Plant Ecology Final Exam 3

Are the two perspectives of how communities are organized in nature unique to plants?

No, they apply to animal communities as well

Mutualisms

- ( + / + ) interactions: both plants benefit from each other Classification of mutualisms: 1: Based on the degree of physical interaction: - Direct = physical contact - Indirect = no contact but via another species (species A affects species C, which affects species B) 2: Based on the degree of closeness: - Symbiotic = partners live closely together - Non-symbiotic = partners don't live close 3: Based on the degree of dependency: - Obligate = each required for survival (if one goes extinct the other will) (ex: orchid which requires specific bee) - Facultative = not required

Facilitation

- Aka commensalism - ( + / 0 ) interaction: one plant is facilitating another plants species, but is not harmed by that interaction benefits - Examples: Epiphytes hang onto an oak tree that is not affected by the interaction - Another example of facilitation involves plants that help other plants get started in life (nurse plants), and the plants that benefit are called the beneficiaries - Example: Saguaro cactus (beneficiaries) use shade from Palo verde tree (nurse plant) to benefit its establishment

Generalist herbivores

- Can tolerate a wide range of host plants - Are more sensitive to secondary metabolites - Employ more general strategies to cope with secondary metabolites, especially those shared by a wide range of plant species (ex: tannins are produced by a lot of plants and are difficult to breakdown, but many herbivores just put up with it)

Phenolics

- Carbon-based water-soluble compounds that are often dissolved in the vacuole of plants (where water is) 1: Simple phenolics - Furanocourmarins - allelochemicals 2: Complex phenolics - Lignins 3: Flavonoids - Anthocyanins - Flavones - Isoflavonoids (phytoalexins) - Tannins Phenol

Criteria for naming plant communities:

- Characteristic species (ex: most abundant or prevalent species) - Habitat or physical features (ex: sand dune community) - Functional groups (ex: shrub community or grass community) - no rules, based on investigator

Plant Adaptations to Herbivores Through Developing Chemical Defenses (best defense against herbivores)

- Chemical defenses referred to as secondary compounds b/c they don't play essential role in primary metabolism (photosynthesis/respiration) - Secondary Compounds first thought of as waste products - Plants divert resources from photosynthesis/respiration to invest in chemical defense (so they may grow slower/shorter). - So most plants won't produce those chemicals unless they're attacked (inducible defenses). - Chemicals that are present in the plant all the time (constitutive defenses) - Three classes of secondary compounds -- Nitrogen-containing compounds (alkaloids) -- Terpenes -- Phenolics

Climate change and threshold

- Climate change is pushing communities and ecosystems to such an extent that they may cross thresholds and become and different ecosystem - ex: if climate change doesn't change there's a concern that the amazon rainforest will cross threshold and become a savanna

Resource Availability Hypothesis

- Coley (1985) - thinking of chemical defenses from the perspective of resource availability there for the plants - argued that what really matters is how many resources plants have and whether they are fast or slow growing plants - "Costs" of defense is a function of growth - Fast-growing plants: produce many "disposable" leaves; invest little in defense; qualitative chemicals b/c there should be less investment in chemical defenses when the leaves are short-lived - Slow-growing plants: produce few "expensive" leaves; invest heavily in defense; quantitative chemicals b/c there should be more investment in chemical defenses when the leaves are expensive and long-lived

Quantitative Descriptions of Plant Communities (Classification)

- Community data = multivariate data - Classification Analysis: mathematical technique borrowed from taxonomist that takes multivariate data and quantitatively arranges them based on how similar they are - Numerical techniques to define communities - Compliments ordination - Cluster diagram

Quantitative Descriptions of Plant Communities (Ordination)

- Community data = multivariate data - Estimate mathematically how similar different sampling places are (stand: each place you put down quadrat; representative piece of vegetation for particular community - Ordination: a mathematical technique where you can take multivariate data and depict them in two and three dimensions to compare the similarity (in terms of species composition) of one stand to another. Usually, correlate data with some aspect of the environment - Mathematical techniques for identifying and depicting communities based on similarity in species composition - Often correlate with environmental variables

Community Data

- Cover: spatial coverage of species within the plot (canopy cover; basal cover: trunk) - Frequency: number of plots in which the species occurs in - Density: number of plants in each plot - Importance Value (IV): sum of the above; reflects the abundance of that species in the community

Direct Mechanisms of Facilitation

- Direct (Active) Facilitation occurs when the nurse plant modifies the habitat to make it more suitable for the beneficiaries - Direct facilitation can modify the soil; ex: soil under a tree is often richer in nutrients or contains more moisture - Direct facilitation can also ameliorate microclimate because it produces shade so its cooler and there's a less extreme variation in temperature

Herbivory and Ecosystems

- Effect of herbivores on primary productivity - Consumption of Primary Productivity by Herbivores Varies with Vegetation Type -- 2-3% Deserts -- 4-7% Forests -- 10-60% Grasslands - Aboveground vs Below-ground herbivory: above ground herbivory by bison can be equivalent to below ground herbivory by nematodes - Effects of herbivores on nutrient cycling: grazing redistributes nutrients through excretion (urine creates nitrogen patches)

Specialist herbivores

- Evolved mechanisms to detoxify or sequester harmful secondary metabolites (e.g., alkaloids) - They often rely on a single food plant and may not be able to eat other plants that differ in their secondary metabolites - In some cases, they use plant chemicals as signals to find host plants or to defend themselves against predators - Ex: monarch butterfly feeds on milkweed and uses their toxic chemicals to make them bitter to birds, so birds won't eat them

Plant Apparency Hypothesis

- Feeny (1976) - tries to predict the type of chemical defenses plants would use from the perspective of the herbivore - argued that from the perspective of the herbivore, there are two types of plants: "Apparent" vs "Unapparent" plants - Apparent plants are very abundant, and easily found by herbivores - Unapparent plants are rare, have more local distributions, and are more difficult to find by herbivores - Apparent plants should use quantitative chemicals for defense b/c they are very difficult to detoxify - Unapparent plants should use qualitative chemicals for defense b/c they are usually hidden

Individualistic View of Communities

- Henry Gleason and Robert Whittaker challenged Clements's superorganism view - They argued that superorganism view counters evolutionary theory where individuals pursue out of self-interest and are not all cooperating and working for the good of another species - The supported individualistic view of community where communities are assemblages of species that have similar environmental requirements that doesn't mean that they are codependent upon each other and will always occur together - No "super-organism" - no evidence of ecotones; continuous variation in species distributions; unable to define communities

Effects of Herbivory on Individual Plants

- Herbivory usually decreases the growth and fitness of plants, but plants vary in responses - Grasses: -- "Decreasers" = highly palatable species -- "Increasers" = less palatable, benefit from other species being eaten (benefits from herbivory) - Compensatory photosynthesis: when one leaf is consumed by the animal, the remaining leaves show increases in photosynthesis to compensate for this loss of photosynthetic material

What is Herbivory?

- Herbivory: the consumption of plant material by animals - Animals = "predators" - Plants = "prey" - Herbivory is a type of predation ( + / - ) interaction: the animal benefits, while the plant has a negative effect

Indirect Mechanisms of Facilitation

- Indirect (Passive) Facilitation usually involves other species - Trees attract animals (birds, small mammals) and they deposit seeds in their understory more frequently than they do away from the trees (enhances and localizes seed dispersal) - Trees can also provide protection from herbivores (especially large ones) - doesn't have to be live; can be fence post, telephone pole

Types of Herbivores

- Leaf eaters: grasshoppers, cows, deer, snails (often loaded with cellulose, which many herbivores cannot digest) - Grazers: grass eaters - Browsers: tree, shrub, and forb eaters - Metabolite feeders (specific feeders): aphids (insect with hypodermic needle mouth that precisely punctures the phloem of the plant, so the sugar can flow from the plant to the aphid) - Below-ground feeders (specific feeders): nematodes - Seed predators (granivores; specific feeders): rodents, insects, and vertebrates who specifically eat seeds - Frugivores: fruit eaters

Terpenes

- Lipids (fatty; not water-soluble) from acetyl CoA via the Mevalonic Acid Pathway - Terpenes are the major components of resin in pines. - Types of Terpenes -- Monoterpenes: essential oils -- Pyrethroids: chrysanthemums produce; deters insect herbivores -- Polyterpenes: latex -- Triterpenes: steroids; affect growth and development of animals

Diversity and Resource Availability (= Productivity)

- Low resource availability = fewer niches = fewer species = low diversity - High resource availability = more species = high diversity

Moderate grazing can increase plant community diversity

- Moderate grazing of grassland increases diversity in the community, so eliminating herbivory may not be helpful if your goal is to conserve biodiversity

Alkaloids

- N-containing compounds that come from amino acids - 20-30% of plant species contain alkaloids - Some alkaloids are highly toxic - Legal and illegal drugs (caffeine, nicotine, morphine, cocaine) - Alkaloids have very specific neurological effects on animals

Community-level Effects of Herbivores

- Overgrazing reduces grass cover which increases the abundance of unpalatable species (shrubs), can change the composition of the community into a shrub land and decrease diversity

Non-symbiotic Mutualisms

- Pollination (best example: periodic benefit when insects pollinate plants and insect receives nectar) - Dispersal - Defense - Bullthorn Acacias are small shrubs that produce thorns with really swollen bases. Ants use the swollen bases as a nest site to raise their young (plant provides housing). The ants also feed on the Beltian Bodies at the tip of Acacias leaves and nectar from outside of the flower (extrafloral nectar). The ants defend the shrub from herbivores and vines

Two Types of Succession

- Primary Succession = succession on land not previously occupied by vegetation (ex: on recent volcanic deposits) - Secondary Succession = succession following disturbance of vegetated land (ex: re-growth of forest following logging, fire, flood)

Two Perspectives on Community Organization Non-equilibrium Perspective

- Recent View - Argues that communities rarely get to their carrying capacities - There's some disturbance or predation that keeps the populations from reaching their carrying capacity, so they are never at equilibrium, and therefore competitive exclusion rarely operates - Emphasis on disturbance and other factors that keep populations from reaching equilibrium - ex: fire as a disturbance increases diversity of grasses in grassland

Examples of Symbiotic Mutualisms

- Rhizobium-legumes (bacteria invades legumes, does nitrogen fixation, and the legumes provide energy and a place for the bacteria to live) - Mycorrhizae (fungi invade roots and both benefit) - Lichens = fungus + alga (formed from fungus and algae; algae are photosynthetic and fungus is not so fungus benefits by receiving energy from photosynthesis and algae are able to live in a terrestrial environment)

Coevolution of Plants and Herbivores

- Selective forces are different for plants and their herbivores - Plants: selection favors individuals that are better at deterring or defending themselves against herbivores - Animals: selection favors individuals that can detect and circumvent plant defenses and use that as a food source - Coevolution = reciprocal evolution of two parties (animal and plant) - ex: plants adapt a new chemical defense, then there's selective pressure on the herbivore to get around that. If they do then selective pressure returns to plants to come up with a new defense (occurs with herbivores and pollinators) - sets up an evolutionary "arms race"

Stages of Succession (Fred & Edith Clements)

- Sere = a recognized successional sequence - Seral Stage = a particular phase (community) along a sere - Pioneer Stage (community first in succession) = earliest seral stage - Climax (the endpoint of succession) = final seral stage - Disclimax = a disturbance-maintained successional endpoint - ex: pine forest remain pine forests b/c fire is frequent. If fire is taken away, eventually the pines are replaced by a hardwood community

Evolution of Plant Defense "Strategies"

- Specialist herbivores: evolved to feed on specific plants - Generalist herbivores: tolerate a wide range of plants

Community Properties

- Species Richness: # of species (rare to find plant community that exists with only one species) - Species Dominance/Evenness (species abundance/evenness) - Species Diversity: Species richness and evenness or sharing of abundance - Shannon Weiner Diversity Index (H'): reduces diversity to a single number so we can quantitatively compare one community to another in terms of diversity (higher the number, higher the diversity)

Contemporary View of Communities

- Stresses individualistic nature of communities - Does not discard notion of interdependencies among species or coevolution among species (b/c if they do cooccur they can potentially coevolve and affect each other) - So boundaries of a community are artificial and determined by the individual that's studying them - However, the community unit view persists for practical reasons: - Conservation Biology (to communicate aspects of habitat) - Utilitarian value in naming and classifying (organizes nature) - Phytosociology = study of naming and classifying plant communities

Why do species and communities replace one another?

- The availability of resources and abiotic environment change during succession - tree species can not live in nutrient and moisture-poor environments which are usually found in early succession stages - Resource availability and environmental condition changes causes the replacement of one species to another b/c there are some species that are adapted to early harsh conditions

Plant Adaptations to Herbivores Through Developing Structural Defenses

- Thorns, spines: good against large herbivores - Leaf hairs (trichomes): good against crawling insects (ex: stinging nettle which releases irritant) - Leaf toughness and silica (grasses accumulate silica which wears down their teeth)

Two Perspectives on Community Organization Equilibrium Perspective

- Traditional view of communities - Communities are fairly undisturbed in nature, so populations operate near carrying capacities (i.e., equilibrium conditions) - Biotic interactions (competition/predation) are intense because there is intense crowding - Argue that communities achieve high levels of diversity because species over time have evolved to use resources slightly differently (resource partitioning) and therefore minimize competition - resource partitioning is critical for high diversity

How do plants sense and respond to insect herbivores?

- Wounding - Insect saliva - Plants know they have been damaged and can increase levels of jasmonic acid (JA), Systemin, and Salicylic Acid in plants, which trigger plant defenses - Damage also causes the release of volatile organic compounds - Volatile compounds are gasses that are released from leaves and move from leaf to leaf, providing a signal to the plant that it is being attacked causing the release of more chemical defenses - Volatile compounds can also attract predators of the insects that are feeding on them

Competitive Exclusion Principle

- a principle that states that no two species can occupy the same niche in the same habitat at the same time - if there are two species in a niche, one species is going to eventually exclude the other (competitive exclusion) - based on the competitive exclusion principle you would expect communities to exhibit low diversity (b/c there are probably a limited amount of niches) - The reality is that there is a high degree of species diversity.

The outcome of succession may depend on the initial conditions (Olson 1958)

- can be multiple endpoints of succession and is highly dependent on initial starting conditions - challenges Clements's view that climax is linear and solely determined by climate

Climate determines climax

- climax represents the endpoint of succession and there is a single climax community for every type of climate - so climate determines what that one endpoint is

Disturbance: The Starting Point of Succession

- disturbance (starting point of succession): any event that disrupts the structure of the community and changes resource availability - the nature and type of disturbance is very important - there are natural and anthropogenic (human caused) disturbances - disturbances vary in their intensity, spacial extent, and frequency

Early and late successional plants are adapted to different light and soil environments. Differences in tolerances, resource requirements and life history strategies lead to replacement.

- early successional trees often have a full canopy that occurs all throughout the tree (multi-layer canopy) b/c their growing all by themselves. Physiologically they exhibit high rates of photosynthesis (sun-adapted) - late successional trees often have a full canopy that occurs at the top of the tree (mono-layer canopy) b/c that architecture is better suited for a crowded forest (shade-adapted)

Positive plant associations (plant clumping) is often indicative of facilitation

- facilitation is a type of interaction that's most important in stressful environments where abiotic stress is high (desert: water stress) - often a central tree where herbaceous plants or shrubs cluster in the understory (occur more prominently associated with the tree, rather than in the open) - Ex: live-oak shrub clusters

Facilitation often transitions into competition

- facilitation is often short-lived b/c once that beneficiary establishes in the understory of the nurse plant, eventually, the beneficiary will get big enough that it starts to have a negative effect on the nurse plant (transitions into competition) - Ex: Pencil cactus produces large stem which tends to fall over, so they use creosote bush to stand upright. As the pencil cactus gets bigger it starts to take water away from the bush and kills it, which leads to the death of the pencil cactus as well

An important conclusion of the Clemensian View of Succession

- if you disturb a habitat it will come right back to where it was (there is one stable state the community will come back to) - This is wrong, there are multiple stable states - Stable state is determined by where the threshold is (when you cross tipping point you move into a new stable state, even if you reduce grazing and frequency of fire

What are plant communities?

- in ecological terms, a community is an assemblage of different species that occur in the same place at the same time - the boundaries of a community are often unclear and usually defined by the investigator - Community Ecology = "Synecology" (used to be used when describing the study of plant communities)

Issues with Plant Apparency Hypothesis

- it's very difficult to classify chemicals as qualitative and quantitative - difficult to define the apparency of herbivores - it was discovered a lot of plants contain both qualitative and quantitative chemicals

Leaf eating is not efficient

- leaf eaters have microbes in the gut to break down cellulose, but there is still a lot that passes through (undigestible) - so leaf eaters have to constantly eat to meet their energy demands

Niche Theory and Community Organization

- niche theory: an ecological community is made up of a limited number of niches, each occupied by a single species. - generalist: a species with a broad niche that is easily adaptable to many environmental conditions - A community of generalists = low diversity - specialist: a species with a narrow niche and incredibly specific needs in order to survive - A community of specialists = high diversity "Species Packing"

Field Studies: Robert Whittaker, Great Smoky Mountains

- selected forest community that grew from the tops of hills to bogs and measured the distribution among the gradient - plotted abundance of tree species (gradient analysis) - Whittaker found no ecotones, meaning there are no sharp boundaries between one community type and another; there's continuous variation (supports individualistic view)

Are Communities Natural Units? The Community Unit Perspective

- the community unit perspective: the traditional way of looking at plant communities - traditionally, plant communities were viewed as discrete, definable entities (almost like species). - Plant communities exist of many co-existing species which all tend to occur in the same community and overtime form tight interdependent relationships and evolve together as a single unit to function like a superorganisms - "Superorganisms": Frederic Clements used to describe plant communities; each species contributes to different aspects of the function of the community - the presence of ecotones (transition zones b/w communities) is important to Clement's view

resource partitioning

- the differentiation of niches that enables similar species to coexist in a community by avoiding competition for resources - allows community to achieve high levels of diversity - minimizes competition - ex: plant species have different root depths, so they get resources in different places to avoid competition with other species (below-ground partitioning of resources) - ex: plants blooming at different types to avoid competing for pollinators (phenological/temporal partitioning of resources)

Is succession a predictable process? Succession on Lake Michigan Sand Dunes: Henry C. Cowles (1901)

- use chronosequence analysis to infer successional changes on sand dunes - most recently formed sand dunes (shifting sand) inhibit dune grasses which are able to stabilize the sand substrate and can grow nutrient cores (pioneer stage) - early-successional tree stage which inhibits fast growing trees and grasses - mid-successional pine forest - late-successional hard wood forest - climax forest: beach maple forest

The traditional view of succession: Clement's View

-succession is linear, progressive & predictable with a single endpoint - go through a number of seral stages starting with the pioneer stage, where different communities replace each other sequentially until the climax is reached - the climax is determined solely by climate (precipitation and temperature) - If climax is disturbed it will restart whole process until climax is reached (predictable)

A Clementsian view of the process of succession

1: Nudation = the disturbance process - The kind of disturbance that sets the stage for succession determines what is leftover following the disturbance (Residuals). Residuals represent the plants that can reseed and repopulate following the disturbance. 2: Migration = dispersal process - Involves dispersing into the disturbed new environment - Initial colonists in succession are species with greatest dispersal capabilities (wind dispersed seeds). Later colonists in succession are species with animal dispersed seeds 3: Ecesis = establishment phase - Recruits must grow and reproduce Initial conditions are often harsh and extreme 4: Reaction = facilitation - Organisms change the environment (add nutrients to soil, add shade, lower temperature) - These changes favor different suites of migrants and residuals 5: Competition = includes all biotic interactions - Plant density increases; increase resource competition - Increased importance of other biotic interactions (herbivory, seed predation, etc.) 6 Stabilization = climax reached and represents a new stable point that is self perpetuated

Plant Community Description: Sampling Techniques

1: Quadrats (square plot) - Best for herbaceous vegetation - Dimensions vary, but 1 m X 1 m is common - quantify abundance of species in plot (area cover) 2: Plotless techniques - Best for woodland/forest vegetation - Several methods, point-quarter is common - set up sampling transect and walk along line stopping at points and measuring distance to closest tree/shrub you find - quantify abundance of species in transects

How do plant ecologists study succession?

1: Repeated sampling of the same site over time - Permanent plots or repeat photographs - Usually a very long process 2: Analysis of chronosequences - Study communities of different ages - Infer successional changes

Succession

Change in species composition of communities over ecological time (ecological time frame: 1-500 years)

Quantitative vs. Qualitative Chemicals

Qualitative Chemicals - have low molecular weight - have specific mode of action; often toxic - b/c of their low weight and simple chemical nature they are easy to detoxify - ex: alkaloids and nicotine Quantitative Chemicals - have high molecular weight - have general effects on herbivores; digestibility, palatability, - more difficult to detoxify (ex: tannins) - their effect is a function of how much is ingested by the animal

Community Stability

Stability can mean different things: - Resistance = ability of a community to remain unchanged during "stress" (late successional forest may be less susceptible to climate change than early successional forest) - Resilience = ability of a community to return to normal following disturbance (early successional stages are more stable; b/c if you disturb weeds they'll grow right back, but it will take centuries for a climax forest to grow back)

changes in the plant community result in changes in the

animal community

Intermediate Disturbance Hypothesis

the hypothesis is that ecosystems experiencing intermediate levels of disturbance are more diverse than those with high or low disturbance levels - if disturbance is too small, this allows populations to get close to their carrying capacity, competitive exclusion occurs, and this reduces the number of species - if disturbance is too strong, this will only allow species that are adapted to survive high levels of disturbance (Ruderals), so diversity will decline - Relationships between species diversity and disturbance are complicated, no definite answer. Maybe there is no universal theory to explain this relationship

ecotones

the transition zone between one community and another