An interesting question to ask someone who has any sort of background in biology is how much taxonomy affects niche. In biology, a group that evolves a feature usually finds one particular niche and stays there. For example, most birds, having evolved wings, have continued to be flying animals, penguins and ostriches notwithstanding. Most members of Ursidae, having evolved the tools to be at least partially carnivorous, maintain a carnivorous lifestyle, the Giant Panda excepted. Different biologists probably have different opinions on the matter, but in general taxons tend to radiate within a niche, only occasionally jumping outside of it. This pattern is more easy to detect in animals, where the form and function are more obviously linked, but it also holds true for plant groups, as well.

That introduction being said, Rosaceae seems to certainly follow the pattern of a family sticking in its niche. There are many different genus in the family, and some of them are speciated beyond counting, such as the genus Crataegus (the Hawthorn), which I have read as having over one thousand species. However, with all this diversity, most groups in the family follow a similar pattern. Almost all are somewhere in size between being large shrubs and small trees. Most have showy and profuse flowers that depend on insect pollination for fertilization (although many members of the family also can undergo parthenogenesis when needed), and producing much fruit that is dispersed by animals, often birds. Their range is mostly in the temperate zone, extending in a few examples to the subtropics and in a to the subarctic. Members of the family don't seem to fill an area or become climax species, instead filling in disturbed areas, such as river and stream banks and in modern time, roadsides. Their seeds are dispersed by birds, meaning they tend to grow wherever a bird happens to rest. They also, for trees and plants, have relatively short lifespans, on about the same scale as a human lifespan. There are probably exceptions to all of these rules, but this has been what I have observed about the more common members of the rose family, in some of their prime territory: the Pacific Northwest and the Rocky Mountains, during many long long walks. Whatever strategy for success the rose family discovered during the double rise of aves and flowering plants, they seem to have stuck to it and modified it for dozens of millions of years.

Within the unity of the family, there is a bit of diversity, and this is the part that often puzzles me the most. In the Willamette Valley of Oregon, for example, where members of the family seem to be one of the most common plants by road and streamsides, there are five main groups that compete with each other for valuable ditch space: the native blackberry, the (invasive) Himalyan blackberry, the wild rose, various types of Hawthorne, and the wild apple tree. I often wonder why one patch of road will be full of the fast growing invasive blackberry, while a little further on the native blackberry is still dominant. Or, when I see rose bushes surrounded by invasive blackberries, I am seeing a state of equilibrium or whether the blackberries are soon going to overtake the roses. Are Hawthorn trees and apple trees growing side by side going to both stay shrubs, or are a few of each going to turn into full fledged trees and confine the other to the shade? I do know there are a few factors that are somewhat predictable, such as the fact that roses seem to do better with cold and dry areas than the invasive blackberries do, but this hardly seems to make a difference when they are growing in a patchwork together. Some of the differences are more easy to see on larger scales: for example, in places both far to the east or far to the north of Portland, the very cold tolerant Sorbus spp. seems to take the place of Craetegus spp.. On the small scale, however, any guesses that could be made about plant tolerances and preferences seems to be slight in predictive power. It is not all a matter of curiosity, either: many species in this family are economically important, and I don't know if there is a simple answer for why the Pear is grown commercially in Oregon's Hood River Valley, while Apples are grown north of the river, on the plains of central Washington.

The only answer to this is that there is a lot to know about botany, and the more you study, the less sense things may make.