Direct and indirect effects of invasive species: Biodiversity loss is a major mechanism by which an invasive tree affects ecosystem functioning

Abstract

1. Global change, such as exotic invasions, dramatically affects ecosystem functioning. However, the mechanisms behind the impacts are often unclear and despite extensive experimental work, we know little about the importance of biodiversity loss as a component of global change effects in real-world ecosystems. 2. We disentangled several mechanisms by which an exotic invader affected ecosystem functioning in East African drylands in Kenya and Ethiopia. We used structural equation modelling to separate direct effects of a woody invader Prosopis juliflora on a range of ecosystem functions from indirect effects mediated through changes in biodiversity (plant species richness) and ecosystem functioning (herbaceous biomass). We analysed effects on ecosystem functions linked to soil biogeochemical cycling and transfer of energy between trophic levels. 3. We found that the mean size of individual indirect effects mediated by biodiversity and herbaceous biomass was about twice as large as the mean size of individual direct effects of Prosopis on ecosystem functions, showing that indirect effects are an important component of the invader's overall environmental impacts. Changes in both herbaceous biomass and biodiversity were approximately equally important as drivers of indirect effects, indicating that we need to expand our view of indirect effects to consider a wider range of mechanisms. Simple univariate models failed to capture some Prosopis invader effects because positive direct effects on soil stability and predatory invertebrates were counteracted by negative indirect effects on biodiversity or biomass loss. The majority of effects were similar in both study systems. Suggestions that woody invaders are able to | 2661 Journal of Ecology LINDERS Et aL.

Rural households’ perception on the effects of Prosopis juliflora invasion: The case of Amibara District of Afar National Regional State, Ethiopia

“…Even if the case was similar with non-infested areas, species diversity and richness of the woody vegetation in Gewane and Amibara were found to be low. As reported by Ilukor et al (2016) and Linders et al (2019), in areas of medium and high P. juliflora invasion levels, native plant species are not available at all; therefore, P. juliflora reduces native biodiversity further. The marginal effect results indicate that, when the household is further away from the nearest bush land by a kilometer, the probability of positive perceptions towards P. juliflora will increase by 0.03%.…”

Global guidelines for the sustainable use of non-native trees to prevent tree invasions and mitigate their negative impacts

“…Climate change could affect the dynamics of invasions of NNTs in many interacting ways, for example: (a) by causing modification in the ecosystems that potentially modify opportunities for establishment, naturalisation, and spread of both native trees and NNTs (e.g., Iverson et al 2008;Bezeng et al 2017;Fei et al 2017;Aubin et al 2018); (b) by favouring individual traits of particular NNTs (e.g., Kawaletz et al 2013;Castro-Díez et al 2014); and (c) by modifying introduction pathways, potentially promoting the increased use of certain NNTs (Lindenmayer et al 2012;Frischbier et al 2019), thereby challenging the recommendation to preferentially use native trees (Rec. 1).…”

Restoration of degraded grasslands, but not invasion by Prosopis juliflora, avoids trade-offs between climate change mitigation and other ecosystem services

“…Prosopis is a so-called transformer species 63 i.e. it can also invade undisturbed habitats 64 . However, the rapid invasion in Eastern Africa is likely to be further promoted by land degradation as well as by regular and seasonal migration of livestock and severe flooding events, which both contribute to seed dispersal 31 .…”

“…For example, Abreu et al 26 showed that fire suppression in savannas of the Brazilian Cerrado increased carbon stocks but was associated with acute biodiversity loss. Similarly, encroachment of degraded tropical grasslands in Baringo by Prosopis increases carbon stocks and the availability of wood, but it threatens biodiversity across multiple trophic levels 64 , increases mosquito densities 68 , reduces the provisioning of herbaceous fodder for grazers 69 and water 65,70 and negatively affects tourism 71 and limits access to water points, pasture, croplands and fishing grounds 72 . In arid and semi-arid regions, the high water consumption by Prosopis is of particular concern, as it decreases the groundwater recharge and thus seriously affects the water available to households in invaded ecosytems 65 .…”

“…Furthermore, Prosopis is highly prolific hence cover densities transition rapidly which suppresses the understory vegetation. Plant species richness in Prosopis-invaded habitats drops to very low levels once Prosopis has reached 50% cover 64 , which has been attributed to competitive exclusion of the slow-growing shade-intolerant herbaceous savanna species by Prosopis, either due to its allelopathic effects or through competition for water during the dry seasons 77 .…”