Predation-Amensalism Summary

• Gause did early predator-prey experiments, and concluded that cycles in nature result from constant migration, because he couldn’t get coexistence in his experiments.

• Huffaker found habitat complexity allowed coexistence • • Holling studied

Functional response

– relationship between prey density and the rate at which an individual predator consumes prey and

Numerical response increase in predator numbers with increases in prey abundance

Predation-Amensalism Summary

• 3 types of functional response curves, I, II, and III • Search image- Only when the prey population increases above some threshold level does the predator form a search image and begin to recognize that prey item as a valuable food source. The predator then focuses on and exploits that food source heavily • Prudent predation occurs without altruism • Predation can cause: changes in size distribution; both decreases and increases in diversity; morphological modifications (spines, mimicry, crypsis)

Predation-Amensalism Summary

• Paine’s exp. led to keystone species concept • Optimal foraging-comcerns types of feeding behaviors that maximize food (energy( intake rate • Inducible defenses are brought on by predation threat and serve to deter predators • Indirect effects of predation: sub-lethal predation; TMII; trophic cascades • Parasites are either ecto- or endo-parasites

Predation-Amensalism Summary

• Parasites exploit host behavior to maximize transmission • Host defenses are behavioral, structural or immunological • Herbivory is the first step in the transfer of energy in food webs; provides for the cycling of nutrients; and can affect the productivity and structure of plant communities. It increases prevalence of species with: – Low nutritive value (low nitrogen) – Chemical Defenses (secondary compounds) – Structural Defenses (calcareous skeletons) – Shifts in functional groups (from fast to slow growers)

Predation-Amensalism Summary

• Secondary compounds can deter herbivores but entail tradeoffs in energy allocation • Mutualisms- usually have one species providing nutrition while the other provides protection or cleaning services. Can be obligatory or facultative • Mutualisms common in tropics and likely evolved from host-parasite relationships • Commensalism and trophic amensalism less common

Review Questions

Community Ecology

• A community is a group of interacting populations, all living in the same place at the same time – the focus is on the interactions between species or populations including competition, predation , succession, invasion, mutualism, predation, etc.

Community Structure and Change

• •

Community Structure -

a description of the community members (species list) and their relative abundances

Community Dynamics -

the changes that occur over time and space in a community. (Even though communities have an underlying structure, the structure may change over time

Emergent Properties

•

Properties not predictable from study of component populations

•

Only apparent at level of community

Why is this important?

Appropriate unit of study: - If the community is more than the sum of its parts, then we must study the entire community

(holistic approach)

- If not then entire picture can be put together from individual pieces

(reductionist approach)

The Study of Ecological Communities

•

Properties & patterns

– Diversity (Number of species) – Species’ relative abundances – Morphology – S uccession

•

Processes

– Disturbances – Trophic interactions – Competition – Mutualism – Indirect effects

Two Views on Communities

• • •

Community as a superorganism (equilibrium community, Clements) Species not replaceable Species need one another to survive

• • •

Community as a group of individual species (non-equilibrium community, Gleason) Species are replaceable Random association of species

Community Dynamics: Succession

• •

Succession - The change in numbers and kinds of organisms in an area leading to a stable ( climax ) community.

•

Pioneer community - the first community to develop in a successional sequence Sere - any successional community between pioneer and climax community

Types of Succession

1. Primary – situation where barren substrate is available for habitation (inorganic substrates= lava flows/ spreading centers 2. Secondary – occurs in areas where communities have previously existed (after fires or hurricane; much more rapid)

Succession in community traits

 increasing size and longevity of organisms  shift from predominantly "r-selected" to predominantly "K-selected" species  increasing biomass  increasing independence of physical/chemical environment

Succession in community traits (2)



decreasing rate of change



increasing species diversity and complexity of physical and trophic structures



increasing habitat modification and buffering of environmental extremes



increasing complexity of energy and nutrient flows



increasingly closed system re-cycling of organic and inorganic materials

Opposing Views of Communities

Superorganism View (Clements, 1916)

-

tightly evolved, interacting

-

functions as a single organism

-

developmental process (succession)

-

homeostasis (self maintaining – stable)

-

underlying “balance of nature” Individualistic View (Gleason, 1925)

-

randomly assembled

-

Similar resource requirements

Types of Species

-

Early successional good colonizers rapid growth short lived

(r-selected)

Early Late Successional

-

poor colonizers slow growth long lived

(k-selected)

Late

Under Equilibrium Models

• Community returns to same position after disturbance • At equilibrium, processes that structure the community produce no net change

Equilibrium Theory

Single stable state Multiple stable states

Outcomes of integrated view

•

Equilibrium assumed (not tested)

•

Explained succession

•

Super-organism concept widely accepted

•

Dominated community ecology until the 1950’s and beyond

Non-equilibrium models

•

Disturbance is the norm rather than the exception

•

Disturbed patches provide opportunities for colonization by dispersive species

•

Patchiness promotes diversity on a larger scale

Evidence for each view:

•

Superorganism:

• remove plants or autotrophs, the community will disappear • • mutualisms and symbiotic relationships are common (example: herbivore gut bacteria)

Non-equilibrium

• high-level consumers can sometimes be removed without major effects on community • disturbances often play a role in determining community structure; these are random

Alternative succession models

Connell and Slatyer (1977) – outlined 3 models: 1. Facilitation – Clementsian succession 2. Tolerance 3. Inhibition Based on effect of initial spp. on subsequent spp

.

Recruitment

Facilitation Model

E L Growth

Early Stand

E E E E Disturbance L L L L L

Late Successionals only

Recruitment Mortality

Facilitation

E L E L E L L

Mixed Stand

Recruitment

Tolerance Model

E L

Mixed Stand

L E E L L Growth Disturbance L L L L L

Late Successionals only

Recruitment Mortality

Tolerance

E L L E L E L

Recruitment

Inhibition Model

E L

Mixed Stand

L E E L L Growth Disturbance L L L L L

Late Successionals only

Recruitment Mortality

Inhibition

E L L E L E

Succession

•

Can occur without invoking the existence of a “Super-organism”

•

Sequential replacement a consequence of individual species properties

Physical disturbance

What are the components of disturbance?

• • •

The frequency of a disturbance The intensity of the disturbance The timing of the disturbance

–

Influences the availability of larvae to recolonize the disturbed area

Intermediate Disturbance Hypothesis

( Connell 1972 ) • Disturbance (e.g., tree falls, storms) creates patchiness and new space to be colonized •Patchwork is created across the landscape with - early

and

late successional species - inferior and superior competitors This theory is a

non-equilibrium

view of how natural communities are structured because landscape is a patchwork of different stages of succession .

Intermediate Disturbance Hypothesis

(2)

Disturbance is critically important in structuring communities because it can prevent competitively dominant species from excluding others.

Weak/infrequent disturbances

competitive exclusion are insufficient to prevent

Intense/frequent

disturbance disturbances exclude species sensitive to Highest diversity might therefore be expected at intermediate frequency or intensities of disturbance

Intermediate Disturbance Hypothesis (Connell)

Top-down vs. bottom-up control

Community structure could be controlled from the

bottom-up

by nutrients: predators herbivores community structure can be changed by manipulating the lower levels autotrophs nutrients numbers of autotrophs are limited by mineral nutrients

Community structure could be controlled

top-down

by predators (trophic cascade model) predicts a series of +/- effects if upper levels are manipulated predators herbivores numbers of herbivores are controlled by predators autotrophs nutrients

Reintroduction and protection of otters has reduced urchin barrens

Trophic cascades

Predation by orcas has increased urchin barrens

Species-area Relationships

 Known for a long time that there is a relationship between the size of an island and the number of species present on the island.  This relationship, which exists for all taxa studied to date, whether on land or in the sea, is known as the Species-area Relationship .

Species-area relationships

• Species-area curve - the larger the geographic area, the greater the number of species • Larger areas have more diverse habitat fig 53.25

• This can be used to predict how habitat loss may affect key species

Species Area Relationships

• As a rule of thumb for every 10x increase in habitat area you can expect a doubling in species number • this relationship is best described by the regression formula S=cA z – where: S = the number of species, c= a constant measuring the number of species/unit area, A= habitat area, and z is another constant measuring the shape of the line relating S & A

Often linearized

• ln (

S

) = ln (

c

) +

z

ln (

A

) –

z

is now the slope – ln (

c

) is now the intercept ln (

S

) ln (

A

)

Top:

Species-area curve for corals in coral reefs on Rasdu Atoll, Maldives, and on Heron Island, Great Barrier Reef. Adapted from Scheer (1978

). Bottom:

Relation of number of species and number of individuals in a sample, based on twenty samples of benthic invertebrates collected from Buzzards Bay,

Why do Species-Area Relationships Exist?

 Habitat heterogeneity - as area increases so will habitat number, and species number  Area per se - extinction rates will go down with increasing area as populations increase  Passive sampling - as area increases there is a larger “target for immigrants to “hit” –  Disturbance - smaller areas will be subject to more disturbance (DI mortality) and species number will be frequently “set back”

Importance of Islands in Ecology

 Islands can provide opportunities for natural experiments because different islands in an archipelago can have different species of potential competitors, or lack certain predators. Thus, the effects of processes such as competition and predation can be easily studied on islands.  Islands are also widespread, even on land, because any isolated patch of habitat is effectively an island (e.g., lakes, coral reefs, kelp beds) for the species living there .



Island Biogeography

Because of the generality of the species-area relationship, Preston (1962) and MacArthur & Wilson (1963, 1967) proposed that islands were supporting as many species as possible.  Since islands continuously receive immigrants, yet species number stays constant, there must be a balance between immigration and extinction.  Preston and MacArthur & Wilson proposed that the number of species on an island is in a dynamic equilibrium between immigration and extinction.

Island Biogeography (

MacArthur and Wilson, 1960’s) The number of species on an island is in a dynamic equilibrium determined by imm. and ext. rates • immigration rate decreases with Sp. N since it becomes more likely that immigrants will not be new species • extinction rate increases with Sp. N because of greater incidence of competitive exclusion fig 53.26a

• equilibrium reached when immigration and extinction rates are equal • equilibrium number is correlated with

area and distance

from mainland

Island Biogeography (2)

 This “dynamic equilibrium” between immigration and extinction was developed into a quantitative theory that was termed The Theory of Island Biogeography.  The theory of Island Biogeography has two major points: the area and distance effects.

Mainland B

Area effect

A

Area Effect

Island size influences immigration and extinction rates because…… • larger islands are more likely to be found by immigrants which increases immigration rate • organisms are less likely to go extinct on larger islands because there is more available habitat • equilibrium number is higher on larger islands because of both higher immigration and lower extinction fig 53.26b

Mainland A B

Distance effect

Distance Effect

Distance from the mainland influences immigration and extinction rates • given islands of the same size, immigration will be higher on near islands since they are more likely to be found by immigrants fig 53.26c

• extinction rates the same (same size islands) • equilibrium number is higher on near islands because of higher immigration

Island biogeography is a simple model and we must also take into account abiotic disturbance, adaptive changes, and speciation events

Latitudinal species richness gradients

• Species richness of many taxa declines from equator to poles Land birds • Why? NOT CLEAR Could be evolutionary or ecological factors, or both?

fig 53.23

Diversity along geographical gradients. Corals from the Great Barrier Reef; copepods from the Pacific; remaining data from all oceans. After Thorson (1957) and Fischer (1970).

Factors Proposed to Explain Latitudinal Diversity Gradients

•

History

(more time permits more speciation) •

Spatial Heterogeneity

(more complex habitats provide more niches and permit more species to exist) •

Competition

(competition favors reduced niche breadth, but competition can also eliminate species!) •

Predation

(predation retards competitive exclusion)

Factors Proposed to Explain Latitudinal Diversity Gradients(2)

• Climate (climatically favorable conditions allow more species to co-exist) • Climate Stability (stable climates allow specialization to occur) • Productivity (Diversity is limited by the amount of energy that can be partitioned) • Disturbance (moderate disturbance retards competitive exclusion= intermediate disturbance hypothesis)

Recent Explanations for Latitudinal Diversity Gradients

 increased area of the tropics  increased effective evolutionary time due to shorter generation times in the tropics

The world’s tropical lands cover about four times the area s the world’s second largest biome, the tundra. Tropical oceans also cover more surface than oceans in other climate zones. From Rosenzweig (1992).

Island Biogeography and Conservation

 In many areas, (1) the total area of natural habitats is shrinking, and (2) formerly contiguous habitats are being fragmented.  In island biogeographic terms, this means that island areas are shrinking and large islands are being broken into archipelagos.

Island Biogeography and Conservation (2)

 Island biogeographic theory allows predictions to be made about the effects of reducing and fragmenting habitats, and to make recommendations for conservation  Areas of application: (1) How large should preserves be? (2)How does isolation affect species number in reserves? (3) What kinds of species will survive if area is reduced?

Application of biogeographic principles to the design of nature preserves. In each pair of figures the design on the left is preferred over that on the right, even though both incorporate the same area. The concepts are: A, a continuous reserve is better than a fragmented one; B, the ratio of area to perimeter should be maximized; C, distance between refuges should be minimized; and D, dispersal corridors should be provided between fragments.

(from Ecology and Evolution of Communities, ed. M. L. Cody and J. M. Diamond, 1975 .

Ecosystems Ecology

Food Chains

• The energy flow from one trophic level to the other is the food chain • A food chain involves one type of organism at each trophic level – Producers (Autotrophs) – Primary Consumers – eat producers – Secondary Consumers – eat the primary consumers – Tertiary Consumers – eat the secondary consumers – Decomposers – bacteria and fungi that break down dead organisms and recycle materials

What is a Food Web?

• Describes which organisms in communities eat other kinds of organisms • Community food web is a description of feeding habits of a set of organisms based on taxonomy, location or other criteria • Webs were derived from natural History approaches to describing community structure

What is a Food Web (2)?

• Food webs portray flows of matter and energy within the community • Web omits some information about community properties – e.g., minor energy flows, constraints on predation, population dynamics

Food Webs: Methods

1. Identify component species 2. Sample to determine who is eating whom 3. Sampling and gut analysis to quantify frequency of encounters 4. Exclosures and removals of species to determine net effects 5. Stable isotopes 6. Mathematical models

Descriptive Food Webs

Interaction or functional food webs depict the most influential link or dynamic in the community

What is a Food Web (cont.): Complexity meets reality

•

Fallacy of linear food chains as a adequate description of natural food webs

– Food webs are reticulate – Discrete homogeneous trophic levels an abstraction or an idealism – omnivory is rampant – ontogenetic diet shifts (sometimes called life history omnivory) – environmental diet shifts – spatial & temporal heterogeneity in diet

What is a food web (cont.)?

•

Modern Approaches to Food Web Analysis

–

Connectivity relationships

–

Importance of predators and interaction strength in altering community composition and dynamics

Are trophic levels useful?

• Even if organisms are not strict herbivores, primary carnivores, etc., as long as they are

mostly

feeding at one trophic level, the concept can have value (e.g., trophic cascade concept).

Energy Flows

Respiration, maintenance Gross primary production (GPP) Net primary production (NPP)

Measuring NPP in nature

• Units: energy per unit area per year – kJ per m 2 per yr, or W per m 2 • 1 g C assimilated = 39 kJ energy – can use plant biomass or CO 2 estimate of energy uptake as an • Ignoring roots – annual aboveground net productivity (AANP), a.k.a., net aboveground primary productivity (NAPP)

Transfer of energy across trophic levels

• All energy used by higher trophic levels originates with primary producers • With each step in the food chain, 80-95% of energy is lost

Primary productivity limits secondary productivity

Consumption efficiency determines pathways of energy flow through ecosystem

Energy allocation

Ecological efficiency

– proportion of the biomass of one trophic levels transformed into biomass at the next higher trophic level

• For heterotrophs, ecological efficiencies average 5-20% • Why?

– indigestible tissues • hair, feathers, insect exoskeletons, cartilage, bone • cellulose, lignin – maintenance costs – loss of energy as heat (entropy)

•

Exploitation efficiency

– proportion of production on one trophic level consumed by the next higher level – usually less than 100% • Not all food consumed by heterotrophs is transformed into biomass

Respiration, maintenance Gross primary production (GPP)

Net primary production (NPP) Ingestion by herbivores Exploitation efficiency = Ingestion/NPP

Gross production efficiency

= (biomass production)/(ingested energy) – 1-5% for warm-blooded animals – 5-15% for insects – up to 30% for aquatic animals

Respiration, maintenance Gross primary production (GPP) Net primary production (NPP)

Ingestion by herbivores Gross production efficiency= Growth/Ingestion

Indigestible Respiration, maintenance

Growth

Assimilation

• Amount of energy actually absorbed from food is

assimilated energy

•

Assimilation efficiency

– proportion of ingested energy actually absorbed by the body – seeds – 80% – young vegetation – 60-70% – grazing/browsing – 30-40% – wood – 15% – animals – 60-90%

Respiration, maintenance Gross primary production (GPP) Net primary production (NPP)

Ingestion by herbivores Assimilation efficiency = Assimilation/Ingestion

Indigestible

Assimilation

• Growth and reproduction in heterotrophs adds biomass •

Net production efficiency

= (biomass production)/(assimilated energy) – the proportion of energy

not

used for maintenance and

not

lost as heat – birds: 1% – small mammals: 6% – cold-blooded animals: 75%

• For plants, net production efficiency = NPP/GPP – fast-growing temperate plants – 75-85% – tropical species – 40-60%

Respiration, maintenance Gross primary production (GPP) Net primary production (NPP) Ingestion by herbivores

Net Production Efficiency = Growth/Assimilation

Indigestible Respiration, maintenance

Growth Assimilation

Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Decomposition Ingestion by herbivores Indigestible Respiration, maintenance Growth Assimilation

Gross primary production (GPP) Respiration, maintenance Net primary production (NPP) Decomposition Ingestion by herbivores Ingestion by predators Indigestible Respiration, maintenance Growth Assimilation

Detritus (dead stuff)

• Assimilation efficiency of herbivores is only 30-70% – most plant tissue is not digested by animals and ends up as detritus • Two independent food chains – herbivores • most important in plankton communities – detritivores • terrestrial communities

•

Residence time

– average time that energy spends on one trophic level = (energy stored in biomass)/(net productivity) •

Biomass accumulation ratio

– residence time based on biomass rather than energy = (biomass)/(rate of biomass production)

Food Webs

• A

pyramid of biomass

is the amount of energy, fixed in biomass, at different trophic levels at a given point in time • The energy available to any trophic level is limited by the energy stored in the level below.

• Because energy is lost in the transfer between levels, there is successively less total energy at higher trophic levels.

Food Webs in the Ocean

• In the oceans the total amount of biomass in algae is usually small. A

pyramid of biomass

for the oceans can appear

inverted

• However, a

pyramid of energy

, which shows rates of production rather than biomass, must have the pyramid shape. Algae can double in days, while zooplankton might double in months, and fish might only reproduce once a year. Thus, a

pyramid of energy

takes into account turnover rate, and can never be inverted.

Decomposition and Mineralization

• Most material is derived from plants • Involves: • • Release of chemical energy

Mineralization

(= organic --> inorganic) • Note

immobilization

= reverse of mineralization •

Net mineralization rate

= mineralization immobilization

Terrestrial communities: Nutrient sources

• Weathering of rock (K, P, Ca and many others) • Fixation of CO 2 (photosynthesis) and N 2 • Dryfall (particles in the atmosphere) • Wetfall (snow & rain); contains – Oxides of S, N – Aerosols • particles high in Na, Mg, Cl, S • produced by evaporation of droplets – Dust particles from fires, volcanoes • Ca, K, S

Terrestrial communities: Nutrient losses

• Release to atmosphere – CO 2 from respiration – Volatile hydrocarbons from leaves – Aerosols – NH 3 (decomposition), N 2 • Loss in streamflow – Dissolved nutrients – Particles (denitrification)

Oceans

• • No outflow Detritus sinks --> mineralization --> nutrients end up 1. Being carried back to surface in upwelling currents, o 2. Trapped in bottom sediments (e.g., phosphorus: 1% lost to sediment with each cycling)

CARBON CYCLE

CO 2 4 PROCESSES

MOVE CARBON THROUGH ITS CYCLE:

1) Biological 2) Geochemical 3) Mixed biochemical 4) Human Activity CO 2

NITROGEN CYCLE

N 2 in Atmosphere Nitrogen-containing nutrients include: 1) Ammonia

(NH 3 )

2) Nitrate

(NO 3 )

3) Nitrite

(NO 2 ) 4) ORGANISMS NEED NITROGEN TO MAKE AMINO ACIDS FOR BUILDING PROTEINS!!!

NH 3 N0 3 & N0 2 -

The nitrogen cycle

PHOSPHORUS CYCLE

PHOSPHORUS FORMS PART OF IMPORTANT LIFE-SUSTAINING MOLECULES (ex. DNA & RNA)

The phosphorus cycle

We’re in the Driver’s Seat - Human Activities Dominate Many Biogeochemical Cycles

Disturbance simplified

•

The greater the disturbance the more habitat that will be opened up

Factors Hypothesized to Influence Biodiversity (Factor/ Rationale) Factor

1. History 2. Spatial heterogeneity 3. Competition 4. Predation 5. Climate 6. Climatic variability 7. Productivity 8. Disturbance

Rationale

More time permits more complete colonization and the evolution of new species Physically or biologically complex habitats furnish more niches a. Competition favors reduced niche breadth b. Competitive exclusion eliminates species Predation retards competitive exclusion Climatically favorable conditions permit more species Stability permits specialization Richness is limited by the partitioning of production among species Moderate disturbance retards competitive exclusion

Source: Modified after Pianka (1988) and Currie (1991).

Time 0 Disturbance opens space; slate wiped clean Time 1 Time 2 Time 3 Model Only certain species can establish themselves in open space; Opportunists, Fugitives, Weeds No special requirements for first colonizers First colonists modify environment so it becomes

less

their further recruitment but

more

suitable for other species suitable for Process continues until residents no longer facilitate recruitment of other species First colonists make environment

less

suitable for their own further recruitment, but this has

little or no

effect on other species First colonists make environment

less

suitable for

all

subsequent species Process continues until no species can invade and grow in presence of residents First colonists continue to hold space and exclude all others (First Come, First Served )

FACILITATION TOLERANCE INHIBITION

Tests of the Island Biogeographic Theory

 Lots of small scale colonization studies were consistent with the Theory  Best know test is the “million dollar experiment” of Simberloff and Wilson  Although the results of this study continue to be cited in support of the Theory, Simberloff says they only provide weak support.

Tests of Island Biogeographic Theory (2)

 This was because many of his extinctions were found to be transients that could not survive on his mangrove islands, or species that visited the islands as part of a larger range (e.g., wasps). Thus, much of the measured turnover was “pseudoturnover”.  He concluded that the Theory still needed verification, as have others since then.

Insect recolonization of four defaunated mangrove islands. The y axis indicates the predefaunation species richness of each island. Most of the islands reached an equilibrium species number after 250 days that was approximately the same as the initial richness. (From Simberloff and Wilson 1969.)

F.E. Clements (1916, 1936) idea of succession • Succession • Sere • Climax • Ecosystem = superorganism

b A b Early Colonizing

Succession

c B c Mid Mixed Time c C c Late Climax

Energy flow through ecosystems

• • •

Energy transfer between trophic levels is not 100% efficient, and energy is lost as it passes up a food chain. Herbivores eat a small proportion of total plant biomass; they also use only a small proportion of plant material consumed for their growth. The rest is lost in feces or respiration Thus, less energy is available for the next trophic levels

Trophic Basis of Production

•

Assimilation efficiency varies with resource

– 10% for vascular plant detritus – 30% for diatoms and filamentous algae – 50% for fungi – 70% for animals – 50% for microbes (bacteria and protozoans) – 27% for amorphous detritus •

Net Production Efficiency

production/assimilation ~ 40%

Marine Ecology: Food Webs

• Ecological efficiency into fish food. is the energy supply available to trophic level N + 1, divided by the energy consumed by trophic level N. You might think of it as the efficiency of copepods at converting plants • In general, only about 10% of the energy consumed by one level is available to the next.=, but this can vary substantially.

• Difficult to measure so scientists focus on measures of assimilation efficiency for selected groups of animals.

Gross primary production (GPP) Respiration, maintenance

Net primary production (NPP)

Decomposition Ingestion by herbivores

Ecological Efficiency = Biomass (higher level)/ Biomass (lower level)

Indigestible Respiration, maintenance

Growth

Assimilation