1. There are lots of factors that can change the ecosystem and can affect populations. But the most common ones are 1 climate changes, 2 changes in resources of land or food or 3 changes in predation. All these 3 factors can alter the food web thus affecting the population of different species.
For example, if suddenly the number of sharks increases, the whole food web is altered. More sharks consume more fishes, so, the population of fishes are changed.
2. Climate changes are different in different part of the world. But in a whole, the main changes are 1 rising temperature 2 rising in sea level 3 increase in rainfall (or decrease in some places) 4 increase occurance of typhoons and floodings
Some of these changes actually bring benefits to certain people and places, for example, some dry areas can now have a longer crop growing period because they have more rainfall now. Or, less people die from coldness in temperate areas.
However, most of these changes also brings problems and harms to life. For example, tropical diseases like malaria or dengue fever are spreading to temperate areas too, or, animal habitats are destroyed due to flooding.
3. When we say human activities, we mining talk about activities like burning of fossil fuels, or deforestation etc. Many of these activities increases the amount of greenhouse gases like carbon dioxide in the atmosphere thus intensifying the greenhouse effect. Greenhouse effect is necessary for earth and lives, but, the thing is humans are intensifying too much. The whole earth is getting hotter and hotter.
When the earth gets hot, ice sheets in polar regions may melt and thus killing animals there like polar bears. It can also promote the occurrence of acid rain and more lives are killed, especially aquatic animals and plants.
