Chapter 54 Community Ecology

A biological community is an assemblage of populations of various species living close enough for potential interaction

Can affect the survival and reproduction of each species 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved Ecologists call relationships between species in a community interspecific interactions Competition Predation Herbivory Symbiosis (parasitism, mutualism, and commensalism) Can affect the survival and reproduction of each species

Competition (–/– interaction) occurs when species compete for a resource in short supply Example?

Competitive exclusion principle **The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place**

Niche Ecological niche- The total of a species’ use of biotic and abiotic resources an organism’s ecological role Ecologically similar species can ONLY coexist in a community if there are one or more significant differences in their niches Why??????????????????????????

Resource partitioning differentiation of ecological niches enabling similar species to coexist in a community Example? Species can change niches when there is overlap with another species Fundamental vs. realized niche

A. distichus perches on fence posts and other sunny surfaces. Fig. 54-2 A. distichus perches on fence posts and other sunny surfaces. A. insolitus usually perches on shady branches. A. ricordii A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei

EXPERIMENT High tide Chthamalus Chthamalus Balanus realized niche Fig. 54-3 EXPERIMENT High tide Chthamalus Chthamalus realized niche Balanus Balanus realized niche Ocean Low tide RESULTS High tide Chthamalus fundamental niche Ocean Low tide

Character Displacement The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species In other words: two species are MORE different from each other when they live in the same area than when they live apart from each other.

Predation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey Predator adaptations include____ Prey adaptations include______

Cryptic coloration- camouflage, makes prey difficult to spot Aposematic coloration- Bright warning coloration that animals with effective chemical defense often exhibit as a warning

Mimicry Batesian mimicry- a harmless species mimics an unpalatable or harmful model Müllerian mimicry, two or more unpalatable species resemble each other

(c) Batesian mimicry: A harmless species mimics a harmful one. Fig. 54-5c (c) Batesian mimicry: A harmless species mimics a harmful one. Hawkmoth larva Green parrot snake

Müllerian mimicry: Two unpalatable species mimic each other. Fig. 54-5d (d) Müllerian mimicry: Two unpalatable species mimic each other. Cuckoo bee Yellow jacket

Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

Fig. 54-6

Symbiosis Symbiosis is a relationship where two or more species live in direct and intimate contact with one another

3 parasitism (+/– interaction) one organism derives nourishment from another organism, which is harmed in the process the parasite the host What do you think is the difference btwn an endoparasite and an ectoparasite?

Why are many/most parasites NOT extremely deadly?

Mutualism (+/+ interaction), is an interspecific interaction that benefits both species Obligate, where one species cannot survive without the other Facultative, where both species can survive alone

(a) Acacia tree and ants (genus Pseudomyrmex) Fig. 54-7 (a) Acacia tree and ants (genus Pseudomyrmex) (b) Area cleared by ants at the base of an acacia tree

Commensalism (+/0 interaction), one species benefits and the other is apparently unaffected Commensal interactions are hard to document in nature because any close association likely affects both species

Fig. 54-8

54.2: Dominant and keystone species exert strong controls on community structure In general, a few species in a community exert strong control on that community’s structure

Species Diverstiy Species diversity of a community is the variety of organisms that make up the community Species richness is the total number of different species in the community Relative abundance is the proportion each species represents of the total individuals in the community

A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% Fig. 54-9 A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10%

H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + …] Two communities can have the same species richness but a different relative abundance Diversity can be compared using a diversity index Shannon diversity index (H): H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + …]

Trophic Structure Trophic structure is the feeding relationships between organisms in a community Food chains link trophic levels from producers to top carnivores REMEMBER WHICH WAY THE ARROWS GO!

A terrestrial food chain A marine food chain Fig. 54-11 Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Herbivore Zooplankton Primary producers Plant Phytoplankton A terrestrial food chain A marine food chain

Food Webs A food web is a branching food chain with complex trophic interactions Species may play a role at more than one trophic level How is a food web diff from a food chain?

Fig. 54-12 Humans Smaller toothed whales Baleen whales Sperm whales Crab-eater seals Leopard seals Elephant seals Birds Fishes Squids Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton

Limits on Food Chain Length Each food chain in a food web is usually only a few links long Two hypotheses attempt to explain food chain length: the energetic hypothesis the dynamic stability hypothesis

The energetic hypothesis suggests that length is limited by inefficient energy transfer The dynamic stability hypothesis proposes that long food chains are less stable than short ones Most data support the energetic hypothesis

Species with a Large Impact Certain species have a very large impact on community structure Such species are highly abundant or play a pivotal role in community dynamics

Dominant species Species that are the most abundant or have the highest biomass Maybe dom b/c most exploitive, competitive Maybe b/c they avoid predation Biomass is the total mass of all individuals in a population

Invasive species (what do you think that means?) typically introduced to a new environment by humans, often lack predators or disease

Keystone species exert strong control on a community by their ecological roles, or niches What’s “keystone” mean to you? In contrast to dominant species, they are not necessarily abundant in a community Often, we figure out “keystone” species b/c when they are removed… whoa changes!

Number of species present Fig. 54-15b RESULTS 20 15 With Pisaster (control) Number of species present 10 Without Pisaster (experimental) 5 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

Otter number (% max. count) Grams per 0.25 m2 Number per 0.25 m2 Fig. 54-16 100 80 Otter number (% max. count) 60 40 20 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 (b) Sea urchin biomass 10 8 Number per 0.25 m2 6 4 2 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain

Foundation Species (Ecosystem “Engineers”) Cause physical changes in the environment that affect community structure beaver dams can transform landscapes on v. large scale

Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community *small distinction btwn “facilitators” and “keystone”

Number of plant species Fig. 54-18b 8 6 Number of plant species 4 2 With Juncus Without Juncus (b)

How are these communities controlled? Bottom-up model Top-down model

Bottom up model Unidirectional influence from lower to higher trophic levels Presence or absence of mineral nutrients determines community structure, including abundance of primary producers

Top-down model Control comes from the trophic level above also called the trophic cascade model Predators control herbivores, which in turn control primary producers

Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control Next slide: biomanipulation is used to fix the effects of pollution

Polluted State Restored State Fish Abundant Rare Zooplankton Rare Fig. 54-UN1 Polluted State Restored State Fish Abundant Rare Zooplankton Rare Abundant Algae Abundant Rare

54.3: Disturbance influences species diversity and composition Decades ago, most ecologists favored the view that communities are in a state of equilibrium Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

Characterizing Disturbance A disturbance is an event that changes a community, removes organisms from it, and alters resource availability Fire is a significant disturbance in most terrestrial ecosystems It is often a necessity in some communities

The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance High levels of disturbance exclude many slow-growing species Low levels of disturbance allow dominant species to exclude less competitive species

Example (FYI) The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

Fig. 54-21 (a) Soon after fire (b) One year after fire

Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance Primary succession occurs where no soil exists when succession begins Secondary succession begins in an area where soil remains after a disturbance

How does succession work? Early arrivals may facilitate appearance of later species by making the environment favorable Early may inhibit establishment of later species Early may tolerate later species but have no impact on their establishment

Example following a retreating glacier Why glacier??

1941 1907 Dryas stage Pioneer stage, with fireweed dominant 1860 Fig. 54-22-4 1941 1907 2 Dryas stage 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Alaska 1760 4 Spruce stage 3 Alder stage

Human Disturbance Humans have the greatest impact on biological communities worldwide Human disturbance to communities usually reduces species diversity Humans also prevent some naturally occurring disturbances, which can be important to community structure

Before…. Fig. 54-24a

…after

54.4: Biogeographic factors affect community biodiversity Latitude and area are two key factors that affect a community’s species diversity Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics (perhaps b/c OLDER species are found in those areas… meaning what??)