Temporal and spatial dynamics of populations

Reading; Smith and Smith Chapters 10-12 (read)

Every Species Has a Range

Range can be determined by physical tolerances, and by the availability of critical resources

– can also be determined by history and geographic barriers preventing dispersal.

In the United states, most species have a range of 4-24 states.– Cosmopolitan species are an exception, they are

worldwide in distribution.

– Endemic species are found in only a small, restricted area.

Example; trees are very sensitive to moisture and temperature– The northern and southern limits of the range of North American

maple species are determined by winter temperatures and summer temperatures

• differences in their ranges reflect different tolerances

– The western limit is imposed by summer drought

– Within the range, each species tends to grow within areas where certain conditions predominate

• Sugar maple-forest species, prefers well developed soil and over 50cm (north) and 100 (south) cm of rain

• Black maple-like silver maple but drier soil, higher Calcium

• Silver maple-moist, well drained soil-river valleys

• Red maple-either wet, swampy soils, or dry, poorly developed soil-where other species will not grow

Distribution Populations of a species occur within areas of

suitable habitat-but the absence of a species does not necessarily mean that the habitat is unsuitable..

– For example-Islands separated from a continental area will harbor some species from the mainland, but not others. Certain birds, arthropods, and reptiles disperse well to islands, but many other species do not.

– The range of a species may be in the process of change, but dispersal has lagged behind climate change. This is the case for certain North American trees, notably the Beech.

– An important geographical barrier may keep a species restricted to a given continent or area, even though suitable habitat exists elsewhere

– Recent human activities have broken down many of these historical barriers to dispersal, resulting in the widespread introduction of exotics to new areas. I.e., the Nile Perch.

Within its range, a species is distributed into one or several populations.– Populations are all the members of a single species,

within an area, that can potentially interbreed.– Populations are frequently defined by areas of

suitable habitat-but, for some species, population boundaries can be rather arbitrary.

– Within these areas, interaction and gene flow is frequently localized-Populations are often subdivided into subpopulations or demes.

– An array of heirarchially structured clusters of individuals is sometimes referred to as a metapopulation

Example: An endemic perennial shrub-Within the geographic range of

Clematis fremontii,individuals are restricted to limestone glades.Within Glades, individuals

are clustered

These are Idealized Population Structures– A) Standard (Levins) metapopulation-Isolated patches

harbor populations that persist for a while, but eventually may go extinct (arrival of a predator or competitor, or simply outstripping its own resources).

– Once empty, patches may be colonized. – Surviving patches send out colonists that colonize empty

patches.– Landscape is a dynamic matrix of empty and occupied patches-

whole system is stable even though isolated populations are not– B) Source-Sink-As above, but some areas are more or less

permanent “sources” and some are “sinks”-areas that cannot sustain permanent populations

– C) ”Patchy Environment or Habitat Network”-more or less permanent corridors link habitat patches

Example-Checkerspot Butterfly Paul Ehrilch studied the Checkerspot butterfly

Euphydryas editha bayensis for over 20 years.– The species occupied serpentine grasslands

(containing high metal content), a patchily distributed plant community at Jasper Ridge, near Stanford, CA.

– Slight changes in solar intensity on the serpentine grasslands regulate moisture and temperature variation that dictate the reproductive success and ultimately determine whether a patch is a “source” or a “sink”.

– Certain patches of habitat are “sources”, which can sustain permanent populations.

– Other patches are “sinks”, unsuitable habitat which cannot sustain populations.

– Still other areas can sustain suitable populations for a while, but ultimately too small to persist, and occasionally go extinct, only to be recolonized.

– For Checkerspots, Weather Affects Suitability of Patches;

• Sunny weather supports increased larval growth

• Rainy weather inhibits larval growth

• Weather also affects host plants-cold weather inhibits the rate of their development.

• During droughts, only larvae on cool slopes survive, because plants senesce before larvae complete development

• During moderately wet years, the population expands to colonize patches on sunny slopes.

• Area C-Large, flat area. Supported a single,widely fluctuating population till 1991.

• Area H. Smaller, but a topographically heterogeneous metapopulation. It was more stable, but ultimately went extinct in 1998.

• The butterfly is currently endangered, but it still may recolonize the Jasper ridge area from the outside.

Example: Mud Daubers The organ-pipe mud dauber, Trypoxylon politum,

builds mud nests-the female stocks her nest with paralyzed spiders of many types, but prefers Neoscona sp.

The species was probably originally restricted to cliff overhangs and caves near water in the Southern United States.– Now it nests wherever a bridge crosses a slow

moving stream, or where a garage is near a source of mud.

– Females are excellent fliers, but once they start building a nest, they tend to stick with it till something kills them.

– Mud daubers in any given area form a local metapopulation, composed of a local subpopulation wherever there is a suitable nesting site.

– Individuals usually disperse from their local subpopulation at birth, Molumby (1995) showed about half of them tend to return.

– These local subpopulations may go extinct at any time floodwaters get high enough to wash away the nests, or whenever the State finds the money to clean the bridges.

– Old populations accumulate parasites, and ultimately become “sinks”. Newer, smaller, and isolated populations are “sources”.

– Because of this extinction-recolonization dynamic, the mud-dauber metapopulation is quite stable, even though the population at any bridge site, shed, or cave, is temporary and might go extinct at any time.

– The parasites of mud daubers, and other species that use their old nests, have similar metapopulations.

Some populations move For example;

– Pacific salmon migrate from the ocean upstream to spawning sites in freshwater pools

– Atlantic eels migrate to breed in the Sargasso Sea, an area of immense seaweed growth in the Central Atlantic. Larvae swim to Europe and colonize freshwater rivers.

– Monarch butterflies migrate to overwintering grounds In Mexico and California, returning to the Midwest the next year. No single individual ever makes the trip, it takes two generations each way.

– Arctic terns migrate from feeding grounds in the Arctic to feeding grounds in the Antarctic every year, breeding on shorelines in the North Atlantic.

Some migratoryvertebrates ofNorth America

Population Growth It is a fundamental characteristic of all living things

that under some conditions, they reproduce and increase in number

The growth of biological populations is of tremendous interest to ecologists-– Many populations of organisms undergo

conspicuous cycles and changes in abundance– The individual propensity for reproduction is the

basis of an organism’s fitness– The ability of a population to grow under a given

set of circumstances reflects its competitive ability.

– Exponential growth is a simple model that describes population growth in an essentially unlimited environment.

– It assumes that the rate of births and deaths are essentially constant, thus;• b=instantaneous per capita birth rate • d=instantaneous per capita death rate • N=population size• dN/dt=bN-dN, since b-d=r; • dN/dt=rN, so calculus fans, you can integrate to

get:

–N(t)=N0ert

• where r is the exponential growth parameter

• N0 is the starting population

• t is the time elapsed

– r=0 if the population is constant, r>0 if population is increasing, r<0 if the population is decreasing. Exponential population growth is only appropriate for

describing the initial colonization of empty habitats.

Ultimately, the growth of every population is limited by some resource.

Actual organisms compete with each other for limiting resources.– As a population becomes more dense, reproduction of

individuals can be inhibited.

– Mortality can increase with increasing population density as well.

– These patterns are called density dependence. Ultimately, all organisms have a carrying

capacity based on limiting resources in the environment-thus they cannot grow forever.

Example-fecundity in Drosophila melanogaster is inversely proportional to population density

– primary mechanism is mortality of larvae

Example-larger populations of mud daubers experience higher rates of parasitism

Example-many plants go through a process called “self thinning”

Mortality rates for seedlings increase with density, because only so many adults will survive-regardless of initial seedling number

The logistic growth model accounts for density dependence by designating a fixed carrying capacity.

– dN/dT=rmaxN(K-N)/K where

K= Carrying Capacity, this is the maximum number of individuals that the population can sustain.

N=The Number of individuals in the population at a given time.

rmaxis the maximum population growth rate

The population growth rate is positive when populations are less than K:

dN/dt = rN(1 - N/K): If N < K, then N/K < 1, so the term inside the

parentheses is positive thereby making the growth rate positive, so the population will increase.

The population growth rate when populations are greater than K: If N = K (when the population has reached carrying

capacity), N/K = 1, and the term in the parentheses = (1 - 1) = 0.

The growth rate is zero when the population is at K, and the population size will not change.

Simple, laboratory populations frequently exhibit logistic population growth.

– For example, if light and nutrients are kept constant, populations of algae, such as Anabaena, will reach carrying capacity

Most natural populations, however, exhibit patterns not predicted by the simple logistic model.– Many undergo periodic cycles. This figure describes population fluctuations in Australian moths.Pp. 190 of Smith and Smith lists four other examples of cyclic populations.

Lag time-oscillations– For all but the shortest-lived organisms, there is a delay between the

growth of a population, and the effects of the increase in population density on the survivorship and/or reproduction of its members.

– This delay is called lag time.

• It is given the symbol t

– Lag time can have major consequences on the dynamics of a population.

• 1. Populations often show overshoots of the carrying capacity or oscillations around K

• 2. The logistic equation will produce oscillations if time lags are introduced.

• 3. The population grows according to a population density prior to the current density of the population

When lag time is incorporated into the logistic equation, the behavior is dymamic-N at any given time depends upon the history of the system.

Essentially, there are three patterns;– r</2-damped oscillations-ultimately the

population reaches a stable size at carrying capacity.

– r<e-1-no oscillations at all, population goes to K

– r> /2-oscillations form limit cycles, with periods from 4 to 5

– very large r produces chaotic behavior-very complex patterns that skyrocket and crash

This is actually a discrete time model, continuous time models look pretty similar, except the parameters are different.

Nicholson Blowfly Experiment A. J Nicholson, an Australian entomologist,

studied the population dynamics of an artificial population of blowflies.– Experiment 1-Provided blowfly larvae with

50grams of ground liver per day (as an oviposition substrate). Adults had essentially unlimited food.

– Monitored numbers of eggs, larvae, and adults for over a year.

– Result-populations of adults fluctuated from over 4000 to 0. Eggs and larvae underwent similar fluctuations.

Nicholson noted that when the numbers of adults were large, they would lay so many eggs that nearly all the larvae died. He postulated this to be the mechanism for the lag time– Experiment 2-Provided blowfly larvae with

50grams of ground liver per day (as an oviposition substrate). Adults had 1 gram/day of food.

– Monitored numbers of eggs, larvae, and adults for over a year.

– Result-populations of eggs, larvae, and adults reached a stable, high population.

Actual populations of organisms vary in their stability-some may persist for thousands of years, others last only a short time.– For actual species, K varies from year to year as well,

because weather and other factors influence available resources-bad years can lead to the extinction of a population (catastrophe).

– For populations with a high r, chaotic behavior can cause a population to crash and go extinct.

– Small populations can also go extinct due to random factors (stochasticity) or inbreeding.

– As we shall see, competition or predation may also drive a population extinct.

Metapopulations and Persistence– 1. Almost all populations are fragmented to some degree

and thus have patch structure • 2. Most ecological processes are strongly affected by the degree

to which populations are subdivided

• 3. Occupation of presumably habitable sites are affected by both ecological processes such as local mortality rate as well as the size of the site and its distance from other sites

• 4. The metapopulation persists as a balance between extinction and colonization of the subpopulation sites.

• 5. The overall population may be stable but the subpopulations are not.

• 6. Local populations that are part of a metapopulation may be unstable in their dynamics

A Simple Model A series of populations linked by dispersal can be

much more stable than any single population.– Simple model p=1-e/c where;

• p=proportion of patches occupied• e=population extinction rate• c=colonization rate

– for e=0, p=1 all patches occupied– for e<p metapopulation is unstable, eventually it

goes extinct (takes a while)– The metapopulation is stable for c>e, if both are

between 0 and 1. In actual species, these assumptions are not likely to be

met-neither e nor c is likely to be constant

Dispersal Nearly every species has evolved some means of

dispersal, but dispersal ability varies from one to the next– Dispersal is dangerous, and for a few, exceptional

species, there is selection against dispersal;• Insects on oceanic islands frequently evolve

the loss of wings.– For species that exist in a matrix of unstable

habitats, dispersal is essential to persistence. • Individuals without dispersal ability will produce

progeny that are ultimately doomed to extinction.

Modes of Dispersal These are a few examples, actual modes of dispersal

are as various as organisms themselves Animals-walking, crawling, directed flight, swarms

pushed by prevailing weather (locusts), attached to animals (parasites), planktonic larvae

Plants-seeds with wings or parachutes, floating seeds (impatiens, coconuts), seeds with spines or prickles that attach to animals, seeds that pass through the digestive tract of animals

Protists, bacteria-resistant stages that can be carried on, or in, other organisms, passive dispersal

Example-roadside weeds in urban areas.– Most people don’t think about them much, but

the urban landscape is a matrix of patches, each of which supports populations of weeds.

– Landscapers, city workers, drought, and development destroy patches on a regular basis, but there is no city that is free of weeds.

– Urban weeds all have one thing in common-excellent powers of dispersal.

• This offsets the high extinction rate by ensuring a high rate of colonization for each species.

The interaction between a population’s capacity for growth, its dispersal abilities, its ecological tolerances, and its competitive ability relative to similar species determines whether a species can live in a particular environment.– Some species are good dispersers, adept at

colonizing disturbed habitats.– Other species do not disperse well, but once

established, are excellent competitors and cannot be displaced by invading species-they have a low extinction rate.

Every species has a life history which has evolved to maximize fitness. Only life histories that are also compatible with the existence of the species have survived.