The research team, led by Askot Mussa Alafi of the Agricultural Research Institute of Mozambique together with Francisco Fernando Munguambe and Hermínio Benvindo of Eduardo Mondlane University, surveyed the scheme systematically, placing sampling points every two kilometers along the Limpopo River, its tributaries, and the network of irrigation channels and drainage ditches that crisscross the plain. At each of 83 sampling locations, the botanists laid out ten one-square-meter quadrats, recording every plant they found, estimating percentage cover, and categorizing abundance on a six-point scale ranging from rare to very abundant. In total, 830 quadrats were inventoried, yielding a dataset of 191 identified plant species belonging to 53 families.

Of those 191 species, 71 were classified as invasive alien plants, distributed across 29 families. Roughly three quarters of the invaders, 53 species, were terrestrial, while 18 species, about 25 percent, were aquatic. Two botanical families dominated the invasive roster: the grasses, or Poaceae, contributed 15 species, and the daisy family, Asteraceae, contributed 11. Together these two families accounted for more than half of all invasive individuals sampled, a pattern the researchers attribute to their enormous taxonomic diversity, prolific seed production, and exceptional dispersal capacity, which allow them to colonize even inhospitable environments. The Fabaceae followed with six species, while Cyperaceae, Convolvulaceae, and Amaranthaceae each contributed four.

The most abundant invaders, measured by an importance value index combining abundance, frequency, and cover, were the common reed Phragmites australis, the pyramidal rosette grass Echinochloa pyramidalis, the African reed Phragmites mauritianus, the water hyacinth Eichhornia crassipes, and the tropical wandering Jew Commelina benghalensis. Among the aquatic invaders, emergent plants, those rooted below the water surface but rising above it, were the best represented group at nearly 56 percent, followed by amphibious species at 22 percent, floating plants at 17 percent, and submerged species at just under 6 percent. On land, the overwhelming majority of invasive species, 83 percent, were herbaceous, with shrubs and trees making up the remainder.

Mapping the invaders’ distributions with geographic information system software revealed a strikingly uneven picture. About 39 percent of the invasive species were widely distributed across the scheme, roughly 21 percent occurred in a random pattern, and just over 40 percent had restricted distributions. The widely distributed group included some of the world’s most notorious weeds, such as Parthenium hysterophorus, Lantana camara, Ricinus communis, and Ageratum conyzoides. The researchers note that many of these species produce abundant seeds and attractive fruits dispersed by wind, water, or animals, and some have propagules that survive digestion, form persistent soil seed banks, and begin reproducing early in life, traits that let them occupy new territory with remarkable speed.

The spatial pattern of the aquatic invaders points to a human fingerprint. The scheme is surrounded by communities practicing agriculture along the riverbanks, and the watercourses receive nutrient inflows from production fields, particularly in areas of large-scale rice cultivation. Water samples analyzed in earlier research by M. N. G. Chilundo revealed nitrogen concentrations in these zones far exceeding those recorded upstream, where only subsistence farming takes place. Consistent with that gradient, the new survey found that water hyacinth, water lettuce Pistia stratiotes, and the submerged coontail Ceratophyllum demersum were more abundant in irrigation channels and drainage ditches near the commercial rice areas than in the Limpopo River itself. Slower water flow in the channels, combined with nutrient enrichment, creates ideal conditions for these plants to explode in number.

To quantify the ecological threat, the team applied the Generic Impact Scoring System, a standardized framework that rates the impacts of alien species across six ecological categories: effects on plants or vegetation, effects on animals, competition, disease transmission, hybridization, and impacts on ecosystem services. Each category is scored from zero to five based on published evidence from anywhere in the species’ invaded range, with the precautionary principle applied when studies disagreed. The literature review revealed that about 92 percent of the 71 invasive species recorded in the scheme are known to cause ecological impacts elsewhere. Lantana camara topped the table with the highest overall impact score of 24, followed by water hyacinth and the leguminous tree Leucaena leucocephala, both scoring 16. A handful of species, including Cyperus articulatus and Ludwigia stolonifera, lacked published impact evidence and went unscored, a gap the authors highlight as a research priority.

The most consequential evidence came from a direct field comparison. When the researchers compared native plant species richness in quadrats invaded by alien plants with that in uninvaded quadrats, the difference was statistically significant, with a Wilcoxon test yielding a p-value of 0.0062. Uninvaded quadrats consistently harbored the greatest diversity of native species, indicating that the invaders are actively suppressing the local flora rather than merely coexisting with it. Competition emerged as the most common impact mechanism among the invasive species, followed by broader ecosystem modification. Many of the invaders also possess allelopathic properties, releasing biochemical compounds into the soil or water that selectively increase mortality among native plants, a mechanism documented in studies of Lantana camara and other notorious weeds that effectively poisons the ground against competitors.

The study’s authors emphasize that the situation is likely to worsen without intervention. Water hyacinth, widely regarded as one of the worst invaders of freshwater ecosystems worldwide, can multiply roughly four-thousandfold from a single pair of plants within one growing season, and its decaying stands add thousands of kilograms of sediment per hectare each year. Submerged species such as Ceratophyllum demersum clog irrigation channels and reduce water flow, imposing real cleaning costs on farmers. The team calls on the managers of the Baixo Limpopo Irrigation Scheme to recognize the problem at every level and to prioritize precautionary eradication of species with restricted and random distributions, before they become as entrenched as the widespread invaders. As the first application of the Generic Impact Scoring System in Mozambique, the study provides a baseline for national policy, and a warning that the country’s irrigation schemes, protected areas, and waterways are all vulnerable to the same creeping green invasion.

Subject of Research: Occurrence, distribution and ecological impact of invasive alien plants in a Mozambican irrigation scheme

Article Title: Occurrence, distribution and potential ecological impact of invasive alien plants at Baixo Limpopo irrigation scheme, Mozambique

Article References: Alafi, A. M., Munguambe, F. F., & Benvindo, H. (2025). Occurrence, distribution and potential ecological impact of invasive alien plants at Baixo Limpopo irrigation scheme, Mozambique. Discover Conservation, 2(1), Article 39. https://doi.org/10.1007/s44353-025-00062-y

Image Credits: AI Generated

DOI: 10.1007/s44353-025-00062-y

Keywords: invasive alien plants, Mozambique, Baixo Limpopo Irrigation Scheme, water hyacinth, Lantana camara, aquatic weeds, biodiversity loss, nutrient pollution, irrigation, ecological impact assessment, species richness, biological invasions