L earning Objective 24

I. STUDY FIGURE 24-A to understand the opposing characteristics of the colonizing plants of Hawaii versus common native plant traits evident in today's flora.

A. The traits most likely for original 272 colonizing plants (the basis of the present day flora) are summarized in the left-hand column of Figure 24-A.

B. In contrast, the traits most common for present-day plants of the Hawaiian flora are summarized in the right hand column of Fig. 24-A. THEY ARE ALMOST DIAMETRICALLY OPPOSED! How can you explain this?
 
 

Figure 24-A:  Characteristics of the Successful Flowering Plant Colonizers of The Hawaiian Islands They arrived, survived and thrived over several million years.   COLONIZER CHARACTERISTICS COMMON NATIVE TRAITS Weedy, Agressive Seeds, Fruits &propagules small Annual, rapidly establishing itself Many armed with prickles, thorns or chemicals such as poisons or strong, smelly oils. Bisexual Flowers (Having functional stamens and ovary in one flower) Easily dispersed by wind, water, or wing. Coastal-adapted, originally from coastal areas "Fragile", easily displaced by introduced plants. Large fruited/seeded Perennial, woody, semi-woody, generally slow growing Thornless, lacking chemical or physical defenses Unisexual flowers with only male or female parts functional Dispersal characteristics lost Dryland plant forms evolved into wet forest forms

 
C. There are several million years between these times, enough for a significant evolutionary "turnabout" or major changes. Certain traits are highly favorable for successful "arriving" colonists to Hawaii, resulting in successful dispersal to the most remote island chain in the world.

However, once here, in order to "thrive", different traits were favorable. Those are influenced primarily by Hawai'i's unique environment, which in turn, was instrumental in an unique evolution of its native endemic plants.

D. Looking at the paired traits, these explanations can be made.

1. It is obvious that the survival of a new arrival to Hawai'i is enhanced by a weedy, aggressive growth behavior. It must survive wherever it lands, whether brought by wind, water or wings, and successfully produce offspring.

a. Weeds, in one definition, are plants growing where they are not wanted. However, some plants are clearly more difficult to remove from an area than others.

b. If these weeds' origin is examined, many come from areas where there has been thousands of years of human "disturbance" and activities.

The Mediterranean is one region where people have been farming for thousands of years. The plants which could survive the farmers attempts to remove them, in favor of the crop plants, developed characteristics of rapid growth, deep-rootedness and other traits, which make them difficult to "weed out." Thus the "Mediterranean weeds," are some of the most difficult plants to get rid of, especially in areas which have had less exposure to widespread human disturbance.

c. In contrast, why would native plants be "fragile", and more easily displaced? Again, imagine the scenario of the arriving plants to Hawaii. Hardly any competition! Only 272 new arrivals over millions of years! The struggle would be for basic survival against the "elements," (that is, the abiotic part of the environment) but there would be very little resistance from other plants or animals. This is quite unlike the situation in the Mediterranean. In pre-human Hawai'i there was little competition from crop plants, or from other plants coming from surrounding continents.

d. Since native plants had little competition from other plants and animals, this legacy allows for their being easily displaced by recently introduced plants. Certain types of plants seem to go wild when introduced to Hawaii or other remote areas. A prime threat presently to all plants in Hawaii is the South American plant called Miconia ( Miconia calvescens). It has already replaced extensive rainforests in Tahiti and could do so in Hawaii.

Miconia Photo by P. Millen  

Two factors allow these threatening introduced plants to massively displace the native plants:

They happen to grow here particularly well but did not get here by natural dispersal means.

Few of their natural enemies -- insects, diseases or herbivores -- are present on these islands.

 

Koster's Curse

2. The second paired characteristics are obvious from earlier discussions. Small propagules are always more dispersible, especially by wind or wings. Imagine a small shore bird, like the Golden Plover or Kolea  , trying to fly 3,000 miles, with an avocado seed in its intestines. Not very likely!  Large seeds can only be effectively dispersed by water, but even the coconut didn't make it due to other factors previously discussed.

 

3. The original colonist would almost have to be an annual.

a. A perennial may take years to establish itself and produce offspring. During such a long time, it could easily be injured or die, failing to reproduce itself.

b. In contrast, there are no true annuals in the Hawaiian flora. A plant closest to that classification in the Hawaiian Poppy, Pua kala (Argemone glauca), which is a short-lived perennial. It also has spines, another trait atypical to native plants. Botanists believe that both those traits can be explained by the pua kala being an recent arrival. It may have come to the islands as recently as two thousand years ago. There is some indication that these colonizer traits are slowly changing. Some populations of pua kala have fewer spines, for example.

c. Why are most native Hawaiian plants perennial or woody today?

It probably has much to do with the environment of Hawaii, much of which is highly favorable to continuous growth because of mild temperatures and sufficient rainfall. Annual plants are well-adapted to the temperate climates, where part of the year is very favorable to plant growth and the colder part is not. A rapidly-growing plant which produces seeds at the end of its short life-time is well-adapted to this seasonal shift. In the environment favourable to continuous plant growth, a short annual life-span is a handicap.

d. Natives also are slow-growing, perhaps because of nutrient/rainfall limitations in many environments. High rainfall tends to wash out nutrients and less rainfall found in many drier areas of Hawai'i limits growth.

4. Native plants lack chemical and physical defenses.

a. These associated traits found in Hawaiian plants are parallel with traits often found in island animals. Wingless insects and birds, both of which were found in Hawai'i and other island faunas, lack common "defenses" found elsewhere. The lack of protections for both plants and animals on islands relate to lack of predators.

b. Island fauna is obviously "disharmonic" (like the plant flora) lacking many insects and large herbivores (like cattle, sheep, deer and goats) found in most other parts of the world. As a result, the poisons, strong odors, thorns, spines and prickles, which are all plant defenses against insects and animals (herbivores), are missing in endemic flora.

c. The "losing" of these traits can happen over a long period of time. It requires extra photosynthetic energy for plants to produce thorns or protective chemicals. If a plant doesn't produce them and also is not assaulted by insects or animals, it can use that energy to grow larger and produce more offspring. Those genetic traits for such protection may well drop out of the gene pool over time.

5. A colonist plant would have had bisexual flowers in order to produce seed on its own and continue its species in the new home. If a plant produces only one type of flower, either male or female, the chances of both those plants dispersing at the same time to the islands, is almost impossible. Concepts around this topic are described in the next Learning Objective, 25.

6. Obviously dispersability is needed to get to islands. The reasons for the loss of dispersability is discussed in Learning Objective 26.

7. Likewise, the pattern of adaptation to coastal areas originally, and moist forests later, is discussed in Learning Objective 26.

  Click here to leave your comments and suggestions.   Learning Objectives

[1 ]	[2 ]	[3 ]	[4 ]	[5 ]	[6 ]	[7 ]	[8 ]	[9 ]	[10 ]
[11 ]	[12 ]	[13 ]	[14 ]	[15 ]	[16 ]	[17 ]	[18 ]	[19 ]	[20 ]
[21 ]	[22 ]	[23 ]	[24 ]	[25 ]	[26 ]	[27 ]	[28 ]	[29 ]	[30 ]

---

[Learning Objectives Numerical Index] [Learning Objectives Descriptive Index] [Main Menu]