Biological Communities

Reading: Smith and Smith, Chapters 21-23

• Biological community- a group of populations of different species that exist in the same place and are, at least in principle, capable of interacting with each other.– The true nature of biological communities is a

subject of intense debate.

Are communities collections of interdependent components, like a wristwatch?

Or, are they ad-hoc assemblages of species that just happen to be able to live together?

• Descriptive Properties of a community

• scale

• species diversity

• species richness

size of community-boundaries are sometimes arbitrary

diversity of species in a community

number of species in a community

• Functional properties-these imply interdependence and some degree of functionality

• trophic structure

• habitat structure– spatial and temporal structure

• stability

• disturbance and succession

resistance of a community to change due to extinction or disturbance

Pattern of energy flow based on who eats whom

Every community has some degree of disturbance, and most are part of a series of communities that replace one another due to the process of ecological succession

A saltwaterfood webfrom SanFranciscoBay, CA

• Biological Communities are frequently nested within one another

Example-pitcher plants, sp. are members of bog communities.

• In practice, communities tend to be defined by their most conspicuous members.– Ecotone-the boundary between biological

communities

This is an ecotonebetween Californiacoastal forest and wetland at the Elkhorn Slough

– Example-dragonflies inhabit freshwater ponds as larvae, emerge and prey upon insects in neighboring forest, wetland, and grassland communities

Some species belong to more than one biological community.

Example-warblers migrate seasonally, moving from temperate forests and grasslands to subtropical forests

• Each of the properties of a community affects the others, for example, a change in species richness will affect habitat structure, which will affect stability and trophic structure

• Example-eliminating disturbance due to fire causes prairie communities to be gradually replaced by woodland

Keystone species-single species that exert a major influence on composition, and function of a biological community

• Example-California sea otters– are predators of marine fish

and invertebrates. – are one of the few important

predators of sea urchins. – Their range formerly

extended along the Pacific rim, from Japan to Baja California

• in Coastal California, sea urchins were hunted nearly to extinction in the eighteenth and nineteenth century. – sea urchins graze rocks of encrusting algae, preventing the

establishment of new kelp plants, and a gradual destruction of the Kelp forests-and the enormous diversity of life forms that live in kelp

– the disappearance of the sea otter is the likely cause of the decline in Kelp forests that occurred during the 20th century-more sea urchins=less kelp.

– With the recovery of the California Sea Otter-densities of sea urchins have dropped (also commercial abalone, however), and kelp communities have stabilized and started to return.

Strongylocentrotus purpuratus

Pacific Kelp,macrocystis sp. forms dense forests underwater

• Introduction of an exotic species can completely change a biological community as well.– Example-Buckthorn; Introduced buckthorn Rhamnus cathartica is a big problem in the midwest.

Originally an ornamental, this species is an incredibly efficient competitor for light, spreading through the forest understory and shading out the other forest-floor species.

– If the trees succumb to disease or old age, a thicket forms, which resists invasion by younger trees.– The result is a much simpler ecosystem, with fewer producers, and the disappearance of specialized

herbivores, pollinators, and mutualists of the trees and plants replaced by buckthorn

In communities dominated by buckthorn, the most conspicuous major food source for birds is an annoying, laxative berry. This spreads buckthorn to other sites

Are Biological Communities Real?

• Some ecologists view communities as ad-hoc assemblages of species that just happen to occur together and be compatible with one another. – This view was promoted by the 20th century botanist, H.A.

Gleason• Other ecologists view communities as interdependent assemblages

of species that actively facilitate and promote each other’s existence.– This view was promoted by the ecologist, F.E. Clements.

• In fact, there may be a continuum between open and closed communities in nature.

• In nature, “closed communities” tend to occur in special environments with tightly defined boundaries.– This leaves it an open question as to whether the species in

these environments are together because they are truly interdependent with specific members of that community, or because their physical tolerances are so specific they just happen to be the only species that can live there.

• They also occur under circumstances where one organism is essential for the survival of every other species-so the physical tolerances of this species determine the distribution of the entire community

Some “closed” communities• Geothermal hot springs-

– organisms that can tolerate high temperature are called thermophiles or hypothermophiles

– may include photosynthetic green and purple sulfur bacteria, bacteria and archaeans that act as chemoautotrophs and others that act as decomposers.

• Most of species are probably unknown to science and cannot be cultured in a lab.

• Majority of species cannot live anywhere else.

– this is a truly ancient assemblage of organisms, the first communities may have resembled these

The archaean, Sulfolobus sp. gains energy by oxidizing sulfur granules around hot springs, and thus lower the Ph.

These communities are often zoned by temperature, with green and red algae growing at the cool edges, cyanobacteria in the middle, and hyperthermophile prokaryotes near the center.

• Cave communities-– includes specialized prokaryotes that grow under low

energy conditions with constant, low temperature and special Ph. Also may include blind cave fish (catfish, tetras, gobies), cave crickets and other specialized arthropods.

– not completely insulated from outside-dependent upon energy and nutrients from outside-for instance, bats have the potential to bring enormous amounts of energy and nurtrients into a cave from outside, and harbor many parasites.

• In other cases, replacement of one species for another occurs along a gradient or continuum. As conditions change, the assemblage of species changes.– Even in the most “open” community, however,

species assume roles -decomposers, producers, herbivores, pollinators, parasites, etc. and depend upon other species for nutrients, food, habitats, and other things needed for survival.

• Gradient Analysis suggests that most major terrestrial plant communities are “open”– open communities might be the rule rather than

the exception

Predators Influence the Diversity of Lower Trophic Levels

• Predators may exert a major influence food web organization.– Paine compared the food webs of rocky intertidal

communities in Washington and the Gulf of California.– In Washington, the sea star Pisaster dominates.– In the Gulf of California, the sea star Heliaster

dominates.– In the Gulf of California, other predators were abundant,

including snails, crabs, and fish.– Herbivore and producer trophic levels were also more

diverse, despite the lower primary productivity there.• These findings motivated his removal experiments.

Removal experiments• Paine is known for two removal experiments that

attempted to elucidate the influence of particular species on the organization of communities. – Both were conducted on rocky intertidal communities in coastal

Washington• Removal of the predatory sea star, Pisaster, vs. unmanipulated control.

– Result; Pisaster removal plots became dominated by mussels, with a decline in the number of other species.

• Removal of “herbivorous” sea urchin, Strongylocentrotus vs. unmanipulated controls– Result, sea urchin removal plots became dominated by a small

number of competitively superior algae, with a decline in the diversity of producers.

Productivity Affects the Diversity and Organization of Communities

• There is a rough correlation between the productivity of communities, and their diversity-

• I.e., tropical rainforests are more diverse for most groups, and more productive than temperate forests, which are in turn more diverse for most groups, and productive than taiga.

– In general, more productive communities are more diverse, but this is not always the case.

• I.e., for many groups, the relatively unproductive desert scrub community is more diverse than the enormously productive salt marsh community

– Does productivity affect diversity?• -experiments where the productivity of an existing ecosystem is boosted

by adding nutrients usually lead to a decrease in diversity

“Top Down vs. Bottom Up”• “Bottom up” control of communities implies that the

diversity and abundance of organisms at each trophic level is determined by the amount of energy coming up from below.

• “Top down” control implies that the diversity and abundance of organisms at each trophic level is determined by the presence or absence of predation from higher trophic levels.– Consumers depress biomass at the level immediately below

themselves, indirectly increasing the biomass at the level below that.

• Real communities show evidence of both.

– In a well known study of ponds by Matthew Leibold, it was demonstrated that the biomass of herbivores (zooplankton) was positively correlated to the biomass of producers (algae), indicating a top down effect.

– He intentionally introduced fish to some ponds, The result was a decrease in zooplankton and increase in producers, indicating a top down effect.

Assembly Rules• A big, unanswered question in community

ecology concerns how species become assembled into biological communities.– Obviously, only some sequences may occur-for

instance, consumers will not persist if they arrive before producers, predators will not persist if they arrive before prey.

– Removal experiments, and laboratory experiments with microcosms suggest that such rules exist, though ecologists have not worked out the details.