Life History Traits
Life history traits involve the timing, amount, and allocation of a plant’s limited resources for the development of different plant traits. The concept of life history traits is used by ecologists to explain how and why plants allocate resources a particular way. They can allocate their resources in a range of ways from reproduction to growth or defense. They create trade-offs in how to be a successful organism and how to cope with limiting resources. Life history traits highlight the importance of recognizing that there is not one solution to a problem imposed by the environment. These tradeoffs are now considered important for understanding the evolution of diversity. One function can compromise another, but plants will only evolve with traits that optimize fitness or increase their ability to successfully reproduce.
Schedule of birth, death, and reproduction
There are many life traits associated with seed germination. A great example of this is scarification were a seed requires physical abrasion of its outer coating in order for it to germinate. Another example of a germination trait is that of cold stratification, which is when a seed needs a period of cold weather in order for germination to be triggered. Perhaps the most familiar seed life history trait is seed dispersal. A plant to disperses its seed from a variety of ways. These methods range from wind blown to animal carried and all have their advantages and disadvantages.
Life history traits for death really encompass the life span of a plant. As a plants evolve to an annual, biannual or perennial species. An annual grows one year, reproduces, and then all parts of the plant die. Bi-annuals grow the first year investing in a root or bulb so next year it can flower for reproduction, but then all parts of the plant die. Perennials grow for several years to reach reproductive maturity then depending on the species and the environment has many years of seed production before dieing.
Life history traits for reproduction cover strategies for flowering and pollination. A plant can have many options for pollination, some have their pollen blown by the wind like pines. Plants can also have a variety of animals pollinate them like bees, moths, and birds. Plants balance advantages and disadvantages of their reproductive methods to optimize their fitness level in their environment. An example of a reproductive life history trait is the time to reproductive maturity. Plants that reach reproductive maturity early in their life are generally colonizers and have fast growth rates. Plants that mature more slowly are more often shade tolerant and climax species. Another trend is how often seeds are produced. It is common for strong plant seed years to occur in several year cycles, rather than using resources to produce many seeds every year.
Life History Tradeoffs Among Traits
Plants balance advantages and disadvantages of their reproductive methods to optimize their fitness level within their environment. One great example of a reproduction trade off is seed size. A plant can produce many small seeds or a few large seeds. When a plant produces small seeds the individual cost for each seed is relatively small. Since little energy is required, many seeds are produced. Due to their small size they are more susceptible to mortality, but the positive tradeoff is the large number of seeds. When a plant produces large seeds they allocate more energy per individual seed, thus a loss of one of these seeds is more costly. The positive tradeoff is the success rate of seedling reproduction. A plant with large seeds uses the seedling success rate to balance the fact they are not able to produce a high number of seeds.
Plants differ in growth rates and in biomass allocation. An example of a life history tradeoff is fast growing vs slow growing. Fast growing plants allocate more resources to quick biomass growth which make them more susceptible to some stochastic events like herbivores, but since they spend less time in that vulnerable state, they are released from many dangers early in thier life cycle. Slow growing plants use less resources for quick height growth, allowing resource energy for quicker reproduction or defense. Thereby still increasing their fitness level within their environment. However, since more energy is used on limbs not primarily for growth, the loss of a branch or leaf is far more costly to them then to a fast growing plant.
Allocation of a plants resources to leaves, steams, roots, flowers or seeds is also described by the concept of life history traits. Depending on the availability of light, water and nutrient a plant has to decide on were these resources will be best used. It could spend more on roots and tap into more water and nutrients. Or it could spend resources on leaves to sequester more carbon. Or perhaps it could invest in the next generation and grow flowers and fruiting bodies.
An important allocation option for plants is whether or not to invest in a defense and what kind of defense it should invest in. Defense mechanisms will evolve on plants where the benefit of the defense outweighs the benefit of growth or another function. A major benefit of defense is a reduction in predation. Predation can kill a parent plant or in many cases kill individual seeds on the plant. If a plant cannot grow at a rate to make themselves less susceptible to predation, defense is the mechanism that will optimize fitness for the plant. Defense is very costly however. A noticeable decline in growth or in some cases seed production will be noticed as a result of the tradeoff between defense and other functions. Even if the result is a decrease in seed production, more seeds will reach maturity due to the decrease in predation.
In a study from Oecologia
species with defense mechanisms were grown in areas free from their natural predators. The results showed that after the enemy release, the species began divert resources from defense growth to other traits. This means that plants only retain defense mechanisms as long as they are of greater benefit then other traits.
Whether you are trying to explain the advantages of seed size or why a plant allocates resources were it does the concept of life history traits is a very useful tool. By looking at were a plant lives when it lives and who it lives next to you can gain a lot of insight into why it allocates resources and develops traits the way it does.
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