All banksias have developed proteoid or cluster roots in response to the nutrient-poor conditions of Australian soils (particularly lacking in phosphorus). The plant develops masses of fine lateral roots which form a mat-like structure underneath the soil surface. These enable it to extract nutrients as efficiently possible out of the soil. A study of three co-occurring species in "Banksia" woodland in southwestern Australia—"Banksia menziesii", "B. attenuata" and "B. ilicifolia"—found that all three develop fresh roots in September after winter rainfall, and that the bacteria populations associated with the root systems of "B. menziesii" differ from the other two, and that they also change depending on the age of the roots. Along with its shallow lateral roots, "Banksia ilicifolia" sinks one or more deep taproots seeking the water table. It is an obligate phreatophyte, that is, it is reliant upon accessing groundwater for its survival; it is more closely tied to the water table than the co-occurring "B. menziesii" and "B. attenuata", and must remain in areas where the depth of the water table is less than below the surface. Recent falls of the water table on the Swan Coastal Plain from use of the Gnangara Mound aquifer for Perth's water supply combined with years of below average rainfall have seen the population and vigour or "Banksia ilicifolia" fall considerably (more so than other banksia species) since the mid-1960s.