Community Ecology Bioe 147/247

Transcription

1 Community Ecology Bioe 147/247 Species Richness 2: Area Effects: Islands and Mainland Themes: o Species Area effects o Mechanisms: area, distance, equilibrium theory o Habitat islands o Is it area? Or.? ISLANDS: True islands (esp. oceanic islands) Biota is special in several ways: 1. Individual level properties 2. Population 3. Community-level properties 4. Evolutionary 1

2 5. Species-Area Patterns: One of the most robust ecological patterns that holds for many different habitats and taxa, is that S increases predictably with the Area of the habitat or sample. This pattern is often associated with islands (see previous handout for other examples), but can also apply to habitats of different areas on the mainland. In fact, this quantitative relationship was first discovered in a plant survey in Surrey, England in the 1850s. In addition, the number of insects occurring on numerous species of trees increases with the area occupied by these trees in Britain. 2

3 AREA EFFECTS A few cases show the species-area pattern without any change in habitat complexity, so area per se appears to be the main factor in at least those examples. Regardless of mechanism, the pattern is very robust. The species-area relationship is generally plotted on a log-log plot because that is a straight line that is easier to interpret and to compare among different habitats, examples, etc. Plotting S vs A gives this power plot. Plotting logs of S and A turns this into a straight line. Species Richness (S) Log S Area (A) S = ca z Log A log S = log(c) + z*log(a) z = slope (rate of accumulation) of S w/ area c = intercept (reflects potential colonists) Q: What is the point of getting an equation for this relationship? Why bother? Answers: Often, z = 0.3, giving a 2x increase in S for each 10x in A: Relative area Formula S if c=10 S if c= c(100).3 ~ 40 ~ c(1000).3 ~ 80 ~ NOTE: This specific relationship holds for a) islands that are about the same distance from a source (so that c is the same); and b) ONLY if z ~.3. If distances or z-values change, then there are different relationships..but because we have the equation, we can calculate the Log Species. 3

4 Log Species DISTANCE EFFECTS: reflect the likelihood of immigration. 1. Far islands accumulate fewer potential immigrants (lower c-value) than on near islands. The intercepts of the lines on the right indicate the effective c-value. 2. Area effects are larger on far vs near islands higher slopes (bigger z-values) on distant islands that are harder to find than near islands. This also reflects the chances that islands nearer the sources can be resupplied (after an extinction) more quickly Rescue Effect source near far log Area Example: Birds on islands near and far from New Guinea. Areas on New Guinea NG itself Near Far 4

5 DISPERSAL ABILITIES VARY: Some taxa disperse more than others. Good dispersers have low z-values, with weak effects of area due to re-supply (Rescue Effect). Poor dispersers have much smaller chances of arriving on an island, so effects of island area are relatively large. Birds/bats: z=.24 Land mammals: z=.48 5

6 I or E Rate I or E Rate I or E Rate S Immigration Rate Extinction Rate THE EQUILIBRIUM THEORY OF ISLAND BIOGEOGRAPHY Time Area Effect Distance Effect 6

7 TESTS OF THE THEORY: Mangrove islets in the Florida Keys DISTANCE EFFECT = Immigration Rates Near islets Far islet Fig Morin AREA Effect = Extinction Rates (w/ reduced island sizes) Fig Strong test of the Equilibrium Theory Patterns: Experiment: Evolution? See reading for section next week. 7

8 MAINLAND EFFECTS 8

9 HABITAT ISLANDS Species-area relations for the number of mammal species found in montane forest habitats in the Great Basin of the western US. Numbers indicated different habitat islands. The dashed line shows the species-area relation within larger continuous areas of montane habitat. (Reprinted from Brown 1971, with permission of the University of Chicage Press). Mainland : Sierra Nevada. Saturated z=0.2 Islands in the Great Basin: E is high I = 0 (since end of last glaciations) z =.43 9

10 CONSTRUCTED HABITAT ISLANDS Species-area relationships in wheat-belt natural reserves in Western Australia. 10

11 NOTE: Are these patterns ALWAYS due to area? Or to something correlated w/ Area?? Does it matter? Species-Area Pattern Species-Habitat Pattern Habitat-Area Pattern 11