First think about fecundity. As humans, we relate to our life cycle generation time of 21 years. When two humans successfully mate, typically only one young is produced, usually in nine months. Our pets, such as a house cat, have a generation time of 10-12 months, and may produce a litter of four to six kittens. With garden pests, the situation is much different.
Aphids are born pregnant, can reproduce without sex, and are fully grown in a week.
Microorganisms, including fungi and bacteria, can double their number in hours or days and produce many thousands of spores in a small area. Many plants can reproduce asexually. Field bindweed spreads by way of underground rhizomes. If you break up one of these rhizomes by spading or tilling, each piece of rhizome is capable of producing a new plant. Pests can also disperse their numbers over time.