Plant Ecology Midterm 2 Oregon State University

growth pattern

when each species was grown alone, populations initially increased exponentially

carrying capacity (K)

growth eventually slowed and stabilized at a maximum populations size, known as ___________, due to resource limitations

result

each species reached a unique carrying capacity determined by resource availability in its environment

population regulation

resource limitations and competition are key factors in regulating population size

ecological principle

Gause's work supports the competitive exclusion principle, suggesting that no two species can occupy the same niche indefinitely if resources are limited

competitive exclusion principle

states that two species competing for the exact same resources cannot stably coexist. If two species have identical niches -- meaning they require the same resources in the same way -- then one will be inevitably outcompete the other, driving it to local extinction. This is because the superior competitor will more effectively use the shared resources, resulting in the decline of the less competitive species

niche differentiation

this helps alleviate this competitive pressure. When species occupy slightly different niches -- perhaps by specializing in difference resources, using the same resources in different ways, or thriving under different environmental conditions -- they reduce direct competition. This allows them to coexist because they are no longer in direct conflict for the exact same resources.

resource partitioning

- is the process through which coexisting species

divide available resources to minimize competition. For example,

different plant species in a forest might grow at varying heights to

access different levels of light, or they might develop roots that

extend to different soil depths to access distinct water or nutrient

sources.

- By occupying different niches and partitioning resources, species

reduce the intensity of competition, which allows multiple species to

coexist in the same area without one outcompeting the others. This

process supports a higher level of biodiversity.

population regulation

the tendency of a populations to approach or to fluctuate, around an equilibrium level

this would be pop growth zero (equilibrium)

N- sub t+1 = N sub t + B - D + I - E

N sub t+1 / N sub t (is the annual rate of increase lambda

random variation

____________ makes it impossible for lambda to equal 1 over long periods of time

for a population to persist, however, it must average lambda = 1 over long time periods, other wise it would go extinct or take over the world

populations can be cushioned from fluctuations by birth and death through...

1) immigration from other areas

2) compensatory changes that might affect fecundity or mortality in a way that compensates for losses. Compensatory changes act in a density dependent way so that overall density is regulated within upper and lower limits.

density dependence

- a regulatory factor is one that causes the growth rate (positive or negative) to decline towards zero as the population size approaches equilibrium (lambda = 1)

- the power, or influence, of the specific factor, is therefore dependent upon population density, or therefore density dependent

- to reiterate, what we mean when we say that a death rate, or seed to seedling transition is density dependent, we mean that the percentage of plants dying from this cause increases as population density increases

- if 80% of seedlings die when there are 10 individuals m^2 and 95% die when they are 100 per m^2, this is density dependence.

why does density dependence matter?

- populations without density dependent processes are doomed to extinction

- imagine a population of 100 doubles or is halved by chance through the flip of a coin.. On average, the population should stay at 100, but occasional random fluctuations mean that occasionally, population will be zero. And, in biological population, you can't recover from zero.

- such a population would drift to extinction without some form of regulation

- in this case, this heads/tails method of population regulation is density independent

self thinning rule

W = CN ^ (-3/2)

(negative line on graph)

3/4 power law

plant species max density follows ____________ that explains relationship between mass and density in animals

similar metabolic constraints, a general law in ecology

Janzen-Connell Hypothesis

_________ proposes that density-dependent effects from specialized natural enemies (such as host-specific pathogens and herbivores) prevent any one plant species from becoming overly dominant. According to this hypothesis, seeds and seedlings that are closer to the parent tree or in high-density patches experience higher mortality due to these natural enemies

- this mechanism creates conspecific negative density dependence, meaning that individuals of the same species are more likely to suffer from pathogen and herbivore pressure as they increase in density near each other

- this effect gives rare or less dense species an advantage, as they face less pressure from specialized enemies, allowing them to survive and maintain their presence within the community

- as a result, the ___________ mechanism promotes species coexistence by reducing the competitive edge of abundant species, allowing a diverse set of plant species to occupy the same area

seed - seedling transition

- the seed to seedling transition is a critical phase in a plants life cycle, representing the period during which a seed germinates and establishes as a young seedling

- a very small fraction of seeds become seedlings

- strongest state of self thinning/density dependence in plant

- this transition is crucial because its a bottleneck stage with high mortality rates due to various biotic and abiotic stresses

- factors influencing survival during this phase include resource availability, competition, herbivory, pathogen pressure, and environmental conditions like moisture and temperature

apparent competition within species

• The Janzen-Connell hypothesis posits that species-specific natural enemies

(like pathogens or herbivores) cause increased mortality in areas where a

species is denser. For example, seedlings or saplings of a tree species that

cluster around a parent tree are more likely to be attacked by pathogens or

herbivores adapted to that species. This increased mortality close to the

parent (or in areas of high density of the same species) can be seen as a

type of apparent competition within the species.

• Here, each individual in close proximity indirectly increases the risk to

others by hosting and propagating the population of shared pathogens or

herbivores. In this case, individual plants are not competing for resources

directly (such as light or nutrients), but instead are experiencing density-

dependent pathogen or herbivore pressure due to their proximity to other

individuals of the same species. This is apparent competition because the

negative effect on nearby conspecifics is mediated through a shared

natural enemy, rather than direct competition for resources

Mechanisms of Apparent Competition

• Within-species apparent competition in this context occurs as follows:

an individual plant acts as a reservoir for pathogens or herbivores,

increasing the abundance and transmission of these enemies in its

vicinity. Nearby conspecifics (individuals of the same species) then

suffer higher rates of infection or herbivory because they are exposed

to a greater density of pathogens or herbivores.

• As more conspecific individuals are present in an area, the

concentration of host-specific pathogens or herbivores increases,

creating a positive feedback loop that intensifies enemy pressure.

This density-dependent enemy effect mirrors the Janzen-Connell

dynamic, where higher density of a species attracts more natural

enemies, resulting in higher mortality rates among conspecifics.

pathogen mediated ND

- experimental study Belize

- seedling grown at low and high densities

- fungicide treatment

- only untreated plots suffered lower survival

- fungal pathogens cause NDD

- common species experience weaker CDD than rare species

Plant soil feedback

• Plants may "condition" the soil in an immediate neighborhood such

that pathogens accumulate in soil beneath the crown of the tree

• Negative feedback means plants perform worse in conspecific than

heterospecific soil

• Does the strength of feedback vary as a function of species

abundance?

PSF vs. CNDD

- mechanisms of plant soil feedbacks

- conspecific negative density dependence (CNDD)

- interconnection of the two concepts

- implications for plant community dynamics

mechanisms of plant-soil feedbacks

Plant-soil feedbacks occur when plants alter the soil environment,

impacting the growth and survival of the same or other plant species. This includes changes in soil nutrients,

microbial communities, and the accumulation of pathogens or allelochemicals, which can either positively or

negatively influence plant growth

Conspecific Negative Density Dependence (CNDD)

refers to the phenomenon where individuals of

the same species (conspecifics) have reduced growth and survival rates when in close proximity compared to

when they are more sparsely distributed. This is often due to increased competition for resources, as well as

increased vulnerability to diseases and pests.

interconnections of the two concepts

The relationship between plant-soil feedbacks and CNDD is evident

when soil changes brought about by a plant species negatively affect the growth and survival of nearby

conspecifics. For example, the accumulation of soil pathogens specific to a species can lead to higher

mortality or reduced growth among nearby conspecifics, exemplifying CNDD driven by negative plant-soil

feedbacks.

implications for plant community dynamics

Both plant-soil feedbacks and CNDD are crucial in shaping

plant community dynamics and biodiversity. They contribute to the regulation of species abundance and

distribution, preventing any single species from dominating an ecosystem. This interplay is especially

important in maintaining species diversity in tropical ecosystems, where both processes are often intensified

due to high biodiversity and intense competition.

strengths of negative plant soil feedback

measured in the shade-house experiment are correlated with adult tree species abundance of the BCI forest

strengths of negative feedback....

measured in the field experiment are correlated with adult tree species abundance of the Gigante forest

simulations indicate that variation in feedback strength predicts tree species abundance

relationships between population size and genetic diversity

- larger populations tent to have greater genetic diversity

- small populations and genetic drift

- bottleneck and founder effect

larger populations tend to have greater genetic diversity

In larger populations, there are more

individuals contributing to the gene pool, leading to a greater variety of alleles (different forms of

a gene). This increased genetic variation enhances the ability of the population to adapt to

changing environments and resist diseases.

small populations and genetic drift

In smaller populations, genetic drift (a random change in

allele frequencies) has a more pronounced effect. This can lead to a rapid decrease in genetic

diversity, as certain alleles may become fixed (present in all individuals) or lost purely by chance,

regardless of their adaptive value.

Bottlenecks and Founder Effects

Events that drastically reduce population size, known as

bottlenecks, or events where a new population is established by a small number of individuals

(founder effect), can lead to a significant reduction in genetic diversity. This reduction can have

long-term effects on the population's viability, as it limits the potential for adaptation and

increases the risk of inbreeding depression (reduced fitness due to mating between closely

related individuals).

R genes in plant disease resistance

- function of R genes in disease recognition and response

- diversity and evolution of R genes

- applications in crop breeding and plant biotechnology

function of R genes in disease recognition and response

R genes in plants encode proteins

that are primarily involved in recognizing specific pathogen-derived molecules, often referred to

as effectors. Upon recognition, these R proteins trigger a defense response in the plant. This

response can include localized cell death (hypersensitive response) to contain the pathogen, and

the activation of broader defense mechanisms throughout the plant

diversity and evolution of R genes

R genes are highly diverse within plant species, a trait that

has evolved due to the co-evolutionary arms race between plants and their pathogens. This

diversity allows plants to recognize and respond to a wide range of pathogens. However, the rapid

evolution of pathogens can lead to the emergence of new strains that can overcome R gene-

mediated resistance, necessitating a continual adaptation of R genes in plant populations.

application in crop breeding and plant biotechnology

Understanding the mechanisms and

specificities of R genes has significant implications in agriculture. R genes are often used in crop

breeding programs to develop disease-resistant varieties. Additionally, biotechnological

approaches, such as gene editing and transgenic techniques, are employed to introduce or modify

R genes in crops, enhancing their resistance to specific diseases and reducing the need for

chemical pesticides.

pN, pS

- comparison of polymorphism and divergence

- detection of natural selection

comparison of polymorphism and divergence

The McDonald-Kreitman

test compares the ratio of non-synonymous (pN, amino acid changing) to

synonymous (pS, non-amino acid changing) mutations within a species

(polymorphism) to the ratio of these mutations between species

(divergence). The basic premise is that under neutral evolution, these

ratios should be similar within and between species

detection of natural selection

If the ratio of non-synonymous to

synonymous mutations is higher between species than within species, it

suggests positive selection; the species have evolved different functions in

the protein due to adaptive changes. Conversely, a lower between-species

ratio indicates purifying selection, where changes are deleterious and

therefore less likely to be fixed in the population.

balancing selection: negative frequency dependent selection in host pathogen systems

- principle of negative frequency dependent selection

- maintenance of genetic diversity in plant populations

- implications for disease management and crop breeding

principle of negative frequency dependent selection

In negative frequency-dependent

selection, the fitness of a phenotype (or genotype) decreases as it becomes more common in the

population. For plant disease resistance, this means that resistance alleles (R genes) are more

advantageous when they are rare. As a particular resistance trait becomes common, pathogens

are more likely to evolve mechanisms to overcome it, reducing the advantage of that resistance

trait.

maintenance of genetic diversity in plant populations

This type of selection is crucial for

maintaining a diverse array of resistance genes within a plant population. It prevents any single

resistance allele from becoming fixed (present in all individuals), ensuring that the population can

respond to a variety of pathogens. This diversity is particularly important in agricultural systems,

where monocultures can be vulnerable to widespread disease outbreaks.

implications for disease management and crop breeding

Understanding negative frequency-

dependent selection is vital for disease management strategies and crop breeding programs. It

suggests the importance of rotating crops or resistance genes to prevent pathogens from

adapting to a single, common resistance mechanism. This approach can prolong the effectiveness

of resistance genes and reduce the likelihood of severe disease outbreaks.

What role do mycorrhizae play in CDD

- enhanced nutrient acquisition

- disease and pest resistance

- altered soil microbial communities

- feedback mechanisms

CNDD in Oregon

predictions: stronger negative effects of local conspecific density on survival or growth in low-elevation forests or with increases in relative humidity

- CNDD is weaker (more positive at higher elevations)

- species suffer less from neighboring conspecifics at higher elevations

the stress gradient feedback hypothesis

relatively host specific above and below ground plant microbe interactions may explain shifts in net conspecific density-dependent feedback across abiotic stress gradients

fire severity feedback hypothesis

predicts that fire reduces the abundance of fungi as a function of fire severity. It predicts that increasing fire severity will reduce feedbacks, both positive and negative as a function of fire severity compared to old growth forests

stronger pathogen defense

ECM fungi provide robust protection for woody plants

antibiotic production

many ECM fungi release antimicrobial compounds in the mycorrhizosphere

physical barriers

fungal mantle and Hartig net physically shield roots

potential for longer root lifespan

ECM trees may have more durable fine roots, potentially increasing resistance to root pathogen (requires further study)

latitudinal gradient in species diversity

_____________ in plant species diversity is a well-documented pattern in ecology, where species diversity generally increases from the poles towards the equator. here are three key points about this phenomenon

- higher species diversity in tropical regions

- decreased diversity towards the poles

- hypotheses explaining the gradient

higher species diversity in tropical regions

Plant species diversity is significantly higher in tropical regions

near the equator compared to temperate and polar regions. This pattern is attributed to a variety of factors,

including the stable, warm climate, high levels of sunlight, and long evolutionary history of these regions,

which have allowed for extensive diversification and specialization of plant species

decreased diversity towards the poles

As one moves towards the poles, there is a noticeable decrease in

plant species diversity. This decline can be attributed to factors such as more extreme and variable climate

conditions, shorter growing seasons, and a more recent evolutionary history (due to glaciations) which has

allowed less time for species diversification.

hypotheses explaining the gradient

Various hypotheses have been proposed to explain this gradient,

including the "time for speciation" hypothesis (tropical regions have had more time for species

diversification due to less climatic disruption), the "energy availability" hypothesis (higher energy inputs in

the tropics support more species), and the "niche differentiation" hypothesis (greater environmental

complexity in the tropics allows for more specialized niches)

BCI Edge Presentation

- a single hectare of Panamanian rainforest contains more tree species than all of Canada

- > 50% of all species on 15% of all land

- 300+ tree species in one 50-ha plot

- 40+ years of continuous monitoring at BCI

- 50 focal species with full genome seq.

- negative density dependence, this prevents dominance and promotes coexistence

plant ecology and evolutionary biology

strive to integrate natural history observations, long term ecological monitoring data, field and greenhouse experiments, genetics and genomic tools, ecological modeling, and ecological and evolutionary theory.

- to understand the mechanisms that generate, maintain, and erode biodiversity in a rapidly changing world

Heliconia Floral Diagram

c = Calyx

p = Petal

a = Androecium

o = Ovary

st = Staminode

Importance of Outcrossing for Genetic Diversity

- maintenance of heterozygosity

- generates novel variation

- increases population resilience

Maintenance of Heterozygosity

outcrossing promotes the exchange of genetic material distinct individuals, increasing heterozygosity and reducing the likelihood of inbreeding depression

generates novel variation

outcrossing produces new genetic combinations that can fuel adaptation and evolution, especially in changing or heterogeneous environments

increases population resilience

improves their ability to resist diseases, pests, and environmental stressors, ultimately contributing to the long-term stability and persistence of plant species

Trait matching

- co-evolutionary adaptations: enhancing nectar extraction efficiency while promoting precise pollen placement and reduction of heterospecific pollen

- mechanical fit and functional integration: drives evolutionary feedback loops that contributes to species diversification and ecological specialization

hub species

Heliconia tortuosa is a _________ in the pollination network

seed set

actually number of seeds per plant/maximum potential number possible per plant

territorial pollinators - generalists

Rufous tailed hummingbirds, scaly-breasted hummingbirds, green brilliants

straight bills, shorter tongues, short distance flyers

traplining pollinators - specialists

violet sabrewings and green hermits

curve billed, long tongues, strong flyers

heliconia pollen loads

80% of successful pollination is by traplining green hermits and violet sabrewings

the nectar drop hypothesis

nectar promotes pollen germination and tube growth on stigmas

_____________ posits that trait matching between specialized hummingbirds and flower morphology increases plant fitness by promoting the growth of pollen tubes from pollen deposited by long distance flying birds resulting in increased fruit maturation, seed set, increased gene flow, and genetic diversity of offspring

treatments

1. hand pollination only

2. nectar extraction

3. nectar drop

bird community data

point count data of all birds across 49 forest fragments for two years

22 fragments overlap with genetic study

modeled occupancy of pollinators of H. tortuosa to determine the effect of area, connectivity, and forest age on bird community composition

forest type and generalists/specialists

traplining birds (specialists) represent lower proportions of pollinators in secondary forest habitat

secondary forests are dominated by territorial birds (generalists)

inbreeding and g/s

inbreeding declines with increased proportion of trapliners observed

dominance of territorial birds is associated with greater inbreeding in H. tortuosa

near neighbor mating

sexual system

distribution of sexual function among individuals (i.e. Dioecious, Monoecious, Hermaphroditic, etc.)

mating system

how genetic material is exchanged (i.e. patterns of outcrossing, selfing, mixed mating, etc.)

breeding system

broader term encompassing both sexual system and mating system, pollination biology, etc.

Angiosperm Sexual System Evolution

Hermaphrodism -----> Dioecy

Hermaphrodism is where pollen and seeds are both male and female (90%) (intersex)

Dioecy is where the seeds are female and the pollen is male (6%)

why have two sexes?

Pros:

promotes outcrossing

optimize resource allocation

Cons:

no reproductive assurance

genetic and ecological constraints

evolutionary dead end?

Process of Hermaphrodism to Dioecy

on the H side there is gynodioecy*, androdioecy, and monoecy.

on the D side there is Distyly and heterodichogamy

Gynodioecy

Female and hermaphrodite coexistence (different individuals)

- arises via cytoplasmic male sterility (CMS) mutations

- maintained only is females produce more/better seeds than hermaphrodites

- correlates:

-- herbaceous growth form

-- temperate distribution

* __________ to dioecy (GD) pathway involves 2 sterilization events

Monoecy

male and female flowers (on same plant)

- correlates:

-- wind pollination

-- woody growth form

-- tropical distribution

- _________ to dioecy:

-- pre-existing flower-level specialization

-- disruptive selection on flower sex ratios causes gradual increase in gender specialization

Evidence of monoecy pathway

- 21% of genera with dioecious spp. also have monoecious, compared to 0.4% dioecious/gynodioecious combination

- Dioecious species often have monoecious sister taxa

- similar correlates to dioecy (wind pollination, woody growth form)

evidence for Gynodioecy pathway

- dioecious males are "leakier" in their sex expression, could suggest gynodioecious origin

- the two sex determining regions found in some dioecious plants like kiwi, asparagus, palm, and grapevine are consistent with gynodioecy pathway (two-step mutation)

- the associations between dioecy and monoecy may be due to monoecy frequently evolving as a breakdown of dioecy

why do different sexual function give rise to different floral traits?

Bateman's principle

- male reproductive success is typically limited by access to mates

- female reproductive success is limited by resource availability

multispecies interactions

mutualisms (+)

- pollination

- seed dispersal

- protection

Antagonisms (-)

- seed predation

- herbivory

- disease

Sidalcea campestris (Meadow Checkermallow)

female and hermaphrodite morphs

- sexually dimorphic difference in...

-- floral display

-- floral reward

-- mating system

pollinator dynamics

do the sexes differ in visitation rate, community assemblages, or pollen limitation levels?

antagonistic interactions

do the sexes differ in seed predation rates?

female advantage

what effects do these difference have on female & hermaphrodite seed production?

pollinator visitation rate (brooklyn lecture)

21 hours of pollinator surveys across 28 sites:

- significant effect of plant sex on visitation rate (p < 0.001)

- hermaphrodites had a 2.24- fold higher visitation rate than females

-- average visits per raceme:

--- female: 0.68

--- hermaphrodite: 1.47

community composition (brooklyn)

- pollinator assemblage was generally similar between females and hermaphrodite morphs:

-- marginally significant community differences (p = 0.07), but plant sex explained only 1.36% of variation.

-- no taxa significantly associated with either sex

seed set and pollen limitation (brooklyn)

- seed set was higher in females, regardless of treatments (p < 0.001)

- both sexes were pollen limited (p < 0.001)

- females were not any more pollen limited (p = 0.218)

- female pollen limitation level did not change with female frequency (p = 0.81)

pre-dispersal seed predation (brooklyn)

- > 1,300 Macrorhoptus sidalceae weevils were reared from collected seeds

- significant effect of plant sex on seed predation (p = 0.002)

- hermaphrodite seed predation 1.3 times higher:

-- female 10%

-- hermaphrodite 13%

pollinators + seed predators (brooklyn)

- the joint effects of pollinators and seed predators give females a 1.76-fold advantage over hermaphrodites

brooklyn lecture recap

- both morphs are generally visited by the same pollinator assemblage, however females have a lower visitation rate

- females have higher seed set

- both morphs are similarly pollen limited, and female pollen limitation is not frequency dependent

- hermaphrodites experience higher rates of pre-dispersal seed predation

-- females achieve the necessary reproductive advantage

-- the factors that regulate population sex ratio remain unclear

mycorrhizae

fungi that have a symbiotic relationship with plant roots

- generally mutualistic, where as the fungus acts as an extension of the root system and helps the plant acquire nutrients

- the plant provides carbohydrates for the fungus

types of mycorrhizal associations

Arbuscular mycorrhizae (AM)

Ectomycorrhizae (ECM)

Ericoid mycorrhizae (ERM)

Orchid mycorrhizae (ORM)

Non-mycorrhizal (NM)

Arbuscular Mycorrhizae (AM)

structures: Arbuscules + hyphopodium

multiple origins (plants): yes

multiple origins (fungi): no

Dominant ecosystem: P limited

organic matter decomposition: none/minimal

host specificity: low

common host plants: herbaceous plants, some trees

degradation capacity: none/minimal

Ectomycorrhizae (ECM)

Structures: mantle + Hartig net

multiple origins (plants): yes

multiple origins (fungi): yes

dominant ecosystems: N limited

organic matter decomposition: yes

host specificity: high

common host plants: primarily trees, some shrubs

degradation capacity: yes

Ericoid Mycorrhizae (ERM)

structures: coils

multiple origins (plants): possibly (1 to 2)

multiple origins (fungi): yes

dominant ecosystems: acidic and low nutrients

organic matter decomposition: yes

host specificity: high

common host plants: Ericaceae + Diapensiaceae

degradation capacity: yes