Lecture Notes

Week 1: Introduction to Class and Materials

I. Go through syllabus and course requirements

A. Course requirements and goals

1. The course is generally aimed at understanding tropical biodiversity and the causative mechanisms that may have produced it

B. Format of facilitation

C. Format of summaries

D. Format of individual research project

E. Format of individual research presentation

F. Potential need to eliminate several days or shorten class times (only if less than four students)

G. Class schedule and modules in the course

H. Daily meeting schedule

II. The tropics are more diverse for nearly all organisms

A. Latitudinal gradient exists for many taxa

1. This was the first pattern that attracted scientific attention to species diversity

2. Diversity for most TERRESTRIAL plants and animals is lowest near the poles and increases toward the tropics (equatorial regions)

3. Highest in tropical rainforests

4. What is the characteristic structure of a rainforest as far as the number of individuals per species? ANS: very few per species, but many species per unit area

5. Actually, if you look at the northern hemisphere (North America, Europe, Asia) many fewer species are being described as new there, in comparison to those in the Southern Hemisphere

a. Nearly all of the insects that are found on England have been described

b. Almost 90% of the species described in the tropics are undescribed

c. This is a hindrance to those interested in doing large-scale faunistic surveys 

d. Example of Steve Ashe’s cloud forest work and best habitat to preserve

6. Originally described from trees

a. Nearly a single species monoculture in the northern tiaga (Northern Spruce)

b. Overwhelming diversity of the rainforest (as we’ve already discussed)

c. Fig. 2.2 - relatively even progression in diversity as you go from north to south - although the isoclines are not absolutely east-to-west in direction (more later)

d. Fig. 2.1 - a relatively neat regression line could be fit to these data, but you’ll notice that there is quite a bit of spread at the lower latitudes -- we’ll come back to this later

7. Also found in many animals and plants (Tables 2.1-2.2)

a. Difference between these two tables is that 2.1 shows all species that were found in a region and how they were distributed north to south

b. 2.2 shows the species present at survey location points (not as thorough on a small scale, but has a much larger area covered and included in the study

8. Why do you think that the organisms don’t show clean isocline lines and that the spread in the number of plants found in the equatorial regions is pretty broad? -- Generally, because there are more forces at work than just latitude and increasing heat in determining climate (what were the others? ANS: Temperature, humidity, wind, sunlight)

a. So - what would you expect is the type of habitat that is at this point (very low equatorial value)? ANS: probably coastal desert (Namib desert off the west coast of Africa)

b. Write their suggestions on the board as to why (at this point) the distributions do not accord absolutely with latitude

III. But not for all (exceptions to the general rule)

A. Exceptions that increase in diversity as you go north or south towards the poles

1. Sea birds - feeding on marine crustaceans and fish

a. Why may this be important? Where do whales feed? Where are whales killed and harvested? Where then is the greatest quantity and diversity of the food on which the whales live? What do whales feed on? Marine crustaceans and fish

b. It has been advanced that it is just productivity that accounts for this disparity

2. Lichens - highest diversity and abundance occurs at high altitude or northern climates, particularly where plants cannot grow

a. Why may this be important? ANS: they are outcompeted by plants wherever plants grow

b. How does this jibe with the sea birds example? ANS: similar reason in that it is available nutrients at the higher latitudes that allows for the Lichen growth -- higher nutrient availability where plants are not there to outcompete the lichens and use all the available resources (primarily light and growing space)

3. Soil Nematodes (primarily soil predators of small arthropods) - highest diversity and abundance in the middle latitudes than in the tropics and in northern latitudes

a. This was attributed to the “partial ecological release due to the absence of tropical competitors” by Proctor 1984-- What is “Ecological Release”? ANS: when the main selective pressures that have been keeping the population in check are removed and the species is “released” and can explode (in the short term in numbers and abundance, but in the long term in the numbers of species)

b. Other possible explanations for this observation? Where is soil thickest? Where is the soil condition ideal for soil nematodes (i.e., not frozen)? 

c. Again it could be thought to boil down to productivity and available nutrients available to the nematodes - more soil in mid latitudes than in either direction, and thus more arthropods

B. General explanation offered to explain why they are contrary to the rule

1. Productivity!!

2. This is the explanation that I lean towards for explaining why the tropics are more diverse than elsewhere

a. However there have been many examples where we observe an INVERSE relationship between productivity and diversity 

b. Anyone remember the term for this seemingly paradoxical situation? ANS: The Paradox of Enrichment

c. When does it occur?

1. Increasing quantities of nutrients in an area (say due to fertilizer or manure effluent)

2. Leads to algal blooms and eutrophication (what’s that?)

d. Seems that the trend towards increasing abundance and species diversity does not continue indefinitely, but rather is highest at intermediate productivity levels

e. Doesn’t eviscerate the explanation, only makes it more precise

3. However, it’s most likely that there are many explanations and they all affect the diversity to some degree.

IV. Hypotheses why they tropics are more diverse than elsewhere -- these will be addressed in greater depth in the third module of the class

A. Age of topics 

1. Increasing age allows for greater number of species to evolve 

2. And for more to disperse to it

3. And for the community to come to a climax community

a. And thus the more rare species that are only found in these types of communities

b. What’s that?

C. ANS: the terminal community that will be found in an area if left undisturbed indefinitely 

d. OR the community where energy coming into the system (sunlight) = energy leaving it (dead organic matter)

B. Constant and large environment (ditto above, plus it increases the number of individuals that can be found in the area of the same species 

1. Why is this important?

2. More individuals, more opportunity and probability of speciation

3. Plus you get a greater target effect -- what’s that? ANS: in island biogeography, the larger the target, the greater the probability that a dispersing organism will encounter your site

4. Plus - you get a greater heterogeneity in habitats available to the organisms

C. Increased disturbances in tropics (just the opposite of the above explanation - usually on a much longer time scale than those advocating the above explanation are addressing)

1. Glaciation did in fact disturb the tropics greatly through climatic changes by expanding savannahs and decreasing the lowland rainforest area

D. Glacial refugia (last gasp hope for the species as they tried to avoid dramatically changing habitat)

E. Increased interactions among species

1. Why would this be important?

2. ANS: increased pressure and ability to specialize on a diversity of hosts, prey, etc.

F. Higher average year-round temperature 

1. Why would this produce an increase in number of species?

2. ANS: faster time through life cycle, more generations per time unit, more opportunities for evolution to rapidly occur