Protecting forests reinforces biotic resistance to invasive species

Ninad Avinash Mungi presents inferences from his recent article: Role of species richness and human-impacts in resisting invasive species in tropical forests. Find out how tropical forest manifest resistance towards invasive plants, when protected from anthropogenic impacts.

As the decade of ecological restoration unfolds, we invest in strategies to restore ecosystems, uncertain about how our degrading planet will respond to these remedies. Amongst the multitude of threats to Earth’s biodiversity, biological invasions are amongst the worst. It not only threatens the survival of native ecosystems, but also escalates risk to human life and economy (e.g. invasion of COVID-19). Governments and multilateral initiatives have made huge investments across sectors to arrest invasions and reinstate native ecosystems. For example, parts of the last remaining hyper-diverse tropical forests are being protected with the hope of restoring self-maintaining forests that can resist invasions. Nevertheless, restoration remains an enigma, as long as the scientific validity of such endeavours is unaddressed.

In our paper, we tested the biotic resistance of tropical forests against plant invasions using plant inventories from 34 protected areas, paired with 34 multi-used areas, spread across tropical forest systems in India. We addressed the confounding externalities using linear mixed models, and independently established the role of native species richness in resisting invasive plants domination. The richness of invasive plants was lower in protected areas, but their abundance was ubiquitous. We did not find evidence of biotic resistance in dry-thorn forests (i.e. savannas), dry deciduous forests, and moist deciduous forests. Nonetheless, protected evergreen and semi-evergreen forests exhibited biotic resistance to invasive plant richness as well as abundance. 

The resistant forests: The wet forest systems exhibited unique biotic resistance towards invasive plants by virtue of their sheer species richness. Importantly, this charismatic trait manifested only in wet forests in protected areas. In the presence of human settlements, forest clearings, frequent forest fires or other extractive human-use, these forests lost their characteristic biotic resistance and became vulnerable to invasion by plants like Mikania micrantha, Chromolaena odorata and Ageratina adenophora.

Tropical wet forests are felled and fragmented to fulfil our unsatiated demand for energy and infrastructure. Engineered mitigation measures, often advocated as ‘green-panacea’, are rarely reckoned to reverse the loss of a forest’s biotic resistance. Nor can we enforce this ability in the plantations that we pursue, in vain, to mimic the lost forests. Turns out, prevention is after all more effective than the purported cure. Merely preserving the native forests from fragmentation and degradation can help control invasions. The study has added another reason to prioritize preservation of existing forests, over any glorified alternative.

The biodiverse tropical wet forests, rich with native species (Photo credits: Abir Jain)

The not-so-resistant forests: The study found dry forest systems being pervasively invaded by hordes of invasive plants. Even the most protected areas in these forests were invaded by Lantana camara, Prosopis juliflora, Parthenium hysterophorus, Xanthium strumarium, etc. These forests have a legacy of human modifications, both ancient and modern. Most of the newly declared protected areas with these forest types lingers under the shadow of historic civilizations, timber-centred colonial forestry practices, and altered disturbance regimes including fire and herbivory. It is thus natural to see a bizarre medley of plants in such novel anthropogenic forests. 

Once humans alter natural regimes beyond their rebound capacity, their dissociation from the system can do little to regain the lost ‘naturalness’. This contravenes our strongly held belief of protecting a forest and expecting it to bounce back with its lost functions. Unlike the protected wet forests, dry forests require continuous interventions to restore native ecosystems with their living functions and control the spread of invasive species. 

Presence of invasive Prosopis juliflora in dry forest systems (Credits: Rajat Rastogi)

A random or even planned group of plants do not constitute a rambunctious tropical forest. A forest is rather a community, exemplified by traits of each constituent species that has co-evolved on the common resources. Once these communities are replaced by monotonous stands of invasive plants, their signature resilience is lost. They not only stand vulnerable to invasive plants, but can also be converted into strong source of invasive plants. This churns a negative feedback loop; while we still lack scientific experiments on managing invasions and restoring our native forests. Only when we graduate from the ceremonial applause of increasing green-cover, would we begin to understand that we are mistaking trees for forest. In this decade, dedicated to ecosystem restoration by the United Nations, collapsing tropics call for action to confront the fierce necessity of “How?” in the fierce urgency of now!

Ninad Avinash Mungi Wildlife Institute of India, Dehradun, India

Read the full article in Journal of Ecology: Role of species richness and human-impacts in resisting invasive species in tropical forests.

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