RISKS OF RARITY: DEMOGRAPHIC RISKS - 2
Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 12: pages 309-323 & 327-331 (Skip Box 21)
Introduction:
Rare species experience some risk of becoming extinct due to population fluctuations. A population that is rare may be well below carrying capacity, and its fluctuations largely density-independent. By chance, might a population fluctuate to zero, hence extinction? A little more exploration of the principles of population dynamics is useful.
Age Structure and Population Statistics:
Birth rates and death rates can be treated as population averages, but it also is useful to know how they vary with age.
Survivorship refers to the probability of surviving from age 0 to age x, or from age x to age x+1. Theoretically we look at three extremes: (a) survival through the full physiological life span, (b) constant probability of mortality throughout life, and (c) high juvenile mortality, followed by low mortality until old age. Eg, well-fed humans, songbirds, and many marine fishes. Carefully collected survivorship data allows us to estimate maximum lifespan, mean longevity (usually much less), and age-related survival bottlenecks.
Birth or fecundity rates refer to the number of female births per year, per female of age x. One can determine an age at which reproduction begins, when reproduction creases, and an age of peak fecundity.
The net maternity function is obtained by multiplying age-specific survival times age-specific fecundity. This estimates the actual expectation of reproduction. It can be used to determine "r".
Body Size and Population Statistics:
Organisms of large body size – elephants and Sequoia trees -- are long-lived and slow-reproducing. Small beasties live short, fast lives. Note relationship between body size and generation time. As body size increases, r declines, and longevity increases.
Conservation Implications:
To summarize some core ideas of populations: Each species has an intrinsic rate of natural increase, which is maximal under environmental conditions that are ideal for that species, and is determined by its survivorship and fecundity. These, in turn, are a function of body size. Rare species may not be much affected by K, if they are rare due to over-hunting, or they may be affected by K is they are rare because their habitat is rare. They certainly are affected by "r". The intrinsic rate of natural increase influences the "bounce-back" from a catastrophe, and the rate of establishment of a re-introduced species.
It is helpful to identify two types of risks to rare populations. Catastrophes, such as a storm, a fire, etc, can easily wipe out a small localized population. The probability might be low – say .02 to .05 – which amounts to an event every 20-50 years. Demographic accidents refer to chance variation in birth rates and other vital statistics. If only a few offspring are born each year, the sex ratio matters a lot.
To best withstand the "risks of rarity", ideally a population would (a) have a high rate of population increase (r), (b) be long-lived (c) not fluctuate a lot in abundance. Unfortunately, correlations with body-size force tradeoffs.
In general, large organisms are long-lived, fluctuate little, have low "r"’. They are vulnerable to a series of catastrophes, because their numbers recover so slowly. They are less vulnerable to a couple of years of reproductive failure, perhaps due to environmental conditions, because the individual organisms are so long-lived. Small organisms are the opposite in every regard.
Example: birds on small British islands (16 islands from 0.7 to 7.7 km2, 100 bird species, 355 popuations). A turnover is the disappearance of a breeding population for one year. Extinction probability = 1/turnover time.
Metapopulations:
We may imagine populations to be evenly distributed over some large area. In reality, most populations consist of a number of smaller, semi-isolated sub-populations, usually because of the patchy distribution of suitable habitat. The term metapopulation refers to the overall population. If small population experience higher extinction risk, and the sub-populations are truly isolated, we may expect them to ‘wink out" one by one, until the species is extinct. If dispersal rates are high relative to extinction rates, re-colonization will regularly "rescue" extirpated sub-populations. As human activity fragments populations and restricts dispersal routes, we increase the risk of extinction.
Transparencies: 1. Survivorship curces 2. Fecundity functions 3. Body size and generation time 4. Bird extinction rates 5. Metapopulation concept 6. Metapopulation and extinction risks 7. Mountain sheep metapopulations 8. Habitat fragmentation creates metapopulations
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