BIOL 4131 Lecture 10 Species diversity Ecological communities differ in species number and composition tropics > temperate remote islands < large islands continents > islands

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

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

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

Measurement of species diversity Shannon-Wiener diversity function 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 s

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

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 50 0.50 -0.693 -0.347 -0.694 0.694

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

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

E = H’ / Hmax E = Pielou evenness 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

Diversity and evenness Community 1 Community 2 s 2 H’ 0.056 0.694 Hmax 0.693 E 0.081 1.000

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

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

Practice problem Community 1 Community 2 s H’ Hmax E

Species diversity indices

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

Commonness, rarity and dominance MacArthur’s broken stick model random breaks in a stick  log normal distribution of pieces results in a few large pieces and many small pieces

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

Fig. 22.1, p. 435: Relative abundance of Lepidoptera captured in a light trap in England (6814 individuals representing 197 species).

Biogeography Observations of relationships between Island biogeography area and number of species distance from source Island biogeography E.O. Wilson and Robert MacArthur

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

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

Fig. 24.14, p. 502: Number of land-plant species on the Galapagos Islands in relation to the area of the island.

Fig. 24.15, p. 503: Species-area curve for amphibians and reptiles of the West Indies.

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

Fig. 24.17, p. 504: Equilibrium model for biota on a single island.

Fig. 24.18, p. 504: Equilibrium model for biota on several islands of different size and remoteness.

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

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

Island biogeography Species:area relationship

Latitudinal diversity gradients Abundance and diversity patterns latitude elevation mountainsides peninsulas

Fig. 22.5, p. 438: Number of tree species in Canada and U.S.

Fig. 22.6, p. 439: Number of species of land birds in North and Central America.

Fig. 22.7, p. 440: Number of species of calanoid copepods in top 50 m of transect from tropical Pacific to Arctic Ocean.

Fig. 22.9, p. 440: Number of species of mammals in continental North America.

Fig. 22.10, p. 440: Species richness of mammals in North and South America in relation to latitude.

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

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

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

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

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

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)

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

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

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

Fig. 22.14a, p. 447. Niche breadth versus niche overlap determined by competition within the community.

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

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.

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

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

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

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

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

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

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

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)

Fig. 22.20, p. 453. Model for intermediate disturbance hypothesis.

Fig. 22.21, p. 453. Effect of periwinkle grazing on algae diversity.

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

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

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

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

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

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

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

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

Food chain model

Figure 23.6, p. 465. Hypothetical food web model.

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

5518 food chain lengths counted Figure 23.8, p. 467. Distribution of food chain lengths in the Ythan Estuary, NE Scotland. 95 species 5518 food chain lengths counted

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

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

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

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

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 X X X Space competitor Producers Producers Producers Producers

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

Figure 23.3, p. 462. Simplified Antarctic marine food web.

Fig. 23.4, p. 464. Food web of boreal forest of northwest Canada.

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

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

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

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