What is the relationship between affluence and ecological footprint?
It depends on the country.
Affluence is a measure of wealth, which does tend to have a large effect on ecological footprint. In general, more wealth means a larger ecological footprint in terms of a country's GDP. You could imagine a mansion with massive lawns, sprinklers, luxurious swimming pools and hot tubs having a massive footprint compared to a lower class apartment or one-room studio.
However, in some cases, we see that lower income countries have a greater per capita carbon footprint because they have to use less efficient methods to generate power, heating, and to feed themselves. Some countries in Africa are facing this because they have to use open wood fires for heating, which is about as inefficient as one can be. Thus, their ecological footprint tends to be higher than, say, a middle-class family living in Germany, which is mostly relying on renewable energy for power.
As wealthier countries increase their renewable energy and make their systems more efficient (a field which the US is leagues behind in) the tables may turn, but for now, the general trend is positive correlation between wealth and ecological footprint.