Community Ecology

Climate and the Distribution of Ecological Communities n Communities are assemblages of large numbers of species that all interact with each other. n Areas with different climatic characteristics contain different ecological communities. n Climate types are classified using the Koeppen Classification System. Categorizes climate types based on annual temperature and precipitation, as well as variations occurring in these two variables. Examples: tropical wet forests, subtropical deserts, temperate grasslands, temperate forests, boreal forests, and tundra.

Climate and the Distribution of Ecological Communities n Productivity is positively correlated with temperature and humidity. n Communities have a characteristic pattern or type of disturbance.

How Predictable Are Community Assemblages? n Two views of community dynamics exist. Clements believed that communities are stable, integrated, orderly, and predictable entities. Gleason believed communities are neither stable nor predictable, but a matter of history and chance. Historical and experimental data support Gleason’s view.

Belém, Brazil Temperature (ºC) JJJFMMAASOND Variation Annual total: HIGH Variation: HIGH Precipitation (cm) Very low variation Average: HIGH Variation: VERY LOW Month Climate characteristics

Appearance

Temperature (ºC) JJJFMMAASOND Annual total: VERY LOW Variation: LOW Precipitation (cm) Variation Average: HIGH Variation: MODERATE Month 10 0 Yuma, Arizona (Freezing) Climate characteristics

Appearance

Temperature (ºC) JJJFMMAASOND Annual total: LOW Variation: MODERATE Precipitation (cm) Average: MODERATE Variation: MODERATE Month 10 0 Denver, Colorado (Freezing) Climate characteristics

Appearance

Temperature (ºC) JJJFMMAASOND Annual total: MODERATE Variation: LOW Precipitation (cm) Average: MODERATE Variation: MODERATE Month 10 0 Chicago, Illinois (Freezing) Climate characteristics

Appearance

Temperature (ºC) JJJFMMAASOND Annual total: LOW Variation: LOW Precipitation (cm) Average: LOW Variation: VERY HIGH Month 10 0 Dawson, Yukon, Canada Climate characteristics (Freezing) –10 –20 –30

Appearance

Temperature (ºC) JJJFMMAASOND Annual total: VERY LOW Variation: LOW Precipitation (cm) Average: VERY LOW Variation: HIGH Month 10 0 Barrow, Alaska (Freezing) –10 –20 –30 Climate characteristics

Appearance

Ponds Plankton species (numbered rather than named, for simplicity) EXPERIMENT TEST ON COMMUNITY STRUCTURE 1. Construct 12 identical ponds. Fill at the same time with sterile water so that there are no preexisting organisms. 2. After one year, examine water samples from each pond under the microscope. Count the number of plankton species in each sample. 3. Plot results. Clement hypothesis: Biological communities have a predictable composition. Gleason hypothesis: The composition of biological communities is largely a matter of chance. REJECTEDSUPPORTED Total species in each pond

Ponds Plankton species (numbered rather than named, for simplicity) Clement hypothesis: Biological communities have a predictable composition. Gleason hypothesis: The composition of biological communities is largely a matter of chance. REJECTEDSUPPORTED Total species in each pond Construct 12 identical ponds. Fill at the same time with sterile water so that there are no preexisting organisms. 2. After one year, examine water samples from each pond under the microscope. Count the number of plankton species in each sample. 3. Plot results. EXPERIMENT TEST ON COMMUNITY STRUCTURE

How Predictable Are Community Assemblages? n Disturbance and change in ecological communities. Disturbance is any event that removes some individuals or biomass from a community. The characteristic type of disturbance found in a community is known as its disturbance regime. Important management decisions hinge on understanding the disturbance regimes of any community.

Giant sequoias after a fire

Fire scars in the growth rings

Reconstructing history from fire scars Years A.D. Number of fires per century

How Predictable Are Community Assemblages? n Succession Succession is the recovery and development of communities after a disturbance occurs. Primary succession removes all organisms and soil, while secondary succession leaves soil intact. A distinct sequence of communities develops as succession proceeds. Succession is greatly impacted by the particular traits of the species involved, how species interact, the short-term weather conditions, and the overall environmental conditions. Glacier Bay, Alaska provides an excellent case study in succession.

Old field Pioneering species Early successional community Mid-successional community Late-successional community Climax community Disturbance ends, site is invaded by short-lived weedy species. Weedy species replaced by longer-lived herbaceous species and grasses. Shrubs and short-lived trees begin to invade. Short-lived tree species mature; long- lived trees begin to invade. Long-lived tree species mature.

Climax community Old field Disturbance ends, site is invaded by short-lived weedy species. Pioneering species Weedy species replaced by longer-lived herbaceous species and grasses. Early successional community Shrubs and short-lived trees begin to invade. Mid-successional community Short-lived tree species mature; long- lived trees begin to invade. Late-successional community Long-lived tree species mature.

Hypothesis 1: Only one successional pathway occurs in Glacier Bay. Glacier Bay Alaska Proposed successional pathway: Soils exposed less than 20 years: willow and Dryas Soils exposed years: sitka alder, scattered cottonwood Soils exposed 100 years: sitka alder, scattered spruce Soils exposed years: dense sitka spruce and western hemlock Glacier Bay Direction of glacial retreat 20 km N

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. Early-mid successional Late-mid successionalClimax Alder Spruce Cottonwood Hemlock PATHWAY 1 Early successional Mid-successional PATHWAY 2 Late-successional Climax No hemlock? ? Early successional Mid-successional PATHWAY 3 Late-successional Climax No hemlock? ?

Hypothesis 2: Three distinct successional pathways occur in Glacier Bay. Early-mid successional Late-mid successional Climax Alder Spruce Cottonwood Hemlock PATHWAY 1

Early successional Mid-successional PATHWAY 2 Late-successional Climax No hemlock? ? Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Early successional Mid-successional PATHWAY 3 Late-successional Climax No hemlock? ? Hypothesis 2: Three distinct successional pathways occur in Glacier Bay.

Recent data indicate that all three pathways exist. n In pathway 1, spruce arrive early and in high densities. Alder never forms continuous groves and western hemlock arrive after spruce. n In pathway 2, alder arrives early and forms continuous thickets. Spruce arrives early as well but develops low-density stands under the alder thicket. Western hemlock is absent. n In pathway 3, alder arrives early and forms continuous thickets. Cottonwood and spruce form a low-density forest int eh alder thickets. Western hemlock is absent.

Species Diversity in Ecological Communities n Quantifying diversity can be simple or complex. n Research has focused on why some communities are more diverse than others and why diversity is important.

Tropical forestBoreal forest Canopy Subcanopy Epiphytes Vines Understory trees and shrubs Canopy Understory shrubs

Species Diversity in Ecological Communities n On a global scale, a latitudinal gradient of species diversity exists for most taxa. Species diversity declines as latitude increases. Several hypotheses have been proposed to explain this gradient, but no simple answer exists.

Latitude (degrees North or South) Number of vascular plant species per 10,000 km 2 Equator 60º 30º 0º 60º 30º

Species Diversity in Ecological Communities n Communities with high diversity are more productive, more resistant, and more resilient than those with low diversity.

Shading indicates burned areas Lake Yellowstone Park boundary