Butterflies and Population Dynamics

Through my office window I can see several different species of butterflies flitting around yellow buddleia flowers. This year's butterfly population is not nearly as numerous as last year's. Last year, for months on end you could see dozens of butterflies at any given time you wandered into the garden. This year, sometimes there are a few around, other times you can't see any. Conditions in the garden have not varied much; in fact, there are probably more flowers available, especially on the buddleias, which they seem to favour. There was less rain over summer than we had last year, but it was still, for us, a milder and damper than usual season. What could explain this great population difference?

I have noticed similar fluctuations in fauna population from year to year with other species that share our  garden space. Some years the lizards that scurry among the pebbles and across the paths are plentiful, other years there are few to be seen.

This applies as well to pest populations. This year has seen an outbreak of 26-spotted ladybirds (the only ladybirds that are regarded as garden pests). In nearly 4 decades of gardening I have never encountered this pest before, now they are prolific. 

You might think, as I did, that these population fluctuations have something to do with seasonal conditions and the absence or presence of predators favouring or disfavouring certain species. This does apply to some extent, but I recently came across the information that population fluctuations obey mathematical rules. It seems that population spikes and troughs follow one of several patterns depending on a mathematical formula, which I am not even going to try to explain here. Anyone interested in this can find out more at the following site: http://members.fortunecity.com/templarser/rhythm.html   

Though it is hard to get your head around, at least, I find it so, it is yet another example of how amazing, complex and fascinating the natural world is.

The photos show some of last year's butterfly bonanza.

Does The World Contain Too Many Mongolian Pear Trees?

Autumn has been mild and sunny, beautiful weather for being outside working, playing or relaxing; until today's foretaste of winter. Deciduous trees are showing off their red, orange and golden foliage displays, which glow in the sunshine.

The main street of my town is planted with Mongolian pears, which have been displaying their flamboyant autumn finery. These trees are planted in many other locations around the town. In early spring they are among the first trees to blossom, their mass of white blooms marking the end of a gloomy winter. They are quite hardy, tolerating both extreme cold and dry conditions. Why then, do I dislike them? Both their spring floral abundance and their fiery autumn foliage are very short-lived. But the same can be said of other trees that don't arouse the same distaste.

I think it is because they are so over-used. They are a 'safe' choice for street trees and avenues. Thus, a street in inland Victoria can look similar to a street anywhere else in the world that has these same trees planted. I would much prefer to see a mixture of the indigenous plants of the area used as street and garden trees so each location reflected its own character and a diversity of leaf and floral interest throughout the year. Such a mixture would also assist in conserving the biodiversity of the area.

I recently attended a talk where a botanist recommended that no one tree species should represent more than ten percent of the street plantings of an area. Does the world contain too many Mongolian pears?