Assessing ecosystem functional impacts from woody invasive species assemblages
Invasive plant species are known to cause negative ecological impacts in many ecosystems, including reductions in biodiversity. Less is known about what drives these ecosystem impacts, and whether the diversity of invaders affects the magnitude of impacts. We sought to 1) determine whether invasion by non-native plants that were functionally similar to extant natives would necessarily cause negative ecosystem impacts, or if additional disturbance precipitated impacts; 2) investigate whether assemblages of morphologically similar invasive species led to the same impacts as single dominant invaders; and, 3) assess the relative effectiveness and ecological outcomes of integrated chemical and mechanical control strategies for the invasive assemblages.
This pilot study was carried out at a preserve near Mystic, CT that provides important habitat for migratory and resident bird species, largely due to a mid-successional woodland dominated by an understory of native vines and shrubs. Transects bisecting the woodland, and nested sampling plots randomly located along the edge of the woodland, were used to test study hypotheses. In sampling plots abundance data were collected to calculate plant community taxonomic and functional diversity indices. Functional capacity to provide avian habitat in plots was assessed using digital imagery analyses and measurements of canopy density.
Preliminary data suggested that not all invasive species that composed resident assemblages caused equivalent impacts. Non-native vines in particular tended to have negative impacts, reducing diversity and richness in some plots by 35-40%. Disturbances facilitated dominance by both non-native and native species, but especially for the invasive vines Ampelopsis brevipendiculata, Celastrus orbiculatus and Cynanchum louiseae. For invasive shrubs the order of treatment did not affect control, but for invasive vines short-term control was 45% higher (P < 0.05) when chemical controls took place before mowing.
We assessed impacts from invasive assemblages on bird foraging and nesting using coarse scale surveys and extrapolation from comparisons of plant functional traits thought to benefit birds. In shaded, minimally disturbed, interior locations invasive species did not negatively impact birds more than analogous native species. In exposed, disturbed locations along woodland edges invasive vines seemingly reduced the habitat’s functional capacity. This finding is concerning as woodland edges are particularly important as stopover locations for migratory birds, and impacts here from invasive vines might have broader consequences. Overall, we found that not all invasive species reduce ecosystem functioning, but that vines in particular may cause potentially significant impacts under certain ecological conditions.