请根据提示，在下面短文中的空格上填入合适的单词或单词的正确形式。Propagule pressure is widely 1(_______________) (recognize) as an important factor that influences invasion success (MacDonald et al. 1989; Simberloff 1989; Williamson 1996; Lonsdale 1999; Cassey et al. 2005). Previous studies 2(_______________) (suggest) that the probability of a successful invasion increases with the number of propagules released (Panetta & Randall 1994; Williamson 1989; Grevstad 1999), with the number of introduction attempts (Veltman et al. 1996), with introduction rate (Drake et al. 2005), and with proximity to existing populations of invaders (Bossenbroek et al. 2001). 3(_____________) (填入一个连词) , propagule pressure may influence invasion dynamics after establishment by affecting the capacity of non-native species to adapt to their new environment (Ahlroth et al. 2003; Travis et al. 2005). Although a successful invasion clearly requires both establishment and spread of the invader, most studies 4(_______________) (look at) just one of these processes (Melbourne et al. 2007). Despite its acknowledged importance, propagule pressure has rarely been manipulated experimentally and the interaction of propagule pressure with other processes that regulate invasion success is not well 5(_____________) (understand) (D’Antonio et al. 2001; Lockwood et al. 2005).