Following the Curve

Curves tell complex, frequently captivating and sometimes shocking stories.  Certainly many of us will be watching the Covid-19 situation unfolding along statistical curves. Curves of infection, of recovery and of deaths. Curves climbing exponentially, peaking, leveling and dropping, and perhaps of second and third waves following the same pattern (take a look at I’ve been reading about the Spanish Flu (1918) and the Black Death (mid-1300) both of which followed a similar pattern. The Black Death killed 40-50% of Europe’s population. It was the poor who died in droves from the Black Death as only the wealthy could self-isolate. The pattern of Covid-19 infection in South Africa may be similar should the disease force its way into densely populated areas.

I am reminded of my studies in population dynamics during my MSc in Conservation Biology, graphs of populations; single curves representing total population size or multiple curves representing births, migration and deaths. Or of different populations influencing one another.  I would like to share with you some of those that I find fascinating.

Predator and prey populations can be mapped to show how they affect one another.  As the hare (prey) population increases so too does the lynx (predator) population until the point where the predators have such an impact on the prey population that this decreases, followed by the predator population experiencing a food shortage. Prey populations increase again, predators follow and so on. This may be a somewhat simplistic look as a number of other factors may influence these population dynamics (Stenseth et. al, 1997), yet the pattern remains.

Rats were inadvertently introduced to southern African coastal islands by passing ships 300 years ago. Rat populations exploded, decimating populations of indigenous flightless insects, birds, small mammals, as well as the island vegetation. Many of these species became locally extinct. You can imagine the curves for this one.

Another example is the effect of populations of wolves and deer on one another and on forest habitat ( Reduced predation as a consequence of wolf extermination allows the ‘escape’ of their prey species with deer populations rising. This is followed by growth in the remaining wolf population. The wolves reduce and limit the deer population allowing forest saplings to grow beyond the vulnerable size zone and for reforestation to occur. Wolves aren’t just effective in reducing deer populations, they fundamentally change how these herbivores behave, where they graze and which areas they avoid. Trees and plants grow again in places that were overgrazed, giving shelter to numerous forest animal species.

And my favourite example, highly complex and dealing with several species.  Sea otters in the Northern Hemisphere have been recognised to be ‘keystone species’, comparatively rare but with a strong influence on the environment.  Sea urchins form a favourite part of the otters’ diet.

Urchins are herbivores and feed on seaweeds like kelp. When urchins are uncontrolled by predators they become larger and more abundant, feeding on kelp until little remains. By controlling the size and number of urchins, sea otters create the opportunity for kelp to grow and flourish, which in turn creates habitat for numerous other kelp forest dwellers. Fish lay their eggs in amongst the kelp.  As a consequence of the seal fur trade, orcas switched their attention to otters causing an abrupt decline in their populations. Urchin populations increased rapidly, kelp forests declined and fish populations dropped as their nursery grounds were damaged (Stevens, 1999)

*please note that this rough diagram is merely illustrative.

Life is filled with examples of abundance and decline. We are experiencing a unique and bewildering situation where we are able to map COVID-19 patterns in South Africa and globally and predict to some extent their outcomes. We are in principle experimenting with lockdowns and other measures for controlling the spread of the virus. May these measures be highly successful and may South Africa’s curves unfold to tell an optimistic story.

  • Stenseth N., Falck W., Bjørnstad, O. and Krebs, C. Population regulation in snowshoe hare and Canadian lynx: Asymmetric food web configurations between hare and lynx. PNAS May 13, 1997, 94 (10) 5147-5152.
  • Stevens, W. Search for Missing Otters Turns Up a Few Surprises. The New York Times, Science Times, Tuesday, January 5, 1999, pp. D1-D2.

Kirsten Neke