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Reminder. Quiz – Chapters 11 and 8: Friday December 17 Mustafa: syllabus. See email. To all: follow your own deadlines please . From populations to communities. Chapter 9. A few points. Populations exist in the context of the whole community Each is within a whole web of interactions
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Reminder • Quiz – Chapters 11 and 8: Friday December 17 • Mustafa: syllabus. See email. • To all: follow your own deadlines please
From populations to communities Chapter 9
A few points • Populations exist in the context of the whole community • Each is within a whole web of interactions • Each responds differently to the prevailing abiotic conditions • …how do biotic and abiotic factors combine to determine dynamics of species populations? • What is the importance of the concept of metapopulations? • What about food webs?
Multiple determinants of dynamics of populations • Why are some species rare and others common? • Why does a species occur at low population densities in some places and in high densities at others? • What factors cause fluctuations in a species’ abundance? • To understand even a single species in a single location – need to know: • Physicochemical conditions • Level of resources available • Organism’s life cycle • Influence of competitors, predators, parasites… • And how all these factors influence abundance through effects on birth, death, dispersal, and migration
‘devil in details’ • A record of numbers alone: What do they tell us? What don’t they tell us? • Thus – need information on age, sex, and size • Correlations with external factors. • High intensities of disease late blight in potato crops occur 15-22 days after a period in which min temp > 10C and humidity > 75% for 2 consecutive days • Correlations – suggest (not prove) causal relationships • Cause requires a mechanism • Pop too large … what could happen? • Correlation does not tell us which of the options could happen.
What does stability mean? • Stability does NOT mean “nothing changes” • Population: may have complex dynamics, flux, and stability • Small, sand-dune plant (androsace septentrionalis): • 150 - 1,000 new seedlings / m2 each year • Mortality: reduced pop by 30% - 70% • Pop kept to same limits • 50 plants always survived to fruit and produce seeds for next seeds • Stability?
Stability or Change? • Do we look for constancy of populations? Or do we emphasize the fluctuations? • If emphasize constancy: look for stabilizing forces within populations to explain why populations do not exhibit unfettered (unrestrained) increase or a decline to extinction • If emphasize fluctuations: look to external factors (weather? Disturbance?) to explain changes • Can the two sides be in agreement?
Stability or Change? • How is abundance determined? How is abundance regulated • Regulation… • Regulation: tendency of a population to decrease in size when it is above a particular level to increase in size when below that level • Can occur only due to one or more density-dependent processes that act on rates of b , d and/or movement (remember: Chapter 5) • Detected in 80% of studies of insects that lasted > 10 years • Determined: • Precise abundance of individuals will be determined by combined effects of all the factors and all the processes that affect a population, whether dependent or independent of density • Weather typically major determinant • Apple thrips: weather accounted for 78% of variation in #
Key factor analysis • Can distinguish between what regulates and what determines the abundance of a population • Then learn how regulation and determination relate to one another • By: key factor analysis (better to name it: key phase analysis) • Calculate k-values for each phase of the life cycle • k-values measure the amount of mortality • The higher the k-value, the greater the mortality • k = killing power • [read box 9.1]
Key factor/phase analysis • How much of the total mortality tends to occur in each of the phases? Mortality = all losses from population • What is the relative importance of these phases as determinants of year-to-year fluctuations in mortality, and thus of year-to-year fluctuations in abundance? [is the toll roughly the same year to year?] • Then phase mortality is low: total mortality is low: pop is large. And vise versa. • A phase with a k-value that varies randomly will – by definition have little influence on changes in mortality • So what is needed: to measure the relationship between phase mortality and total mortality: regression coefficient of former on latter • Larger regression coefficient: key phase causing pop change • Regression coefficient at zero: random phase mortality
What about density-dependency? • Plot k-values for each phase against # present at start of the phase • For density dependency: k-value would be highest when density is highest. Right? • For beetle population • Key phase: summer adults • Density-dependent: older larvae
Wood frog (rana sylvatica) • Key phase determining abundance: larval period, due to year-to-year variations in rainfall • Density-dependent: adult phase (due to competition for food)
Dispersal, patches, and metapopulation dynamics • Migration can be a vital factor in determining and/or regulating abundance • Disease : important role when populations are fragmented and patchy (as many are) • Abundance of patchily distributed organisms: determined by properties of two features: ‘habitable site’ and ‘dispersal distance’ • Metapopulation: if pop comprises a collection of subpopulations, each one of which has a realistic chance both of going extinct and of appearing again through recolonization
American pika • Metapopulations: N, middle, and S network of patches • Northern network: high occupancy throughout study period (1972-1991) • Middle network: more variable and much lower occupancy • Southern network: steady and substantial decline • When isolated (in simulation): northern network was fine, but middle and southern networks crashed • When entire network simulated as a single entity: middle network – stable, southern network: periodic collapses • Consistent with data • So? – northern network acts as a net source of colonizers that prevent middle network from collapsing -> delaying extinction -> allow recolonization of southern network; southern network: transient behavior
Temporal patterns in community composition • Patch dynamics. Disturbances open up gaps • 2 fundamentally different kinds of community organization • Founder-controlled • When all species are good colonists and essentially equal competitors; Species are approximately equivalent in their ability to invade gaps and can hold gaps against all comers during their lifetime • Examples: tropical reef fish; vacant living space: limiting factor • Competitive lottery • dominance-controlled • Some species are strongly superior competitively • Community succession • Opportunistic, early-succession species with poorer powers of dispersal mid-succession climax stage; # of species increases (colonization) then decreases (competition)
6) Climax Community: Finally, those plants (and animals) populate the area that are the best suited for growth This kind of community is very stable and so the process of succession slows down drastically Except for a natural disaster, this kind of community, called a Climax community, will stay for a long time BECAUSE IT CAN MORE SUCCESSFULLY COMPETE FOR AVAILABLE RESOURCES WHEN OTHER ORGANISMS COME INTO THE SAME AREA. 5) Intermediate Community: The process continues: other plants and trees begin to grow and eventually completely replace the older species of life that had grown there. This also applies to animal life. One species replaces another only because it is better suited for growth (because of better use of nutrients, more access to light, etc) than the current inhabitants of the area. 4) Intermediate Community: -Larger plants begin to grow among the small shrubs and herbs and if better suited to the environment, grow better than the shrubs and herbs, eventually choking them out of the area Weathering continues. Trees that are planted in small areas on rocks begin growing in the rocks. The roots begin to grow into the rock, and as they get bigger split the rock in various pieces, helping the physical weathering and soil formation process. 2) Intermediate Community: Wind brings the spores or germinating bodies of small plants like lichens and moss which attach and grow on the rocks. Lichens and mosses produce acids which cause the rocks to weather (break into small pieces) Combined with physical weathering, the first soil is formed Insects and small animals begin to inhabit the area. 3) Intermediate Community: Seeds of small shrubs and other plants (herbs) are carried by the wind and eventually replace the lichen and moss community BECAUSE THESE GROW BETTER THAN THE LICHENS AND MOSS. Different kinds of animals move into the area, often competing better than the prior animals for resources. 1) Pioneer Community: -After a volcanic eruption or after a glacier recedes from an area, a barren, rock-filled but lifeless region exists
Plants and animals in succession • Just because plants dominate succession – does not mean that animals necessarily follow • Sometimes: animals determine nature of plant community • Heavy grazing. Trampling. [box 9.4]
Succession in a patchwork: size and shapes of gaps • Has climax been reached? A matter of scale • Many successions take place in a mosaic of patches, with each patch, having been disturbed independently, at a different successional stage • Large gaps • Centers: colonized by species that travel great distances (relatively) • Small gaps • Colonized by established individuals around periphery of gap
Food webs • Let’s examine systems with at least 3 tropic levels • Plant – herbivore – predator • Let’s then consider direct and indirect effects that a species may have on others on the same or other trophic levels • Effects of a predator on individuals of its prey. Clear. What about effects of the predator on the prey’s resources? Or on other predators of that prey?
Food webs: indirect and direct effects • What would happen when a species ire removed from a community? • Increase in abundance of a competitor • Increase in abundance of a prey • Or • Competitor decrease in abundance • Decrease in abundance of a prey • Why? Direct effects < indirect pathways
Cats, rats, birds • Feral cats threaten local birds • Cats eat rats and birds; Remove cats; Exotic Rats have less pressure on them; Rats attack the endangered flightless parrot (Of 21 chicks that hatched between 1981 and 1994, nine were either killed by rats or died and were subsequently eaten by rats) ; parrots translocated to an island w/o the exotic predators
Cat control in 1982 arrested a sharp decline in Kakapo numbers, and they have recently increased under the Kakapo Recovery Plan. Red arrows indicate breeding years. Numbers become less precise before 1995, with the 1977 figure perhaps out by 50 birds. The 2009 figure is as of April 11, 2009.
Trophic cascade • Predator reduces abundance of its prey impacts trophic level below prey’s own resources increase in abundance • Or • Predator reduce intermediate predator -> increase herbivore decrease plants (4 trophic levels)
2 year experiment: predation by birds – Glaucous-winged gulls and oystercatchers excluded from large areas in which limpets were common • Excluding the birds increased overall abundance of 1 of the limpet species (L. digitalis) , but a second limpet species became rarer (L. strigatella), and a third did not vary in abundance – the third (L. pelta) is the most commonly eaten by the birds. Hmm…how? • L. pelta is the birds’s favorite food and it is did not increase with the removal of the birds?
L. digitalis – light colored; occurs on light-colored goose barnacles • Dark L. pelta – on dark California mussels • Predation by birds reduces areas covered by goose barnacles. So removing predation? • Increasing barnacles -> decrease in area covered by mussels [more competition from barnacles] • Third species (L. strigatella) is inferior competitively to ohers; increase in L digitalis decrease in L. strigatella released pressure on L. pelta so remain unchanged
Birds -> limpets (and barnacles/mussels) – and algae! • Birds eat limpets. Limpets eat fleshy algae. • Birds eat barnacles. More space for algal colonization. • Birds excluded. Algae cover… • Decreased
Food webs • Top-down? • Predators controlling prey • Bottom-up? • Plants can limit herbivores • “Why is the world green?” (Hairston et al, 1960) • Because top-down control predominates? Green plant biomass accumulates because predators keep herbivores in check • Or “is the world prickly and tastes bad” (Murdoch, 1966; Pimm, 1991) • A world controlled from the bottom up may still be green
To be continued • Page 313