Community Dynamics

What is a Community? Ecological communities are composed of populations which share a defined area and interact. Community ecology examines how species interactions influence community structure and function.

Communities are Dynamic Community structure is not static, but changes over time as a result of things like the arrival and loss of species, and the effects of outside forces such as fires and floods.  Dynamic – constantly changing

Disturbance & Succession Disturbance and succession are two common causes of change in community structure.

Disturbance Disturbance is "any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.” This broad definition covers nearly anything that creates open space, including hurricanes, volcanoes, floods, footprints, and last but not least, fire. Disturbances result from forces originating outside the community.

Succession Succession is the repeatable change in community composition through time following a disturbance.

Fire in Yellowstone Fire is a natural occurrence in the Yellowstone ecosystem. Many, if not all, of the species in these communities are fire-adapted and slowly die out if fire is excluded from the landscape.

Succession in Yellowstone After 1988 fires, studies of succession past & present. In 1988, most trees were the same age – regrowth after a big fire in the 1700s. Regrowth follows a pattern - succession 1988

Succession in Yellowstone 1991

Succession in Yellowstone 1992

Succession in Yellowstone 1997

Succession in Yellowstone Lodgepole pines require fire to reproduce. Cones don’t open to release seeds until exposed to a fire. Burned areas soon have lodgepole seedlings sprouting. Annuals common early on, shrubs & trees take longer to grow, but eventually the pines are dominant once more.

Processes Driving Succession Self-thinning – lots of seedling lodgepole pines begin growing, naturally thin out as the trees grow.

Processes Driving Succession Life history trade-offs Early succession traits Roots or seeds that survive or even require fire. Light seeds float on the wind into burned area. Late succession traits Slow growers May require conditions created by early successional species.

Life History Traits The differences in physiology and behavior that make each species well suited to grow in particular environments are called the life history traits of the species.

Life History Traits There are often trade-offs involved with life history traits. For instance, it may not be possible to have a physiology that allows rapid growth in both low and high-light environments.

Life History Traits Species that do well in early succession often have different life history traits than those that appear in late succession.

Types of Succession Primary succession – Starting from bare rock. After volcano erupts After a glacier recedes Secondary succession – Growth has been removed, but soil & seed bank is intact. Abandoned agricultural field Fires

Three Models for Succession Facilitation Inhibition Tolerance

Facilitation Barren ground is uninhabitable by all but the most stress-tolerant of colonists (small, yellow flower). Over time, early stress-tolerant colonists make the environment more suitable for successive species (such as the purple flower) by increasing nutrient availability, developing soils, reducing pH, or providing shade from the sun and shelter from the wind. This sequence continues until the most competitively dominant species (here the tall orange flower) no longer facilitate the invasion and growth of any other species.

Inhibition All species arriving on an unoccupied site can survive. Thus, the initial community composition is simply a function of who gets there first. Once a colonist becomes established, it inhibits growth of subsequent arrivals by monopolizing space and/or other resources. Only when space and/or resources are released through the death or decay of dominant residents can new colonists invade and grow. Because short-lived early species die more frequently, succession slowly progresses from short-lived to long-lived species.

Tolerance All species arriving on an unoccupied site can survive. Thus, the initial community composition is simply a function of who gets there first. Species that appear later simply arrived later or arrived early but grew more slowly. Late arriving species tolerate the presence of early species and grow despite the presence of early- successional species because they are better competitors for light and nutrients. Over time, late-successional species exclude other species. Early-successional species have no effect on late-successional species.

Climax Community If there are no more disturbances, the community will eventually reach a stable state – the climax community. An equilibrium state Something almost always provides a disturbance – keeps the community out of equilibrium.

Fire Creates Habitat Diversity Smaller burned patches were colonized more quickly and tended to have greater forb, grass, and total cover then more severely burned sites. These results indicate that rare years characterized by widespread fires of different sizes and intensities create much of the habitat diversity in the Park.

The intermediate disturbance hypothesis predicts that the greatest diversity in the community will be found with intermediate levels of disturbance.

Food Chains & Indirect Effects

Food Chains Species can be grouped according to where in the community they obtain their food, and groups can be linked together to form a food chain, where each link comprises a different trophic level. 

Food Chains Primary Producers – Plants & algae that make food through photosynthesis. Herbivores – Feed on primary producers Carnivores (predators) – Feed on herbivores. Omnivores – Feed on plants & animals.

Yellowstone Food Chain Aspen Elk Wolves Wolves removed Elk hunted, but then allowed to rebound Reintroduce wolves to keep elk populations in balance Controversy

Aspens Quaking aspens are the primary hardwood in forests around Yellowstone. They have been declining. Why? Fire suppression – fires may provide ideal conditions for aspens to sprout. Elk herbivory – too many elk may feed on saplings before they have a chance to get established.

Top-Down Control Elk controlling density of aspen is an example of top-down control. Population is controlled by the trophic level above.

Top-Down Control Marine example: Sea urchins feed on kelp Adding urchins reduces kelp density Removing urchins increases kelp density

Trophic Cascade Adding a third link in the food chain: Otters feed on urchins Urchin population declines Kelp increases

Trophic Cascade Adding a fourth link in the food chain: Orcas feeding on sea otters Otters decline Urchins increase Kelp decline

Behavioral Cascade In a traditional trophic cascade, predators actually reduce the population of herbivores. Wolves reduce the elk population somewhat, but mostly they change elk behavior. Hide & feed in pine forest rather than being in open feeding on aspen.

Predation Risk Prey species, like elk or gerbils, will forage more in areas where they feel safe from predation.

Ecosystem Engineers Animals at different trophic levels effect community structure through feeding. Ecosystem engineers affect the ecosystem itself – indirect effects on community structure.

Ecosystem Engineers In order for an organism to be considered an ecosystem engineer, it must alter the availability of environmental resources. Corals are considered autogenic ecosystem engineers because their physical structure alters ocean currents and siltation rates, creating conditions that many reef communities require.

Ecosystem Engineers Ecosystem engineers can also alter the environment by directly redistributing or transforming living and non-living materials.

Beavers & Willow - Yellowstone In Yellowstone, elk herbivory also reduced willow density. Beavers disappeared. Would returning beavers be beneficial?

Top-Down vs Bottom-Up Control Some ecologists have suggested that communities are not so much structured by consumers as by resource availability. Support for this suggestion comes from farms where additions of nutrients like nitrogen and phosphorus have successfully increased crop yields. Ecologists taking a bottom-up perspective hypothesize that organisms on each trophic level are resource limited.

Bottom-Up Control When leaf litter in the stream was limited, primary consumers and predators declined.

Food Chain Length What limits the size of food chains? Productivity- more producers = more links Ecosystem size – bigger lake = more links Productive-space – both aspects important.

Community Stability

Resistance How resistant is the community to disturbance? How big of a change did the disturbance cause? The term resistance is used to describe how much the community changes due to a particular disturbance. More resistant communities will change less.  Disturbance specific

Return Time How quickly did the community recover from the change? Return time is the amount of time it takes for the community to stop changing (reach an equilibrium) after the disturbance. 

Resilience How closely did the post-recovery community resemble the pre-disturbance community?

Response to Disturbance Communities differ in their response to disturbance in each of the three ways you saw in the simulation. The response can also change depending on what the disturbance is. For instance, the size of a disturbance affects recovery time. The larger the disturbance, the longer (in general) you expect recovery to take.

Persistence The overall degree to which a community stays the same over time, especially (but not exclusively) after disturbances, is called the community's persistence.

Alternative Stable States Different potential communities in the same place are called alternative stable states and occur when more than one type of community can exist in a particular environment.

Why are some communities more stable than others? The diversity-stability hypothesis argues that species-rich communities are more stable.

Why are some communities more stable than others? Food web theory suggests that communities with higher  connectance(i. e., with many species interacting trophically) should be more stable.

Keystone Species Keystone species, which have a larger impact on community stability than would be expected based on relative abundance.

Dominant Species Dominant species, which have a large impact due to their high abundance.