The animal product industry is directly responsible for significant environmental destruction. “Protecting the environment” may be an unpersuasive argument for some. But it's worth noting that effects on the environment are not solely important for those who find nature inherently valuable – given “nature's” direct impact on humanity, the following can be seen as humanitarian concerns as well. By reducing the demand for animal products, it's possible to benefit both the environment as a whole, and the human beings that it directly impacts.
1. The livestock industry is a major driver of global climate change.
As a byproduct of their four-stomached digestive system, cattle produce and emit significant quantities of methane, a greenhouse gas that is 23 times more potent by quantity than CO2. Given this methane production, livestock-related deforestation, and other greenhouse gas-heavy practices, the livestock industry is thought to be responsible for a whopping 18% of all manmade greenhouse gas emissions – more than that of all of the world’s non-livestock-related transportation, including all cars, planes, and ships.
2. Animal waste from factory farms has been known to leak into local water supplies, spreading harmful bacteria to those who live nearby.
Livestock produce fecal waste – and at the current levels of livestock production in the developed world, there is a lot of feces to deal with. Livestock waste, known in its solid form as manure, or, when mixed with water, as “slurry,” is typically either left in large open-air lakes called “slurry pits,” or spread onto fields as fertilizer. While the existence of slurry if not itself a problem, and using animal waste as fertilizer can be an efficient food-production practice, bacteria and other contaminants from this waste often makes their way into the local water supply, posing significant health risks to nearby residents. 
3. Animal products have a significantly higher water footprint than their plant-based equivalents.
As many parts of the world, including the western United States, face dwindling fresh water resources, the demand for animal products fuels massive water usage in the livestock industry. Estimates indicate that the production of 1kg of beef requires about 43,000L of fresh water, including both raising the livestock itself and growing the crops needed to feed it. For comparison, 1kg of grain only requires 1,000L. In terms of calories, that’s a 20 times greater water footprint for beef as compared to cereals, and a 6 times greater water footprint for beef as compared to its protein-equivalent in pulses (such as beans, peas, and lentils). While cattle raised for beef is by far the worst offender, other animal products consistently fare worse than their plant-based nutritional equivalent. Given this immense water usage, one study estimates that “meat contributes 37% towards the food-related water footprint of an average American citizen.” Of course, these are largely comparisons of meats and their energy equivalents in grains and pulses - a comparison that included fruits and vegetables would certainly find many non-animal products to be water-intensive and deserving of a decrease in consumption as well. Nevertheless, by reducing one’s meat consumption one can likely reduce one’s own impact on our fast-depleting fresh water resources.
4. The demand for livestock is one of the major causes of deforestation, particularly of the Amazon rainforest.
Raising livestock requires a great deal of land – both for the animals themselves and for growing the large quantities of food required to feed them. As demand for animal products rises, farmers and ranchers must find more land, often cutting down forests in the process. The Amazon rainforest in particular has borne the burden of this search for land, as cattle ranching and soybean farming have been major drivers of deforestation in the region. Notably, while soy is often associated with a plant-based diet, about 83% of all of the world’s soybean production is used for animal feed, a less food-efficient use than its direct consumption. Therefore, switching to a more plant-heavy diet will help lower the demand for cattle and the soy that feeds them, slowing the rate of Amazonian deforestation.
5. While the majority of these reasons apply only to livestock raised on land, the seafood industry has many of its own environmental consequences.
Perhaps the most direct effect of high demand for fish and other seafood was already mentioned in the Food System section – the current rates of fishery depletion are unsustainable, leading to significant collapses in global fish populations. Already, the world is seeing declining rates of catch, predicted only to worsen if current levels of consumption continue.
As populations have decreased, making fishing more difficult, the fishing industry has turned to more intensive practices, such as trawling, to make up the difference. But as nets are trawled, or dragged along the ocean floor, they cause significant damage to the natural habitat, affecting coral reefs in particular. Coral reefs, vital in maintaining fish populations and oceanic biodiversity, face considerable, long-term damage from this method of fishing.
Another effect of most fishing methods, including trawling, is bycatch – a term for marine life (and often birds) that are accidentally caught as a byproduct of fishing practices. Bycatch affects and threatens the population of many species, including sea turtles and sea mammals such as dolphins, porpoises, and seals.
The natural response to these negative impacts of the wild seafood industry is to promote its alternative: seafood farming. However, while the effects of farming vary greatly with the species raised, aquaculture has its own host of negative impacts. Aquaculture has long been known to produce environmental waste, including that which contributes to eutrophication – a process whereby an influx of nutrients, often from agri- and aquaculture runoff, ends up in local waterways where phytoplankton populations then bloom, using up the system’s oxygen resources and killing other aquatic life. Aquaculture is also associated with the destruction of valuable wetland habitats, such as salt marshes and mangrove forests. Finally, and counterintuitively, aquaculture may contribute to depletion of world fisheries – the farming of carnivorous fish requires the input of other fish, which are often themselves caught in the wild. While the environmental impacts of seafood are different than that of land-raised livestock, and therefore difficult to compare, they are doubtlessly significant. By eating less seafood, lowering demand for these products, it is possible to help reduce these adverse consequences.
6. Combining all of these factors, diets that are heavy in animal products cause significantly more harm to the environment than more plant-based diets.
According to multiple life-cycle studies, which test the total environmental impact of a product, from its start to its finish, diets high in animal products have a significantly higher negative impact than nutritionally equivalent vegetarian or vegan diets.
 Shih, J.-S., Burtraw, D., Palmer, K., & Siikamäki, J. (2008). Air Emissions of Ammonia and Methane from Livestock Operations: Valuation and Policy Options. Journal of the Air & Waste Management Association, 58(9), 1117–1129. doi:10.3155/1047-32126.96.36.1997
 Charmley, E., Stephens, M. L., & Kennedy, P. M. (2008). Predicting livestock productivity and methane emissions in northern Australia: development of a bio-economic modelling approach. Australian Journal of Experimental Agriculture, 48(2), 109–113.
 FAO Livestock Environment and Development. (n.d.). The Role of Livestock in Climate Change. Retrieved from http://www.fao.org/agriculture/lead/themes0/climate/en/
 McMichael, A. J., Powles, J. W., Butler, C. D., & Uauy, R. (2007). Food, livestock production, energy, climate change, and health. The Lancet, 370(9594), 1253–1263. doi:10.1016/S0140-6736(07)61256-2
 Steinfeld, H., Gerber, P., Wassenaar, T., Castel, V., Rosales, M., & De Haan, C. (2006). Livestock’s long shadow. FAO Rome. Retrieved from http://www.globalmethane.org/expo-docs/china07/postexpo/ag_gerber.pdf
 Burkholder, J., Libra, B., Weyer, P., Heathcote, S., Kolpin, D., Thome, P. S., & Wichman, M. (2007). Impacts of Waste from Concentrated Animal Feeding Operations on Water Quality. Environmental Health Perspectives, 115(2), 308–312.
 Campagnolo, E. R., Johnson, K. R., Karpati, A., Rubin, C. S., Kolpin, D. W., Meyer, M. T., … McGeehin, M. (2002). Antimicrobial residues in animal waste and water resources proximal to large-scale swine and poultry feeding operations. Science of The Total Environment, 299(1–3), 89–95. doi:10.1016/S0048-9697(02)00233-4
 Hooda, P. S., Edwards, A. C., Anderson, H. A., & Miller, A. (2000). A review of water quality concerns in livestock farming areas. Science of The Total Environment, 250(1–3), 143–167. doi:10.1016/S0048-9697(00)00373-9
 Mallin, M. A., & Cahoon, L. B. (2003). Industrialized Animal Production—A Major Source of Nutrient and Microbial Pollution to Aquatic Ecosystems. Population and Environment, 24(5), 369–385. doi:10.1023/A:1023690824045
 Ramos, M. C., Quinton, J. N., & Tyrrel, S. F. (2006). Effects of cattle manure on erosion rates and runoff water pollution by faecal coliforms. Journal of Environmental Management, 78(1), 97–101. doi:10.1016/j.jenvman.2005.04.010
 Vidal, M., López, A., Santoalla, M. C., & Valles, V. (2000). Factor analysis for the study of water resources contamination due to the use of livestock slurries as fertilizer. Agricultural Water Management, 45(1), 1–15. doi:10.1016/S0378-3774(99)00073-6
 Guru, M. V., & Horne, J. E. (2000). The Ogallala Aquifer. Kerr Center for Sustainable Agriculture. Retrieved from http://www.kerrcenter.com/wp-content/uploads/2014/11/ogallala_aquifer.pdf
 Pimentel, D., Berger, B., Filiberto, D., Newton, M., Wolfe, B., Karabinakis, E., … Nandagopal, S. (2004). Water Resources: Agricultural and Environmental Issues. BioScience, 54(10), 909–918. doi:10.1641/0006-3568(2004)054[0909:WRAAEI]2.0.CO;2
 Mekonnen, M. M., & Hoekstra, A. Y. (2012). A Global Assessment of the Water Footprint of Farm Animal Products. Ecosystems, 15(3), 401–415. doi:10.1007/s10021-011-9517-8
 Downing, T. E., Hecht, S. B., Pearson, H. A., Garcia Downing, C., & others. (1992). Development or destruction: the conversion of tropical forest to pasture in Latin America. Retrieved from http://www.cabdirect.org/abstracts/19936716728.html
 Soyatech. (n.d.). Soy Facts. Retrieved from http://www.soyatech.com/soy_facts.htm
 Kaimowitz, D. (1996). Livestock and Deforestation in Central America in the 1980s and 1990s: A Policy Perspective. CIFOR.
 Thornton, P., & Herrero, M. (2010). The Inter-Linkages Between Rapid Growth in Livestock Production, Climate Change, and the Impacts on Water Resources, Land Use, and Deforestation (SSRN Scholarly Paper No. ID 1536991). Rochester, NY: Social Science Research Network. Retrieved from http://papers.ssrn.com/abstract=1536991
 Mullon, C., Fréon, P., & Cury, P. (2005). The dynamics of collapse in world fisheries. Fish and Fisheries, 6(2), 111–120. doi:10.1111/j.1467-2979.2005.00181.x
 Pauly, D., Alder, J., Bennett, E., Christensen, V., Tyedmers, P., & Watson, R. (2003). The Future for Fisheries. Science, 302(5649), 1359–1361. doi:10.1126/science.1088667
 Pauly, D., Christensen, V., Guénette, S., Pitcher, T. J., Sumaila, U. R., Walters, C. J., … Zeller, D. (2002). Towards sustainability in world fisheries. Nature, 418(6898), 689–695. doi:10.1038/nature01017
 Pauly, D., Watson, R., & Alder, J. (2005). Global trends in world fisheries: impacts on marine ecosystems and food security. Philosophical Transactions of the Royal Society B: Biological Sciences, 360(1453), 5–12.
 Althaus, F., Williams, A., Schlacher, T. A., Kloser, R. J., Green, M. A., Barker, B. A., … Schlacher-Hoenlinger, M. A. (2009). Impacts of bottom trawling on deep-coral ecosystems of seamounts are long-lasting. Marine Ecology Progress Series, 397(279-294), 40.
 Hall–Spencer, J., Allain, V., & Foss\a a, J. H. (2002). Trawling damage to Northeast Atlantic ancient coral reefs. Proceedings of the Royal Society of London B: Biological Sciences, 269(1490), 507–511.
 Thrush, S. F., & Dayton, P. K. (2002). Disturbance to Marine Benthic Habitats by Trawling and Dredging: Implications for Marine Biodiversity. Annual Review of Ecology and Systematics, 33, 449–473.
 White, A. T., Vogt, H. P., & Arin, T. (2000). Philippine Coral Reefs Under Threat: The Economic Losses Caused by Reef Destruction. Marine Pollution Bulletin, 40(7), 598–605. doi:10.1016/S0025-326X(00)00022-9
 Chuenpagdee, R., Morgan, L. E., Maxwell, S. M., Norse, E. A., & Pauly, D. (2003). Shifting gears: assessing collateral impacts of fishing methods in US waters. Frontiers in Ecology and the Environment, 1(10), 517–524. doi:10.1890/1540-9295(2003)001[0517:SGACIO]2.0.CO;2
 Lewison, R. L., Crowder, L. B., Read, A. J., & Freeman, S. A. (2004). Understanding impacts of fisheries bycatch on marine megafauna. Trends in Ecology & Evolution, 19(11), 598–604. doi:10.1016/j.tree.2004.09.004
 Read, A. J., Drinker, P., & Northridge, S. (2006). Bycatch of Marine Mammals in U.S. and Global Fisheries. Conservation Biology, 20(1), 163–169. doi:10.1111/j.1523-1739.2006.00338.x
 Secchi, E., Ott, P., & Danilewicz, D. (2006). 9 Effects of fishing bycatch and the conservation status of the franciscana dolphin, Pontoporia blainvillei. Books Online, 2006(5), 174–191.
 Wallace, B. P., Lewison, R. L., McDonald, S. L., McDonald, R. K., Kot, C. Y., Kelez, S., … Crowder, L. B. (2010). Global patterns of marine turtle bycatch. Conservation Letters, 3(3), 131–142. doi:10.1111/j.1755-263X.2010.00105.x
 Goldburg, R., Elliott, M. S., Naylor, R., Commission, P. O., & others. (2001). Marine aquaculture in the United States: environmental impacts and policy options. Pew Oceans Commission Arlington, Virginia. Retrieved from http://www.iatp.org/files/Marine_Aquaculture_in_the_United_States_Enviro.htm
 Cao, L., Wang, W., Yang, Y., Yang, C., Yuan, Z., Xiong, S., & Diana, J. (2007). Environmental impact of aquaculture and countermeasures to aquaculture pollution in China. Environmental Science and Pollution Research - International, 14(7), 452–462. doi:10.1065/espr2007.05.426
 Piedrahita, R. H. (2003). Reducing the potential environmental impact of tank aquaculture effluents through intensification and recirculation. Aquaculture, 226(1–4), 35–44. doi:10.1016/S0044-8486(03)00465-4
 Páez-Osuna, F. (2001). The Environmental Impact of Shrimp Aquaculture: Causes, Effects, and Mitigating Alternatives. Environmental Management, 28(1), 131–140. doi:10.1007/s002670010212
 Naylor, R. L., Goldburg, R. J., Primavera, J. H., Kautsky, N., Beveridge, M. C. M., Clay, J., … Troell, M. (2000). Effect of aquaculture on world fish supplies. Nature, 405(6790), 1017–1024. doi:10.1038/35016500
 Baroni, L., Cenci, L., Tettamanti, M., & Berati, M. (2006). Evaluating the environmental impact of various dietary patterns combined with different food production systems. European Journal of Clinical Nutrition, 61(2), 279–286. doi:10.1038/sj.ejcn.1602522
 Gill, M., Smith, P., & Wilkinson, J. M. (2010). Mitigating climate change: the role of domestic livestock. Animal, 4(03), 323–333. doi:10.1017/S1751731109004662
 Leitzmann, C. (2003). Nutrition ecology: the contribution of vegetarian diets. The American Journal of Clinical Nutrition, 78(3 Suppl), 657S–659S.
 Marlow, H. J., Hayes, W. K., Soret, S., Carter, R. L., Schwab, E. R., & Sabaté, J. (2009). Diet and the environment: does what you eat matter? The American Journal of Clinical Nutrition, 89(5), 1699S–1703S. doi:10.3945/ajcn.2009.26736Z
 Nijdam, D., Rood, T., & Westhoek, H. (2012). The price of protein: Review of land use and carbon footprints from life cycle assessments of animal food products and their substitutes. Food Policy, 37(6), 760–770. doi:10.1016/j.foodpol.2012.08.002
 Pimentel, D., & Pimentel, M. (2003). Sustainability of meat-based and plant-based diets and the environment. The American Journal of Clinical Nutrition, 78(3 Suppl), 660S–663S.
 Reijnders, L., & Soret, S. (2003). Quantification of the environmental impact of different dietary protein choices. The American Journal of Clinical Nutrition, 78(3 Suppl), 664S–668S.