Community Dynamics

Test 2

species composition

the list of species that make up the community

species richness and evenness

total number of species and how those species are distributed

relative abundance

the proportion of each species compared to the total number of individuals

trophic structure 

how species are connected through feeding relationships, i.e. food webs and energy flow 

spatial and temporal patterns

how species are arranged in space or how the community changes over time

factors that influence community structure

environmental gradients/environmental heterogeneity

resource availability

species niches, tolerance, and life history strategies

species interactions (competition, predation, mutualism, coevolution, etc.)

food webs (direct or indirect interactions)

disturbance (frequency and intensity)

Disturbance

natural and anthropogenic

how community composition can change through time

species influence

modification of habitat

interactions

How community composition can change through time

succession

complex, non-linear process shaped by species traits, resource ratios, disturbance, spatial dynamics, and chance

gradual, non-seasonal, directional, somewhat predictable pattern of species replacements over time

Driven by: effects of species activities on habitat, and changing geological, physical, or chemical conditions

Autogenic

change in community composition driven by biological factors

a successional change

Red mangroves trap sediment with their prop roots, building up soil and allowing black and white mangroves to establish farther inland 

Allogenic

changing geological, physical or chemical conditions promote species replacement over time

a successional change

sea-level rise alters salinity and changes salt marsh communities

Climax community 

The final stage of autogenic and allogenic changes, communities evolve towards equilibrium. 

remains essentially unchanged 

only minor changes in taxa occur through time 

primary succession

colonization of a site with no existing community

occurs on newly exposed substrate

Biological & physical forces shape succession (autogenic / allogenic)

secondary succession

colonization on site that was previously vegetated

occurs after disturbance

Biological & physical forces shape succession (autogenic / allogenic)

degradative 

type of succession 

colonization of dead organic matter (logs, dead leaves, dead animals)

Xerarch

type of succession

primary succession on sand with typical endpoint being forest

1. dune grass (hold sand in place with underground rhizomes, stems)

2. cottonwood seedlings establish next (decaying leaves promote soil formation)

3. pine trees replace cottonwood (further increase soil acidity)

4. Oaks replace pines

Hydrarch succession

type of succession 

aquatic succession with a typical endpoint being wetland

1. submersed and floating leaved aquatic plants 

2. rooted emergent plants (cattails) 

3. willows and shrubs

4. mesic forests (oaks, breech, maple, etc)

what maintains wetland hydrology 

precipitation and groundwater influx 

Monoclimax Hypothesis

Each region has one climatic climax community

All succession pathways in a given area ultimately lead to this stable endpoint determined by regional climate.

After disturbance (like fire), the community would recover toward that same climax state, not develop a new one.

Example: In the Midwest, Clements argued the climax would always return to an oak–hickory forest, regardless of local variation.

Polyclimax Hypothesis

Region might support many different climax states depending on: Local climate, Soil & topography, Fire or other disturbance

For example, coastal oak hammock and salt marsh communities occur in the same region

Initial Floristic Composition Hypothesis 

The course of succession depends on which species are present or arrive first.

Species replacement is not strictly orderly or sequential.

Early colonizers can suppress or exclude other species through competition or resource use.

When those initial species die or decline, new species can establish and shift community composition.

Differences in life history traits (growth rate, lifespan, shade tolerance) determine which species dominate at different times as conditions change.

Facilitation

early species modify the environment to make it suitable for later species

inhibitation

early species prevent others from establishing until they die or are removed

tolerance 

species neither help nor hinder others; those best suited to the current conditions dominate over time

early succession

light is abundant, but soil nutrients are scarce (especially nitrogen)

favored species are fast-growing, light-demanding pioneers (grasses, herbs)

Later succession

as plant biomass accumulates, shading increases and decomposition enriches the soil

Favored species are shade-tolerant, nutrient-efficient competitors (shrubs, trees)

trade off characteristics 

What succession is based on that enables plants to compete for light and nitrogen (essential resources) 

Interactions among species

What succession depends on

Paleoecology

The study of the distribution and abundance of ancient organisms and their relationship to the environment.

Fossils provide information on the composition of past communities as bone, insect exoskeletons, plant impressions, and pollen grains

species migrated north at different rates due to differences in

temperature tolerance

seed dispersal

interspecific interactions