Impact of the Alien Aquatic Plant Ludwigia hexapetala on the Native Utricularia australis: Evidence from an Indoor Experiment

Abstract

Ludwigia hexapetala is an alien aquatic plant considered highly invasive in Europe since it alters freshwater habitats by forming dense mats both in water and along banks, outcompeting natives. Ludwigia effects on the native carnivorous plant Utricularia australis were investigated here. A 21-day indoor experiment was performed by setting up some separate tests in which Utricularia was made to grow both alone (control tests) and together with Ludwigia (arena tests). Water chemical and physical parameters and growth and morphological traits of Utricularia and Ludwigia were analysed weekly. Water samples were also analysed by UV-visible spectra to verify allelochemical (quercitrin) production by Ludwigia. In arena tests, oxygen concentration and pH were lower and conductivity higher than in control tests. Utricularia grew significantly less in arena tests in both shoot length and internode number, and its fresh weight, trap number and internode length decreased more than the control. Quercitrin was found only in arena tests as an allelochemical product released by Ludwigia. Overall, this study demonstrated Ludwigia significantly alters water parameters and negatively affects the growth of Utricularia, showing aggressive and competitive behaviour against this native species. Such evidence suggest that the widespread of L. hexapetala can represent a serious threat to the conservation of native plant diversity occurring in the freshwater habitats it invades.

Impact of the Invasive Alien Macrophyte Ludwigia hexapetala on Freshwater Ecosystems: Evidence from Field Data

“…Indeed, this study shows that the presence of dense floating populations of L. hexapetala exerts an indirect impact on the growth of other aquatic plants by reducing the availability of light and heat and the levels of dissolved oxygen in the water. In addition to this type of impact, as shown in several studies, Ludwigia is capable of producing and releasing allelopathic substances into the water that strongly inhibit the seed germination and growth of other plants [ 22 , 23 , 24 ]. In contrast, the alien aquatic species Alternanthera philoxeroides (Mart.)…”

“…This result suggests that L. hexapetala, with its aquatic morphotype, adversely affects water oxygenation, due to the production of dense floating mats that interfere with the gas exchange at the water-air interface, leading to lower oxygenation in the water column beneath these populations [51]. In addition, it is able to produce pneumatophores that effectively capture most of the oxygen dissolved in the water [18,24,26,52], reducing the amount of oxygen available for other aquatic organisms. Regarding the dissolved oxygen in the water, the different waterbodies explained 30% of the variance, as they showed inherently differently oxygenated waters.…”

“…The invasiveness of L. hexapetala is mainly due to its ability to produce dense populations in water, negatively affecting water quality and establishing anoxic conditions that are unfavorable for other aquatic organisms [ 18 , 19 , 20 , 21 ]. Moreover, it can release allelopathic substances into the surrounding environment that hinder the seed germination, growth, and reproduction of other plant species [ 22 , 23 , 24 ]. The invasiveness of L. hexapetala is also related to its high ecological and morphological plasticity, which allows it to colonize both aquatic and bank habitats.…”

“…At a later stage, when it has colonized the bank habitat, it switches to a terrestrial morphotype, with elongated vertical flowering stems and lanceolate leaves [ 21 , 26 , 27 ]. It also produces two types of roots: branched roots, which grow downwards, anchoring the plant to the sediment, and spongy white roots, which grow upwards, acting as specialized structures (pneumatophores) for the aeration of tissues submerged in water [ 24 , 25 , 27 , 28 ].…”

“…Thiébaut et al [ 29 ], and, more recently, Pelella et al [ 24 ], have investigated the impact of L. hexapetala on single aquatic native plants, such as Mentha aquatica L. and Utricularia australis R.Br., respectively. However, the evidence from these studies comes from laboratory experiments and, thus, suffers from some limitations, such as short time frames, small volumes, and less complex biotic and abiotic interactions than those occurring under natural conditions [ 3 , 30 ].…”