Ecology and Evolution of Invasive Species

Species impacts in native and introduced habitats, character displacement, and life history change following invasion by House Finches and other species. Ecological impacts of recovering species.

Exotic species introductions and endangered species recoveries are topics of great conservation interest and also provide exceptional opportunities to understand the ecological and evolutionary consequences of species interactions over large spatial and temporal scales.  I and my students have studied several aspects of species invasions.

An early and continuing interest of mine is the ecological and evolutionary implications of the House Finch (Carpodacus mexicanus) introduction from western North America into eastern North America.  This species exhibited explosive spread throughout eastern North America following their release by cage bird dealers from Long Island, New York, in the 1940’s.  Using data from both the Breeding Bird Survey and the Christmas Bird Count, I documented the geographic spread, population growth and associated population changes in several species of birds potentially competing with House Finches (Wootton 1987a).  These results showed that both the congeneric Purple Finch (Carpodacus purpureus) and previously-introduced House Sparrow (Passer domesticus) declined as the House Finch invasion front reached different areas.  One intriguing pattern was the shape of the competitive relationship as the invasion proceeded:  rates of per-capita competitive impact declined as House Finch populations increased.  Such a pattern is consistent with possible ecological and evolutionary changes in the system that may have increased niche partitioning.  In the case of Purple Finches, this species specializes in conifer-dominated habitats, whereas House Finches avoid these habitats.  Prior to the House Finch invasion, however, Purple Finches were also found in deciduous habitats in eastern North America, and House Finches appear to be competitively excluding them from these areas.  The case of House Sparrows may be related to differences in feeding performance on different types of seeds:  at feeding stations, House Sparrows prefer smaller seeds such as millet whereas House Finches prefer sunflower seeds.  This combination diet differences and declining competitive effect raises the possibility of an interesting evolutionary response:  character displacement.  Prior studies of the evolution of morphological characteristics of the House Sparrow following its introduction to North America demonstrate that it can evolve rapidly.  I have collected beak and leg measurements from most major collections of North American House Sparrows prior to the House Finch invasion, and plan to compare them to measurements of populations exposed to House Finches for different periods of time.  There are surprisingly few specimens of modern House Sparrows in museum collections, but results from the few I have found are tantalizing: House Sparrow beak depths are significantly smaller in recent individuals than in individuals collected prior to House Finch invasion; a shift away from the mean beak depth of House Finches.  Further measurements and studies of the relationship of morphology to feeding performance are needed to nail down this effect.  I have also studied the House Finch introduction for changes in life history attributes under different density conditions (high density ancestral populations vs. low density introduced populations), and showed substantial increases in clutch size in the low density population (Wootton 1986b).  The House Finch system offers a wealth of other opportunities to study ecological and evolutionary responses to species invasion.  For example, recent work at Cornell University has shown a disease, avian conjunctivitis, is sweeping through the eastern House Finch population, causing strong population declines.  The implications of this new species interaction have yet to be fully fleshed out.  Similarly, House Finches are poor hosts for Brown-headed Cowbirds, a generalist brood parasite, which may disrupt nest parasite patterns affecting other species.  Finally, Dylan Maddox and I have been using this and other invasions to probe “dynamic macro-ecology”, for example exploring how range size and abundance are linked.

Other aspects of my research also relate in species invasions.  I and my colleagues have documented changes in the seabird community of Tatoosh Island, based on our long-term bird population records there, that arise from both direct predation and through indirect effects as Peregrine Falcons (Falco peregrinus) have made a recovery from near extinction following protection under the Endangered Species Act (Paine et al. 1996).  This and other observations of changing ecology following species recovery (sea otters, wolves, etc.) indicate that many endangered species, rather than being insignificant minor players as is often assumed, had historically substantial roles in many ecosystems.



I am also studying experimentally the role that the Signal Crayfish (Pacifastacus leniusculus) plays in river food webs on the Olympic Peninsula of Washington state, where it is a native.  This species is now an invasive in other areas, and may be driving native crayfish to extinction.  Understanding its role in its native food web may help to anticipate and counteract its effects in non-native areas.


Several of my graduate students have studied the ecology of invasive species in marine and terrestrial habitats.  Their research is described in my Graduate Student lab page.