Chapter 53 Community Ecology

What is a Community? A biological community is an group of populations living close enough for interaction Animals and plants surrounding a watering hole in southern Africa are members of a savanna community

Figure 53-01

Interactions Relationships between species in a community = interspecific interactions Interspecific interactions affect species survival and reproduction Examples: competition, predation, herbivory, symbiosis = (parasitism, mutualism, and commensalism), and disease

Table 53-1

Competition Interspecific competition occurs when species compete for a resource that is in short supply Strong competition can lead to competitive exclusion (one killing the other) or local elimination of a competing species

Competitive Exclusion Principle = Gause’s Law States that two species competing for the same limiting resources cannot coexist in the same place –The strong one will overpower the weaker one and eventually will die

Ecological Niche An organisms Niche = its Ecological Role = It’s NICHE =“job or profession” Similar species can coexist in a community if there are differences in their niches Competition can make a species’: –fundamental niche (no interference from other organisms) differ from its –realized niche (narrower – interference from other populations)

LE 53-2 Chthamalus fundamental niche High tide Low tide Ocean Chthamalus realized niche High tide Low tide Ocean Balanus realized niche Chthamalus Balanus

Resource partitioning Resource partitioning = using different spaces in an ecosystem –enabling similar species to coexist in a community –… think of this as the “Ghost of Competition Past”

LE 53-3 A. insolitus usually perches on shady branches. A. ricordii A. insolitus A. christophei A. cybotes A. etheridgei A. aliniger A. distichus perches on fence posts and other sunny surfaces.

Character displacement This is a trend for traits to be more diverse in sympatric (geographically overlapping) populations of two species than in allopatric (geographically separate) populations of the same two species An example is variation in beak size between populations of two species of Galapagos finches

LE 53-4 Beak depth Sympatric populations G. fuliginosa G. fortis Santa María, San Cristóbal Los Hermanos Daphne G. fuliginosa, allopatric G. fortis, allopatric Beak depth (mm) Percentage of individuals in each size class

Predation An interaction where one species, the predator, kills and eats the other = the prey Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

“On Defense” Prey display various defensive adaptations Behavioral defenses include hiding, fleeing, self-defense, and alarm calls Cryptic coloration = (camouflage), makes prey difficult to spot

Figure Canyon Tree Frog

“Offensive Trick Play” Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration Predators are particularly cautious in dealing with prey that display such coloration

Figure Poison Arrow Frog

Mimicry In some cases, a prey species may gain protection by mimicking the appearance of another species In Batesian mimicry, an edible or harmless species mimics an unpalatable or harmful model

LE 53-7 Hawkmoth larva Green parrot snake

Mimicry #2 In Müllerian mimicry, two or more unpalatable species resemble each other

LE 53-8 Cuckoo bee Yellow jacket

Herbivory Refers to an interaction in which an herbivore eats parts of a plant. It has led to evolution of plant defenses –Thorns, poisons, toxins, spines

Parasites = one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process Diseases are similar = bacteria, viruses, or protists

Symbiosis Mutualism (+/+) : is an interspecific interaction that benefits both species

Figure 53-09

Commensalism One species benefits and the other is unaffected Commensal interactions are hard to describe in nature – because one interaction often affects the other – even if not noticable.

Figure 53-10

Interspecific Interactions / Coevolution? Coevolution is reciprocal evolutionary adaptations of two interacting species Examples: Flowering plants & Insects

Keystone Species A species that exerts strong control on that community’s structure Two fundamental features of community structure are species diversity and feeding relationships

Diversity / Richness / Abundance Species diversity of a community is the variety of organisms that make up the community It has two components: species richness and relative abundance Species richness is the total number of different species in the community Relative abundance is the proportion each species represents in the community

LE Community 1 A B C D A: 25% B: 25% C: 25% D: 25% Community 2 A: 80% B: 5% C: 5% D: 10%

Trophic Levels Trophic structure is the feeding relationships between organisms in a community Food chains link trophic levels from producers to top carnivores It is a key factor in community dynamics

LE Quaternary consumers Tertiary consumers Carnivore Secondary consumers Carnivore Primary consumers ZooplanktonHerbivore Primary producers Phytoplankton Plant A terrestrial food chain A marine food chain

Food Web A food web is a branching food chain with complex trophic interactions

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

Food Webs – Simplified Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community

LE Zooplankton Fish larvae Fish eggs Sea nettle Juvenile striped bass

Keystone Species In contrast to dominant species, keystone species are not necessarily abundant in a community They exert strong control on a community by their ecological roles, or niches Field studies of sea stars exhibit their role as a keystone species in inter-tidal communities

LE Without Pisaster (experimental) With Pisaster (control) 1963 ’64’65 ’66 ’67 ’68’69 ’70 ’71 ’72 ’ Number of species present

Observation of sea otter populations and their predation shows how otters affect ocean communities

LE Sea otter abundance Otter number (% max. count) Sea urchin biomass Grams per 0.25 m Total kelp density Number per 0.25 m Year Food chain before killer whale involvement in chain Food chain after killer whales started preying on otters

Ecosystem “Engineers” ( Foundation Species ) Some organisms exert influence by causing physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale

Figure 53-18

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers

Top-down model Proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers Long-term experimental studies have shown that communities can shift periodically from bottom-up to top-down controls

Pollution – Bioremediation Pollution can affect community dynamics Bio-manipulation can help restore polluted communities

Disturbance – Good or Bad? Decades ago, most ecologists thought that communities were in a state of equilibrium Recent evidence leans towards a non- equilibrium model, where communities are constantly changing after disturbances

LE Before a controlled burn. A prairie that has not burned for several years has a high propor- tion of detritus (dead grass). During the burn. The detritus serves as fuel for fires. After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

Intermediate disturbance hypothesis Suggests that moderate levels of disturbance can yield higher diversity than low levels of disturbance The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

LE Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. One year after fire. This photo of the same general area taken the following year indicates how rapidly the com- munity began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

Humans – the bad guys? Humans are the most widespread agents of disturbance Human disturbance to communities usually reduces species diversity Humans also prevent some naturally occurring disturbances, which can be important to community structure

Succession / After Disturbance Early-arriving and later-arriving species may be linked by one of three processes: –Early arrivals may help later species by making the environment favorable –They may also make it harder for later species –They may tolerate later species but have no impact on their establishment

Ecological Succession Succession = A disturbed area is colonized by a species, which are gradually replaced by another species, and so on, and so on… –Primary Succession : A lifeless area: (example right after volcano =no soil) –Secondary Succession : Soil still in place (example: after clearing / or forest fire)

RSJDshttps:// RSJDs

LE Pioneer stage, with fireweed dominant Dryas stage Spruce stage Nitrogen fixation by Dryas and alder increases the soil nitrogen content. Successional stage DryasPioneer Alder Spruce Soil nitrogen (g/m 2 )

Age Structure One important demographic factor in present and future growth trends is a country’s age structure Age structure is the relative number of individuals at each age It is commonly represented in pyramids

LE Rapid growth Afghanistan Age Male Percent of population Female –49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0– –84 75–79 70–74 65–69 60–64 55–59 50–54 Slow growth United States Age Male Percent of population Female –49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0– –84 75–79 70–74 65–69 60–64 55–59 50–54 8 Decrease Italy Male Percent of population Female