1610 Prelim 2

Interactions

defined based on how 2 species influence each other’s fitness. Can be negative, positive, or no effect.

Amensalism

• (-/0)

  • A flower is stepped on by an elephant 

Commensalism

• (+/0)

  • A vulture eats the scraps of a lions kill

Competition

• (-/-)

  • A sparrow and a cardinal eat the same seeds 

Exploitation

•  (+/-)

  • Predation (four categories): a spider eats a fly 

Mutualism

•  (+/+)

  • A butterfly and a flower 

Neutralism

• (0/0)

  • 2 species that don’t influence each other 

Competition

Can be for any limited resource (mates, locations, food, etc.). Noone ever wins in competition.

Intraspecific competition

 occurs among individuals of the same species.

Interspecific competition

occurs between individuals of different species.

Exploitative competition (aka Resource or Scramble Competition):

 competition between individuals by reducing availability of shared resources 

Interference competition (also called contest competition)

direct competition between individuals for scarce resources by one impeding or denying access to the resource by another 

when pop size increases what happens to intraspecific comp?

typically becomes greater

Resource utilization curves

 the more overlap the greater the competition

Competition exclusion

 when 2 species are very similar they may not be able to coexist, because competition is so strong. One species may consume all the resources leaving little for the other. (if one species is a poor competitor it can go locally extinct, if 2 species are very similar one may randomly go locally extinct) 

How can species coexist?

Time has been insufficient to allow exclusion, there is immigration, the environment is temporally variable, the environment has spatial variation, the environment has spatial variation, there are multiple resources

To coexist with other species, the species needs to be the best at something, sometime, somewhere, somehow, etc. need their own niche. 

Niche

 the range of environmental conditions and resources within which individuals of a species survive, grow, and reproduce. Niches have many dimensions (temp, precipitation, soil grade, etc.). 

Narrow niche breadth- specialist 

Wide niche breadth- generalist 

Niche is an N-dimensional hypervolume

Concept of a niche is based on a species tolerance and use of a series of n environmental factors and resources 

Can define multiple (n) biotic and abiotic resource axes, each utilized with a certain frequency distribution 

Fundamental niche

 the full hypervolume or range of environmental factors permitting a species to survive and reproduce (the abiotic)

Realized niche

the conditions under which an organisms actually exists, after limitations by factors such as competition, disease, and predators (smaller than the fundamental niche)

ways to coexist

 resource partitioning (behavioral) or niche differentiation (evolutionary) 

niche

how are exploitative interactions categorized?

intimacy and lethality

Photoperiod

 used by many plants and animals to predict seasonal changes (to respond with things like coat color). Time immemorial (does not respond to climate changes like temperature).

animal defense against predators

color matching (crypsis), toxicity (aposematism, venoms), behavior (running away, hiding, aggregation, vigilance), physical armament (turtle shell), size refuge

Herbivory

relationship in which a heterotrophic organism eats tissues or internal fluids of plants or algae 

Importance of herbivores:

• Remarkably diverse in size, feeding style, taxonomy 

• Major conduit of energy transfer

• Populations fluctuate (increasing and declining) 

• Can impact plant individuals, populations, communities, ecosystems, and pests in agriculture 

• “Deep time” antagonism resulting in coevolution

How do plants defend against herbivores?

• toxicity/chemical defense 

• Physical armament (spines, hairs, bark)

• Phenological escape 

• Bodyguards 

• Behavior (moving away)

• Color matching (crypsis) 

Mutualism

association between two species in which each partner benefits (fitness, population growth, or persistence across the ecosystem) from the association 

“reciprocal parasitisms”

Levels of association in mutualism

 obligate (required) and facultative (beneficial)

Defensive mutualisms (mostly facultative)

species receive food or shelter from partners in return for defending partners against herbivores, predators, or parasites 

By-product

not costly, a benefit that comes from the “regular” activities of the partner (ex. Aphid excrement (ants, some pollination, mixed species flocks))

Investment

costly product or service for the partner, typically not needed for self (nectar produced by plants, nitrogen fixed by rhizobia) 

Purloin

costly product taken/stolen from the partner (plant pollen, fed to bee larvae) 

source of the benefits to the partner

byproduct, investment, purloin

Conditionality is common when

• Interaction is facultative 

• Interaction is indirect (has multiple partners)

• density/spatial distance of partner matters 

Energy Rate Maximization

• Many different optimal foraging models/measures 

• One of the most common measures of optimal foraging 

• Animals should forage such that they gain the most calories per unit time to maximize their fitness (subject to constraints) 

Giving Up Density (GUD)

measures when to stop foraging

• Used with animals that have diminishing returns while foraging 

• Quitting Harvest Rate: rate of resource gain where an animal quits foraging because benefits= costs, Quit when the costs = the benefits (predation is also a cost) 

Cost benefit analysis with predation

• Most organisms are prey for others 

• Many organisms face a fundamental tradeoff between food and safety 

• Death is bad 

• Why would an organism ever risk the huge negative effect of death for the small benefit of food? 

organisms will trade off food for safety

Energy rate maximizers would still need to consider predation by incorporating that cost (the risk of death) into decisions. Organisms will quit foraging when the benefits = the costs. 

Most organisms are involved in some exploitation interaction where they are the - 

Prey organisms constantly must balance costs and benefits in order to maximize fitness. 

Ecology of fear

predators reduce prey fitness by eating them but ALSO change prey’s physiology and behavior, which also has an impact on fitness and populations 

Optimal foraging (context matters)

 individual differences (not all individuals are the same, weigh less, more willing to risk) 

“Rules of Thumb”

• Not all optimal foraging decisions are complex 

  • If you are about to die of starvation go out to get food, no matter what 

  • Forage in the patch with the greatest return 

  • Eat prey items that are closer to you

  • Eat the prey item that has the most calories (largest)

  • Quit foraging when the costs = the benefits 

  • Forage in the location with the fewest competitors 

IT MATTERS WHAT OTHERS DO, NOT JUST THE INDIVIDUAL 

Game Theory 

Economics meets ecology 

Calculate skipping or cooperating (example)

You cooperate: 

(75% chance partner skips x 12 hours) + (25% chance partner cooperates x 4 hours) = 10 hours 

You skip: 

(75% chance partner skips x 8 hours) + (25% chance partner cooperates x 0 hours) = 6 hours 

CHEATERS WIN

Evolutionary Stable Strategy

 behavioral strategy that is adopted by a population that cannot be invaded by another strategy (all members of the pop. adopt the strategy, no other strategy will yield a greater benefit to individuals over the long term)

Skipping is the ESS. it cannot be invaded. The best solution is to skip even though is on average everybody cooperated there would be less total time on the project. 

Ecological modeling, game theory, optimal foraging theory, are all full of assumptions. 

Elinor ostrom

argued that basic game theory and the tragedy of the commons is an oversimplification for humans. Trust is important and community members are not “trapped by greed”. Chronicled numerous examples of community success in overcoming the tragedy of the commons. 

8 Principles for Managing a Commons: 

1. Define clear group boundaries 

2. Match rules governing use of common goods to local needs and conditions

3. Ensure that those affected by the rules can participate in modifying the rules

4. Make sure the rule-making rights of community members, for monitoring members behavior 

5. Develop a system carried out by community members for monitoring members behavior 

6. Use graduated sanctions for rule violators 

7. Provide accessible, low cost means for dispute resolution 

8. Build responsibility for governing the common resource in nested tiers from the lowest level up to the entire connected system 

Community

 understanding diversity and relative abundances of different species occupying the same place, and their interactions 

Super-organism view

community members tightly bound and integrated, due in part to shared evolutionary history (Fred Clements) 

Individualistic view

 species are distributed independently of others, interactions are generalized or replaceable (Henry Gleason)

Clements

 envisioned that some species are interdependent and group together with competition causing distinct, sharp boundaries (ecotones), coincident species ranges, coevolution between species is prominent 

Ecotones

 regions of rapid replacement of species along an environmental gradient. Represent zones of transition between discrete communities

Gleason

Independent no ecotones, boundaries diffuse. Independent species ranges, uncommon and diffuse coevolution between species    

Fundamental to community structures are properties such as

 species richness, relative abundance, and distribution (together = diversity) 

Assemblages

 taxonomically related group that occurs together 

Species richness

 number of species present 

Evenness

degree of similarity (equality) in relative abundance of different species 

Shannon Diversity Index

 incorporates both species richness and evenness, gives more weight to common species

H’ = value of Shannon diversity index

s= number of species in community 

pi = proportion of the ith species 

ln (pi) = the natural log of pi 

 

Spatial Scales of Diversity (Richness)

alpha, beta, gamma

Alpha diversity

 local diversity within a habitat # species within habitat

Beta diversity

among-habitat diversity 

• Measured as species turnover between habitats 

• Calculated as gamma/alpha diversity 

Gamma diversity

 number of species in all habitats within a region (ex. Regional diversity) # species across all habitats 

why is world green

top down or bottom up controls

Food webs

 complex and realistic representation of how species feed on each other. Typically represent direct effects: a direct interaction between two taxa (food webs may have HUNDREDS of direct connections) 

Trophic levels

different levels of the food web 

Connectivity food web

 draw lines for predator-prey relationships; straight-forward connections 

• Lumping often occurs by guild (organisms using the same resource), functional group, or taxonomic relationship 

• Data easy to collect, but ecologically important interactions 

• Assumes all links have equal weight

Flow food web

measures rates of energy flow through food web (ex. Mass or energy per unit area per unit time). VERY LABOR INTENSIVE to collect data. Thickness of arrows reflects frequency of interaction 

Indirect effect

: when two species interact through an intermediate species (often shown using a dashed arrow)

Trophic cascades

when indirect effects occur across 3+ trophic levels, ONLY OCCURS TOP-DOWN

Bottom-up control:

organisms on each trophic level are resource limited. If more energy or limiting resources are available and moving through a community, then each trophic level should be able to support more biomass 

Not all interactions are equal

species can interact strongly or weakly

Strong interactions = one species has great influence on the other species abundance 

Weak interactions = species have limited influence on each other

Keystone species

a special case of strongly interacting species, a species whose impact on the community is large and disproportionately great relative to its biomass

predator increase or decrease biodiversity

Increase diversity:

• If they feed on a competitively dominant species 

Decrease diversity: 

• Feed on competitive subordinates first 

• Novel predator where native species have not evolved defenses 

types of shapes of relationships between diversity and ecosystem productivity

negative, positive, no relationship, hump shape 

Intermediate-productivity hypothesis

diversity greatest at intermediate levels of ecosystem productivity 

Diversity is low with low resources due to poor conditions, limiting nutrients, harsh environment 

Diversity is low with high resources because increased competition only a few species will dominate (ex. Fertilization increases competitive shading) 

Habitat diversity supports species diversity

• More diverse habitats within a community → more niches → more species 

• Resource (or niche) partitioning = differential use among organisms of resources such as food and space 

• More resources = greater species diversity via niche partitioning

Ecosystem engineers

organisms that directly or indirectly influence resource availability for other species by creating, modifying, and maintaining habitat structure 

Stability

 ability of community to maintain a particular structure and function over time, despite disturbances 

Function

concerned with the role that different species play in ecological processes (ex. Energy or nutrient flow)

Structure

measured by species richness, evenness, composition

Aldo Leopold

all ethics so far evolved rest upon a single premise: that the individual is a member of a community of interdependent parts 

primary metrics of stability

resistance and resilience

Resistance

 the amount that a community changes in response to disturbance

Resilience

how quickly a community returns to its original state following a disturbance

Diversity-Stability Hypothesis

proposes this positive relationship between diversity and stability 

Robert May (1972)

 mathematical models with randomized food webs

• Increased food web linkages and species → more extinctions and strong competitive interactions 

• Does not support diversity-stability hypothesis 

• Repeated by other mathematical models

does diversity support stability?

Debated but:

• Strong interactions can destabilize food webs (due to predation or competition) 

• Abundance of weak interactions may stabilize the community when it is perturbed (ex. Alternative food resources, portfolio effect) 

• Need more direct studies manipulating community assemblages and measuring 1) interaction strength and 2) community and ecosystem responses 

disturbance

A relatively discrete event in time and space that changes

• The structure of species composition usually killing or damaging dominant species 

• Resource availability (light, nutrients, space)

• The physical environment 

Outside the “normal range” of a system’s perturbations

features of disturbance

occur on a continuum of frequency and intensity

sources of disturbance

abiotic or biotic, natural or anthropogenic (human origin)

succession

a gradual change in community structure (ex. Species composition) and ecosystem function (processes) over time, initiated by a disturbance

Primary succession

succession on newly exposed mineral substrate, or after disturbances that remove virtually all traces of the prior ecosystem (living species and organic matter) (ex. Lava flows, landslides, or after glacial melting) (begins with no life, no soil present, new area, lichen and moss come first, biomass is low) 

Pioneer stage (0-20yrs)

• Dominated by cyanobacteria, lichens, liverworts

• Includes few forbs, shrubs, or trees 

Dyras stage (30+ yrs) 

• Dominated by dyras drummondii, a N-fixing matforming dwarf shrub 

• Scattered seedlings, most notably willow, cottonwood, alder, and spruce 

Alder growth (50+ yrs)

• Dense alder thickets appear

• Dyras disappears

Spruce stage (100+ yrs)

• Spruce seedlings mature, overtop alder 

Old Growth (about 250-600 yrs)

• Dominated by large sitka spruce and western hemlock and/or mountain hemolock 

• Climax community

Secondary succession

succession after disturbance that kills most species but leaves behind some species, propagules (ex. seedbank), or organic matter (soil or sediment) from the prior ecosystem (ex. Post fire, hurricane, ice storm, logging, agriculture) (follows removal of existing biota, soil already present, old area, seeds and roots already present, biomass is higher) 

How to measure succession?

• Direct observation through time 

• Chronosequence: a group of related communities that differ in development due to differences in age (“space for time substitution”) 

Chronosequence

• a group of related communities that differ in development due to differences in age (“space for time substitution”) 

Mechanisms of Succession 

• Founder controlled communities

• Priority effect 

• Dominance controlled communities

• Climax community

• Succession patchworks

Pioneer community/early species

• The first community in a successional sequence of communities to be established following a disturbance (usually in primary succession, but can occur later) 

Climax community/late species

A community that occurs late in succession whose populations remains stable until disrupted by disturbance