Biology

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Community Ecology I Stability, Resilience

WFC 10 – D. A. Kelt

A biological community is defined by the species that occupy a particular locality and the interactions among those species.

A Primer of Conservation Biology, 3rd ed. R. B. Primack 2004

Community Ecology is the study of biological communities. In what ways are communities organized, structured, predictable? In what ways are they not?

Note the difference between “habitat” and “community.” The former refers to a physical location, whereas the latter refers to constituent species.

Many communities may appear very similar.

Coniferous Forest near Mt. Rainier

central Oregon

King’s Canyon National Park

Sandy Desert

Sahara Desert

Simpson Desert (Australia)

Death Valley, California

Thus, there may be great variation from point to point in these communities

One major way in which they differ is in composition – the particular species that occur at a site.

Example: Burrowing mammals

N. Amer. - Gopher

Asia - Zokor

Australia – Marsupial mole S. Amer. – Tuco tuco

Africa – Mole rat

Ecologically similar species in different regions with different evolutionary origins.

N. Amer. - Gopher

Asia - Zokor

Australia – Marsupial mole S. Amer. – Tuco tuco

Africa – Mole rat

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Often true at smaller spatial scales as well . . .

Geomys Eastern Pocket Gophers

Cratogeomys Yellow-faced Pocket Gophers

Pappogeomys Southern Pocket Gophers

Thomomys Western Pocket Gophers

4 genera of North American pocket gophers

From a conservation perspective we are interested in how stable a community is in the face of anthropogenic abuses.

Stability – often portrayed in simple cartoon fashion as follows:

So, given all this variation, how are communities structured, and how do they respond to disturbance?

Global Stability Local Stability

Stability may be measured by a community’s fluctuation over time.

Communities often remain stable over time.

However, they may be perturbed by some external force.

What happens then?

The American chestnut (Castanea dentata) made up >40% of trees in mature eastern deciduous forest.

Chestnut blight – introduced to New York City in ca. 1900

By 1950 only 1 remaining large tree in North America

What impact did this enormous loss have on the biota of eastern North America?

Perhaps surprisingly, essentially no impact.

Eastern deciduous forests are very diverse – maples, oaks, hickories, catalpa, etc. Loss of American chestnut led to NO major changes in animal or plant communities.

Black bears may have suffered from loss of mast.

Thus, this was a relatively minor perturbation from the perspective of the community – it evidently shifted to a different local stable point.

Seven butterfly/moth species were specialists on American chestnut, and have gone extinct.

Another 49 Lepidopterans simply shifted their hosts.

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Pollution – another perturbation that can result in ecological deterioration on large scales.

Are such trends reversible?

Not always, but sometimes yes.

Puget Sound

Lake Washington

Lake Washington – ca. 18 mi long, 3 mi wide; requires about 2.3 yr to flush.

Greater Seattle Region

Portage Bay

UW (Go Huskies)

I-5

Lake Washington

Lake Washington – ca. 18 mi long, 3 mi wide; requires about 2.3 yr to flush

In early phases of urban development, Lake Washington was used for disposal of raw sewage.

This stopped between 1926-1936 when treatment plants were built.

However, as the population grew, more sewage treatment plants were built between 1941-1959, and released more sewage to Lake Washington.

What is the problem? What is in sewage?

Ecologically speaking – nitrogen and especially phosphorus.

Consequences of this “perturbation”?

Phosphorus input over time (mostly in effluent)

Phosphorus in lake water

Response of algae

Transparency of water

By 1955, problems were becoming obvious Citizens began diverting sewage (to Puget Sound!)

By 1967, most sewage being diverted

Effects of diversion?

With sewage addition to lake, sedimentation tripled!

Phosphorus input (metric tons/yr)

Phosphorus content (micrograms/liter)

Algal density (mm3/yr)

Water transparency

Daphnia populations rebounded in ca. 1975. Small crustaceans, eat algae, keeping water clear.

During “polluted phase”, however, a dominant alga (Oscillatoria) fouled the feeding apparatus of Daphnia.

Since 1976, water has remained clear because Daphnia keep algal levels low

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Take home message from this?

1. Ecological systems can bounce back from perturbation.

2. Certain elements – in this case phosphorus – can have strong influences on ecosystems (especially aquatic ones!)

Where do we find much P?

Laundry detergents – led to strong push in 1960s for phosphate-free detergents.

View from above Lake 226 in August 1973. The two basins of this lake were separated by a plastic curtain. The lower basin received additions of carbon, nitrogen and phosphorus; the upper basin received carbon and nitrogen only. The bright green color results from bluegreen algae (Cyanobacteria), which are growing on phosphorus added to the near side of the curtain.

Aerial view of Lake 227 (foreground) in 1994. Note the bright green color caused by algae stimulated by the experimental addition of phosphorus for the 26th consecutive year. Lake 305 in the background is unfertilized. (photo by Karen Scott)

http://www.umanitoba.ca/institutes/fisheries/eutro.html

+C +N

+C +N +P

+P

Some communities require perturbation

Chaparral

Monterrey pine Cones open only after fire

Chamise re-sprouting

Consider the Scottish moors . . .

Might a system shift to an alternative stable state or configuration?

What if the perturbation is extreme?

Calluna vulgaris (heather)

Today, heather dominates the moors. Originally found principally in open parts of pine, birch, and oak woodlands on acid soils.

Calluna vulgaris (heather)

• Periodic fires set to maintain habitat for sheep.

• Combination of grazing and burning reduced survival of tree seedlings . . .

• Now, after many hundreds of years of this, there are virtually no seeds left.

• Even if grazing were to stop, no trees would grow here.

Heather required disturbance that provided clear areas.

• Significant forest destruction began in Scotland about 3000 B.C. (human needs for fire, housing, agriculture).

• Simultaneously, climate cooled and moistened.

• Grazing animals (esp. sheep) introduced to open areas – sheep stocks increased greatly b/t 1700s-1800s.

A new stable state

Another example – kangaroos Numbers of larger kangaroo species have increased since British colonization.

In spite of intensive hunting as pests and for food and leather.

Evidently, ranchers have improved habitat of kangaroos, in 3 ways:

1. Increased water for sheep (‘roos drink too).

2. Clearing of timber, production of grasslands.

3. Removal of a major predator – the dingo.

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3. Removal of a major predator – the dingo

SOUTH AUSTRALIA

WESTERN AUSTRALIA

NORTHERN TERRITORY

QUEENSLAND

VICTORIA

NEW SOUTH WALES

The dingo fence

One way to evaluate the impact of grazing is to remove grazers, and follow the response of the plant community.

Grazing animals can have substantial impacts on ecosystems

European rabbits – in Britain for >1000 yrs, agricultural pests for >100 yrs.

In 1954, a virus (myxomatosis) decimated rabbit populations.

By 1960, rabbits began developing resistance (natural selection), and populations increased.

In response to low rabbit numbers: • Grasses increased in height. • A spectacular increase in flowers – even orchids previously known from only a few leaves! • Woody plants also increased – brambles, heather, tree seedlings.

A rabbit’s-eye view of the world: Preferred edible

• decline under grazing. Not-preferred but edible

• likely decline under grazing. Inedible

• increase under grazing.

Another example of the impact of rabbits is provided by a fenced rabbit exclosure, put in place in 1946, and censused for plant diversity through 1960.

European rabbit

H = Hieracium

= Festuca

Fencing installed

Hieracium pilosella

Festuca ovina

In the absence of grazing, 2 plants are especially competitive.

Sheep’s fescue grass (Festuca ovina)

Hawkweed (Hieracium pilosella)

Fescue dominated under grazing pressure.

Hawkweed increased upon rabbit removal.

Note that this plant became senescent and was beginning to be replaced again by the grass in 1960. Is this a long term cyclic relationship?

A final example of how perturbations alter communities

- Lake Erie -

Detroit

Buffalo

Cleveland

Historically (1800) bordered by large stands of forest and extensive marshes.

Chicago

Limited soil erosion, clean runoff water, healthy aquatic vegetation.

By 1870 this was an agricultural region.

By 1910, nearly all marshes and swamps had been drained.

Woodlands cleared, swamplands drained.

Silt runoff covered spawning beds.

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Photo: Rutherford B. Hayes Presidential Center (Fremont, Ohio)

Back to 1800 – Lake Erie hosted a great variety of fish.

By shore’s edge: Largemouth Bass Smallmouth bass Muskellunge Northern pike Channel catfish

In open lake: Lake herring Blue pike Lake whitefish Lake sturgeon Walleye Sauger Freshwater drum White bass

At eastern end of lake, Lake trout

Back to 1800 – Lake Erie hosted a great variety of fish.

By shore’s edge: Largemouth bass Smallmouth bass Muskellunge Northern pike Channel catfish

Back to 1800 – Lake Erie hosted a great variety of fish.

In open lake: Lake herring Blue pike Lake whitefish Lake sturgeon Walleye Sauger Freshwater drum White bass

Back to 1800 – Lake Erie hosted a great variety of fish.

At eastern end of lake, Lake trout

Today, Blue pike, Sauger,

Lake trout . . . . . . are gone

Greatly reduced populations of sturgeon, lake herring, whitefish, muskellunge

Present fish community dominated by: yellow perch

white bass

channel cat freshwater drum

carp

goldfish

rainbow smelt

As a general pattern, commercially valuable species have been replaced by less valuable species

Introduced

Introduced

Introduced

Introduced

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The commercial catch from Lake Erie over past 150 yr has exceeded that of other four Great Lakes combined!

However, the fishery has been a major stress on the community.

Commercial catch began after the War of 1812 and developed rapidly.

Through 1850 the catch grew by about 20% per year.

First casualty: Lake sturgeon • large (to over 80 kg (=170lb)) • covered with heavy scales • tore nets • fishermen got heavier nets,

caught sturgeon in large numbers, stacked them like cordwood, and burned them with oil

In 1860s an immigrant arrived with knowledge of how to smoke sturgeon and make caviar from its eggs.

By 1870 it was rare from over-fishing.

Fishermen turned to Lake trout in 1880s . . .

Fishermen turned to Lake trout in 1880s . . .

and then Lake whitefish in 1890s.

In late 1800s, concern about dwindling stocks grew.

Two solutions advocated.

1. regulate stringently 2. use a new technique from

France – fish hatcheries

Between 1867-1920 about 18 hatcheries were constructed. All but one have been closed now; virtually no fry survived.

When all else fails . . .

3. Stock new species.

Most introductions failed; a few succeeded.

Rainbow smelt – 1931

Smelt live in open waters and are eaten by lake trout, whitefish, blue pike, sauger.

Initially great, but . . . Over-fishing of these species allowed increased numbers and greater survival of smelt.

Older smelt feed upon young fry of lake trout, whitefish, blue pike, and sauger.

Thus, desirable species have thus been driven even further down.

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Another factor in Lake Erie is pollution.

Sewage and industrial wastes feed blue-green algae now dominate the plankton.

 die and fall to the bottom to decompose.

 decomposition uses oxygen . . . dead zones.

The western basin is shallow and has very valuable spawning grounds

Detroit

and Toledo

It also has . . .

Central and eastern basins are deeper, cooler. summer sanctuaries for important cold-water

fishes – herring, whitefish, blue pike.

Increased pollution means that even these basins are becoming more “productive,” and less oxygenated.

Prognosis? -- partial recovery is possible if we drastically reduce our discharges to the lake.

However, the presence of smelt will preclude recovery of some species, and further introductions – Pacific salmon (to please sport fishermen) – further complicate issues. Recovery rests directly on the need for continuous oxygen-rich bottom waters in the central basin.

Prognosis? -- partial recovery is possible if we drastically reduce discharges to the lake.

However, the presence of smelt will preclude recovery of some species, and further introductions – Pacific salmon (to please sport fishermen) – further complicate issues.

Recovery rests directly on the need for continuous oxygen-rich bottom waters in the central basin.

Current status?

In fact, partial recovery has occurred.

US and Canada have invested >$7.2 billion since 1972 to treat wastewater.

Reduced P discharge from 15,260 tons (1972) to 2449 tons (1985).

Reintroductions of native predatory fishes has also helped.

Total phosphorus discharge from Detroit Wastewater Trt. Plant (1966-2003).

A 2010 report by Healing Our Waters – Great Lakes Coalition* refers to a “sewage crisis” in the Great Lakes. Five cities dumped 41 billion gallons of untreated sewage and dirty storm water into the Lakes in 2009.

Estimate that the Buffalo Sewage Authority, as one example, will need to spend up to $500M to eliminate sewage overflows.

Heart of the problem – an old system that combines sewage and storm runoff in common pipes; high storm runoff leads to overflows.

Is this reparable? Heck yes – it just takes resolve!

* http://www.buffalonews.com/incoming/article96926.ece/BINARY/sewagereport.pdf

Are there any attributes of communities that allow them to resist such perturbations?

Complexity begets Stability

Resilience – a measure of the ability of a system to persist in the presence of perturbations (e.g., weather, physical and chemical factors, other organisms, human activities).

Data slowly accumulating . . .

Study sites with more species were more resistant to drought (measured as biomass after vs. before a major drought).

Recent work shows that plots with MORE SPECIES not only had MORE BIOMASS, but that the effect of more species greatly exceed the effect of water or fertilization!