Species diversity Ecological communities differ in species number

Species diversity • Ecological communities differ in species number and composition – tropics > temperate – remote islands < large islands – continents > islands 1

Species diversity • Comprised of – species richness: number of species present – heterogeneity of species • equitability or evenness • relative abundance of each species present in the community 2

Measurement of species diversity • Species richness – number of species present in community – first and oldest concept of diversity – simplest estimate of diversity – only residents are counted – treats common and rare species with the same weight 3

Measurement of species diversity • Heterogeneity of species – uses relative abundance to give more weight to common species – possibilities in a 2 -species community: Species A Species B Comm 1 99 1 100 Comm 2 50 50 100 4

Measurement of species diversity • Shannon-Wiener diversity function s H' = - (pi) [ln(pi)] H’ = Shannon-Wiener index of species diversity s = number of species in community pi = proportion of total abundance represented by ith species 5

Shannon-Wiener diversity index Community 1 Species N A 99 B 1 pi ln(pi) pi[(ln(pi)] Community 2 Species N A 50 B 50 6

Shannon-Wiener diversity index Community 1 Species N pi ln(pi) pi[(ln(pi)] A 99 0. 99 -0. 010 B 1 0. 01 -4. 605 -0. 046 100 1. 00 -0. 056 H’ 0. 056 Community 2 Species N pi ln(pi) pi[(ln(pi)] A 50 0. 50 -0. 693 -0. 347 B 50 0. 50 -0. 693 -0. 347 100 1. 00 H’ -0. 694 7

Measurement of species diversity • Shannon-Wiener diversity function – values range from near zero to ? ? ? – increased values indicate increased diversity – index has no units; value only as comparison between at least two communities 8

Species diversity • What increases species diversity (H’)? – increasing the number of species in the community (s) – increasing the equitability of the abundances of each species in the community 9

Evenness • Measurement of equitability among species in the community • Pielou evenness E = H’ / Hmax E = Pielou evenness H’ = calculated Shannon-Wiener diversity Hmax = ln(s) [species diversity under maximum equitability conditions] – values range from near zero to 1 10

Diversity and evenness Community 1 Community 2 s 2 2 H’ 0. 056 0. 694 Hmax 0. 693 E 0. 081 1. 000 11

Practice problem Community 1 Species N A 62 B 97 C 110 D 84 E 16 pi ln(pi) pi[(ln(pi)] 12

Practice problem Community 2 Species N A 88 B 10 C 0 D 211 E 27 pi ln(pi) pi[(ln(pi)] 13

Practice problem Community 1 Community 2 s H’ Hmax E 14

Species diversity indices 15

Commonness, rarity and dominance • Preston’s log normal distribution model – a few common species with high abundances – many rare species with low abundances 16

Commonness, rarity and dominance • Mac. Arthur’s broken stick model – random breaks in a stick log normal distribution of pieces – results in a few large pieces and many small pieces 17

Commonness, rarity and dominance • Community organization – model 1 • a few very common species • many rare species – model 2 • a few very common and very rare species • most species of intermediate abundance 18

Fig. 22. 1, p. 435: Relative abundance of Lepidoptera captured in a light trap in England (6814 individuals representing 197 species). 19

Biogeography • Observations of relationships between – area and number of species – distance from source • Island biogeography – E. O. Wilson and Robert Mac. Arthur 20

Island biogeography • Island communities: well-defined, captive • Variables – size – degree of remoteness – elevation • Simple community structure • Increase in area increase in number of species 21

Island biogeography • Habitats considered as “insular” because they are isolated from other communities – caves – mountain tops – some peninsulas – wildlife or game preserves 22

Fig. 24. 14, p. 502: Number of land-plant species on the Galapagos Islands in relation to the area of the island. 23

Fig. 24. 15, p. 503: Species-area curve for amphibians and reptiles of the West Indies. 24

Island biogeography • Relationship between remoteness and number of species – increase distance from mainland decrease number of species – number of species present is dependent on immigration from mainland • rate is a function of the number of species already present on the island • number of species present = balance between immigration and extinction 25

Fig. 24. 17, p. 504: Equilibrium model for biota on a single island. 26

Fig. 24. 18, p. 504: Equilibrium model for biota on several islands of different size and remoteness. 27

Island biogeography • Small species are found on more islands than are large species • Number of herbivore species > carnivores • Number of generalist herbivore species > specialist herbivores 28

Island biogeography • Species: area relationship – log : log relationship – 10 -fold decrease in area 50% decrease in number of species 29

Island biogeography • Species: area relationship 30

Latitudinal diversity gradients • Abundance and diversity patterns – latitude – elevation – mountainsides – peninsulas 31

Fig. 22. 5, p. 438: Number of tree species in Canada and U. S. 32

Fig. 22. 6, p. 439: Number of species of land birds in North and Central America. 33

Fig. 22. 7, p. 440: Number of species of calanoid copepods in top 50 m of transect from tropical Pacific to Arctic Ocean. 34

Fig. 22. 9, p. 440: Number of species of mammals in continental North America. 35

Fig. 22. 10, p. 440: Species richness of mammals in North and South America in relation to latitude. 36

Latitudinal diversity gradients • Tree species – Malaysia (4 acres): 227 – Michigan (4 acres): <15 • Ant species – Brazil: 222 – Trinidad: 134 – Cuba: 101 – Utah: 63 – Alaska: 7 37

Latitudinal diversity gradients • Snake species – Mexico: 293 – U. S. : 126 – Canada: 22 • Fish species – Amazon R: >1000 – Central American rivers: 450 – Great Lakes: 172 38

Latitudinal gradient hypotheses • • History (time) Spatial heterogeneity Competition Predation Productivity Environmental stability (climate) Disturbance 39

Latitudinal gradient hypotheses • History (time) hypothesis – tropical habitats older, more stable – support for • geological past of temperate less constant than tropics due to glaciation • all communities diversify with time – argument against • as glaciers moved in, species moved south to escape • history hypothesis can not be tested 40

Latitudinal gradient hypotheses • Spatial heterogeneity hypothesis – higher diversity in tropics due to increase in number of potential habitats – environmental complexity moving away from equator • macro level: e. g. , topographic features • micro level: e. g. , particle size, vegetation complexity 41

Latitudinal gradient hypotheses • Spatial heterogeneity hypothesis – Hutchinson’s n-dimensional niche specialization – types of diversity defined by spatial heterogeneity • within-habitats ( diversity) • between-habitats ( diversity) 42

Diversity defined by spatial heterogeneity Between habitat diversity ( ) Temperate Tropical No. species per habitat 10 10 No. different habitats 10 50 Within-habitat diversity ( ) Temperate Tropical No. species per habitat 10 50 No. different habitats 10 10 43

Latitudinal gradient hypotheses • Competition hypothesis – less competition in temperate and polar environments compared to tropics because these populations are more regulated by extreme environmental conditions than by biological factors – populations maintained <K due to weather, etc. and major sources of mortality are abiotic – since population sizes small, decreased competition for resources 44

Latitudinal gradient hypotheses • Competition hypothesis – no weather extremes in tropics, populations can increase to densities at which competition for resources is necessary – promotes species diversity through specialization resource partitioning – and diversity higher in tropics due to organisms being more specialized to habitats 45

Fig. 22. 14 a, p. 447. Niche breadth versus niche overlap determined by competition within the community. 46

Latitudinal gradient hypotheses • Predation hypothesis – increased species diversity in tropics is function of increased number of predators that regulate the prey species at low densities – decreases competition among prey species – allows coexistence of prey species and potential for new additions 47

Fig. 22. 16, p. 449. Janzen-Connell model for increased diversity of tropical rainforest trees: seed predation versus distance of seed from tree versus seed survival. 48

Latitudinal gradient hypotheses • Predation hypothesis – there is more selective pressure on prey evolving avoidance mechanisms than in becoming better competitors – cropping principle • remove predators and prey start competing • predation increases diversity by reducing intraspecific competition among prey species 49

Community anchored by keystone starfish Heliaster in northern Gulf of California. 50

Latitudinal gradient hypotheses • Predation hypothesis – cropping principle in lakes • top predators (fish) feed on zooplankton • if fish are removed community diversity decreases, becomes dominated by a few species of large, grazing zooplankton • add fish diversity of small zooplankton and their invertebrate predators increases 51

Latitudinal gradient hypotheses • Productivity hypothesis – tropics support a greater number of species because more resources are available, allowing for more specialization – in general: production diversity – exceptions • marshes: high production, relatively low diversity • deserts: low production, high diversity 52

Latitudinal gradient hypotheses • Environmental stability (climatic) hypothesis – annual climate in tropics more stable than temperate or polar climates – constant climate finer specializations and adaptations, shallower niches – tropical species number of broods / year potential for evolutionary change rate of speciation 53

Latitudinal gradient hypotheses • Environmental stability (climatic) hypothesis – high diversity habitats generally found in stable climates; low diversity habitats associated with severe and/or unpredictable climates 54

Latitudinal gradient hypotheses • Disturbance hypothesis – if community disturbance frequency is very high local extinction of species diversity – if community disturbance frequency is very low competitive exclusion by dominant species diversity 55

Latitudinal gradient hypotheses • Disturbance hypothesis – intermediate disturbance hypothesis • moderate disturbance maximizes diversity • leads to patches at local level – intermediate disturbance high species diversity in some communities (not all) 56

Fig. 22. 20, p. 453. Model for intermediate disturbance hypothesis. 57

Fig. 22. 21, p. 453. Effect of periwinkle grazing on algae diversity. 58

Fig. 22. 21, p. 453. Effect of periwinkle grazing on algae diversity. Community dominated by one algal species Predator limits number of possible algal species 59

Basic concepts related to energy flow and trophic structure • Energy moves through community and is lost as heat • Nutrients move through the community in cycles and are retained 60

Basic concepts related to energy flow and trophic structure • Niche – sum of all parameters that enable an organism to live in its biotic and abiotic environments • competition, food gathering, predator escape, mate location, reproduction, etc. • temperature, moisture, nutrients, soil structure, salinity, etc. – Hutchinsonian niche: n-dimensional hypervolume 61

Basic concepts related to energy flow and trophic structure • Trophic level – Lindeman (1942) • classification of animals according to location in lake • lake trophic groups – – benthic demersal plankton nekton 62

Basic concepts related to energy flow and trophic structure • Trophic level – Lindeman (1942) • described food chain with primary producers at base and other trophic levels of animals based on feeding relationships • more accurately described as food web, since few organisms other than plants occupy only one feeding level 63

Food webs and energy flow • Trophic levels – ecosystem feeding levels – biomass and usable energy as level – most systems support only four trophic levels – aquatic communities have slightly longer food chains than terrestrial communities – ultimate food chain length limited by inefficiency of energy transfer from one trophic level to the next 64

Food webs and energy flow • Food chains – sequence of organisms where each is the food source for the next • Food webs – represent energy flow through ecosystem 65

Trophic levels Tertiary consumers (top carnivores) Secondary consumers (carnivores) Primary consumers (herbivores) Primary producers (plants) 66

Food chain model 67

Figure 23. 6, p. 465. Hypothetical food web model. 68

Food web terminology • Top predators: species eaten by nothing else in the food web • Basal species: species that feed on nothing within the food web • Intermediate species: species that have both predators and prey within the food web • Trophic species: groups of organisms that have identical sets of predators and prey • Cycles within food web: which species eat which other species • Interaction: any feeding relationship within food web • Connectance: number of actual interactions in food web divided by number of possible interactions • Linkage density: average number of interactions per species in the food web • Omnivores: species that feed on more than one trophic level • Compartments: groups of species with strong linkages among group members but weak linkages to other groups of species 69

Figure 23. 8, p. 467. Distribution of food chain lengths in the Ythan Estuary, NE Scotland. 95 species 5518 food chain lengths counted 70

Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966). Producers 71

Rocky intertidal community food web (Paine’s 1966 study) • (Producer level omitted from original figure) • Level 1 – herbivorous gastropods and chitons – filter feeding bivalves – suspension feeding barnacles and brachiopods • Levels 2 -4: carnivorous gastropods • Level 5: top carnivore – Heliaster starfish 72

Keystone species • Usually the top carnivore • Presence or absence determines community structure and composition 73

Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966). Top carnivore Producers 74

Food web of a rocky intertidal community, northern Gulf of California (after Paine 1966). Top carnivore X Species outcompeted in absence of keystone species X X Space competitor Producers X Producers 75

Keystone species • Paine (1974): Pacific rocky intertidal community – dominated by Pisaster starfish – remove starfish → mussel Mytilus californiensis ↑ → excludes all other invertebrate species – Mytilus becomes numerically dominant – Pisaster feeds on Mytilus → prevents Mytilus domination of community → ↑ community diversity 76

Figure 23. 3, p. 462. Simplified Antarctic marine food web. 77

Fig. 23. 4, p. 464. Food web of boreal forest of northwest Canada. 78

Generalizations about food webs • Size of animal increases with increase in trophic level • Abundance decreases with increase in trophic level • Large animals can not exist on small animals as prey • Small carnivores are limited to prey that can fit into their mouths 79

Which trophic level is most important? • Studies by Charles Elton in two square miles of Wytham Woods • Which species could be removed without changing the community? – top carnivore, except keystone species – lower levels are food source for higher levels – importance of top carnivores <<< herbivores 80

Which trophic level is most important? • Dependent on complexity of community – increased number of interconnections in community → increased complexity of food web → increased stability of community structure → alternate food sources should one be removed – redundancy model versus rivet model 81

Which trophic level is most important? • Determining species importance – species with highest biomass – where nutrients accumulate – where energy accumulates 82