The Equilibrium Theory of Island Biogeography. Reading: MacArthur & Wilson

Transcription

1 The Equilibrium Theory of Island Biogeography Reading: MacArthur & Wilson

2 DiversificaCon or Why are there so many species? Lecture 17 Recap Jürgen Haffer

3 Theodosius Dobzhansky DiversificaCon or Why are there so many species? Any organism which lives in a temperate or a cold climate is exposed at different periods of its life cycle or in different genera:ons to sharply different environments. The evolu:onary implica:ons of nature's annually recurrent drama of life, death, and resurrec:on have not been sufficiently appreciated. In order to survive and reproduce, any species must be at least tolerably well adapted to every one of the environments which it regularly meets. No mader how favored a strain may be in summer, it will be eliminated if it is unable to survive winters, and vice versa. Faced with the need of being adapted to diverse environments, the organism may be unable to adain maximum efficiency in any one of them. Changeable environments put the highest premium on versa:lity rather than on perfec:on in adapta:on. - Dobzhansky

4 DiversificaCon or Why are there so many species? Theodosius Dobzhansky All of these quotes come from a great paper called Evolu:on in the Tropics (posted on the website) Dobzhansky viewed the stability of the tropical climate as one of the main drivers of diversificacon Under a stable climate, there is Cme for mutualism, parasicsm and other complex coevolved relaconships to emerge (also sympatric speciacon)

5 DiversificaCon or Why are there so many species? Jürgen Haffer 1. Known climate fluctua:ons during the Pleistocene 2. Reconstruc:on of rainforest refugia - Rainfall paderns - Current bird distribu:ons (endemic & close habitat associa:ons) 3. Bird specia:on processes can be quick k years

6 DiversificaCon or Why are there so many species? Jürgen Haffer

7 DiversificaCon or Why are there so many species? Jürgen Haffer SCmulated a number of studies debacng the number, locacon, and extent of rainforest refugia Current evidence shows most of the speciacon events occurred in the late Pliocene, before the climacc shixs of the Pleistocene However, we know far more abut the speciacon in the tropics thanks to Haffer s hypothesis

8 Natural Laboratories Natural systems where key factors vary so that their effects can be isolated

9 Natural Laboratories Sea Surface Temperature Wave Height

10 Natural Laboratories Sea Surface Temperature Wave Height

11 Natural Laboratories Temperature Wave Height

12 Natural Laboratories High Temp/ Low Waves High Temp/ High Waves Temperature Low Temp/ Low Waves Low Temp/ High Waves Wave Height

13 Islands serve as perfect natural laboratories for biogeography 1. Numerous 2. Varied geographical circumstances - Distance from mainland - Age - Size 3. Tractable biotas

14 Theories obtained from these island systems are then applied to the concnents SCmulated the field of conservacon biology, especially as it relates to habitat loss and fragmentacon Processes and mechanisms are not unique to islands, it is just that islands have provided the context for their isolacon and analysis

15 Biogeographers have gained important insights from island systems throughout the history of the field (Galapagos, Malay Archipelago) In the 1950s through the mid- 1970s, modern work on marine (and concnental) islands led to theories on the dynamics of species richness

16 Olof Arrhenius Species vs. Area Rela:onship Looked at plant communices on islands off the coast of Sweden Found that as area increased, there tended to be more species First to describe this relaconship mathemaccally

17 Olof Arrhenius Species vs. Area Rela:onship

18 Olof Arrhenius Species vs. Area Rela:onship

19 Olof Arrhenius Species vs. Area Rela:onship Log- log plots are called Arrhenius plots Also nocced that the slope of the line appeared to differ systemaccally between islands and non- isolated areas on concnents

20 In oceanic islands, species richness declines with distance from a mainland source TradiConal explanacon was impoverishment with distance, which held that Cme had been insufficient for remote islands to fill up ImplicaCon that over Cme the species richness of these islands would increase further This is a non- equilibrium explanacon

21 Wilson & MacArthur An Equilibrium Theory of Insular Zoogeography The authors argued for an equilibrium model of species richness along a distance gradient from the mainland sources, and that this was also a funccon of island size This theory represents a dynamic steady state due to the offseing effects of immigracon (influenced by distance) and excnccon (influenced by area)

22 Wilson & MacArthur Mainland

23 Wilson & MacArthur Mainland

24 Wilson & MacArthur Mainland

25 Wilson & MacArthur Mainland

26 Wilson & MacArthur Mainland

27 Wilson & MacArthur Mainland

28 Wilson & MacArthur Mainland

29 Wilson & MacArthur Mainland

30 Wilson & MacArthur Mainland

31 Wilson & MacArthur Much less likely that an organism from the mainland source populacon is going to reach a remote island relacve to a near island Also the relaconship between the number of species and new immigrant species the more species an island has, the less likely a new immigrant is going to represent a novel species that does not already exist on that island

32 Wilson & MacArthur The relaconship between the number of species and new immigrant species the more species an island has, the less likely a new immigrant is going to represent a novel species that does not already exist on that island ImmigraCon Rate Number of Species Present

33 Wilson & MacArthur Near Island Much less likely that an organism is going to reach a remote island relacve to a close island from the mainland with the source populacon ImmigraCon Rate Far Island Number of Species Present

34 Wilson & MacArthur As the number of species on an island increases, the rate of excnccon will also increase, as there are more species to possibly go excnct (given that all species are equally likely to die out), there is also less space overall for each species which means smaller populacons A smaller island will have greater excnccon rates than a larger island for the same number of species as there is less space and can support a small populacon sizes

35 Wilson & MacArthur As the number of species on an island increases, the rate of excnccon will also increase, as there are more species to possibly go excnct (given that all species are equally likely to die out), there is also less space overall for each species which means smaller populacons ExCncCon Rate Number of Species Present

36 Wilson & MacArthur A smaller island will have greater excnccon rates than a larger island for the same number of species as there is less space and can support a small populacon sizes Small Island ExCncCon Rate Large Island Number of Species Present

37 Wilson & MacArthur Near Island Small Island ImmigraCon Rate Far Island ExCncCon Rate Large Island Number of Species Present

38 Wilson & MacArthur 1. An island which is farther away from the source of colonizacon will have fewer species, because the immigracon curve will be lower and hence intersect the excnccon curve farther to the lex 2. ReducCon of the species pool of immigrants will reduce the number of species on the island 3. If an island has a smaller area, or more severe climate, the excnccon curve will rise and the number of species will decrease

39 Wilson & MacArthur 4. If you have two islands with the same immigracon curve but different excnccon curves, any given species on the island with the higher excnccon curve is more likely to die out 5. The # of species on an island far from the source will grow more rapidly with island area than will near islands 6. The # of species on large islands decreases with distance form the source of colonizacon faster than does the number of species on small islands

40 Wilson & MacArthur Near Island Small Island ImmigraCon Rate Far Island ExCncCon Rate Large Island Number of Species Present

41 Wilson & MacArthur Main Points Islands represent natural laboratories to study biogeographic panerns The impoverishment with distance explanacon is a non- equilibrium theory The equilibrium theory represents a dynamic steady state due to the offseing effects of immigracon (influenced by distance) and excnccon (influenced by area)

42 QuesCons on the reading?