Community Ecology (Lectures 17-23) Flashcards
what is a community?
> a group of populations coexisting in a given location.
- patterns of species diversity at a local regional scales
how do you study communities?
> scale up from two-way interactions to multi-species interactions - all the same types of interactions - predator/prey dynamics - competition - facilitation > scale down from ecosystem factors - primary productivity - climate - biogeography
how do you categorise communities?
> theoretically: set of species actively interacting
- direct competition/predation/facilitation
- indirect interactions
- gene flow
practical/ operational: group of organisms in an area smaller than a landscape that share clear physical/compositional characteristics and/or clear boundaries.
- species on a sand dune
- species in a hot spring
- species in a meadow
how do you define communities?
> closed or open systems
trophic levels
food webs
describe trophic levels for defining communities
> transfer of energy through a community
- measured by which types of species eat other types of species
- chain is kind of outdated.
describe food webs
> explore interactions among species across the trophic levels
- connectivity through predation/herbivory
- who eats who
- transfer of energy
wwhat are some issues with food webs?
> not all species fit into set categories or fit into more than one - omnivores - detritivores - parasites - pollinators > does not generally incorporate processes occurring within trophic levels - intra-specific competition - interspecific competiton
what are interaction networks?
Include: > trophic interactions - vertical links > competitive and facilitation - horizontal links - don't involve eating eachother - pairs of species interacting
what are some common ways of selecting a subse of species to study?
> taxonomic groups
- mammals, reptiles, plants
trophic guilds
- group of organisms that use the same resources
functional groups
- subset of a community that function in a similar way
types of interactions
- organisms across trophic guilds that form interaction webs through a single mechanism
- pollination, mycorrhizeal interactions, decomposition.
describe community structure
> a community has attributes that differ from those of its components
- number of species
- relative abundance of species
- nature of species interactions
- physical structure
‘fit’ of organisms to the physical environment + species interactions explain the ecological processes that control community structure
evolution equally important for understanding which set of species are found in a given community
what is species diversity?
= the number and abundance of species in a defined area
> important components of diversity
- relative abundance = proportion of individuals in a community from a single species
- species richness = the number of species in the community
- composition = the identity of the species in the community
- species evenness = tells us about the commonality and rarity of species
what is the shannon index?
> common measure of diversity
s = number of species
pi = relative abundance of each species
what is pielou’s evenness
> low evenness if one or two species dominate
> high evenness if all species have similar abundances
describe diversity and scale
> diversity will vary depending on the scale at which it is measured and the effort put in to measuring it.
species accumulation curves
higher number of samples = higher diversity
species interactions in communities
> are all species of equal importance to a community?
- Diversity?
- function?
- stability?
studies of diversity provide limited information about the processes resulting in the diversity and composition we see in given communities
describe community assembly
> understanding mechanisms of community assembly is a central goal of community ecology research
- species interactions with their environment
- the importance of the niche in understanding biological communities
- species interactions
- direct interactions
- indirect interactions
- intraspecific interactions
what are three niche theories?
> eltonian niche: the functional attributes of a species (including its trophic position).
- the environmental tolerances of a species
- a species’ relationship with food and enemies
- classic def.
grinnellian niche
- the niche of a species is determined by the habitat it lives in and behavioural adaptations to the environment
hutchinsonian niche
- N-dimensional hypervolume
- dimensions are environmental conditions and resources that determines a species’ way of life.
describe the fundamental niche constraints on community structure
> all living organisms have a range of environmental conditions under which they can successfully survive, grow, and reproduce
this range of environmental conditions conditions is not the same for all organisms
species pool - how many species are around to fill a community?
- biogeography
- evolutionary history
- ecology
describe the species pool
> same types of organisms often occupy similar environmental space
compare and contrast intra- vs. interspecific interactions
> intraspecific interactions:
- interactions among the individuals of a given species
interspecific interactions:
- interactions among individuals of a different species
the strength of these interactions can be used to understand why some species can coexist in a community and others cannot
describe direct interactions
> positive, neutral and negative interactions within or between specie that only involve the individuals (of the species) involved in the interaction
- competition for resources
- pollination
- facilitation
how do you asses interactions?
> consider direction, magnitude and stability (the net effect or long term outcome)
describe the different types of competition/facilitation
> interspecific competition -/- - resources - food, shelter etc > mutualisms +/+ - resources - services (pollination) > commensalisms +/0 - resources - habitat
how do we measure impacts of species on other species?
> relative changes in: - biomass - reproductive success - population size > models allow us to assess the effects of one species on another and then scale up to populations
what are keystone species?
> key ecological role filled by 1 species
removal of these species impacts many other species
typically not the most abundant species
impacts of removal typically disproportionate to abundance
punch above their weight
what are dominant species?
> the most common species in the community
often defined separately for each trophic level
- e.g., dominant predator, dominant producer etc
dominance is usually assumed to result from competitive exclusion
this happens in two pathways:
- transitive competition - linear hierarchy of competition
- intransitive competition - circular network of competiton
what are ecosystem engineers?
> species that create particular environmental characteristics of an ecosystem
- without them, the ecosystem changes fundamentally
are all species equally important? describe interaction strength
IS = ln((C/E)/I))
- IS = per capita interaction strength
- C = the # of target individuals in the presence of the interactor species
- E = the # of target individuals in the absence of the interactor species
- I = the # of interactor individuals
IS = (C-T)/(T*M)
- IS = per capita interaction strength
- C = mean density of target species in the presence of the interactor species
- T = mean density of a target species in the absence of the interactor species
- M = mean density of the interactor species
what is a network analysis?
> analytical approach to studying interaction webs while accounting for biological complexity.
- incorporates information on:
- strength of interactions
- direction of interactions
- commonality of interactions
common approach to the study of food webs and pollinator networks
- easiest if examining 1 type of interaction
describe ecological network analysis
> quantify links among species in a community
allows assessment of many different direct interactions among species in communities
provides insight into mechanisms driving and maintaining diversity
doesn’t require as much detailed information as interactions strength analyses
What are the terms associated with network analysis?
> modules - linked subset of species within communities with more interactions ith each other than between modules > Peripherals (specialists) - a species with few links within its own module and rarely any to other modules > Generalists: species that interact with lots of other species - connectors: species that link modules together through their interactions with species in multiple modules - module hubs: lots of interactions within its own module, forms the main structure of a module - network hubs: is a 'super generalist' holding together both its module and the whole network.
how do you quantify ecological networks?
> modularity, nestedness, asymmetry, other features of networks
modularity of a community (extent of module organisation in a community) can by caused by:
- habitat heterogeneity
- divergent selection regimes
- phylogenetic clustering (groups of related species)
structure of network modules reflects:
- outcomes of coevolutionary processes
- community resilience: capacity for community to maintain its functionality under disturbance/change
- long term stability
what are indirect interactions?
> positive, neutral and negative interactions between two species that indirectly impact a third species.
- all the same types of interactions as involved in direct interactions
- in complex communities indirect interactions are often just as important as direct interactions
the net impact of species interactions on a focal species
the enemy of my enemy is my friend
what are the types of indirect faclitation?
> trophic cascade
- predator eats herbivore, therefore primary producer has less herbivory stress
predator of competitor
prey of commensal host
prey of mutualist
indirect facilitation mediated by directly helpful interactions
- communalism chain
how can ecosystem engineers have indirect effects?
> species that modify their environment to indirectly impact a third species
what is a simple model of community assembly?
> community assembly
- the set of processes involved in determining the diversity of species in a community
- assumed to drive multi-species stable coexistence
many processes involved
different processes are more or less important for different communities
what are the three steps in community assembly process?
1) dispersal and the regional species pool
2) environmental filtering and niche compatibility
3) biotic interactions/ coexistence dynamics
describe dispersal ability
> different trees have different dispersal ability of seeds that limits the areas it can reach
the traits of seeds, e.g., bigger if carried by animals etc
access to communities
what is an environmental filter?
> an environmental condition that restricts which species ca persist in a particular location or community
- generally an abiotic factor
- water availability
- temperature range
- food/nutrient availability
- light
strong filtering leads to more similar species from the regional pool
what is the limiting similarity theory?
> concept that more similar species will compete more strongly with each other and thus stable coexistence is less likely among similar species
- required resources
- life history
- approches for resource acquisition
used as an underlying assumption in most models of community assembly
- increasingly replaced with modern coexistence theory
what are functional traits?
> any morphological, physiological or phenological feature of a species
- wind dispersed
- drought resistance
- nitrogen fixation
- leaf area
- heat tolerance
traits considered ‘adaptive’ if they give a plant an advantage in a specific habitat
traits considered ‘ecologically important’ if they have direct links to ecological processes
describe modern coexistence theory
> how can ecologically similar species coexist without competitively excluding each other?
limiting similarity assumes 1 type of competitive difference
- differences in competitive ability as measured by fitness differences
- what species does best when under direct competition?
modern coexistence theory: stable coexistence is mediated by two types of ‘ competition’-based species differences:
- competitive ability/fitness differences and
- niche differences
COEXISTENCE can occur as long as niche differences are greater than fitness differences
explain fitness differences
> limit diversity by driving he best adapted species to a given environmental context (niche) to drive other species to local extinction
not density dependent
- species A is more effective at obtaining N than species B
measured as competitive effect of one species on another
explain niche differences
> differences in species’ niches that promote diversity
they reduce negative effects of interspecific competition, allowing species to increase from rarity
also limit population growth at high densities (negative frequency dependence)
measured as relative strength of intraspecific competition.
what are the mechanisms of coexistence?
> stabilising effect of niche differences:
- niche differences can prevent competitive exclusion by the strongest competitor by reducing the intensity of interspecific competition relative to intraspecific competiton.
negative density dependence
- stabilising effect depends on the density of intra-specific individuals
- intraspecific competition is stronger at higher density than low, so at low densities, populations increase but at high densities niche differences
what are the types of stabilising mechanisms?
> variation-independent mechanisms - resource partitoning - predator partitioning - frequency-dependent predation > storage effect - species are effected differently by spatial or temporal variation in the environment
describe the different ways organisms can partition resources
> niche partitioning
character displacement
- co-occuring species evolve traits’ or strategies that allow them to avoid direct competition with other species
behavioural resource partitioning
- pragmatic
- responding to the competitor pool at the given location
- avoid direct competition
describe the storage effect
> stable coexistence of similar species by avoidance of stronger competitors through space and or time.
- mediated by environmental heterogeneity
- density-dependent population growth rates
- buffered population growth rates
how does storage effect differ from niche partitioning?
> reflects distinct responses of species to variable environmental conditions
how can coexistence occur in relation to three population level phenomena?
- every species has a different optima or preferred set of environmental conditions, making no one place (or time) the best for every species
- species living at low densities experience less competition than species living at high densities (stabilising niche differences)
- at low densities competition doesn’t restrict growth in populations
- at high densities species experience greater intra-specific competition and thus more limited population growth
- this effect relates to the environment because competition outcomes co-vary with the environment - “buffering” allows net positive growth rates across heterogeneous environments by waiting out bad conditions.
explain covariance between environment and competition
> competitive success is related to the environmental conditions in a location
the magnitude and sign of covariance varies depending on the density of a species and its competitors
- dominant species have strong positive covariance between environment and competition
- species at low density don’t suffer from intraspecific competition
- low density species are outcompeted by dominant species under the most common environmental conditions
the most favourable environmental conditions for a dominant species are not the most favourable conditions for less abundant species in a community
OUTCOME: population growth occurs for species found at low densities when they are favoured by the environment
Explain buffering
> refers to obtaining a positive growth rate by averaging across different conditions
- microenvironments (space)
- growing seasons (time)
species buffer by maintaining the bulk of their life in the most stable, least ‘bothered’ life stage:
- seed banks and other storage phases in short lived organisms
OUTCOME: populations can have positive growth rates over time, even if some years/places are no good, as long as on average growth is positive.
what is n ecosystem function?
> ecosystem processes stemming from biotic communities
- productivity - biomass production
- nutrient cycling
- water purification
- soil creation and retention
- any ecological process - pollination, gene flow
What is a BEF?
> biodiversity ecosystem function
core hypothesis: diversity is positively correlated with ecosystem functioning
four hypotheses about ho diversity relates to ecosystem function
- complementary hypothesis
- redundancy hypothesis
- driver and passenger hypothesis
- driver and passenger hypothesis plus overlap
describe the complementarity hypothesis
> as species richness increases, there will be a linear increase in community function
- each new species has a unique and equally incremental impact on community function
explain the redundancy hypothesis
> as species richness increases, there is a positive but bound effect on community function
- when diversity is low, new species tend to add unique value to the community function but that positive effect levels off and reaches a cap.
- functional additions are still thought of as equal in extent
- species can have redundant value to community function
explain the driver and passenger hypothesis
> the strengths of the impacts each species had on community function varies a lot
- driver species have a large effect on community function
- passengers have small to no effect on community function
combined impacts result in a stair step increase in function
explain driver and passenger hypothesis with overlap
> combines the Driver/passenger and redundancy hypotheses
- species have variable impacts on community function, causing stepped increases in function with added species
- there is a cap or upper bound on the addition of new functionality
what are 4 key concepts in succession?
- agents of change: abiotic or biotic, act at all spatial and temporal scales
- succession results from agents of change action on communities
- succession mechanisms are context-dependent
- successional trajectories are diverse and display alternate stable states
what are some abiotic agents of change?
> related to physical environment
temperatures, rainfall, etc
STRESS: reduced fitness of individuals
- draught -> fewer seeds per individual in plant population
DISTURBANCE: kills individuals or populations
- fire wipes out entire plant population
what are biotic agets of chnage?
> related to species interactions
- competition, facilitation, predation, disease
how are agents of change described?
> in terms of:
- intensity: what is the severity of the storm?
- frequency: how often do storms of this severity occur?
- extent: what spatial area of the reef was affected?
describe primary succession
> starts off as bare rock, maybe from a volcanic erruption
the pioneer species come in and break up the rock, and die, leaving organic matter behind for the
intermediate species to come in
then the climax community is what is the eventual state of the community.
happens over hundreds and hundreds of years
describe secondary succession
> happens over 150+ years (so quickly)
the disturbance does not fully remove organic matter from the ground, still has soil etc.
disturbance and stress can reset the community at any stage.
what are the 3 different models of sucession?
- facilitation model: early species enefit later species
- inhibition model: early species hinder later species
- tolerance model: interactions not important, turnover due to innate species traits and life histories.
> no one model is correct, a mix of all three is seen
> misture depends on the study system, the temporal and spatial context of study.
describe what happened at glacier bay
> the study showed that overall a combination of tolerance, facilitative and inhibitive
tolerance and facilitative model dominant in EARLY SUCCESSION
inhibition model more dominant in LATER SUCCESSION
what are alternative stable states?
> climax community composition not always predictable
alternate stable states: climax communities can end up differing in composition from one another, even in the same area
hysteresis may occur when moving from one alternate state to another
ecosystem change may be difficult, costly, or impossible to reverse, may be stuck in alternative stable state
restoration can be successful with knowledge of how communities naturally change
describe global forest diversity
> species richness and composition varies with latitude: more species at low latitudes
species richness an composition varies from continent to continent even when latitude is similar
same biome type on different continents vary in richness or diversity depending on location
what are the diversity indices?
gamma (y) diversity: species pool for an entire region
Alpha (a) diversity: species pool for a local community (subset of gamma)
Beta (B) diversity: species turnover. several ways to calculate, generally: greater B means greater variability among communities, uniqueness of ommunities
why do the tropics have more species? give 3 hypotheses
Latidudinal diversity gradient:
- tropics are a cradle of diversity
- high speciation rates relative to extinction rates
- greater land area, probability of speciation - tropics are a museum of diversity
- climate stability relative to polar regions -> less disruptive to diversification
- once species originate, they stick around - productivity and energy hypothesis
- more resources in tropics -> higher productivity and carrying capacities
- faster molecular evolution and mutation rates
give a recap of diversity in the world
> diversity is a result of processes interacting at a variety of scales, from global to local
relationship between local and regional richness reflects whether local factors (e.g., competiton) or larger factors (e.g., environment) are more dominant in determinging local species richness
current species distributions often reflect paleographic conditions
several hypotheses exist for latitudinal diversity gradient (cradle, museum, productivity/energy)
explain how local and regional species processes interact
> if regional species richness is in a 1:1 ratio with local species richness, than all species in the region are present in each community
if ratio is local to regional if the line is asymptotic, local processes determine local richness, e.g., competiton
describe the species -area relationship
> you will find more species as you sample larger areas
influenced by isolation
the number of species increases with area sampled and proximity to source populations
balance of immigration and extinction
explain the equilibrium theory of isalnds
> immigration rates decline as more species added
- fewer individulas added are of a new species
extinction rates increase as more species are added
- greater probability of extinction and negative interactions
isolation influences immigration rates
size of island influences extinction rates
THEREFORE:
near, large islands have more species (high immigration, low extinction)
small, far islands have the fewest species (low immigration, high extinction)
number of species found in a given location positively related to size of area sampled and negatively related to isolation from source populations
