Soil fertility as a mediator of interactions between an introduced specialist beetle and a native generalist nematode on an exotic invasive plant and its native congener

1. Invasive plants are often attacked by both introduced specialist and
native generalist natural enemies in new ranges. Soil fertility can
potentially alter the interactions of these natural enemies with native
versus invasive plants in ways that have largely unexplored implications
for biological invasions and biological control.  2. Common
garden experiment was conducted to compare the performance of an
introduced specialist flea beetle, Agasicles hygrophila, and / or
a native generalist nematode, Meloidogyne incognita, on invasive alligator
weed, Alternanthera philoxeroides, and its native congener sessile
joyweed, Alternanthera sessilis, under different levels of soil nitrogen
(N) and phosphorus (P).  3. At a relatively low or moderate N and
P level, the flea beetle and the nematodes were not significantly affected
by each other. Under these conditions, alligator weed responded
plastically by producing more branches and biomass, as well as longer
stolons, in response to attack by the flea beetle and / or the nematode,
compared to sessile joyweed responses to similar levels of damage.
However, under a relatively high N and P level, nematode infestations
significantly reduced flea beetle damage on alligator weed, resulting in
significantly greater above- and below-ground biomass and longer stolons
than plants without herbivory. In contrast, beetle herbivory significantly
increased the level of nematode infestations on sessile joyweed, resulting
in significantly fewer fine and coarse roots, and lower above- and
below-ground biomass compared to plants without herbivory.  4.
Synthesis and applications. Our findings illustrate the importance of soil
fertility in mediating interactions between specialist biocontrol agents
and native generalists on native versus invasive plants. High phenotypic
plasticity seems to be an important attribute that contributes to the
success of invasive plants like alligator weed in relatively nutrient-poor
environments. However, nutrient-rich environments could potentially confer
greater growth benefits on invasive plants than on native ones by changing
herbivore-herbivore interactions on plants differently. There is a great
need to fully investigate the direct and indirect interactions between
biocontrol agents and generalists across food webs following classical
biocontrol releases. Nutrient measurements of both soil and water bodies
should also be incorporated into all stages of biocontrol programs.

1. 入侵植物在新分布区常受到引入的专性天敌和本地广食性天敌的共同攻击。土壤肥力可能改变这些天敌与本地植物及入侵植物间的相互作用，其对生物入侵和生物防治的意义尚未得到充分探索。
2. 本研究通过普通园圃实验，比较了引入的专性跳甲Agasicles hygrophila和/或本地广食性线虫Meloidogyne incognita在入侵物种空心莲子草Alternanthera philoxeroides及其本地同属种莲子草Alternanthera sessilis上的表现。
3. 在较低或中等氮磷水平下，跳甲与线虫之间无显著相互影响。在此条件下，与本地同属种莲子草相比，入侵植物空心莲子草对跳甲和/或线虫的攻击表现出表型可塑性反应，产生更多分枝和生物量，且匍匐茎更长。然而，在较高氮磷水平下，线虫侵染显著降低了跳甲对空心莲子草的损害，使其地上和地下生物量及匍匐茎长度显著高于未受植食的植株。相反，跳甲取食显著增加了本地莲子草上线虫的侵染水平，导致其细根和粗根数量显著减少，地上和地下生物量低于未受植食的植株。
4. 综合与应用：本研究结果表明，土壤肥力在介导专性生物防治剂与本地广食性天敌之间、以及它们与本地植物和入侵植物间的相互作用中具有重要意义。高表型可塑性似乎是空心莲子草等入侵植物在低养分环境中成功定殖的重要特性。然而，高养分环境可能通过差异性改变植物上的植食者间相互作用，为入侵植物带来比本地植物更大的生长优势。亟需充分研究经典生物防治释放后，生物防治剂与广食性天敌在食物网中的直接和间接相互作用。此外，土壤和水体的养分测定应纳入生物防治项目的所有阶段。