COS 251-2 - Effects of alien and native neighbors on population growth are often positive, but differ between populations and species of closely related native plants varying in breeding system
Alien species are considered one of the primary threats to native plant populations and their control is often prominent among proposed management actions. While negative alien effects are well documented, there are also many ways that alien plants can have positive effects on native plant populations that may actually contribute to their persistence. Moreover, the effect of alien plants can change in magnitude and direction over varying abiotic conditions. I quantified the effects of neighboring alien and native plants on all demographic rates in a several populations, including four species, of the Hawaiian endemic genus. I performed biannual censuses for 4 years in one population to encompass relatively harsh and as well as benign seasons and years and annual cenuses in several other populations. I regressed annual growth, survival, and reproduction (fruiting and recruitment) against size and cover of both alien and heterospecific native neighbors. I used these demographic rate regressions to construct Integral Projection Models of population growth for each year in each population at different levels of alien and native cover and compare population growth. I compared the effects of alien and native neighbors on population growth based on characteristics of each population and species.
Results/Conclusions
The effects of alien and heterospecific native plant neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien and native neighbors benefit native plants through multiple mechanisms, such as nurse plant effects and associational resistance. There were significant effects of alien cover on 34, and of native cover on 39, of the 56 demographic rates examined across all populations, and the vast majority of these effects were positive. There were significant, positive effects of native and alien plants on at least one vital rate in every population and year (14 total) included in this study. The effect of alien and heterospecific native neighbors on population growth was generally positive—in every population and year, at least one and often both types of neighbors drove increased population growth. These mixed, but largely positive, effects of alien and native neighbors highlight the need to consider potential benefits of alien, as well as native, species in the management of at-risk native plant populations, and that those benefits may be altered by changing abiotic conditions, as indicated by differing effects across (and within) years and populations in this study.
