Week 4 Lecture Notes - Sept. 20

Paper topics and mangrove report were due.  There will be a quiz next week!

Review WHY is there a latitudinal diversity gradient? 

Environmental stress and Complex Physiognomy

Other things that add to the complexity (Ch 6 trop eco, ch 3 neotrop)

Disturbance = means just what it sounds like – the ecosystem is disturbed by something.

I. Human Disturbance

1. Clearing – for agriculture

2. Grazing – removes plants

3. Burning – frequent fires will convert the tropical forest to a savannah, one of the biomes we mentioned the 1st day of class.

        (Savannahs : 

• Edge of hot desserts, on the tropical side. Spiny shrubs, small trees loose leave during dry winter.   
• Where it is grazed by large animals we have savanna – grasses with scattered trees.  – 
• AFRICA with giraffes, antelope, etc.)

4. All of causes of deforestation.

5. Conclusion – humans have a huge impact on nature – enough to change it into a different biome.

II. Natural Disturbance

1. Gaps  = openings created in the forest that allow sunlight to penetrate to the forest floor.

      A. Size

           1.  Small – leaf fall, branch fall

           2. Large – hurricanes, landslides in wet season

      B. Creates complexity both horizontally and vertically

            1. Reduces the biodiversity at 1st

            2. Then biodiversity increases as new plants move in

      C. Shade tolerance plays a large role

            1. Shade tolerant sp are slow growing

            2. Shade-intolerant (sun-loving) will grow quickly if there is a disturbance that allows sun onto the forest floor.

            3. Vines and non-woody herbaceous plants grow quickly

       D. Conclusion – not as severe as the disturbance created by humans, thus the ecosystem can recover.

2. Fire  (p 210 tropical ecology textbook)

      A. Lightning strikes

      B. Scientists can tell when a forest burned by looking at charcoal deposits.

Leads to Succession = old-growth - - - >  disturbance - - - >   2nd growth

Succession = an ecological process in which fast growing plants colonize and area that has been disturbed, to eventually be replaced by slower-growing species that remain indefinitely (until the next disturbance)

Secondary succession – what we see in tropical forest.  There is already a plant community, then it is disturbed and a 2nd community comes along.

Primary succession – occurs where there are no plants – the rock breaks down into soil and slowly plants move in.  Requires 100s of year.

What we are talking about in this class is secondary succession.

General steps

1. Disturbance opens up the forest floor

2.Rapid colonization by fast growing species (pioneer species = vines, herbaceous, non-woody) 1 to 3 years

3. Fast growing, light-loving trees form a canopy 10 – 30 yrs

4. Shade-tolerant plants begin to grow 75-100 yrs, eventually they…

5. Shade-tolerant plants become the canopy – now looks like an undisturbed forest.  Lasts until the next disturbance.

This also illustrates how long it takes for a forest to recover from human activities.

pioneer species – the first to colonize previously disrupted or damaged ecosystems.

     heliconia (Heliconiaceae),

     piper (Piperaceae),

     legumes (Fabaceae) – trees, shrubs, vines, herbs,

     cercopia (Urticaceae)

Canopy – the uppermost layer of vegetation in a forest.

Succession differences due to natural vs human disturbance –

1. Disturbance is usually more severe and in a larger area.

2. Natural disturbance, unless it’s a hurricane or wind storm, destroys individual trees.

3. There may not be seeds left to recolonize the area.

Seed bank = seeds laying dormant in the soil.

Amazon has 500 – 1000 seeds/m2

Seedling bank – seedlings that need light to continue growth

Plate 9-1 - A large gap created by a landslide.

Fig. 6-9 - Succession in a tropical rain forest.