Equilibrium Theory of Island Biogeography

Transcription

1 Equilibrium Theory of Island MODULE: 04 EQUILIBRIUM THEORY OF ISLAND BIOGEOGRAPHY UNIT: 01 CONCEPTUAL FOUNDATIONS Objectives At the end of this series of lectures you should be able to: 1. Define a bunch of terms. Terms and concepts are noted in the module synopsis and highlighted in the presentation. Important for all course objectives 2. Explain the role that ETIB had in the development of modern ecology. Course objective Describe the Species-Area and Species-Isolation relationships. Course objective Explain and demonstrate the ETIB. Course objective 3. 1

2 Objectives 5. Discuss how can be applied to different types of problems. Course objective Contrast ETIB with metapopulation theory Course objective 3. Warning The same set of ideas may appear in different classes. The approach or importance of the topic will differ between classes. The usage of the ideas within the different classes will also differ. 2

3 Ecology in the 20 th Century Until the 1960s Closely tied to natural history and systematics Uniqueness of study sites and environment No general statements that could be applied to other systems the systems were unique Descriptive Minimal use of descriptive statistics and no real use of inferential statistics Static systems Ecology in the 20 th Century After the 1960s Distinctly different discipline from systematics A role for natural history the fight continues The impact of professor s education Hypothetico-deductive approach Phliosophy of Science Popper Equilibrial/Dynamic equilibriums/nonequilibrial systems Hypothesis testing/inferential statistics 3

4 Ecology in the 20 th Century The role of ETIB during this transition: Emerged in the 1960s Tension between earlier and later forms of ecology Article 1964 Monograph 1966 Early flag bearer for the new ecology Took considerable heat from the old guard Influential Amazon review Static Theory of Islands Community structure on islands was fixed Evolutionary events (overemphasized) Unique immigration events (no general pattern) Unique extinction events (no general pattern) Either a species had already colonized an island or it never would. No general explanations or models Each island was a unique and required a unique explanation (special pleadings). Dominated the study of and Ecology until the mid-1960s. 4

5 Dynamic Equilibriums Opposing forces maintain constancy in some characteristic of a system despite continual changes in other properties of the system. Homeotherm example. Paradigm shift Sought general patterns Sought general ecological explanations to those patterns Less emphasis on explaining the idiosyncratic or exceptional observations. Island Patterns The number of species tends to increase with island area The number of species tends to decrease with increased isolation of the island. 5

6 Species-Area Relationship The larger the area that we sample the more species we find. Schoener -- one of community ecology s few laws Most general, best-documented patterns in nature. Species-Area Relationship Arrhenius equation / Power model S = Species richness c = A fitted constant (taxonomic group and region species source) A = Area z = A fitted constant (region the type of the area sampled) 6

7 Species-Area Relationship Log transform of the power model We can now estimate c and z from simple linear regression on the log transformed species richness and log transformed area. 7

8 Species-Area Relationship c is frequently interpreted as the intercept The actual intercept of the power model is 0 z is frequently interpreted as the slope z describes the relationship between Log S and Log A not the relationship between S and A The relationship between S and A (how quickly S increases as A increases) is influenced by both c and z. Species-Area Relationship Most arguments related to species-area relationships focus on comparisons of the slope z as if z was the entire story. z varies only slightly Typically between 0.15 and 0.35 Continents tend to 0.15 Islands tend to 0.35 Theoretical value 0.25 c varies considerably By only comparing z-values we are missing the point. 8

9 Species-Area Relationship Why does the species-area relationship exist? Not entirely sure. Some great possible explanations. Beyond what we need to discuss now. Species-Isolation Relationship Islands far from the continent support fewer species than islands that are nearer to the continent. Assuming that pattern is the result from a decline in dispersal rates with isolation the relationship between species richness and isolation should be: Negative exponential function Negative sigmoidal function 9

10 Species-Isolation Relationship Less general than the species-area relationship Include broad ranges of differences in area, but only limited ranges of differences in isolation. Relevant measures of isolation are challenging. 10

11 Species richness Incorporates the different island patterns that we have discussed. Species-Area relationship Species-Isolation relationship Species richness on islands can be explained by two factors. Island area Island isolation 11

12 Area influences species richness on islands by controlling population size A strong inverse correlate of extinction. (Use of the word extinction) (Extirpation is not the correct word see dictionary) The smaller the island the smaller the population that can be supported on the island. The smaller the population size the more likely it will become extinct. Extinction Probability The greater the number of species in any community the greater the probability that any one species will go extinct. Numerical demonstration. Graphical representation. 12

13 Now we can combine the two concepts related to extinction: Islands with smaller areas have greater extinction probabilities. The more species on the island the greater the chance that there will be an extinction. 13

14 All species have some limit to their dispersal ability. Some species will be able to disperse long distances. Some species will able to disperse only short distances. The islands very close to a continent will have all of the species (species capable of only limited dispersal and those capable of long distance dispersal) Islands island very far a continent will have very few species (only those species capable of long distance dispersal). The number of species on an island will be inversely related to its distance from a continent. 14

15 Immigration Probability The more species on an island the less likely that any immigrant arriving at the island will represent a new species. Therefore the immigration rate declines with increased species richness on an island. Numerical demonstration. Graphical representation. 15

16 Now we can combine the two concepts related to immigration: Islands with greater isolation have fewer immigrants. The more species on the island the lower the probability that an individual arriving on the island will represent a new species to the island. 16

17 Any given island is described by its area and its isolation. Because extinction rate (a function of area) and immigration rate (a function of isolation) both influence species richness, we can combine them into a single graph. 17

18 Dynamic equilibrium Go back a slide Not all islands have the same area or isolation. Predictions on how species richness should vary with isolation and area of different islands. 18

19 Island biogeography theory assumes: No evolution Operates in ecological time Area controls extinction Isolation controls immigration Area and isolation are independent Equilibrium Species identity does not matter 19

20 Advantages Graphical presentation no need to do all of the math General Model had clear testable predictions Most mathematical models in ecology are not testable Predicts trends in species richness Predicts turnover rates Limitations So simple that it obscures patterns and processes Many (most?) islands are not in equilibrium Identities and characteristics of species are ignored Interactions among species are not included Immigration and extinction are treated as independent processes Measuring area and isolation Difficult to identify sources of islands colonists a priori Evolution on islands violates assumptions Disturbance on islands Area is only an indirect measure of an island s ability to maintain species. 20

21 Equilibrium/Dynamic Equilibrium/Nonequilibrial Hierarchy of Observation Variables can appear in equilibrium (constant) if they change very slowly or very quickly. Both probably occur in island biogeography Islands distant from the mainland may change very slowly. Islands near the mainland may change very quickly Rescue effect. Example Relaxation Application of ETIB Islands are a form of patch. Water is a form of matrix. Inhibits immigration. ETIB heavily influenced the early development of Landscape Ecology in North America. Largely replaced by metapopulation theory. 21

22 ETIB vs. Metapopulations ETIB Focuses on species richness a community level property. Metapopulation Theory Focuses on species distribution a species level property. However, at the end of the next lecture we will discuss metacommunities. 22