This can increase the number of pests in the area, affecting crops. It could also lead to an increase in disease carrying insects, such as mosquitoes, leading to higher disease rates among other animals, including humans. This could affect populations of these other animals, affecting populations of other species that prey on or are preyed upon by these species, and so on. Ecosystems are incredibly complex things; the interconnections between the disparate elements are so intertwined and so complex that it is almost -- perhaps completely -- impossible to know the effects of changing or removing a single element. Even more complex and unknowable are the indirect effects on portions of an ecosystem from other sources. The acidification of the Hubbard Brook is almost certainly a result of human pollution. Carbon dioxide can cause acidity in the atmosphere, which is collected by rainwater as it falls through the levels of the atmosphere and eventually enters the groundwater and, to an even larger degree, surface waterways such as rivers, streams, and lakes. Thus, humans are almost certainly responsible for the acidification of the Hubbard Brook in the 197-s, and the resultant decline in the number of fish species present there. Other effects of this acidification, such as the possible rise in disease vectors like mosquitoes, which has a direct effect on humans, are all related back to human's pollution of the...

This shows how it is impossible for any constituent of an ecosystem, even (and perhaps especially) humans to escape the effects of their actions upon a given ecosystem.
Other species, too, are affected by the change in fish populations, as are other parts of the water system which did not experience acidification. When fish populations change upstream, it affects all of the ecosystems downstream, too. A small population change upstream could have exponentially greater affects downstream, as populations dwindle faster due to less movement. The fish populations almost always disappear downstream before they do upstream; it is not that the acidification necessarily occurred downstream before or to a greater degree than upstream, in fact the exact opposite may be the case. Regardless, change in one are has a huge impact in other areas as well.

The interconnectivity and unpredictable relationships of ecosystems goes even farther than the above paragraphs suggests. There are no distinct boundaries to ecosystems, so each one affects those surrounding it. This is a lesson it has taken a very long time for humans to learn; one might argue, in fact, that it still has not been learned. The fact that this study appeared almost forty years after the fact is a good indicator that we are only now truly beginning to understand the importance of ecosystems and our actions in relation to them.