I was wondering whether someone has an idea about how to investigate community changes of plants/animals along an urban-rural gradient? I have a large data set consisting of plant/animal abundance and urban variables such as % impervious surface. I thought about Jaccard similarity index as response and urban variables as explanatory variables. But I assume there are better approaches. I would appreciate any shared experiences in this. Many thanks!
I take a great interest in plants, birds and insects of the British countryside, so I may be able to make a few observations which will help you with your study. Over the last 50 years and particularly in the last 20,bird populations both rural and urban have generally declined, though there are some exceptions to this. The urban and suburban decline is due in part to:
1) Concreting over front gardens to provide parking space for vehicles. 2) Urban renewal and "improvement", which gets rid of waste ground and mini-habitats. 3) People are becoming more house proud and less in touch with nature than they used to be,gardens more manicured, shrubberies removed,staging etc added. 4) Modern buildings are less friendly to nesting birds, especially swifts. 5) Residents illegally destroy house martin nests because bird droppings soil their cars. One man I know went so far as to shoot the nesting birds. 6) Intensive agriculture has reduced weed, bird, and invertebrate populations on city outskirts. 7) Reduction of weeds and wild flowers means reduction of seeds and insects, and therefore a reduction in small birds.
There are some interesting exceptions to the general rule. Blackbirds might be expected to decline, because lawns and therefore worms are fewer, but blackbirds are as common in suburbia as ever they were. The same is true of urban wood pigeons. Strangely, song thrushes, which used to be common in Hereford 20 years ago have now almost vanished, and reports suggest that this decline is general all over southern England. Perhaps a reduction of garden snails is to blame. Magpies were always common in the countryside, but over the last 20 years there has been a great increase in suburban magpies. Urban colonies of herring Gulls and Lesser Black backs are becoming commonplace, and there has been one in the centre of Hereford for the last 20 years. Collared doves are a new species which began to colonise the London area 70 years ago and are now common in suburbia all over southern Britain, but rarely seen in deep countryside. The same thing is happening with ring-neck parakeets, which threaten to become an agricultural pest. House sparrows and starlings are said to be getting scarce in S.E. England, but are still common in Hereford.
Regarding wild flowers, they are still to be found in towns and countryside, but not in their former numbers, though dandelions and some other weeds are as common as ever. The decline in weeds has probably been one of the causes of a decline in butterflies and bumble bees. The number and variety of butterflies coming to my Buddlia bushes in suburban Hereford is far less then it was 40 years ago. Painted Ladies are summer migrants, so are less affected than resident species, and they are abundant this summer. Large whites and a few other species are as common as ever. Garden ponds encourage dragonflies and other invertebrates, and I occasionally see a few though I have no pond of my own.
As you very likely know, there has been a huge increase in suburban foxes prowling around houses in search of food, and they are sometimes encouraged by friendly wildlife watchers. If there is an outbreak of rabies, as one day there must be, they will become a danger to the public if it gets into the fox population. Grey squirrels too have become increasingly common, but their presence is damaging to other wildlife, as they eat the eggs and young of small birds. The same is true of magpies. So you see that the issue of wildlife populations in urban, suburban areas and the rural perimeters of towns is very complex. The general picture is one of decline, but there are exceptions and they are sometimes difficult to explain. Climate change has to be one cause of changing bird populations and especially the arrival of new species, of which I mention only a few.