Unit III: Community Ecology & larger-scale ecology

Unit III: Community Ecology& larger-scale ecology • A. Introduction: Determinants of community structure & species diversity (biodiversity) • B. Spatial patterns in community structure: latitudinal gradients in biodiversity • C. Temporal patterns in community structure: response of ecosystems to disturbance • Introduction to concepts ecosystem response to disturbance • Examples of succession: start thinking about mechanisms • Summarize observations: hypotheses on what causes successional changes • Ecosystem attributes as they change through time

Ecological response:Two “types” of succession • Primary succession [definition]: • A substrate that does not have any living organisms or legacy of former living organisms Large disturbance [but below biosphere disturbance] • Secondary succession [definition]: • A substrate has been disturbed but contains a legacy of organisms that lived their before Smaller disturbance

Succession applies to community structure and all ecosystem processes • Succession is the response to disturbances that affect single ecosystems or communities within them. • Succession does not involve evolution. • Succession is relatively predictable. • Succession is causes by suites of ecological processes and can be predicted by ecological principles. • Succession normally occurs at a demographic time scale, but the time scale in some ecosystems overlaps with long-term climate change (no simple equilibrium) Disturbance time

Large, rare disturbances start primary succession

Mt. St. Helens Before After

Raw devastation right after the eruption Nothing alive left here! All parent material (pre-soil)

Mt. St. Helens: a model system for studying primary & secondary succession 5 years later “At first glance, this ash covered clear cut…appeared completely lifeless following the eruption.” Fireweed one of the first colonizing species at Mt. St. Helens (any significance to this name??) All living things dispersed from somewhere else: Life history strategies?

Primary/secondary? succession on Mt. St. Helens “Blow down” forest 1983 (3 years after eruption).

Primary/secondary? succession on Mt. St. Helens “Blow down” forest 1989 - 6 years later

Primary/secondary? succession on Mt. St. Helens “Blow down” forest 1994 - 11 years later

First species to appear on Mt. St. Helens Alders Lupine Nitrogen fixing plants are among the first to grow on volcanic substrates-(think about mechanisms)

Glaciers also start primary succession As this glacier in Alaska retreats, it leaves a bare surface for primary succession

Succession starts when the glacier retreats Glacial morraines and till Nothing left alive; no legacy of “life” after a glacier has passed over. “Till” is composed of large gravel rocks = “parent material, not “soil”.

Global warming has caused many glaciers to retreat quickly in the past 100 years. Same outcrop Succession has proceeded quickly in 60 years here.

Toboggan Glacier 2000 1909 Rapid succession

If we could get in a time machine, we’d see all these changes over about 500 years. Instead of waiting around for 500 years, an ecologist works with a geologist to age different surfaces (in time since glacial retreat). Parent material ages “weathers” Glacier melting Pioneer species establish Different species replace each other over time When does ecosystem change stop?

Hypotheses about succession • Basic ecological questions: • When can a population “invade” and establish when rare? • Mathematically, can dNi/dt > 0, when Ni 0? • How long can a population persist? What are the mechanisms of persistence? • Mathematically, can Ni* > 0, Ni*  Ki for large T? • Can a population resist invasion by other species?Mathematically, can Nj < 0 or Nj*  0 when Ni  Ki? • Where Niand Nj are population densities of the ith and jth species • When we know that the environment (abiotic and biotic) are changing with time, T. • We want to know what the steady state is--not only attributes of community structure and ecosystem processes, but also names of the species.

“pioneer” “climax” mid successional early successional late successional disturbance A SERE = a sequence of plant communities in succession at one location seral stage Abundance of each species No change after this point Time Community composition as a balance between the ability of new species to invade and established species to resist invasion. Think about attributes of species that will be successful as time passes.

Mechanisms of succession • Allogenic: Change comes from progression of physical (abiotic) processes: • Weathering of “parent material” (lava, ash, glacial till, talus, etc.) by wind and water • Autogenic: Change comes from biotic processes • Soil is created by first living organisms interacting with weathered parent material. • includes life history traits or interspecific interactions as populations establish through time. • Primary succession starts with allogenic, proceeds to autogenic. • Secondary succession starts with autogenic.

Autogenic processes • Facilitation: some species arrive earlier and make it easier for species to invade later. • Inhibition: some species arrive earlier and make it harder for species to invade later. • Tolerance: no interactions, some species tolerant early conditions, some species tolerate late conditions--may depend on life histories, but NOT on species interactions.

Summary of autogenic models of succession Figure 20.20 Primary succession Secondary succession Initial floristic composition Floristic relay Floristic relay Facilitation Tolerance Inhibition

Clement’s “floristic relay” Clements 1916 Inhibition predominates Facilitation predominates

Egler’s Initial Floristic Composition Egler 1954 Tolerance model predominates All species are always present--they live out their lives independently

Glacier Bay, Alaska Detailed example in Chapter 20

Primary succession:Using physically separate locations with different histories to infer patterns and mechanisms of succession Alders Dryas Primary succession post-glacial Retreat in Glacier Bay, AK (Chapin et al. 1994) Spruce “pioneer” forbs

Mechanisms of succession - Glacier Bay Effect on spruce seedlings through time 

Ecosystem attributes change with time Species richness and diversity increase Figure 20.3

Primary succession: Early stages allogenic and then autogenic-facilitation C horizon = parent material Soil depth Soil development begins with “weathering” of parent material; Proceeds with development of organic components = true soil

Soil properties through time N & water holding capacity increases…. Figure 20.11 Moisture Organic content Nitrogen Phosphorus & pH decrease N limiting in early stages, P in later Phosphorus

Ecosystem attributes through time: Nitrogen Nitrogen fixers

Ecosystem attributes through time: Nitrogen v Phosphorus Nitrogen fixing plant species common in early succession Organic accumulation results in increasingly more nitrogen available to plants Facilitation Phosphorus becomes depleted with time bc of competition Inhibition

Hurricanes can start primary succession or secondary succession Hurricane Katrina

Andrew’s damage in the Everglades

Old field succession Crop monoculture -- poor at resisting invasion and succession. Takes energy and chemical input to stop succession High disturbance system Low diversity, but high productivity, which is what humans want!

Climax community Old growth forest Very resistant to invasion; is self replacing Also highly vulnerable to disturbance Biodiversity might be low but includes species found nowhere else: Spotted Owl

Species richness increases at first and then levels off Figure 20.5: Number of bird species Figure 20.4: Number of woody plant species

Fire disturbance maintains grasslands in the central US & Canada and central Asia Fire is an essential component to maintain the natural structure of tall grass prairie communities. Fire eliminates trees and cycles nutrients into the prairie “sod” the mixture of grasses and wildflowers. Humans like to use grassland biomes for their agriculture. Humans replace fire as a disturbance with agricultural activities like plowing and harvesting.

Tall grass prairie Big Bluestem Tallgrass prairie has high biodiversity of perennial grasses and herbaceous plants (wild flowers).

Fire promotes biodiversity Fire disturbs the winners of competition Fire makes new space and releases C,N and other nutrients used by the competitors

Bison & fire interact Fire and grazing are NOT interchangeable: Effects of fire are patchy because of grazing by bison Urine (N) deposition

Intermediate Disturbance HypothesisOften applies well to succession Humans can cause both high and intermediate disturbance disturbance-caused extinctions competitive exclusion disturbance-mediated coexistence Early Succession Late Succession Mid-Succession Disturbance frequency

Algae species in a stream fit the IDH Algal species diversity H’ Figure 20.8

Shorelines are constantly starting primary succession Energy from waves is constantly transporting and depositing parent materials (sand) New surfaces Succession begins

Plate 5.8 Dune succession in the Algarve region of Portugal First plants establish First plants stabilize shifting sand, add organic matter. Next set of species can invade.

Shoreline dune succession: Clements’ Floristic Relay fits most dune seres well Endpoint depends on regional climate r-selected life histories K-selected life histories Question: do early species affect the ability of later species to establish, survive and reproduce? Disturbance! Good competitors, resist invasion: long lifespan, low reproduction, low range of dispersal Short lifespan, small size, high reproduction & high dispersal ability. Low biodiversity, high productivity. Higher biodiversity, high standing crop

World famous ecologist studying plant succession at Mono Lake Exposure of new shoreline 500 yr former shoreline Pioneer plants 10 yr. shoreline Studying seedling establishment on 50 year shoreline Prof Toft with students

Primary succession is commonly dominanted by facilitation in early stages. Pioneer “greasewood” facilitates establishment of other species by building dunes. 50 year shoreline still dominated by pioneers After weathering of dunes, rabbitbrush can establish Rabbitbrush can only establish after pH of sand goes “down” to 8 or 9.

Ecosystem development: shoreline dunes& old field succession Species “turnover” high in early stages “Productivity” high r-selected life histories early Biomass = “standing crop” accumulates through time Diversity often highest at intermediate time K-selected life histories later Eventually competition for resources occurs at “climax” steady state

Productivity vs. efficiency Productivity increases at first, may level off or decline Biomass continues to increase Low “turnover” of biomass in later stages = efficiency

Summary of disturbance & succession • Biodiversity increases with time, overall, in succession. • Moderate disturbance can provide the greatest biodiversity because it prevents competitive exclusion. • Succession’s endpoint depends on the climate ultimately (Biodiversity varies with the climate). • The endpoint of succession is ecological stability because the community structure persists through time. [biodiversity and ecological stability are related]

Unit III: Community Ecology & larger-scale ecology