Once animal populations become so endangered that only a handful of individuals are left, it becomes very difficult to reestablish a viable population from the limited genetic pool available
Natural populations are threatened by various factors, such as pollution, invading species, developing infectious diseases, habitat loss and changes, direct death from exploitation, and climate change. An estimated 1 million species of plants and animals are at risk of going extinct in the next several decades due to these widespread and worldwide threats. Genetic variables are at the heart of the problems facing animals, and threats to wildlife populations frequently interact with one another. A population’s ability to adapt to novel conditions brought on by invasive species or climatic change, for example, may be hampered by genetic inbreeding and loss of genetic variation because of population decreases and fragmentation.
Results of severe inbreeding when people selectively bred for white tigers
Inbreeding results in the loss of genetic fitness, increased mortality in young animals, reduced fertility and depressed growth. This is commonly referred to as inbreeding depression. The number of breeding animals present in the herd or social groups is vital, as this will influence the effective population size and the rate of inbreeding in each generation. Based on the information obtained from studies on domesticated herbivores, 50 breeding animals are the effective population size that is required to maintain an inbreeding coefficient, or degree of inbreeding, of below 1% per generation. The sex ratio also plays an important role in the flow of genetic material in a population, because it affects the random variation of the gene frequencies between one generation and the next. An effective population size of 50 breeding animals can be structured according to sex with the help of the following equation:
Ne = [4(Nm Nf)] ÷ [(Nm + Nf)]
Where:
|
Ne = |
The effective population size. |
|
Nm = |
The number of effective breeding males. |
|
Nf = |
The number of effective breeding females. |
The size of a breeding herd does not only influence the effective population size, but it also influences the rate of inbreeding per generation. Suppose that a wildlife rancher wants to produce the African savanna buffalo. To be able to keep the inbreeding coefficient below 1%, 50 breeding animals are required. Suppose the sex ratio under natural conditions is 1:1. Therefore, when the above equation is used, 25 sexually mature cows and 25 sexually mature bulls will be necessary for the population to keep the breeding coefficient below 1%. However, suppose that the rancher has a surplus of six males that he wants to utilise for trophy hunting, while at the same time, he wants to increase the productivity of the herd and apply genetic conservation. Then the equation can be modified as follows:
The number of breeding bulls required = (3 ÷ 8) x 50 effective breeding animals = 19
The number of breeding cows = (5 ÷ 8) x 50 = effective breeding animals = 31
Therefore Ne = [4(19) (31)] ÷ (19 + 31) = 2356 ÷ 50 = 47 animals.
Here the natural parity of the sex ratio of the herd is changed to three males for every five females. This leaves 19 breeding males and 31 breeding females, which are substituted in the above equation to give an effective breeding herd size of 50 animals. The degree of inbreeding will therefore now be little more than 1%, with an effective population size of 47 animals. Now suppose further that effective breeding animals make up 40% of the total herd. Then the minimum necessary herd size to apply genetic conservation will be a total of 125 animals. The generation interval influences the rate of inbreeding in the following way: with a generation interval of five years, the rate of increase in inbreeding will be (1.5 ÷ 5) = 0.3% per year, with an inbreeding coefficient of 1.5.
To maintain the genetic integrity of a wildlife population, the sex ratios are very important to ensure sufficient breeding of animals in the herd. (Source: pixelstalk.net)
One should try to achieve equal sex ratios in breeding populations where an equal number of males and females participate in breeding to maximise genetic diversity in the following generation. Additionally, one should try to reduce the variance in the number of offspring produced by the various females. By doing this, it is possible to guarantee that genetic variation is maintained to the greatest extent possible from one generation to the next and that no single individual contributes disproportionately more to the following generation. If one individual contributes significantly more to the next generation—either a single male mating with all females or one of the females who produce the majority of individuals—it will reduce the genetic diversity in that generation because the genetic material of a sizable population is not passed on. This is one risk associated with the practice of some wildlife ranchers of selective breeding, which aims to develop animals with greater trophies, a particular colouring, or other morphological variations.
Elephants should ideally be kept in large conservancies where a breeding herd of 50 individuals are necessary to keep the inbreeding coefficient < 1%.
Elephant breeding herds should be a minimum size of 50 individuals to keep the inbreeding coefficient below 1%
Bottleneck Effect
The bottleneck effect displayed
The Bottleneck Effect in genetics occurs when an original population has been significantly reduced in number and an entirely new population has been bred from the surviving population which leads to the loss of valuable genetic material. Over a short period, valuable genetic material is therefore lost. Usually, these sharp declines are due to human interference, natural calamities, environmental damage or drastic climate changes, limited resources and even hunting and not as a result of death due to disease as this is seen as part of natural selection.
When a population is drastically decreased in number this restricts the genetic variety of the species since there are fewer individuals with whom to mate. When the bottleneck occurs, the few remaining individuals end up mating with relatives. Inbreeding then further reduces the size of the available genetic pool which leads to declined genetic variability and even possible mutations. The remaining population further struggles to adapt to changes in the environment and have a lower survival rate.
A. The Cheetah
Conservation of the cheetah is important due to its vulnerable status and low reproductive rate
The Cheetah population is believed to have gone through two historical bottleneck events which have significantly reduced their population size and gene pool. The first event might have been a result of the rapid range expansion of the cheetah population which caused small groups of cheetahs to be isolated over large areas of land. This restricted their ability to interchange genes between populations.
As a result of their shallow gene pool, their gene pool limits their survivability in nature. Unfortunately, they have a low reproductive rate, which means fewer offspring. This inhibits their ability to grow in population size and adapt to environmental changes. Other factors such as human activities, habitat loss and climate change further place pressure on the survival of this species.
B. The Black Wildebeest
The black wildebeest species was on the brink of extinction in the late 1800s
The Black wildebeest who once roamed the grasslands of southern Africa by the hundreds of thousands survived near extinction in the late 1800s. Consequently, the species underwent a genetic bottleneck in the 1930s which did bring the species back from the brink of extinction. Fortunately, their population was able to recover quite well to a point where they are not even on the list of vulnerable or threatened species.
Private farmers and conservation authorities initiated actions which led to the drastic recovery of the black wildebeest numbers. Today, the problem lies with these same actions which once saved the species and are now threatening the black wildebeest species. These injudicious translocations have brought the black wildebeest into the same vicinity which is occupied by the blue wildebeest. As a result, hybridisation has occurred and further places pressure on the genetic integrity of the black wildebeest population which has already undergone one significant genetic reduction event.
C. White Rhinoceros
Although the white rhino nearly faced extinction, it is now one of the most common rhino species. (Source: Stephen Temple)
Although the white rhino population is quite strong today with numbers reaching 18 000 individuals or higher, their population was threateningly low with merely 200 individuals left in 1970. Overexploitation between the 1890s and 1960s in the form of hunting for meat and sport was the cause of the drastic decline in population size for the white rhino.
The survival of this species is due to the astonishing conservation work done by Dr Ian Player, a famous South African conservationist. In 1960, about 100 white rhinos were discovered in the Umfolozi Game Reserve. Conservationists jumped to action to conserve this species and bring it back from the brink of extinction. The white rhinos were captured and relocated to protected areas within the country. Dr Ian played a fundamental role in the conservation of the white rhino as he established a breeding programme and sold the animals to zoos and safari parks or nature reserves across the world to protect the species. Over the coming 30 years he was able to enlarge the population size to the now around 18 000 white rhinos alive. Today, thanks to these magnificent conservation efforts, the southern white rhino is the only species not endangered and also the most numerous.
Ironically, the market for legally permitted white rhino trophy hunting was what aided the species’ recovery. Private game ranch owners were given sufficient financial motivation and legal protection to permit breeding and population control of the animals on their properties. However, the problem today still lies with the illegal trade of rhino horns.
As you can imagine, reestablishing an entire population of more than 18 000 individuals from a founding population of about 100 individuals resulted in quite a genetic bottleneck effect. Detailed studies have shown that this did affect the genetic diversity of the species and that it is lower than what would have been expected from a natural population.
There are many other wildlife species with which this has also occurred. Some of them were able to return and revive the populations while others unfortunately died out.
Minimum Social Herd Size
Every species has a minimum number of individuals which is necessary for a herd to ensure normal social behaviour
For normal social behaviour, a certain number of animals are necessary. A few factors affect the minimum viable herd sizes of wildlife populations. The size of the ranch, as well as social, economic, and genetic factors, play a role. The absolute minimum social group for re-establishment is three males and five females. Such a breeding herd will usually consist of six adult animals, of which one is a bull and five are cows, plus two young males. Any young males that are born into this herd will join the other young males in a bachelor herd when they leave the breeding herd later because of social pressures.
It is important to remember that animal numbers should always be in accordance with the size of the farm. Establishing a population which is larger than what the area can accommodate will have detrimental effects on both the plant and animal spheres. In case of limited area, it is therefore advised to have fewer species diversity but larger herd sizes of the selected species.
We will briefly take a look at some recommended herd sizes for some of the wildlife species which will help maintain social behavioural structures for each species.
A. Elephants
For social behaviour, elephants require a herd size of 15 individuals. (Source: Rudi van Aarde)
In East Africa, the mean herd size of elephants is 3.1 individuals for the bachelor herds and 11.6 for the breeding herds. An average herd of 15 animals could serve as the general norm to maintain social behaviour in an elephant population.
B. Black and White Rhino
To maintain social behaviour, both black and white rhinos require a minimum herd size of at least 6 individuals. (Source: AZ Animals)
Rhinoceroses which are being relocated are recommended to be a minimum of 6 animals per ranch to maintain social behaviour integrity along with the age and sex ratios.
C. Lions
To maintain social behaviour, lions form prides with at least 4-6 individuals and separate male coalitions of up to seven males can hold tenure over the females
The pride, which comprises several related females (none dominant) and their cubs, is the main social structure in the matrilocal society of lions. Pride size is measured by the number of adult females. Arid environments have the smallest pride size and otherwise range from 4-6 adult individuals. Young females who are not yet part of a natal pride may form unrelated female prides.
A single male or a coalition of males can include up to 7 individuals who hold tenure over one or more female prides. This prevents other males from siring offspring with their females.
D. Blesbok
Blesbok ewes form a herd of a minimum of 10 individuals and separate bachelor herds will form as well
The ewes form breeding herds which consist of a minimum of 10 individuals per ram and separate bachelor herds are found.
E. Blue Wildebeest
The minimum social herd size of blue wildebeest includes six adult cows per bull and a separate bachelor herd. (Source: African Sky)
These antelopes form social structures which consist of a territorial bull and a few cows and their calves with a mean herd size of 13.7 animals and a minimum of six adult cows per bull. Separate bachelor herds will also form with a mean of 6.4 individuals.
F. Buffalo
The smallest suggested herd of buffalo should consist of 8 individuals
The natural herd sizes of buffalo tend to be extremely large and the mean herd sizes which occur in the Kruger National Park far exceed the amount of animals most other wildlife ranches would be able to support. The minimum herd size for buffalo reestablishment is therefore suggested to be 8 individuals of which five are breeding cows and three are bulls.
G. Hippopotamus
Due to the aggressive nature of hippos, a minimum of five individuals are suggested on small game ranches
Due to the aggressive nature of hippos, the re-establishment of hippos onto small wildlife ranches is not suggested and expert evaluation of the environment and area should be done before introducing these animals onto any wildlife ranch. For social behaviour, hippos older than five or six years should be considered for reestablishment and a minimum of five individuals are suggested. However, for a viable population, more individuals should be introduced. Usually, they have a sex ratio of 1:1 but on a smaller wildlife ranch, only one adult bull per herd is best.
The mean herd sizes for some herbivores in the Kruger National Park. (Source: Unpublished reports of the South African National Parks)
|
Animal |
Mean Breeding Herd |
Mean Bachelor Herd |
Percentage Annual Increase |
|
Blue wildebeest |
13.7 |
6.4 |
17.4 |
|
Burchell’s zebra |
5.3 |
4.1 |
12.4 |
|
Giraffe |
5.7 |
3.1 |
5.2 |
|
Greater kudu |
8.0 |
3.6 |
14.8 |
|
Impala |
21.9 |
6.4 |
11.6 |
|
Nyala |
4.1 |
2.5 |
17.5 |
|
Reedbuck |
3.7 |
– |
15.7 |
|
Tsessebe |
7.7 |
3.0 |
25.7 |
|
Warthog |
3.0 |
2.2 |
12.3 |
|
Waterbuck |
9.9 |
4.4 |
13.4 |
Minimum Surface Area
The minimum surface area of a reserve should be considered when stocking the reserve with wildlife. Different habitat and vegetation types will also affect how many of each species will be able to survive irrelevant of the total surface area. (Source: Welgevonden Game Reserve)
Suppose the same example that was used above for the African savanna buffalo is used again, and assume now that the wildlife rancher has the following objectives:
- Grazing capacity is 10 ha per Large Animal Unit.
- The buffalo must form 15% of the total biomass of herbivores on the ranch.
- The total buffalo population is 125 animals.
- An African buffalo equals 1.0 Large Animal Units.
∴ We need to therefore determine what the minimum size of the ranch should be to accommodate the 125 buffalo at a 15% stocking rate of the total biomass.
Therefore:
The number of animals x conversion factor = Large Animal Units in buffalo, or Large Animal Units present = 125 x 1.0 = 125 Large Animal Units in buffalo.
The size of the area that is required for these buffalo = number of Large Animal Units x grazing capacity required per Large Animal Unit = 125 Large Animal Units x 10 ha per Large Animal Unit = 1250 ha.
Therefore, 15% of the total stocking rate is equivalent to 1 250 ha, and the total size of the ranch that will be needed for a herd of 125 African savanna buffalo is (100 ÷ 15) x 1 250 ha = 8 333 ha.
In conclusion:
For the wildlife rancher to keep the buffalo population of 125 animals at a 15% stocking rate of the total biomass of the ranch, he will need a ranch which is 8 333 ha in size.
Below is a table of the Large Animal Units for a few of the common wildlife species:
Large Animal Unit Conversions (Grossman 1991)
|
Species |
Mass in Kg |
L.A.U |
|
Blesbok |
61 |
0.22 |
|
Bontebok |
55 |
0.21 |
|
Buffalo |
495 |
1.07 |
|
Bushbuck |
30 |
0.13 |
|
Common Duiker |
19 |
0.09 |
|
Eland |
500 |
1.08 |
|
Elephant |
4500 |
10 |
|
Gemsbok |
210 |
0.56 |
|
Giraffe |
830 |
1.58 |
|
Hippopotamus |
1340 |
2.24 |
|
Impala |
50 |
0.19 |
|
Klipspringer |
13 |
0.07 |
|
Kudu |
200 |
0.54 |
|
Nyala |
60 |
0.23 |
|
Rhino: Black |
900 |
1.65 |
|
Rhino: White |
1800 |
2.75 |
|
Roan Antelope |
250 |
0.64 |
|
Warthog |
70 |
0.25 |
|
Waterbuck |
180 |
0.50 |
|
Wildebeest: Blue |
180 |
0.50 |
|
Zebra |
260 |
0.66 |