Who can explain the negative effect of speculation in financial markets?

Thanks for your answers so far. To make my question more specific: I would like to know about the "dirty" investment strategies that speculators use, which cause economic crises and impoverish others.

"Pump and dump" is probably one of them, but this strategy won't affect any serious investor, will it? Generally any investor should make up their own opinion about the value of stocks, and be very careful when listening to random people's recommendations, because any stock market recommendation carries a risk of conflict of interest. So those speculators with "pump and dump" strategy will fool only those investors that only look at the price curve instead of looking at the real value of a stock. Those deserve to be fooled, I'd say.

Then, there was a debate about speculation of agricultural futures, and its negative impact on nutrition in Africa. So let's assume speculators artificially inflate prices of futures on agricultural products. Hig

My comment got cut off. So about speculation with agricultural futures, and its negative impact on nutrition in Africa. So let's assume speculators artificially inflate prices of futures on agricultural products. High prices for agricultural futures will encourage farmers to invest in increasing their crop yields. At some point, the inflated futures will reach their due date, and speculators will be sitting on hundreds of tons of physical wheat, corn, etc., which they will have to sell before it turns bad, otherwise they'd make huge losses. Selling under time pressure will push prices down, so isn't it that those speculators are actually making losses at the benefit of the distributors and end customers?