Lecture 2 Key Ideas and take Home Messages (1) Defining invasive and invasion is hard. (2) Most agree that there are some key differences between the natural processes of range expansion and dispersal. - Rate of arrival - The crossing of biogeographical barriers independent of species biology / ecology / physiology. - In other words, humans are bridging natural limitations on range boundaries and moving species incapable of moving themselves and doing so at a frequency far greater than such events occur naturally (3) Invasion process A stepwise process that removes species through a series of filters between each transitional step (Transport – Establishment – Spread – Impact) The original pool of species becomes progressively smaller as species fail to cross each of the filters so that only a few species go on to become what we’d think of as important invaders. (4) Invasion models - Reaction-diffusion has a time and space component. Species increases its range while simultaneously increasing its population density. Range expansion is limited by the species own capability of dispersal - Human vectored – Humans move individuals from the founder population beyond the limitations of the species own dispersal capability to create new population foci. - Stratified – incorporates both processes. The result is a coalescing of populations expanding through their own dispersal with those created by human aided long distance transport.