SUB-POPULATIONS

Raw, undifferentiated population is not especially useful for analysis or prediction. Instead, it is common to divide a population into such subsets as age groups ("cohorts") and gender. Consider the following largely hypothetical case: a starting (Time A) population of twenty total, of eleven females and nine males at various ages (blue for pre-reproductive, yellow for reproductive, and purple for post-reproductive ages). Assume that this is a closed population (no immigration or emigration) of mobile organisms. Also notice that there is an imbalance in gender among the youngest cohorts, an excess of juvenile females but a shortage of infant females.

By Time B the population has grown to twenty-two overall. There have been three new infants (the previous three are now juveniles), but also one male death, for a net change of +2 to the original twenty. The gender imbalance among juveniles has disappeared, although a slight shortage of infant females persists. The apparent shortage of infants could reflect a stressed population, or alternatively this might be a species having a typically prolonged maturation of pre-reproductive individuals.

At Time C the population has jumped a third to thirty-three total. This is wholly the result of new infants, with no deaths; living conditions apparently are quite favorable. The post-reproductive subpopulation has increased five-fold, and the increase of reproductive age females is especially noticeable, which may be indicative of a species for which females mature more rapidly than males. The large number of reproductive individuals, and also the large evenly-balanced "crop" of infants, suggests that the population should increase further, so long as the habitat remains able to sustain it.

Time D appears to confirm the prediction of Time C; the population has increased again (but less rapidly) to thirty-nine total. Not only is the post-reproductive cohort now the largest, but that it also is forming a linear cluster, almost to the exclusion of the other cohorts. This might indicate a spatial preference along some linear habitat feature of particular use to the older (and perhaps increasingly infirm) subpopulation, such as closer proximity and access to a river. The inverted pyramid form of the graph indicates an aging population, although there seem ample reproductive and near-reproductive individuals to add to the population, so long as the habitat remains able to sustain it. The relative paucity of infants may reflect a temporary deterioration of habitat conditions, or it may indicate the population approaching the habitat's permanent carrying capacity.

Time E; disaster! The total population has crashed to only a quarter of Time D, but the subpopulation balance indicates that males survived more than females. Older (and presumably more self-sufficient) individuals did also; infant conception either was very low or infant mortality was total. No infants still survive, and only adults (presumably the fittest) do. Among adult survivors there also is a reversal of dispersion, with the younger cohort seeming to have displaced the older from the linear cluster of Time D, such as if aggressive mature adults outcompeted the elderly or infirm for water.

The gender balance is extremely unhealthy for this population now. Only a fifth of the total population is female of reproductive age, and for many species a 70% male population is likely to either exhaust or competitively exclude these females rather than successfully propagate. In extreme situations the females can completely die off; such was the case of the now extinct dusky seaside sparrow (Ammodramus maritimus nigrescens), whose final population consisted of five males in the Disney World zoo in 1987.

Think the above example fictitious? Not entirely. Although for simplicity I have excluded immigration and emigration and "geometrized" the "range", these patterns are of the rise and fall of the African elephant (Loxodonta africana) at the Aruba River in Tsavo National Park (Kenya) between approximately 1950 and 1970 (see Sheldrick 1977). Droughts, devegetation, disease, and most especially poaching led to this population crash.

RATES OF CHANGE

Not only do populations change, but they tend to do so in characteristic fashions that differ between species and habitats. Although the reality varies for every population, and is likely to do so at different times also, the two extremes of change rate patterns are the "J curve" and the "S curve".

J curve populations typically experience "boom and bust" cycles, usually in parallel with fluctuating habitat conditions. Such populations grow slowly at first, then ever more rapidly until they exceed the habitat's carrying capacity (K; the "minimum number of individuals supportable by the habitat under normal 'most adverse' conditions). The population then crashes through mortality and/or emigration, and the cycle begins anew--so long as the residual population does not fall below the critical minimum (CM). If the population does fall below critical minimum, that organism at that place suffers extirpation ("local extinction"). Creatures following this population change rate tendency tend to have relatively low resilience ("ability to recover") following habitat disturbance.

S curve populations typically exhibit greater (but NOT total) stability, so long as the habitat remains able to sustain them. These populations grow slowly at first, then more rapidly, then slowly until they reach the habitat's initial carrying capacity (K1). The population then fluctuates near K until habitat conditions change, when the population adjusts to a revised carrying capacity (K2). Creatures following this population change rate tendency tend to have relatively high resilience following habitat disturbance.

It is possible, and even likely, that an organism's populations may shift between the extremes of J and S curve growth tendencies. This is especially likely if a new and sustained element of population control becomes available. A famous example is of the moose (Alces alces) population in Isle Royale National Park, Michigan. Moose had completely vanished from the island during the 19th Century as a result of hunting and habitat alteration. By uncertain means, a small "starter stock" reappeared about 1905, and with new land management practices (abandonment) this colonizing population soon exhibited J-curve growth in response to favorable vegetation. By 1930, however, moose had exceeded the island's carrying capacity and began to decline. Wildfire losses of vegetation in 1936 accelerated the decline. But during World War II, while the island received minimal utilization and favorable vegetation regrowth, the moose population again exploded, exceeding K again by 1945.

To this date, moose exhibited classic J-curve growth tendencies; only habitat supply limited their increase. The winter of 1948, however, was extraordinarily cold, with complete ice-over of the channel separating Isle Royale from the mainland. Across this came a small pack of grey wolves (Canis lupus). Within two years the wolf population had doubled, and their moose prey had dropped below K_moose. Over the next twenty-five years both the moose and the wolf populations stabilized near their respective carrying capacities. The addition of a predator control over the moose population caused a shift from J-curve to S-curve tendencies.

Not evident on the graph above, but confirming the role of predator control, is the consequence of canine distemper that broke out among the Isle Royale wolves during the late 1970s. As wolf numbers declined, moose populations reverted to J-curve characteristics. Wolves have since recovered, and the moose population has returned to S-curve tendency.

References:

Brown, J. H. and Gibson, A. C. Biogeography. LOC QH84.B76 1983 ISBN 0-8016-0824-4

Harris, A. and Tuttle, E. Geology of National Parks. ISBN 0-8403-2810-9

Odum, E. Population Ecology. 1977

Robbins, Chandler S., Bruun, Bertel, and Zim, Herbert S. Birds of North America. LOC 66-16454 ISBN 0-307-47002-4

Seddon, B. Introduction to Biogeography. 1971. ISBN 06-496140-0

Sheldrick, D. The Tsavo Story. 1975

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