The photograph shows monarch butterflies landing on plants where there are thousands of other monarch butterflies. these butterflies feed on milkweed plants as they travel south for the winter, covering distances of 5,000 km or more each year. however, the population of these butterflies has been steadily declining over the last 20 years. what is the most likely reason the population of these butterflies has been declining?

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.