ーThe number of people riding their bicycles to work has a tendency to increase because people are more concerned about the environment these days.
「最近は環境を心配する人が増えたので、自転車通勤する人の数は増加傾向にある」
to have a tendency to increase で「増加傾向にある」と言えます。
ーThe number of people watching TV these days seems to be declining because there aren't as many good shows on TV anymore.
「あまりテレビで面白い番組をやっていないので、最近テレビを見る人の数は減少傾向にある」
seems to be declining で「減少傾向にある」と表現できます。
ご参考まで!