We have exactly one planet, which has been around for more than 4 billion years. Life arose on our planet surprisingly quickly, and evolution has produced tens of millions of species. These species have evolved in response to each other and their physical environments.
Ecosystems are sets of species and their physical environment that are inter-dependent. Every species is adapted to a particular kind of climate and physical conditions (e.g. soil structure, water salinity). In turn, biological species alter the physical environment, such as when ants tunnel into soil and provide air for plant roots. Therefore, preserving species requires that we preserve their ecosystems as well.
Humans are an increasingly important component of every ecosystem on earth. Urban landscapes exclude some species (e.g. moose, redwood trees) and provide habitats to which other species have adapted (roaches, dandelions). Humans also have cleared large swaths of ecosystems to enable agriculture, have sent novel chemicals into our atmosphere and oceans, and moved species to new areas where they out-compete native plants and animals.
Regardless of your politics or feelings about conservation, you cannot escape the conclusion that humans have a profound influence on the environment. It also is inescapable that some of those influences have adverse effects on our own health and well-being. Air pollution in 1950s London caused an estimated 12,000 fatalities (Bell et al. 2004); the rising incidence of asthma in urban areas can be traced to allergens in the home (Gautier and Charpin 2017); farmers face increasing crop losses from invasive species pests (Paini et al. 2016). Traces of human activity can be found in the remotest locales, including the great plastic swirl in the Pacific and polychlorinated biphenyls in Antarctica.
A cumulative effect of human activity is loss of species, and the rate of extinctions has accelerated rapidly in the past few decades (Ceballos et al. 2015). The passenger pigeon, the dodo, American chestnut trees — all are gone, and their demise can be clearly traced to human activity. Extinctions can happen quickly, especially in ecosystems that are isolated and highly specialized such as those on islands.
Governments around the world act to protect their native species, with many adhering to guidelines from the International Union for the Conservation of Nature (IUCN). In the United States, the Endangered Species Act identifies species that scientists have confirmed as heading towards extinction, and puts in place legal safeguards for their habitats. This Act has become a political hot potato: while it has indeed averted extinctions, including that of the bald eagle, it has also placed restrictions on environmental exploitation (mining, logging, grazing) that some view as burdensome and economically indefensible.
The field of environmental economics places monetary value on ecosystem services (Costanza et al. 2014). By providing clear air and water, shade, recreation, and other benefits, healthy ecosystems contribute more to human well-being than all other economic activity put together. The challenge, though, is that individuals take these services for granted but not their paychecks. Thus perceived conflict between an environmental issue and personal needs are difficult to resolve.
Environmental science is a broad discipline that uses mathematics, chemistry, physical sciences, and biology to understand how ecosystems work and to inform policymakers who wrestle with difficult questions of public goods and scarce dollars. Understanding the nature of ecosystem services and how human activity can negatively affect those services is essential to making wise policy decisions that will serve generations to come.
For further reading:
Barnes, D. K. A.; Galgani, F.; Thompson, R. C.; Barlaz, M. (2009). "Accumulation and fragmentation of plastic debris in global environments". Philosophical Transactions of the Royal Society B: Biological Sciences. 364 (1526): 1985–98.
Bell, M. L., D. L. Davis and T. Fletcher (2004). "A retrospective assessment of mortality from the London smog episode of 1952: The role of influenza and pollution." Environmental Health Perspectives 112(1): 6-8.
Ceballos, G., P. R. Ehrlich, A. D. Barnosky, A. Garcia, R. M. Pringle and T. M. Palmer (2015). "Accelerated modern human-induced species losses: entering the sixth mass extinction." Science Advances 1(5): 19 June.
Costanza, R., R. de Groot, P. Sutton, S. van der Ploeg, S. J. Anderson, I. Kubiszewski, S. Farber and R. K. Turner (2014). "Changes in the global value of ecosystem services." Global Environmental Change-Human and Policy Dimensions 26: 152-158.
Fuoco, R., M. P. Colombini, A. Ceccarinni and C. Abete. 1996. "Polychlorobiphenyls in Antarctica. " Microchemical Journal 54 (4) :384 - 390.
Gautier, C. and D. Charpin (2017). "Environmental triggers and avoidance in the management of asthma." J. Astham Allergy 7 March.
Paini, D. R., A. W. Sheppard, D. C. Cook, P. J. De Barro, S. P. Worner and M. B. Thomas (2016). "Global threat to agriculture from invasive species." Proceedings of the National Academy of Sciences of the United States of America 113(27): 7575-7579.