Goals: look at natural populations and their spatial distribution; contrast landscape and metapopulation approaches to the problem.  Consider species that can be studied using a metapopulation approach, causes of local extinction and recolonization and the equilibrium number of individuals in a metapopulation.  Discuss the importance of studying source, sink populations.

The lecture:  No species is evenly distributed over its entire geographic range.  Species have higher density in some areas, lower in others, and often occur in patches with gaps with no individuals of the species separating the patches.

Two approaches to studying this spatial variation:

• landscape ecology: determine how variation in aspects of topography (physical structure) and vegetative structure of an area are important in organismic distribution, abundance, species diversity.
• metapopulation approach: study of species as spatially structured groups of local breeding population with movement of individuals among populations having some impact on what happens within local populations

The metapopulation view of species:

• The environment consists of patches: areas of habitat that contain requirements for persistance of a species, separated from each other by area of unsuitable habitat.
• Each habitat patch could potentially have a deme:  ("natural population" or "local population"): a population of individuals of the species that occur within the same patch, and interact, reproduce most with each other.
• Since habitat patches are separated by unsuitable habitat, most likely for individuals to stay within their own patches.  There is expected in most cases, however, to be some level of dispersal: movement of individuals from their original deme to breed in a new deme
• Some or all of the demes in the metapopulation are subject to local extinction: dying out, leaving habitat patches that do not contain the species (although habitat is suitable) for a period of time of at least several generations
• The presence of suitable habitat patches that may not have the species creates the potential for recolonization: individuals of the species dispersing into the patch and starting a new population.
• Metapopulations may reach, or approach, an equilibrium number that is a balance between local extinction and recolonization. An exception to this is a so-called "non-equilibrium metapopulation" in which there is no ability for individuals to disperse to new patches, so no potential for colonization; such populations are expected eventually to die out.

To understand metapopulations, we need to understand what affects these two processes, recolonization and local extinction.

Factors affecting recolonization: clearly, the landscape, which may have barriers to movement, and the ability to disperse will affect recolonization.  In addition, recolonization will be affected by:

• distance from currently existing demes to empty patches.
• size of currently existing demes
• dispersal behavior.

Dispersal behavior:

• costs to dispersal: risky, may not find new habitat, increased vulnerability to predation
• benefits to dispersal: locate mates, avoid competition, find better habitat, avoid inbreeding.

Some examples of species for which these genetic methods have been used include catfish, for which different drainages have different populations with genetic differences from each other, indicating low gene flow (low dispersal), pocket gophers in which there can be some genetic difference between neighboring fields and are larger differences between more distant areas, indicationg low gene flow and low dispersal, and red-winged blackbirds and song sparrows, for which most populations across the country have high genetic variation and are genetically similar to each other, indicating high gene flow (high dispersal).

Dispersal is not the same as recolonization.  A species could have high dispersal, but low recolonization, if the main benefit was mate location, so that dispersal went primarily to areas that already had members of the species.  Another situation with high dispersal but low recolonization occurs if almost all dispersers are the same sex.  In many mammals, for example, it is primarily males that disperse.  This could help avoid inbreeding.  It will not promote recolonization since males, by themselves, can't start a new population.

Some experimental methods focus more directly on recolonization than dispersal.  These can involve release of individuals in areas followed by studies of where those individuals go.  One example of such a study: lizards were released at borders of areas of good habitat, some maintained with conspecifics and others without, dispersed most frequently into the areas with conspecifics.  This result would suggest that recolonization might be low even if dispersal is high.

Four main reasons small local populations go extinct:

• demographic stochasticity: random events associated with births and deaths may lead to a point at which no new individuals are born; predicted to apply only to very small populations
• environmental stochasticity refers to random, unpredictable events in the environment that cause high mortality of individuals of the species being studied.  In a small population, such events could lead to loss of the entire population.
• deterministic threats: non-random changes in the environment that create unsuitable environment.  Examples: human destruction of natural habitats; ecological succession (natural change in the species composition of communities over time.)
• loss of genetic variation: this is predicted to lead to extinction relatively slowly, so may be less important than other effects, but can impact them.  Losss of genetic variation occurs through genetic drift: evolution that occurs because of random fluctuations in the proportions of alleles in a population.  Over time,  alleles become lost by chance.  This decreases the potential for natural selection, so populations have less chance to adapt.  Associated with genetic drift in small populations is inbreeding depression: decreases in production of surviving offspring because of reproduction between close relatives.  In small populations, it is likely that relatives will reproduce with each other.  This means that harmful recessive alleles are likely to be expressed in offspring of relatives; these individuals are unlikely to survive, so overall numbers of surviving offspring decrease. Inbreeding may have less impact on populations that have historically occurred in small patches.  In such populations, past inbreeding may have rid the population of most harmful recessive alleles, so that inbreeding has ittle impact now. In contrast, populations that have historically occurred in large demes may be more subject to problems from inbreeding if demes become small since there has been no history of inbreeding so the population is predicted to have recessive harmful alleles.

Local extinction depends on population size; smaller populations are more likely to go extinct.  Patch size will affect deme size and this will affect what happens in metapopulations, so metapopulations have been considered to fall into a continuum between the following extremes:

• classic metapopulations: all demes are small and equally likely to go extinct
• mainland-island metapopulations:. some demes are very large and likely to persist, but are surrounded by small demes that undergo local extinction

Up to this point, we have been assuming all habitat areas that could have a deme are equal in quality.  This will never be true.  One particular difference in habitat is that there may be some areas of good habitat in which more individuals are born than die (birthrate, b, is greater than deathrate, d) but other areas of poor habitat in which deathrates are greater than birthrates.  From what we have considered looking at single populations we would predict the former to grow and the latter to die out.  Such areas may, however, remain relatively constant in size if individuals tend to move out of the first kind (emigrate) but move into the second kind (immigrate.)  Models that consider birthrates, deathrates, immigration rates, and emigration rates are called BIDE (for birth, immigration, death, emigration) models.  Based on them, we can define two kinds of population:

Source populations have higher birthrates than deathrates and have higher emigration (moving out of the population) than immigration (moving into the population.)  They are predicted to occur in areas of good habitat from which individuals emigrate when the areas get crowded.

Sink populations have higher deathrates than birthrates and have higher immigration than emigration.  They are predicted to occur in areas of poor habitat.  Individuals immigrate into these areas when sources become overcrowded.  Sinks are maintained because of immigration -- if no individuals moved into the populations, the fact that d is higher than b would mean that they would die out (or, think of them as sinks, they'd go "down the drain.")

To distinguish source from sink populations, we would need to determine whether b was greater than d or d was greater than b.  We could potentially do this using life table estimates of r (remember r=b-d) if we could keep track of individuals even if they left the area.

The existance of source and sink populations shows another reason that we need to consider more than one population to understand natural populations.  In addition, it affects our understanding of some ecological principles.  One example of this is the distinction between the fundamental and realized niches of a species.  We had defined the fundamental niche as the range of factors required to maintain a viable population, and the realized niche as the range of factors within which a species actually occurs, which we had considered to be typically narrower than the fundamental niche because of biotic interactions such as competition and predation.  If there are sink populations, however, we should note that these are in areas that do NOT have the range of conditions required to maintain the species since populations die out there without immigration, so they are outside of the fundamental niche.  They are within the realized niche, though, since populations do occur in sinks.  Thus, sink populations are an example of a situation that makes the fundamental niche narrower than the realized niche -- the opposite of the typical situation.

The existance of source and sink populations must be considered when setting aside areas of habitat as nature reserves for species.  It is crucial to set up nature reserves that maintain source populations -- if the source populations are not maintained and we set up reserves in areas where there are sinks, then the species we wanted to preserve will die out in the absence of the sources.