Mod 13 (Ch 16) - Communities

included on exam 3, updated for spring 2026

population

a group of individuals of a single spp that live in a particular area at the same point in time

living organisms occur in these

these occur in communities

community

populations of different spp living and potentially interacting in a certain area at a given time

types of interactions between spp that shape communities

predation (+/-)

commensalism (+/neutral)

parasitism (+/-)

mutualism (+/+)

competition (-/-)

can be direct or indirect

how biological communities are often differentiated

based on physical (abiotic) or biological (biotic) characteristics

physical characteristics

abiotic characteristics (climate, H2O characteristics, soil type, location, other habitat features, temp, precipitation, substrate type)

similar types of characters used to define biomes

determine which spp occur in a community

only those spp with the necessary adaptations for survival and reproduction under the particular abiotic conditions can occur

examples of communities defined by physical characteristics

all the spp that occur in a: desert, stream, pond, cave, tropical rainforest

biological communities defined based on biological characteristics

biological = biotic

implies the importance of particular abundant spp

all organisms in the community are involved with a certain biotic factor somehow (e.g. use it for food or shelter)

e.g. kelp forest community, longleaf pine community, coral reef community

subset of a spp in community

what most ecologists study in a community

(too many spp to study simultaneously, too difficult)

may be selected based on taxonomic similarity, guild, functional group (helps define a “community” being studied)

taxonomic similarity

relation due to similar evolutionary lineages

one way to define a community

e.g. bird community, insect community, fungal community, algal community

guild

a group of spp that use the same resources, even though they may be taxonomically unrelated

one way to define a community

e.g. nectivore community, plankton community, piscivore community

functional group

a group of spp that function in similar ways but may or may not use the same resources

e.g. piercing mouthpart community, aquatic photosynthesizers, nitrogen-fixing plant community, predator community, decomposer community, pollinator community

what a community ecologist may study

which organisms occur in a community (community structure)

interactions among organisms in a community (e.g. food chains and webs)

how interactions affect community composition

2 aspects of community structure

spp diversity

spp composition

spp diversity

spp richness (# of spp in an area) and evenness (relative abundance) combined into one index

allows comparison of communities

can be compared with a diversity index

spp richness

the # of spp in an area

spp evenness

the relative abundance of spp in an area

Shannon-Wiener index

a diversity index allowing comparison of diversity between 2 or more communities

H = -sum [p_i*ln(p_i)]

pi = proportion of each spp (as a decimal)

H represents diversity; higher H value indicates higher spp diversity

assumes that the actual number of spp evaluated is accurate and correctly known

note: the - is important, otherwise you’ll end up with a negative number, which is incorrect (take the negative of the negative)

spp accumulation curves

spp richness plotted as a function of total # of individuals counted

accounts for additional individuals or spp in the community that may not have been found

help determine when all of the spp in a community have been observed

when additional sampling effort no longer results in new spp, likely near total # of spp in the community

can vary greatly for different spp

factors affecting how many spp will be observed when sampling a community

# of individuals sampled (more individuals sampled = more spp observed)

area sampled (more area sampled = more spp observed)

heterogeneity of area sampled (more homogeneity of area = more spp observed) (e.g. flat sandy beach vs intertidal zone or coral reef)

proportion of rare spp in sample area (more rare spp in area= fewer spp observed)

spp composition

identity of spp in a community

which spp occur in a community determines the types of interactions

can be very different even for two communities with identical diversity values

trophic structure

feeding relationships among organisms

i.e. how the organisms in a community obtain energy

determines the pattern of energy movement through an ecosystem

can be illustrated with food chains or webs

2 ways to consider trophic structure

food chains

food webs

food chain

one way to illustrate trophic structure

follows one path of a food web

shows trophic levels

trophic level

groups of spp that have similar ways of obtaining energy

e.g. primary producers, primary consumers, secondary consumers, tertiary consumers, quaternary consumers

food web

one way to illustrate trophic structure

illustrates many paths, shows links between food chains

arrows follow path of energy transfer (from the eaten to the eater)

difference between food webs and interaction webs

food webs only illustrate feeding interactions

interaction webs include both trophic and non-trophic interactions, e.g. competition and mutualism

^interactions can be direct or indirect

direct interactions

interactions in which two individuals interact with each other plainly

include both trophic (i.e. predation/herbivory) and non-trophic (e.g. competition) interactions

indirect interactions

a relationship between 2 spp is mediated by a third (or more) spp. Affect population growth rates (i.e. survival probability or reproductive rate)

types: trophic cascade, trophic facilitation

trophic cascade

rate of consumption at one trophic level results in a change in spp abundance or composition at lower trophic levels

e.g. otters; decreased urchin consumption indirectly lowered kelp abundance, increased urchin consumption indirectly increased kelp abundance

trophic facilitation

when a consumer is indirectly helped by a positive interaction between its prey and another spp

e.g. aphids, Iva, and Juncus

aphids eat Iva. Juncus allows Iva to grow (shades soil surface, decreases evaporation and salt buildup; oxygen from Juncus roots moves into soil and gets used by Iva)

Juncus increases Iva growth → more Iva = more food for aphids → aphid population growth, because of Juncus

Juncus indirectly helps aphids by helping its food source grow

this relationship perseveres because Iva cannot survive without Juncus (positive of improved soil outweighs cost of aphid attraction)

interaction strength

magnitude of the effect of one spp on the abundance of another spp

= C/E

C = # of target individuals with interactor present

E = # of target individuals with interactor absent

generally measured by removing interactor spp from the community and observing the effect on the target spp

may depend on environmental factors (e.g. sea stars preying on mussels, interaction strength was higher in protected areas; sea stars were less efficient predators in waves. control is caged without waves)

per capita interaction strength

how spp interactions vary in direction

effect each individual interactor has on each target spp individual

effect density of interactor on target spp population

= ln[(C/E)/I]

(divide C by E, then divide that by I, then take the ln of that final number)

I = # of interactor individuals (capital i, not lowercase L)

important spp in community

dominant spp

keystone spp

dominant spp

most abundant or have highest biomass

some are ecosystem engineers

e.g. large trees, coral

ecosystem engineers

species that create, modify, or maintain physical habitat for themselves and other spp

e.g. some dominant species

e.g. some trees

keystone spp

a spp that exerts strong community control by its ecological role (not by its abundance)

elimination dramatically affects survival of many other spp

e.g. otters, Pisaster sea stars

a spp can be a keystone spp in some communities but not in others

examples of spp that are both keystone spp and ecosystem engineers

beavers (dams create marshes with wetland plants, which increase regional biodiversity)

gopher tortoises (burrows are used by >400 spp for habitat and protection, enhance plant growth by loosening soil and redistributing soil nutrients, dung provides nutrients, food, and habitat for spp and seed dispersal)