Essay #1 Grading

20 pts 25 pts

– A, A-: 19.5-20 24-25

– B+, B & B-: 18-19 23-24.5

– C+, C & C-: 15-17.5 18-22.5

Remaining Schedule• Essay #2 (undergraduates)

– Correspond with Jen Palacio <jenpalacio@gmail.com>– You may schedule an appointment (in person, by phone, by skype) to meet with

Jen to discuss your draft

• Term papers (graduate credit) – Your paper is assigned for grading to either Jen jenpalacio@gmail.com or

Wenfei wtong@fas.harvard.edu – You may schedule an appointment (in person, by phone, by skype) to meet with

Jen or Wenfei to discuss your draft

• Dec 10th: last lecture

• Sunday, Dec 13th: Review for final exam– Streamed live, then available on the course website– Distance students email questions to Anne ammcguir@fas.harvard.edu in

advance or during review– Review questions sent to you next week

• Dec 17th: Final exam– Similar format as midterm– Distance students: need to arrange proctor

Patterns & Determinants of Species Richness• Diversity indices• Alternative hypothesis for species richness: resources, niche breadth and niche

overlap– Productivity and resource richness– Competition and species richness (chap. 6.4)– Predation intensity– Spatial heterogeneity

• Temporal variation and species richness– Climatic variation– Disturbance– Evolutionary time

• Theory of Island Biogeography• Gradients in species richness

– Latitudinal– Altitude & depth– Community succession

• Patterns in species richness through time– Fossil record– Human disturbance

• Community assemblage is determined by:

- dispersal constraints

- environmental constraints

- internal dynamics

Description & Properties of Communities

Descriptors- list of species- graphs of species richness- indices of species diversity (incorporate relative abundance)

Diversity Indices & Rank-Abundance Diagrams

The Rothamsted pasture experiment (1856-1949)

Shannon-Weaver diversity index, H, = sum of the PilnPi

- H confounds richness & evenness

Rank-Abundance curves show increasing domination over time by the commoner species (steeper curves)

A model based on niche overlap & breadth

-higher species diversity because:a) > range of resources, R

Competition dominates:(b) more specialized (smaller average niche breadth, n)

Predation may keep species below their K’s:c) > resource overlap, obut… predation may alternatively exclude prey

d) R more fully utilized

Species richness may increase with productivity because: 1) wider range or resources; 2) higher rate of supply; &/or 3) rare resources increase to support additional specialized populations

Productivity & NA tree species richnessa) Potential Evapotranspiration (PET) b) PET & Rainfall (PET reflects energy availability: water potentially evaporating or transpiring) Points are 336 quadrats N of Mexico defined by lines of lat. & long.

NA Vertebrate Species Richness & PET (= temp): ectotherms might need more energy, endotherms use less energy/individual?- > E for growth -> larger populations -> narrower niches

Productivity & Diversity

a) + correlations for ants & rodents in SW Amer. Deserts

b) fish in NA lakes

(but recall Rothamsted! “the paradox of enrichment may lead to competitive exclusion, so expect humped curves, as per: b) phytoplankton in NA lakes&c) desert rodents in Israel

but cannot generalize across taxa and communities:

d) humped curves more common in plant comparisons, but more positive relationships with animals

Guild structure in 9 spp. Amphiprion anenome fish-using 10 spp. Heteractis (PNG)

- anenome sp., zone location & size (food size?)

Niche complementarity(of niche dimensions)

Niche differentiation among seven Bumble bee species in Colorado: resource partitioning by flower corolla length

-Resource diversity encourages species-rich consumer guilds

-niche complementarity: similar species differ in other niche dimensions (e.g., Bombus occidentalis)

Niche partitioning in plants - share similar light energy & mineral resources, so possible niche dimensions few- expect competition is overwhelmingly important process

-often involves both leaves & roots (clover & skeleton weed)

Vary in:- crown illumination index (sun --> shade)& soil type (clay-rich vs. sand-rich)

- therefore, related to vertical dimension (crown height) and spatial dimension

Niche differentiation in 11 coexisting Macaranga species: separation in space

Tree species niches distinguished by:-maximum height-% trees in high light-% trees on clay-rich soils

Pinus resinosa

Niche differentiation: Vertical separation of 26 species of ectomycorrhyzal species in pine forest litter layer

Predator-mediated coexistence of prey species:predation or grazing can interrupt competitive exclusion

0= no grazing,1=light, etc.4=very heavy grazing

Intermediate densities of periwinkles maintain high algal diversity in New England tide pools - Enteromorpha alga competitively exclude other species at low densities- At high densities, periwinkles broaden diet

- effect is due to preference for competitive dominant

Keystone Predators:Paine’s Pisaster starfish exclusion experiments

Mytilus mussels outcompeted other space-holding spp, reducing species diversity from 15 -> 8

Starfish prefer to feed on the dominant mussels & barnacles

The hemiparasite Rhinanthus minor suppresses growth of hosts & prevents competitive exclusion by dominants

Applying exploiter-mediated coexistence to manage grassland species richness

Diversity & Environmental Spatial Heterogeneity

a) plant diversity weakly increases with index of topography & soil near Hood River, Canada

b) removing needles from Doug fir branches reduced spider species diversity

c) fish diversity was higher in Wisconsin lakes with more diverse vegetation types

d) arboreal ant richness higher with Brazilian savanna tree diversity

“harsh environments” must limit the number of species that are adapted to them…but is low species richness due to low productivity?

a) Alaskan tundra plant spp & pH b) Stream inverts, Ashdown Forest, England

The Intermediate Disturbance Hypothesisa) algae on intertidal boulders; b) NZ steam insects;

c) German lake phytoplankton

Species-area relationships

Lack’s hypothesis:Larger islands have more habitats &

resources Left: plants on cays near Bahamas

Right: birds on Florida lakes

Left: bats in Mexican caves

Right: fish in Australian desert springs

…but McArthur & Wilson (1967) proposed a very different explanation…

Equilibrium Theory of Island Biogeography

• The accumulation of species on islands is stochastic (= probablilistic) and not deterministic--- it is uncertain which species colonizes an island (establishes a population) or goes extinct

• The rate of arrival of lucky breeding individuals, pairs or groups should depend on both island area (bigger target) and on distance from the colonizing source (continent or bigger island)

• As species accumulate on the island, competition for resources increases, so the average sizes of these populations will decline

• Populations fluctuate, and the annual risk of extinction depends on population size; therefore, as species accumulate, the rate of extinction will increase

sourcefar

Colonizing pair

MacArthur & Wilson’s Theory of Island Biogeography- rates of either a) immigration or b) extinction vary with no. resident species on island- curve of immigration declines with island area & distance from colonizing source- risk of extinction depends on population size, so related to habitat area-no. of species on the island fluctuates; equilibrium occurs when two rates are equal

Simberloff’s test of the equilibrium theory:isolating effect of area from habitat diversity- Arthropod species changes on mangrove islands off the coast of Florida- islands 1 & 2: reduced in size each year; #3 only after 1969; control unchanged

Integrating effects of habitat diversity and area:top: herbivorous (circles) and carnivorous (triangles) beetles of Canary Islands (note: habitats are plant species)

proportion of variance in spp richness explained by different factors for four animal taxa of the Lesser Antilles islands

Distance from a colonizing source also influences the equilibrium number of species: birds on S. Pacific Islands

Colonization takes time: increase in species on the new volcanic island of Surtsey, formed in 1963 near Iceland

Each island is plotted as ratio of its bird species to island of similar size but near to New Guinea source (the “saturated island”)(Diamond 1972)

The Tropics are rich in species in most taxa

left: marine bivalvesright: swallowtail butterflies

left: N. Am. mammalsright: N.A. trees

hypotheses:a) >predation & reduced competitionb) climatic stabilityc) productivityd) evolutionary time…. unclear

Latitudinal Gradients in Species Richness

Not all tropical taxa are more specialized

(ambrosia & bark beetles combined)

Altitudinal Gradients

a) Himalayan breeding birds

b) Plants in Mexican mountains

c) Ants in Spring Mtns., Nevada

d) Plants in Nepalese Himalayas

Species Richness Gradient with Depth in Lakes and Oceans

- large lakes- deep, dark and cold water poor in productivity & in species- marine: plants restricted to shallower photic zone, but can have high diversity on ocean floor

However, on continental shelf ocean floor (benthos), diversity peaks at about 1 km deep-> environmental stability there?

(Megabenthos SW of Ireland)

Species richness increases during early succession

a) birds after shifting cultivation (India) b) Bugs in old field succession

Taxon richness through the fossil record

top: marine invertebrates vascular land plants insectsbottom: amphibians reptiles mammals

Pleistocene extinctions % genera herbivorous mammals % large animals surviving going extinct in last 130,000 yrs & arrival of human hunters

mammals mammals

Africa

Australia

N. America

Madagascar& New Zealand