The world's biotic diversity, formed from the ‘dust of the earth' (geological matter) and powered mainly by solar-derived energy, is strongly influenced by climatic conditions. Distribution of ‘life-forms' is determined by the character of the substrate in combination with the climatic factors of light, temperature and moisture.
Activity of the various ‘life-forms' is stimulated by climatic conditions, modified by interactions within each particular community. Behaviour patterns have developed for optimum survival of each ‘form'. Many animal species make use of extreme conditions to aid their survival.
We have examples of polar bears (in the north) and penguins (in the south) breeding in the earths coldest areas, while crab plovers and certain other species head for the hottest parts of the earth. These species would not adapt well in the opposite extremes. Areas of greatest biological diversity are generally located in the most climatically stable regions of the earth, often in the coldest seas and in the hottest jungles.
A vital requirement for life is available moisture. Where moisture is limited, as in ice fields or desert, diversity is greatly reduced. The degree of ‘water' and ‘light' (affecting temperature and food production) become controlling factors for physical life as we know it.
Within this sub-tropical region of Africa we are accustomed to loosely defined seasons of - warm/wet - cool/dry - and - warm/dry/windy weather. Throughout the world the seasons are affected by the annual tilt of the earth influencing day length and temperature. Winds are generally opposite in the northern and southern hemispheres and this has a varying effect on precipitation.
The adults of many species are adapted for survival in climatic extremes and prolonged periods without food and water. One fairly constant requirement for all species is the availability of nutrition for development of the young. The availability of suitable ‘nesting material' and adequate cover are among other limiting factors that must also be considered. Seasonal climatic conditions become a determining factor in the behavioural activities of most species.
Seasonal And Inter-Related Behaviour
Many plant and ‘insect' species are very sensitive to temperature and humidity. In ‘summer' day length increases and atmospheric temperatures rise. As ground temperatures rise plant growth is stimulated, usually assisted by increased moisture. Rising temperatures and plant growth encourage ‘insect' activity.
Plant and insect production provides food for birds, reptiles and mammals with an influence on higher predators. Starting with sensitive plants and insects, the opposite occurs as it becomes dry and cool again.
Bearing in mind the vital necessity of food for young who lack the tolerance of adults, breeding will cease when the food supply diminishes. If climatic conditions continue to sustain food production then breeding will continue for as long as possible. Fast breeding species adapt very quickly while large, slow breeding species can not quickly adjust their behaviour.
The information given in our ‘natural history' books is based on records of known distribution and behaviour. They can not take all unusual weather conditions into account although many cover a wide range of conditions. With changing weather patterns we are very likely to find changed distribution and breeding patterns for many species.
Observed changes may be more closely related to the indirect behaviour of other associated species rather than the effect of prevailing weather conditions. Inter-related behaviour is better explained by a few examples. Deeper soils are attractive to antbears (Aardvarks) that dig sizeable holes.
The occupied holes become havens for many insect species. The vacated holes become homes for a sequence of many other medium sized mammals such as porcupines, civets, hyenas, wild dog and jackals, each with their own ‘insect' community. Hole nesting birds burrow into the walls of the main hole and many reptiles use them for shelter.
The mound building and other activities of various termite species have an important effect on plant growth and the behaviour of many other animals. In some soils they are the most important factor for animal survival within that region. Trees pushed over by elephants make browse available to lower animals, remove perches and nesting sites for larger birds and form ground cover for game birds and small mammals.
Dead trees are important for woodpeckers and barbets. I have mentioned previously that spider web is an important item in the nest building of many bird species, without which many birds will not breed. Swallows will not breed unless there is mud available for nest building. So it continues - with the actions of certain species affecting the behaviour of other species - stimulated by but not solely subject to climatic conditions.
Under normal circumstances the interactive behaviour of the many species is synchronised but it is pretty obvious how one missing ‘link' in the chain of events can vitally effect many other species.
There are no species that are useless or expendable in nature and each should be granted the right to exist within a naturally balanced ecosystem. The catastrophe of extinction is not only the loss of one species but the chain effect on all inter-related behaviour and function of other species.
After several dry years we have had above average rainfall this season (2005/2006). Falls have been variable throughout the area and against the escarpment we have recorded a total of around 1000 mm to date. Apart from some light falls in November, most of the rain has been concentrated in three months from the middle of January to the end of March. Humidity and temperatures were high for a long period before the main rains arrived.
The concentrated falls have resulted in saturated soils and accelerated erosion. Plant growth, flowering and insect activity was stimulated early but went through an uncertain dormant period before mid January.
Plant growth, flowering, insect food species, spider web construction and a whole lot of activities were delayed, resulting in a delay in nest construction and breeding by many bird species. Some birds managed an early attempt at nesting but most were delayed into the new year. Heavy falls and high winds disrupted many of the late attempts.
In the present stable conditions some species are nesting much later than previously recorded. The effect of cooler temperatures is now becoming apparent. The last comparable rainfall was that of the 1999/2000 season when we recorded a little over 1200 mm.
In that season the rain started in August and ended in late June 2000 - almost 11 months of wet conditions. Insect activity and food production was extended and many birds completed several successful breeding cycles before temperatures dropped.
No matter how hard we try to ‘box' and classify animal behaviour, there will always be unexpected and interesting departures from the observed norm. There are many incidences in the animal world but I will stick to some illustrations from birdlife.
Cut-throat finches regularly use abandoned weaver nests for breeding late in the season. blue waxbills normally build their own grass ball-shaped nests - often near a wasp's nest for added protection. This year they have bred later than normal and some have used the old nests of red-headed weavers.
Some years ago, in the Hwange National Park, I had cut-throat finches using an easily observed sparrows nest above my office desk. In each of five breeding cycles they laid six eggs. In the first cycle they reared six young. During the second cycle I found a young chick on my desk and returned it to the nest.
The parents would not accept it and it was promptly ‘kicked out' again - not to survive. In the third cycle two chicks were discarded, followed by three, four and five, respectively, in the fourth, fifth and sixth broods.
Although they regularly laid six eggs they increasingly evicted chicks that they were unable to feed. When they could only rear one chick they ceased their breeding attempts.
Regulation of the number of eggs (clutch size) has more often been recorded in response to food supply. We recorded barn owls successfully rearing 13 chicks (normally 3 - 5) and blackshouldered kites rearing 9 chicks (normally 2 - 4) in natural response to a seasonal out break of ‘multi-mammate' mice.
A jacobin cuckoo laid 21 eggs in the nest of a black-eyed bulbul. The incident only became apparent after the eggs began falling to the ground beneath the nest which was piled high with white cuckoo eggs. This type of behaviour is difficult to explain. We don't know all the answers and would welcome reports of any unusual animal behaviour - apart from those concerning folk who get out of their vehicles in the middle of the game reserve !