Plant Ecology Exam 2

What is a life history in plants?

The pattern of growth, reproduction, and survival (mortality) over a plant’s lifespan, which ultimately determines fitness

What are life-history trade-offs?

Situations where investing resources in one function (growth, reproduction, survival) reduces the ability to invest in others due to limited resources

What are the three main energy allocation categories in life-history strategies?

1. Growth

2. Maintenance (survival)

3. Reproduction

What is the key trade-off between current vs. future reproduction?

• Investing in reproduction now reduces future survival/growth

• Waiting to reproduce risks dying before reproducing
  → Depends on environmental predictability

What is environmental uncertainty and why does it matter?

The unpredictability of future conditions; it influences whether organisms should reproduce early (risky environments) or delay reproduction (stable environments)

Define annual, biennial, and perennial plants.

• Annual: Complete life cycle in <1 year

• Biennial: Complete life cycle in 2 years

• Perennial: Live and reproduce for many years

What is the difference between semelparity and iteroparity?

• Semelparity: Reproduce once, then die (annuals)

• Iteroparity: Reproduce multiple times (perennials)

What are survivorship curves?

Graphs showing the probability of survival at different ages in a population

Describe Type I survivorship curve.

• Low mortality early in life, high mortality late

• Example: humans, large mammals

• Rare in plants

Describe Type II survivorship curve.

• Constant mortality rate across ages

• Common in perennial plants and birds

Describe Type III survivorship curve.

• High mortality early in life, low mortality later

• Common in plants (e.g., trees, grasses)

Why are grasses often more Type III than forbs?

Grasses experience high early mortality, but survivors live longer → contributes to dominance in grassland ecosystems

Why do deserts (e.g., Death Valley) have many annual plants?

• Environments are unpredictable

• Annuals reproduce quickly before death
  → favors “live fast, die young” strategy

Why are tropical rainforests dominated by perennials?

Stable environments favor long-term survival and repeated reproduction
→ investment in growth and defense is beneficial

What is bet-hedging in plant life histories?

A strategy to reduce risk across unpredictable environments, such as:

• Seed dormancy

• Reproducing across multiple years

What is Grime’s C-S-R model?

A framework classifying plant strategies into:

• C (Competitive): High resources, low disturbance

• S (Stress-tolerant): Low resources, harsh conditions

• R (Ruderal): High disturbance, short-lived environments

What characterizes competitive (C) plants?

• Fast growth

• Strong competition for resources

• Found in stable, resource-rich environments

What characterizes stress-tolerant (S) plants?

• Slow growth

• Long lifespan

• Adapted to low-resource environments (e.g., tundra)

What characterizes ruderal (R) plants?

• Short lifespan

• Rapid reproduction

• Thrive in disturbed environments (e.g., deserts, fields)

What is the seed size vs. number trade-off?

• Large seeds: Better survival, fewer produced

• Small seeds: More numerous, lower survival
  → Trade-off between offspring quality vs. quantity

What is fitness in the life-history game/activity?

Fitness is measured as lifetime reproductive success, or the total number of offspring produced

What is the main trade-off demonstrated in the energy allocation game?

The trade-off between growth, maintenance (survival), and reproduction, where investing in one reduces resources available for the others

Why is early investment in growth beneficial in stable environments?

Investing in growth early increases future energy availability, allowing for greater reproduction later and higher overall fitness

How should life-history strategy change in unpredictable vs. stable environments?

• Unpredictable environment: prioritize early reproduction (risk of not surviving)

• Stable environment: invest in growth first, then reproduce later

What strategy led to the highest fitness in the activity and why?

A balanced strategy—early growth followed by later reproduction—because it maximizes energy over time while still producing offspring efficiently

What does the intrinsic growth rate (r) represent in population ecology?

It represents the rate at which a population increases or decreases over time, based on the difference between birth and death rates

What happens to a population when r > 0?

The population grows exponentially, meaning it increases at an accelerating rate over time

What does it mean when r = 0?

The population size stays constant because birth rates equal death rates, not because births and deaths stop

Why doesn’t exponential growth lead to the same number of individuals added each year?

Because exponential growth increases by a constant proportion, not a constant number, so the number of individuals added each year increases over time.

Under what conditions will a population grow exponentially?

When r is positive and resources are unlimited, with no environmental constraints

What is a stage-structured population model?

A model that divides a population into different life stages (e.g., juveniles, adults) and tracks transitions between them using probabilities

What does the vital rate F represent in a plant population matrix?

The number of new small juveniles produced per adult per time interval, including reproduction processes like flowering, seed production, and germination

What do transition probabilities like P21 and P33 represent?

• P21: probability that a small juvenile survives and grows into a large juvenile

• P33: probability that an adult survives and remains an adult in the next time step

What is the main goal of Adler, HilleRisLambers, and Levine (2007) in “A niche for neutrality”?

The paper aims to resolve the niche vs. neutral theory debate by reframing it using classical coexistence theory, showing that both niches and neutrality are part of a continuum rather than opposing explanations for biodiversity

How does the paper redefine the niche vs. neutral debate?

Instead of treating them as opposites, the paper argues that community diversity depends on two axes:

1. Stabilizing mechanisms (niches)

2. Fitness differences (neutrality vs. inequality)
   Neutrality is just the special case where species have equal fitness and no stabilizing mechanisms.

What are “stabilizing mechanisms” in Chesson’s framework?

Processes that make species limit themselves more than they limit others, leading to negative frequency dependence, which promotes coexistence by preventing competitive exclusion

What is negative frequency dependence and why is it important?

It occurs when a species’ per capita growth rate decreases as it becomes more common.
This is a signature of stabilizing mechanisms and helps maintain diversity by preventing any one species from dominating

How does fitness inequality affect coexistence?

Fitness inequality describes differences in species’ average per capita growth rates in the absence of stabilizing mechanisms.

• Large differences → strong stabilizing forces required for coexistence

• Small differences → weak stabilization can still allow coexistence

What is the key insight about niche vs. neutral theory in this paper?

Coexistence is not determined by niche OR neutrality alone, but by the interaction between stabilizing mechanisms (niches) and fitness differences (neutrality axis).

What is shown in the conceptual framework (Fig. 1 and 2 idea)?

• X-axis: strength of stabilization (niches)

• Y-axis: fitness inequality (neutrality differences)
  Coexistence depends on the balance between these two. Strong fitness differences require strong stabilization to maintain diversity

What empirical approach do the authors propose to study niches vs neutrality?

They propose measuring per capita population growth rates across species frequencies to:

1. Quantify stabilization (frequency dependence slopes)

2. Estimate fitness differences (growth rates without stabilization)

3. Compare their relative strength in real communities

What are the three main analyses proposed in the paper?

1. Quantify stabilization (measure frequency-dependent growth)

2. Compare stabilization vs fitness inequality (fit population models)

3. Remove stabilization experimentally or in models to test its effect on extinction and coexistence

What is the paper’s main conclusion about niche vs neutral theory?

The debate is misleading as a strict either/or question. Instead, real communities exist along a continuum where coexistence depends on both niche stabilization and fitness similarity, and both must be measured together to understand biodiversity.

What is the main research question of this study Mary N. Van Dyke1 , Jonathan M. Levine?

The study investigates how relatively small changes in rainfall (specifically a 20% reduction in precipitation) affect plant community coexistence by altering not just individual species performance, but more importantly, the competitive interactions between species in a California annual grassland. It asks whether climate-driven changes in species interactions can reshape long-term community composition even when direct effects on species growth are minimal.

What is the main hypothesis or idea being tested?

The authors hypothesize that changes in precipitation will affect plant coexistence primarily through changes in species interactions (competition strengths, niche differences, and fitness differences), rather than through direct effects on individual species’ growth or fecundity. In other words, indirect ecological interactions are expected to be more important than direct physiological responses to rainfall reduction

What system and experimental setup did the researchers use?

The study used a California annual grassland ecosystem at Sedgwick Reserve. Six annual plant species were grown in field plots under two rainfall conditions: ambient rainfall and a 20% reduced rainfall treatment. Plants were grown both alone and in pairwise competition, and their germination, survival, and fecundity were measured over a full growing season to quantify competitive interactions

What was the key experimental manipulation?

The key manipulation was a controlled rainfall reduction of approximately 20% using rainout shelters. Rain was excluded during storms and then redistributed to ensure consistent treatment differences. This created two conditions: normal rainfall and reduced rainfall, allowing comparison of how altered water availability affects plant competition and coexistence

What theoretical framework did the study use to analyze coexistence?

The study used modern coexistence theory, which separates species interactions into stabilizing niche differences and fitness differences. Stabilizing niche differences promote coexistence by reducing direct competition between species, while fitness differences reflect inherent competitive advantages of one species over another. Coexistence is predicted when niche differences are strong enough to overcome fitness differences

What were the main findings regarding individual species responses to drought?

Most species showed little or no change in fecundity when grown without competitors under reduced rainfall. Only two of the six species showed significant declines in seed production. This indicates that the direct physiological effects of reduced rainfall on individual species were relatively small

What was the main finding about species interactions under reduced rainfall?

Despite weak direct effects on individual species, reduced rainfall caused major shifts in competitive interactions. Ten out of fifteen species pairs changed their predicted coexistence outcomes (switching between coexistence and exclusion). This shows that rainfall strongly reshaped species interactions even when individual performance changed little

What did the study find about the mechanism behind changes in coexistence?

The study found that changes in competitive outcomes were driven more by changes in competition coefficients (species interactions) than by changes in demographic potential (intrinsic reproduction or fecundity). This indicates that altered rainfall mainly affected how strongly species compete with each other, rather than how well they grow individually

How did functional traits relate to changes in competition?

Species that were more functionally similar (based on traits like leaf structure, rooting depth, and water-use efficiency) experienced smaller changes in competitive outcomes. In contrast, more functionally dissimilar species showed larger shifts in fitness differences under reduced rainfall. This suggests that trait differences help predict sensitivity of species interactions to climate change

What is the main conclusion and ecological implication of the study?

The study concludes that even modest changes in rainfall can strongly reshape plant community structure by altering species interactions rather than direct species performance. This means that predictions of climate change impacts that only consider species’ individual responses may be incomplete. Understanding and predicting ecological change requires accounting for how climate alters competition and coexistence dynamics within communities

What is fitness equivalence in species coexistence?

Fitness equivalence refers to how similar species are in competitive ability; higher equivalence means neither species has a strong advantage

What is stabilization and how does it promote coexistence?

Stabilization refers to niche differences that reduce competition, allowing species to recover when rare and coexist

What does negative frequency dependence mean?

It means a species has higher growth when rare and lower growth when common, which helps maintain coexistence

Why is there a tradeoff between fitness differences and stabilization?

Larger fitness differences require stronger stabilization to prevent competitive exclusion and allow coexistence

When does competitive exclusion occur despite strong stabilization?

It occurs when fitness differences are too large for stabilization to overcome, causing one species to outcompete the other

What is the main question of the study and how was it tested?

The study asks how changes in rainfall affect plant coexistence through species interactions. It was tested using rainfall manipulation (ambient vs. reduced) and pairwise competition experiments, then applying coexistence theory to predict outcomes

How does coexistence theory explain the effects of rainfall on niche and fitness differences?

Stabilizing niche differences promote coexistence, while fitness differences promote exclusion. Rainfall changes resource availability, which can shift both. Coexistence occurs when niche differences outweigh fitness differences

What were the key results for species grown alone vs. in competition?

Most species showed little change when grown alone under reduced rainfall. However, 10 of 15 species pairs changed their competitive outcomes, showing strong effects on interactions

Why is the difference between individual performance and competition important?

It shows that indirect effects (species interactions) matter more than direct effects of climate. Even small environmental changes can strongly alter coexistence through competition

What is a limitation of the study and a follow-up idea?

The study only tested pairwise interactions in one season. A follow-up could test multi-species communities over multiple years to better reflect real ecosystems

What is the central argument of the paper “Facilitation and the niche: implications for coexistence, range shifts and ecosystem functioning”?

The central argument of the paper is that facilitation, which involves positive interactions between species, can expand a species’ realized and sometimes fundamental niche by reducing environmental stress and improving habitat conditions. The authors argue that these positive interactions play a major role in shaping species distributions, coexistence, biodiversity, ecosystem functioning, and responses to climate change, and therefore should be incorporated into major ecological theories alongside competition and predation.

How does facilitation influence a species’ niche according to the paper?

Facilitation influences a species’ niche by allowing the species to survive in environments that would otherwise be too stressful or unsuitable. Foundation species such as macroalgae, mussels, or nurse plants reduce stresses like heat, desiccation, or lack of resources, which broadens the areas where other species can persist. As a result, facilitation can expand both the realized niche and, in some cases, the fundamental niche of associated species.

What evidence does the paper provide to support the idea that facilitation broadens species’ niches?

The paper provides evidence from both marine and terrestrial ecosystems to support the idea that facilitation broadens species’ niches. On rocky intertidal shores, canopy-forming algae and mussels reduce heat and desiccation stress, allowing many organisms to survive in upper shore environments that would otherwise be too harsh. In alpine ecosystems, nurse plants improve environmental conditions and allow other plant species to grow at higher elevations. The paper also discusses coral-algae symbioses and fungal mutualists in grasses as examples of positive interactions that help species tolerate stressful climatic conditions.

How can facilitation both increase and decrease biodiversity and species coexistence?

Facilitation can increase biodiversity by reducing environmental stress and creating new habitats or microhabitats that allow more species to survive. However, facilitation can also increase niche overlap because multiple species may begin using the same favorable habitat created by the facilitator. When niche overlap becomes too great, competition between species may intensify, especially if one species is competitively superior. Therefore, facilitation can either promote coexistence or contribute to competitive exclusion depending on environmental heterogeneity and differences in competitive ability among species.

Why is this paper important for understanding climate change and ecosystem functioning?

This paper is important because it explains how positive species interactions can influence ecosystem responses to climate change and biodiversity loss. The authors argue that facilitators may help species survive warming temperatures and extreme environmental conditions by acting as refuges or stress buffers. The paper also explains that facilitation can affect ecosystem functioning by altering productivity, nutrient cycling, stability, invasion resistance, and species diversity. Overall, the paper highlights that understanding facilitation is necessary for predicting how ecosystems and species distributions will change in the future.

How does facilitation change across environmental stress gradients?

Facilitation increases as environmental stress increases because organisms benefit more from positive interactions in harsh conditions. Competition is more common in mild environments.

Why is facilitation strongest during the seedling stage?

Seedlings are more vulnerable to stress and depend on nurse plants for shade, moisture, and protection. Adults are usually more stress tolerant

What is the difference between pioneer and late successional species?

Pioneer species grow quickly after disturbance and tolerate harsh conditions. Late successional species grow slowly and dominate stable environments

How does fire affect ecological succession?

Fire removes existing vegetation and allows pioneer species to colonize first. Later, slower-growing species gradually replace them as succession continues