Community Ecology

Question 1

What’s an ecological community?

Answer 1

• A group of populations that occur together in space and/or time and generally share the same resources
• Tend to refer to within same trophic level
• Implicit in this definition is the idea of competition…

Question 2

What’s a resource

Answer 2

Resource = something that individuals take up and use, and can deplete (ie. NOT temperature!)
• Species compete for 1 or more limiting resources (eg. space in a rock pool)

Question 3

give an example of ‘diversity gone mad’

Answer 3

Brazil - 400 species in a single hectare of tropical rainforest in Brazil

Tropical reef systems also have huge species diversity

• Estimated to be ~830,000 species of animals and algae on coral reefs
• Nearly a 1/3 of all named marine species occur on reefs

By contrast, only 700 tree species in total in USA & Canada

Question 4

Explain Gause’s principle of competitive exclusion

Answer 4

Gause said that ‘complete competitors cannot coexist’. Paraphrasing, this means that for two species to coexist they have to be different in ecologically relevant ways,

e.g. they might use different resources or they might use the same resources but have spatial or temporal separation in resource use.

Gause’s theory does not include any measure of just how different species need to be in order to coexist. Formal theory, including Chesson’s coexistence framework tries to address this.

Question 5

What does Tilman’s R* look at

Answer 5

When the two isocline lines DON’T cross and 2 species can’t coexist

Tilman said, if two species compete for the same resource the species which deplete the resource to the lowest level will dominate

Question 6

what mathematical model can you use to look at coexistence/ competitive exclusion?

Answer 6

Lotka-Volterra

• the logistic model is a single species population model that includes density dependence (ie. intra)
• you can add competition coefficients into the LM so that it takes interspecific competition from competitors into account
• comp coefficients tell us what would happen to pop growth rate of one species if you add in one individual of the other species
• allows it to be asymmetric
• combining the isocline lines of dN/dt=0, you can plot species 1 vs species 2 and determine if intra>inter
• for species to coexist in a stable equilibrium (ie. regardless of starting conditions) the k1/alpha and k2/beta must be greater than the k1 and k2

Question 7

Give the barnacle example of coexistence

Answer 7

Connell’s barnacles (1961) – zonation on a rocky shore. Competition for space, spatial niche partitioning.

o Able to coexist because of how they interact with the environment
o Due to changes in tide, the surfaces are each covered in water some of the time.
o Connell put down new tiles, found larvae don’t settle in the same range as the adults. Larvae of both species settle everywhere.
o On the upper shore the animals are exposed to desiccation and have much less time to feed. On the lower shore the animals are covered with water for longer and hence more exposed to predation by the dog whelk.

Chthamalus
 Adults tend to be found in upper shore
 Because - it is more tolerant of desiccation than Balanus as it invests in a more robust shell, so is able to persist at the top of the rocky shore despite being a worse competitor

Balanus
 Adults found in lower shore
 Because - any Balanus that settled at top died from desiccation
 However - Balanus is a better competitor and can withstand predation by the dog whelk better, so Chthamalus is competitively excluded from the lower shore by Balanus

Question 8

Give the Gause’s Paramecium example of coexistence

Answer 8

o Both Paramecium aurelia and Paramecium bursaria eat bacteria
o Gause put the two species together in a beaker to see if one would outcompete.

Paramecium aurelia
 Regular paramecium

Paramecium bursaria
 Has chlorella algal cells inside - symbiosis gives protection from predators to bursaria, despite their growth being restricted by it

Aerobic conditions, well-stirred mixture
o When grown together P. aurelia outcompetes P. bursaria.

Anaerobic conditions, not stirred creates anoxic layer
o Both species persist, although P. bursaria is mostly confined to the bottom of the beaker
o This is because anoxia sets in at the bottom of the beaker, and P. bursaria, with its photosynthetic algal symbionts, generates oxygen internally.

Question 9

What was Tilman trying to test with his diatoms?

Answer 9

R* is a predictive theory for competition for a single limiting resource

Tilman was trying to test if we can predict winners and losers - ie. which species will outcompete the other - using R*

Question 10

Explain and illustrate how R* theory can be used to predict the outcome of competition for a single limiting resource.

Answer 10

o Conditions for R* theory to work: species must compete for a single limiting resource that is labile (moves easily over long distances) and in a system that is well-mixed. This is important because species must be able to deplete the resource globally.,
o If these conditions are met, then we predict competitive exclusion, not coexistence. This is because whichever species is the best competitor for the resource will win.
o R* theory, formulated by David Tilman, allows us to predict who is the best competitor. It is the species that, when grown in monoculture, can drive the availability of the resource down to the lowest level. This is because this level of the resource will not support positive population growth of the competing species.
o Diagram of two species of diatom grown alone, showing that, as population size increases, then the level of the resource declines, but to different levels in the two species
o When the population reaches its equilibrium, the R* value is reached for the resource. Then show how you can predict which one will win (lower R) when you put the two together
o Experiments with R have not been done often – diatoms and prairie grasses – are the two best examples. The diatoms were competing for silica and the grasses for nitrate: two systems in which the assumptions seem to be met
o Good students might comment that R* is really one of the few mechanistic predictive models that we have in community ecology, hence it is very valuable. However R* theory is rather simplistic. That it is rare for a system to only have a single limiting resource.

Question 11

What causes zonation on a rocky shore? (legit simple answer)

How can this be measured?

Answer 11

Increasing emersion with increasing height on the shore. Driven by a combination of tides and exposure

The amount that the shore is exposed to the waves determines the type of assemblages seen

Exposure can be measured using a Ballantine scale

Question 12

Orielton typical zonation!

Answer 12

Low shore

• Kelp
• Sea lettuce (green algae)
• Porphyra (red algae)
• Subtidal invertebrates such as lobsters, edible crabs

Middle shore

• Barnacles
• Starfish
• Mussels
• Fucoid algae (esp. bladder wrack)
• Dog whelk
• (+ limpets & periwinkles)

Upper shore

• Limpets
• periwinkles gastropods
• lichen

Question 13

Describe the main species that make up a typical temperate rocky shore food web

Answer 13

Common animals :-
Grazers - top shells, limpets, periwinkles
Sit and wait predators - beadle anemones, snake locks anemones
Scavengers - shore crabs, prawns
Predators - Whelks, swimming crabs
Filter feeders - Barnacles, seasquirts, sponges

Common algae :-
Brown algae e.g. Fucus spp, Green algae e.g. sea lettuce
Red algae eg. Porphyra, mainly below low tide

Also lichen

Question 14

Food web at bottom of rocky shore

Answer 14

Primary producers (green and brown algae), grazed when just settled by grazing molluscs, keeping seaweed densities low.

Grazers such as winkles and limpets fed on by crabs and especially dog whelks.

Question 15

what are the problems with living inter-tidally?

Answer 15

• Desiccation
• water dilution of concentration
• high temperatures
• inability to feed or move (e.g. limpets)

Extreme place with lots of zonation according to heights above low tide.

Question 16

trophic cascades in rocky shores example

Answer 16

Predatory starfish eat mussels, allows barnacles to settle where dead mussels were

Top down grazing (herbivory) and predation maintain high community species richness.

Question 17

What is zonation?

Answer 17

o Zonation is concept that describes the change in assemblage composition that occurs as horizontal bands as you move vertically up the rock

It is nearly universal feature of the intertidal zone globally

Question 18

What is the intertidal/ littoral zone?

Answer 18

the section of the shoreline that is between the high and low tide marks

Question 19

What determines which zone a species occupies on a rocky shore

Upper and lower distributional limits?

Answer 19

Difference species have different tolerances to different environmental conditions and therefore occupy different ecological niches distributed across the intertidal zone

Upper distributional limits are generally caused by physical factors (i.e. salinity, temperature change, and desiccation)

Lower limits are determined by biological interactions such as predation and competition.

Question 20

What are diatoms?

Answer 20

o Photosynthetic, planktonic organisms
o Diatom cells are contained within a unique silica cell wall (ie. they need lots of silica to survive)
o Can be grown in culture such that silica is the limiting factor, limits pop growth

Question 21

What happens when you add diatoms of different species to a culture that has plenty of resources (including silica)

How did this experiment lead to Tilman’s R* theory?

Answer 21

 When you initially add diatoms, conc of silica is high and population of diatoms increases
 As diatom growth depletes silica, population growth rate slows and population stabilises
 Species do not depleting silica to the same level – one species left lots of silica and was not using all of it
 Tilman predicted that when you put two species together, the species with the lowest R* (ie. that can drive the silica down to an intolerable level for the other species) will outcompete the other species and win

Question 22

Give the Tilman’s diatoms example of coexistence

Answer 22

• grew diatoms of diff species in monoculture
• they each reached a stable population size
• however, one needed a higher silica conc in order to be stable and survive
• therefore when you put the species together, the one with the lower R* (ie. lower resource requirements) was able to outcompete the other

Question 23

assumptions of Tilman’s R* theory

Answer 23

Assumption 1: Species are competing for a single limiting resource
Assumption 2: the resource is labile and the system is well-mixed.

Question 24

what does R* actually mean

Answer 24

The level to which the concentration of a limiting resource is reduced by an equilibrial monoculture of a species is called R*.

Alternatively, R* is the minimum resource concentration a species requires for positive population growth.

Question 25

why is R* still useful

Answer 25

it’s the only predictive theory of competition that is shown to work!!

Question 26

very very short - explain another example of R* that’s not tilman’s diatoms

Answer 26

Grasses in Minnesota

• N is limiting
• R* (nitrate in soil req. for stable growth) measured from monocultures grown for several years
• hierarchy based on R* accurately predicts competition, whereas biomass hierarchy does NOT.

Question 27

2 examples of competitive exclusion and 2 examples of coexistence

Answer 27

Competitive exclusion

• when grown together in aerobic, well-stirred conditions, P. aurelia outcompetes P. bursaria
• tilman’s diatoms - when you drive silica conc down only better competitor exists

Coexistence

• Chthamalus on US and Balanus on LS
• when grown together in anaerobic, not stirred conditions, both species persist, although Paramecium bursaria is confined to the bottom of the beaker (under anoxic layer!)

Question 28

conditions of the LV competition models that give stable coexistence

Answer 28

if the alphas are both 1
𝐾1/𝛼12 > 𝐾2 AND 𝐾2/𝛼21 > 𝐾1

or if you wanna put it how lindsay did,
𝐾1 > 𝐾2𝛼12 AND 𝐾2 > 𝐾1𝛼21

If K1=K2, coexistence occurs when the interspecific competition coefficients (alphas) are <1

intra > interspecific competition

Question 29

conditions of the LV competition models that give competitive exclusion

Answer 29

if one of the alphas is&raquo_space;1

Question 30

NICHE THEORY

What is the importance of ecological niches to whether intra>inter? and coexistence can occur?

Answer 30

if each species has it’s own ecological niche, intra likely to be > inter and species are able to persist

eg. if species use different food or prefer different habitats, this concentrates competition onto members of their own species

this means niches are one of the most important but also the vaguest and most misused concept in ecology

types of niche partitioning include spatial, temporal, resource. Also morphological and conditional differentiation.

Question 31

NEUTRAL THEORY

• who
• what does the model assume

Answer 31

Stephen Hubbell (2001)
A new way of explaining the diversity of species in ecological communities.

The model assumes

• all species have the same competitive ability (alpha)
• all species are identical, with identical per capita birth and death rates
• perfect equalising mechanism and no stabilising mechanisms

Clearly not true, but model works well in some systems.

Has sparked controversy - maybe null models are important for understanding processes?

Question 32

How can you reconcile niche and neutral theory?

Answer 32

Chesson (2000) recognised that two mechanisms are important for determining the long-term persistence of diversity in competitive communities.

Emphasises that there are both fitness and niche differences between species and these lead to equalising and stabilising mechanisms respectively

• Equalising mechanisms - Fitness differences between species
• Stabilising mechanisms - Niche differences between species. Introduces density-dependence (because niches mean intra>inter?)

• In neutral models there are no fitness differences and no stabilising mechanisms.

Question 33

Why was Hubbell motivated to come up with neutral theory?

Answer 33

• He struggled with the idea of the incredible diversity he was surrounded with eg. tropical rainforests and coral reefs
• He couldn’t get his head around each species having a niche.

Question 34

Mechanics of neutral theory

Answer 34

• adult holds site until it dies
• juveniles compete in lottery to occupy vacant sites
• death hits sites at random so per capita death rate of each species is the same. We can therefore work out the probability that the individual that died is from each species
• birth - replacement takes place via unbiased lottery so all species have same per capita birth rate
• therefore the trees all have identical fecundity, and the only factor that determines probability of taking over a site is the number of individuals of each species to begin with! Translates into % of seeds produced by one species over total seeds produced

Question 35

Essentials of neutral dynamics

Answer 35

• Common species are more likely to die, but they are also more likely to capture the vacant site. (be replaced)
• The change in population size is thus entirely uncoupled from current abundance
• There is no disadvantage when common, and critically, no advantage when rare
• It’s entirely unpredictable who is going to win – drift dynamics

Question 36

Features of neutral dynamics

Answer 36

• The birth and death process is entirely stochastic.
• This means that the only force acting on populations is drift.
• Eventually all species except one will drift to extinction – this is the only possible long term outcome
• BUT in a large community this process can take a very long time.
• There’s also a speciation process going on in background

Question 37

How robust is neutral theory?

Answer 37

• Neutral theory produces wonderfully plausible patterns…
• IF species are truly identical
• But it is not robust to the inclusion of fitness or niche differences, no matter how small…
• If the fecunidity is not exactly the same, the result is entirely deterministic. If you have an advantage in a neutral model, you will win every time. As soon as you violate the initial assumption of the neutral model, the whole thing collapses.
• Produce more seeds, harder wood so live longer, fewer predators etc are all examples of fitness differences

Question 38

What’s a Chessonian niche?

Answer 38

Chessonian niche - Introduce a stabilising mechanism to induce density-dependence by niche differences!

Eg. we could make the birth rate and/or the death rate of each species dependent on the density of same sp. individuals, instead of being fixed. Depending only on intraspecific competition.

Question 39

what’s the relationship between fitness and niche differences?

Answer 39

Larger fitness differences require stronger stabilising mechanisms (dd niche differences) to keep diversity there

o Fitness differences are fundamental ‘how good you are at being a tree’ and there is density dependence – balance of these 2 determines diversity level.

Question 40

How do you add density dependence to the neutral model?

Answer 40

Modify the birth rate (fecundity) to add in dd.

𝐹𝑖 = 𝐹𝑚𝑎𝑥 / 1 + 𝑁𝑖

where 𝐹𝑚𝑎𝑥 = max fecundity of species i

• As Ni gets larger, Fi gets smaller and smaller (as you get more common there is a disadvantage, rare is an advantage)
• Notice that the fecundity of species i is only affected by the number of conspecific individuals (Ni).
• Notice that fecundity increases very steeply as the species becomes rare
• This greatly increases its chance of winning a site.

Adding dd fecundity to the neutral model stabilises dynamics!!

Question 41

What’s the result of adding density dependence to the neutral model?

Answer 41

• By including dd we have regulated the populations and we get a dynamical effect, and rarely see extinctions.
• Every time you get a completely different result, but with all 3 species coexisting over time. That’s what niches give you – stabilised communities (dynamic)
• However, there’s still no fitness difference, we have only put a niche difference in

Question 42

How can you accommodate fitness differences into the neutral model?

Answer 42

•Adding density dependence also allows us to accommodate fitness differences (as well as niche differences!)
o Even when one species has higher fitness, it still experiences density dependent regulation
o The species with higher fitness will have higher average abundance, but density-dependence helps to prevent the extinction of other species.

Therefore this model shows how species that have differing competitive abilities and fitnesses are able to coexist!

However, if the fitness differences are too large, then dd is not enough to prevent extinctions of the less fit species.
o You still get diversity persisting, and some species are more or less abundant over time (stably!)
o However, there is a limit to it
o Fitness differences are fundamental ‘how good you are at being a tree’ and there is dd – balance of these 2 determines diversity level.

Question 43

How do equalising mechanisms contribute to stable coexistence?

Answer 43

• The equalising/stabilising framework is due to Chesson (2000)
• Chesson tried to unite neutral and niche theory to understand how species coexist!
• He recognised that two mechanisms are important for determining the long-term persistence of diversity in competitive communities.
• Fitness differences (equalising mechanisms) and Niche differences (stabilising mechanisms) are both lacking in the neutral model.
• Equalising mechanisms tend to reduce fitness differences among species. This is important for coexistence, as if fitnesses are highly unequal, then one species will quickly exclude the others (think alpha»1 leads to competitive exclusion!!) For example, if one species had much higher fecundity than another species (and everything else was equal) the outcome would be rapid competitive exclusion.
• Niche mechanisms in contrast are frequency (or density)-dependent. Density dependence in a single-species population means that per capita growth rates decline as population size increases. This introduces a feedback that stabilises the population. Similarly, in a multi-species community, frequency dependent mechanisms are those that cause the per capita growth rate to fall as the frequency of any species increase. Hence, species tend to have high per capita growth rates when rare, and low per capita growth rates when common. Niche mechanisms introduce stability into a community. Specifically, niche mechanisms cause species to limit themselves more than they limit others.

Notice that this is not the same as ‘generalised density dependence’; the total community size could remain constant, and frequency dependence could still operate. Such negative frequency dependence is widespread in ecological communities, even in hyper-diverse systems, e.g. Barro Colorado Island.

• Why do we need both? Naively niche mechanisms could be held solely responsible for stable coexistence, but in fact the equalising mechanisms (fitness differences) are important. This is because it’s much harder to stabilise a community when the species have highly unequal fitness. And hence the niche mechanism needs to be much stronger.
• Ecologists thus often refer to a continuum where communities go from having small fitness differences, and weak stabilisation at one end to having large fitness differences and strong stabilisation at the other. We don’t really know where real communities lie.

The neutral model is a single ‘vanishing point’ on this continuum, where species have exactly equal fitness, and there are no stabilising mechanisms at all. There has been one nice experiment – with California annuals – where the niche mechanisms were removed, and diversity was seen to collapse quite rapidly. This suggests that strong niche mechanisms are present in at least some real ecological communities.

Question 44

Difference between a traditional and a Chessonian niche

Answer 44

Niches are vital for stabilising communities!!

Traditional - emphasise the biological ‘role’ of an individual

Chessonian niche - Introduce a stabilising mechanism to induce density-dependence by niche differences!

Eg. we could make the birth rate and/or the death rate of each species dependent on the density of same sp. individuals, instead of being fixed. Depending only on intraspecific competition.

Question 45

the two vital things you need for a lovely stable community

Answer 45

niches! - are necessary to stabilise communities

fitness differences!! - small fitness differences make it much easier to stabilise

Question 46

Why does having a unique ecological role (niche) induce density-dependence and stabilise dynamics? How are they the same thing?

Use the example of MacArthur’s warblers

Answer 46

• MacArthur observed warblers for 1000s of hours and recorded where they were feeding
• appears to be niche partitioning within the trees! eg. Cape May Warbler only eats from very tops of trees

How does this feeding niche partitioning create density dependence?

• if the pop size of Cape May warblers increases, other Cape May warblers (rather than other warbler species) will suffer the most because food availability will ONLY be reduced at the tops of trees
• lower food availability for CM warblers means fecundity decreases, driving drop in pop size
• this is the dynamical effect of niches (disadvantage of being common stops competitive exclusion and from things going extinct)

Question 47

Plant niches

• why is it hard to see what they might be
• what are they (lol)

Answer 47

Most plants only need a few essential resources – all the plants need all of them so how are niches divided??

Life history trade-offs are responsible!

• trade off between growth rate and defence
• everything falls along the line because resources are limiting and it’s stupid to be slow growing and undefended
• The resulting trade-offs offer potential niche axes along which species can be differentiated

small scale heterogeneity might be able to promote diversity within habitats!

Question 48

Heterogeneity

- how does this impact life history strategies in different habitats?

Answer 48

Difference in environmental conditions (environ. heterogeneity!) in different habitats mean

Diff habitats might lie on diff points along the trade-off line (btwn growth rate and defence) and have different life history strategies because of it

eg. in the desert, high growth rate is impossible due to scarcity of resources, and massive cost of tissue loss, therefore DEFEND
eg. forest, high growth rate essential to avoid shading by other things, resources are plentiful so cost of tissue loss is low, therefore GROW

Question 49

SMALL SCALE
Heterogeneity
- how does this impact life history strategies within a habitat?

Give an example of a trade off and the species involved

Answer 49

Small scale heterogeneity might be able to promote diversity within habitats by introducing stabilising mechanisms (niches)

eg. here is the example of heterogeneity in light environment, created by the trees themselves

Pioneers and shade tolerators are at either end of a general trade off between growth rate and lifespan. They can stably coexist by partitioning the light environment, resulting in a stable equilibrium between the prop of pioneers and shade tolerators (this doesn’t mean they have equal fitness! Forests are usually dominated by beeches (shade-tolerators) but because the fitness difference isn’t too massive it’s OK)

PIONEER = Birch
Growth rate = It produces lots of seeds and grows quickly so it can take advantage when gaps appear.
Life span = Short-lived. It can’t regenerate in the shade.
When it dies its place is taken by the shade-tolerator.

SHADE-TOLERANT = Beech (can be planted under a birch and will be fine!)
Growth rate = Slow growing
Life span = It is long-lived.
When it dies, it creates a gap, which is captured by the pioneer.

Another example of small scale heterogeneity is the Janzen-Connel effect, which stabilises populations by across-trophic level effects

Question 50

How does heterogeneity promote coexistence (short sentence)

Answer 50

If different species specialise on different patches in the environment, they have different life history trade-offs, and the stabilising mechanisms of niche partitioning mean you get stronger intra>inter and species can coexist

Question 51

Hubbell’s view on trade-offs and the mechanism by which they allowed coexistence

Can life history trade-offs be equalising?

Answer 51

• Hubbell thought we had been misinterpreting trade-offs
• Hubbell knew that species weren’t the same, and he suggested that trade-offs might be equalising (ie. to do with fitness) rather than stabilising (to do with niches and dd eg. pioneer and shade-tolerant species)

HE WAS WRONG -
• Life-history trade-offs are very unlikely to be perfectly equalising
• When you change conditions, you inevitably introduce fitness differences
• Without a stabilising mechanism (niches), extinction will follow

Question 52

Seed size vs seed number trade off

• is it equalising (does it help reduce fitness differences) and is that the thing that allows coexistence?

Answer 52

o On average, individuals of different species produce the same total mass of seeds (despite some producing many small seeds and some fewer large seeds)
- Maybe seed size is drifting (equalising) rather than due to niches?

you can make a model to determine which strategy is best

• sand dunes as patchy habitats where many species compete to get a share of the resources within each patch
• seeds are dispersed
• each species (i) wins a share of resources (Pi) in proportion to the total mass of their seeds (niSi) that have landed in that patch
• Mathematically this means Pi=niSi/∑niSi
• niSi is constant for all species as there’s a perfect trade off between no. of seeds and size of seeds (giving same overall mass of seeds per individual)

theoretical outcome

• species with the smallest seeds always win and large seed individuals driven quickly to extinction (didn’t behave neutrally!!)
• the neutral theory doesn’t work because the patchiness of sand dunes means that small-seeded species have an advantage because they distribute their seeds more evenly. Average is the same, but variance is not the same! This is better in nature because the resources available in each patch are capped. Overkill in one patch doesn’t compensate for not having tickets in the other lotteries

THEREFORE NOT PERFECTLY EQUALISING. When you make things different (seed size!) you almost inevitably introduce fitness differences. And without a stabilising mechanism (niches!) extinction will follow

Question 53

What’s the Janzen-Connell Effect

Answer 53

A powerful way of stabilising populations!!
Shows you can’t ignore other trophic levels

• offers an explanation for the maintenance of tree species biodiversity in tropical rainforests
• host-specific herbivores, pathogens, or other natural enemies make the areas near a parent tree (the seed producing tree) inhospitable for the survival of seedlings.
• this mechanism prevents each species from re-occupying a site it had previously occupied, and introduces powerful dd regulation
• this prevents one tree species from becoming too common, as if it is, there will be few ‘safe’ places where its seeds can grow without being eaten (negative feedback is introduced)
• it’s another stabilising (niche, dd) method of maintaining biodiversity.
• another example of tiny scale heterogeneity

Question 54

Summary - how do biological mechanisms allow stable biodiverse communities (such as rainforests) to exist?

Answer 54

• Neutral theory (Hubbell) ain’t enough - it shows persistence but only because it assumes
• all species have the same competitive ability (alpha)
• all species are identical, with identical per capita birth and death rates
• perfect equalising mechanism and no stabilising mechanisms

Therefore in real species-species interactions, to have biodiversity you need both stabilising (niche) and equalising (fitness) mechanisms (Chesson’s coexistence framework)

Introduction of niches (VITAL)

• Life-history trade offs constrain species to a set of possible strategies that are likely to be more or less successful in different environments (have to be a pioneer or a shade-tolerant)
• Environmental heterogeneity then offers niches for different LH strategies
• LH trade-offs are unlikely to be perfectly equalising (fitness of each strategy will not be equal)
• Janzen-Connell Effect offers a powerful way of stabilising populations by small scale heterogeneity (you can’t ignore other trophic levels)

Small fitness differences (FAVOURABLE)