution of smaller adult sizes, earlier times to maturity, higher fecundity, and shorter life span.

During the past 2 decades, researchers have begun to investigate whether large-scale fishing can cause unintended evolution of fish life histories and they have confirmed that commercial fishing imposes substantial selection on fished populations. In keeping with the requirements for evolution, there is sufficient heritability present in fish populations for the selection to cause a change in subsequent generations. For example, in the 1930s and 1940s, the northeast Arctic cod (Gadus morhua) had a median age at maturity that ranged between 9 and 11 years. By the 1960s and 1970s, age at maturity ranged between 7 and 9 years. The data collected by commercial fishing boats does not typically contain information on other life history traits such as fecundity and longevity. However, based on our knowledge of common life history trade-offs it is reasonable to assume that an increase in adult mortality and a decline in age at maturity imposed by fishing practices coincide with increases in size-adjusted fecundity and declines in longevity.

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One challenge in determining the effect of fishing pressure on life history is that it can cause other significant changes. As we noted earlier in this chapter, environmental changes—such as resource levels—can affect life history traits. For example, fishing from commercial fishing boats could reduce competition among the remaining fish. We have seen that reduced competition can allow a faster time to sexual maturity. In some cases, researchers cannot differentiate between the environmental induction of life history changes from reduced competition and the evolution of life history changes. In other cases, however, scientists have been able to document that a fish population held at low numbers for several decades continues to exhibit life history changes over time. In such cases, the changes in life history are more likely to be the result of evolution by artificial selection.

The impact of human selection on natural populations is not limited to fish; similar impacts have been found in hunted mammals and some plants. In all of these cases, identifying the factors that naturally cause the evolution of life histories and how various life history traits trade off with each other has helped fisheries managers understand how human harvest of wild populations can have unintended consequences.

SOURCES: Law, R. 2000. Fishing, selection, and phenotypic evolution. CIES Journal of Marine Science 57: 659–668.

Darimont, C. T., S. M. Carlson, M. T. Kinnison, P. C. Paquet, T. E. Reimchen, and C. C. Wilmers. 2009. Human predators outpace other agents of trait change in the wild. Proceedings of the National Academy of Science 106: 952–954.