The hоrizоntаl аxis is lаbeled Species Present, and the vertical axis is labeled Nitrоgen Influx in kilograms per year. On the horizontal axis, from left to right, there are evenly spaced tick marks labeled N, B, S, and S B. The vertical axis has evenly spaced tick marks labeled 300 through 600,in increments of 100. The pointover N is at about 375. Its error bar extends from about 350 to about 425. The pointover B is centered at about 375. Its error bar extendsfrom about 360 to about 405. The pointover S is centered at about 430. Its error bar extends from about 410 to about 470. The pointover S B is centered at about 510. Its error bar extendsfrom about 500 to about 530. The caption on the figure reads: Figure 1. Mean nitrogen influx plus or minus 2 times the standard error of the mean as a function of species present. N equals neither salmon nor bears present, B equals only bears present, S equals only salmon present, and S B equals both salmon and bears present. Figure 1. Mean nitrogen influx ±2SEẍ as a function of species present. N = neither salmon nor bears present, B = only bears present, S = only salmon present, and SB = both salmon and bears present. Pacific salmon and black bears have often been cited as examples of keystone species. Pacific salmon spawn in freshwater streams but spend most of their lives at sea. When mature salmon return to the freshwater streams to spawn, they are preyed upon by bears and other predators. When salmon migrate from their marine habitat to the freshwater streams, they bring nitrogen and other marine-derived nutrients that subsequently remain in the areas surrounding the streams—a process called nitrogen influx. In an investigation, the relationship between black bears, salmon, and influx of marine nitrogen into the area around a southwestern Alaskan stream was studied. The investigators established several test plots of the same size along the stream with the following species composition: no salmon or black bears (N), bears but not salmon (B), salmon but not bears (S), and a plot where salmon and bears interact (SB). Nitrogen influx in the different sampling areas was measured as a means of assessing the impact of the different species on the health of the ecosystem. The data are plotted in Figure 1. Which of the following statements is best supported by the data?