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Transcript
Population dynamics: eruptive and logistic growth The main difference between J and S is the pattern of “adjustment” to system carrying capacity eruptive (malthusian) Eruptive (fastest) Logistic (slowest) # logistic time 1 Fates of eruptive “crashes” 2 # 3 1 time Fig 3-21 (I&II)/3-22 (III&VI) 1. Ecosystem not seriously impacted: population recovers and repeats the J-crash sequence 2. Post-crash, population comes into a lower Stype equilibrium 3. Ecosystem seriously impacted by the overshoot: population is extirpated or persists only at a very low level 2 Take-home message: populations are never static! In the language of KR#8 n “overshoot” = biotic potential n “dieback” = environmental resistance Each population in the system may be a different place in the population cycle, while the carrying capacities of the system for different species may also vary! 3 Are there differences between populations that grow eruptively and those that grow logistically? Eruptively growing populations (more J-curvish) tend to be: • Generalists with broader environmental niches • Do well in unstable/rapidly fluctuating environments • Tend to be short lived species that reach sexual maturity early in life • Produce numerous offspring • Provide little parental care • Populations predominately regulated by extrinsic or (density independent) factors 4 Logistically growing populations (more S-curvish) tend to be: • Specialists with narrower environmental niches • Do well in stable environments, rarely tolerate rapidly fluctuating abiotic situations • Have long life spans reaching sexual maturity relatively later in life • Produce few offspring • Provide extensive parental care • Populations primarily regulated by intrinsic or density dependent factors 5 While the shape of the curve differs, growth of all populations follows the same “rules” r dN/dt =rN(1-N/K) r= intrinsic rate of increase K = carrying capacity 6 From the basic model of population growth, ecologists have taken the labels r and K for these 2 patterns or “life history strategies” n n Eruptive (generalist or density independent or r-selected) strategies work for species that can tolerate a broad range of environmental conditions, move quickly into disturbed environments, grow rapidly, mature early, produce numerous young to which they provide little parental care. Logistic (specialist or density dependent or Kselected) strategies work for species adapted to specialized, stable environmental conditions, grow slowly, are late maturing, have few off-spring for which they provide extensive parental care. 7 Why are there two “strategies” for population regulation, r and K, or why do some species opt for the eruptive pattern while others follow a logistic pattern? Answer derives from the environmental conditions in a particular habitat: n In habitats with extremes of environmental conditions or conditions that fluctuate rapidly, density independent factors (which are generally abiotic) are the stronger regulating factor n Conditions do not remain the same for sufficiently long periods of time to favour specializations – successful species (those leaving the most offspring) are genetically equipped to reproduce early, often n In more stable habitats, density dependent factors which are generally biotic are more important. n “success” is favoured by traits that lead to specialization, habitat partitioning, co-existence, symbiotic relationships 8 Time plays an important role n n n n n Density dependent factors act over successive generations and long periods of time (evolutionary time) We have a very poor understanding of factors controlling the distribution of K strategists and hence poor predictive power However it is not surprising that increasing the disturbance regime of an environment leads to losses of the more specialized K-selected species in that environment or at a minimum, reductions in their abundances. With a particular species loss, symbionic partners may also be lost Those losses open up carrying capacity (light, nutrients, water, space, etc.) for more r-selected (generalist) species 9 Current patterns of anthropogenic land use, e.g. forestry, agriculture, urbanization - not to mention climatic warming - change environments: • making them less “predictable” • better suited to opportunistic r-strategists (rats, cockroaches, flies, pigeons, starlings, dandelions, tree-ofheaven) • less hospitable to K-strategists (oak and maple trees, hawks and owls, butterflies and a host of species many urban dwellers have never seen) • We may think we’re “ok” but what are the implications for ecosystems’ abilities to adapt to an unknown future? • Humans appear to more K than r-selected – will our own actions make our environment too unpredictable for human persistence? • When might stress push ecosystems to new regions of attraction? • What are the implications for production of the ecosystem services we require to survive as a species? homogenization of landscapes 10 Ecological Succession Often subtle, on-going changes that result in transitions from one type of biological community to another The experimental approach: Suppose you stopped mowing your lawn or in this case, a plowed field is simply left to see what will happen http://natl.ifas.ufl.edu/oldfgall.html 11 The nature of disturbance (and response): Scales of space and time 1. 2. 3. Totally unpredictable to species, rapid, rare, short-lived: tsunami, earthquake, landslide Uncommon but within the lifetimes of the system’s longest lived species: ENSO, fire, floods Unpredictable but unremittingly constant or permanent: climate change or other human interventions such as agriculture/urbanization 12 Mt. St. Helens n n n Mud slides and the largest debris avalanche in recorded history. Blast waves leveled trees 11 km from the explosion. Succession (primary? secondary?) 13 n The eruption occurred on an early morning in May • survivors included nocturnal animals that were in their burrows, • plants that had not yet broken winter dormancy under the snow, • migratory bird populations that had not yet returned. 1984 Timing is everything! “Gophers came up through the ash, and succeeded in mixing the soil with the ash layer, with seeds and parts of plants." n In areas with less than 25 centimeters of ash, gophers "were really important to the recovery." n 25 years post-eruption, scientists have counted more than 150 species of wildflowers, shrubs and trees, with an average of ten new plant species re-gaining a foothold every year. n - Virginia Dale (Oak Ridge National Laboratory) 2004 14 Fire n n n n n Before european contact, large fires burned through a variety of NA montaine landscapes (like Yellowstone) every 250 - 400 years. Large fires also burned central NA prairie grassland systems every 25 - 60 years and southeastern pine systems perhaps every 100 years Lightning starts an average of 22 fires each year. 80% of which go out by themselves. Plants in these systems are adapted to fire. Yet until the Yellowstone fires, park managers believed they had to extinguish fires to preserve park resources (Smokey the Bear and all that). See the disturbance and resilience section 15 Over time, fire suppression n n n Led to the replacement of grasses by woody shrubs (remember the Australian example from KR # 4E?) Forests were increasingly “cluttered” with woody debris that became a haven for wood-boring insects that then attacked living trees “Valuable” commercial conifer species (e.g. redwood) failed to regenerate as various species of deciduous trees increasingly dominated the landscape 16 Yellowstone n n n n n Yellowstone had implemented a natural fire policy in 1972 From ‘72 to 1987, 235 fires were allowed to burn 33,759 acres (13,354 ha) ~2250 acres.y-1 All fires were naturally extinguished Public response was good, and the program was considered a success. 1988 was radically different 17 n n n n n n 1988 turned out to be the driest in the park’s recorded history By July 15, 8,500 acres had burned Due to continued dry conditions, the decision was made to suppress all fires – it didn’t work Within a week, fires had encompassed nearly 99,000 acres By the end of the month, the extant fuel load and high winds made the fires uncontrollable On the worst single day, Aug 20, winds pushed the fire across 150,000+ acres. 18 More than 25,000 firefighters, as many as 9000 at one time, worked the Yellowstone fires at a total cost of about $120 million. n Yellowstone’s fire management policy was the topic of heated debate, from the bars in park border towns to the US Congress. n By September snow dampened the fires that the US’s largest fire-fighting effort could not. n Hence the joke: n • “How do you put out the Yellowstone fires?” • “Pour on a hundred million $ and wait for snow” 19 Ecosystem wide . . . 36% of the park burned n 345 elk (of an estimated 40,000-50,000), 36 deer, 12 moose, 6 black bears, and 9 bison died (most were trapped as fire quickly swept down two specific valleys) n Some small fish-kills occurred as a result of heated water and/or fire retardant in the streams n Less than 1% of soils were heated enough to burn below-ground plant seeds and roots. n 20 What we learned from Yellowstone: n n n n n n n Yellowstone’s grasslands returned to pre-fire appearance within 8-10 years. Forest ecosystems continue to recover. The grasses elk feed on were more nutritious after the fire – as evidenced by increased fecundity of elk. Bears were seen grazing more frequently at burned vs. unburned sites. The fires had no observable impact on the number of grizzly bears in greater Yellowstone. Cavity-nesting birds, such as bluebirds, had more dead trees for their nests; birds dependent on mature forests, such as boreal owls, lost habitat. No fire-related effects have been observed in the fish populations or the angling experience in the six rivers that have been monitored regularly since 1988. 21 Compare Yellowstone with the “stump barrens” of Michigan In late 19th and early 20th century Michigan, selective logging for pine and hemlock left enormous debris piles in the what was left of the forest When fires inevitably hit, they were extremely hot due to the combustion of the resinous slash They burned deeply destroying the organic matter in the soil and incinerating residual seeds These areas have proved extremely resistant to rehabilitation/restoration for over 100 years 22 or the Sonoran Desert fires of 2007 n n n n n n n Desert plants have evolved to deal with heat and drought Fire has never been sufficiently frequent or wideranging to force adaptation So, unlike plants that evolved with fire, desert plants have not developed thick bark or cork layers to protect delicate tissue and are easily killed An invasive weed known as Sahara mustard has moved in to desert areas formerly dominated by creosote bushes Mustard is not grazed by indigenous herbivores so dead plant material is leading to high fuel loads . . . . . and fire Plant ecologists fear the formerly saguarodominated landscapes may morph into something that more closely resembles savannah as native plants with no defense to fire are killed 23 Consider flooding . . . and flood dependent ecosystems Furbish's Lousewort is an endangered species. n The only known place in the world that this plant grows is the northern St. John River Valley (Maine and New Brunswick) n n An endemic species restricted to just a few locations on Earth is important as a unique reservoirs of genes – Lousewort is called a “Lazarus” species as it was thought to be extinct Lousewort does not compete well with other plants. It would likely soon be crowded out by taller, more aggressive plants without the spring floods along the river. Ice jams and subsequent flooding mean shards of ice scour the shore, removing most of the vegetation on the shore each spring – resetting early successional conditions 24 Interrupting flooding with dams Sediment build-up behind the dam changes downstream morphology Changes in stream temperatures Losses of fisheries Failure of downstream riparian vegetation to regenerate The Dickey-Lincoln dam, a $227 million hydroelectric project proposed for the upper St. John River in 1974 was blocked by the U.S .Congress in 1986 after years of study, because the dam would have flooded 360 km2 of Maine forest and severely reduced the habitat for Furbish’s Lousewort (only “re-discovered” during the environmental assessment for said dam) The Eau Claire River as it enters Lake Altoona, a reservoir in Wisconsin The 70 m high Matilija dam in Ventura Co Calif has virtually completely filled with sediment over its 62 year “life” n n The capacity of the Matilija Reservoir has been reduced by over 90% Estimates are somewhere around 300 million $ to remove the dam and associated sediments 26 What we “think” we know on disturbance 1. Natural environmental variability (disturbance) is important in structuring systems The more frequent the disturbance, the more rselected species we expect to find (compare the plant communities of prairie ecosystems that experience fire on a decadal scale with those for forests that burn on a scale of centuries) 2. 3. As disturbance becomes more infrequent, biological interactions and K-selected species become the more significant element structuring ecosystems There are more types of biological interactions than we have covered so far – what do we know about how they operate? Our topic for Thursday! 27