Ecosystem disruption
Fragmentation can reduce habitat area and connectivity, changing gene flow and extinction risk. An introduced species becomes invasive when it spreads and causes ecological harm, not simply because it arrived from elsewhere. Excess nutrients can stimulate algal growth; decomposition of resulting organic material can lower dissolved oxygen. Climate change can shift ranges and seasonal timing, potentially separating interacting species. Persistent pollutants can accumulate within organisms and become concentrated at higher trophic levels when their chemical and biological properties permit.
Earlier flowering without a matching shift in pollinator activity can reduce overlap and reproduction. Test both timing and reproductive output before claiming a complete causal chain.
Do not say fertilizer directly sucks oxygen out of water. Trace nutrient input → production → dead organic matter → microbial respiration → lower oxygen.