Litt and Pearson (full citation at the end of the blog) are trying to improve scientists’ ability to predict the impact of biological invasions. Their goal is to predict which organisms will be winners, which losers, in the face of anthropogenic ecosystem change.
They focus on exotic plant invasions, because they are ubiquitous. Furthermore, plant invasions affect ecosystems by reassembling the plant community in ways that affect the niches used by native animals and hence the animals’ success under the new conditions. After determining the differences between the traits exhibited by invasive plants vs. the native plants they are displacing, scientists can then identify which native animals are most likely to be affected, as well as how and why they might respond to exotic plant invasion. [Note that Doug Tallamy is looking at similar issues.]
Litt and Pearson have developed a framework to assess how plants’ traits might affect associated wildlife. Applying the framework requires certain baseline information about the ecosystem in question.
This knowledge is applied in stepwise fashion:
1) Identify the fauna of interest and their linkage to the native plant community. This association might be food or habitat values such as shelter. Then the researcher determines the relevant plant traits of importance to that animal and approximates the strength of the animal’s dependence on these traits. Note that the focus is on plant traits relevant to the animal users, rather than specific plant species.
2) Determine overall importance of the plant traits for the area under study by (a) averaging dependence of a representative subsample of individuals to obtain a community-level value for each plant species or functional group and (b) quantifying the relative abundance of the plant functional group in the community (e.g., cover or biomass).
3) Plot the way the animal species’ abundance changes with resource abundance.
4) Understand how the invasive plants will alter the distributions of the native plants’ traits and potentially introducing novel traits that might alter the faunal community.
Litt and Pearson reviewed earlier studies to test how well this framework explained the responses of three groups of fauna to plant invasions in different ecosystems.
Spiders in invaded grasslands
Intermountain grasslands of western Montana are heavily invaded; non-native plants already comprise 25–60% of average total plant cover.
One group of native spiders construct their irregular webs entirely within a single plant. A second group – orb weavers – suspend their larger webs from multiple plants. The former depend on the architectural complexity of individual plants; they can build larger webs in plant species possessing greater branching and/or longer branches of the flowering stalks. Orb spiders depend more on the complexity of the overall plant community.
Plant architecture is closely tied to the plant’s functional groups, that is, whether they are grasses or forbs.
These grasslands are generally dominated by perennial grasses. The irregular-web spiders can use grasses, but strongly favor forbs, particularly those with the most complex flowering structures. Orb weavers are generalists, incorporating multiple plant species; but they also tend to favor forbs, presumably because they are more robust.
Invasive plants in the Western Montana grasslands are of two types: an annual grass, cheatgrass (Bromus tectorum), and numerous perennial and annual forbs. Cheatgrass largely replaces the dominant native grasses with a similar architecture – although cheat is shorter. The exotic forbs, which can collectively invade at levels comparable to cheatgrass, tend to be taller and more complex structurally than the native forbs. Thus, invasion by exotic forbs strongly shifts the community-level distribution of the key trait toward greater structural complexity by replacing the dominant, but structurally simplistic, native grasses, and the more diminutive native forbs. These changes increased the abundance of both spider groups, but especially the specialist irregular web weavers. They find the new conditions meet their needs. Both spider groups appeared to expand their realized niches in response to invasion, i.e., they are able to use a broader range of plant architectures than was available in the native system.
Rodents in semi-desert grasslands invaded by Lehmann lovegrass
In the semi-desert grasslands of the American southwest, native grasses and forbs provide food and habitat for a variety of rodents. This vegetation influences which species of rodents are present in two ways: the size of the plants’ seeds and the density of vegetative cover. Litt and Steidl examined both. They divided the rodents into separate guilds based on diet and preferred vegetative cover. The two sets of guilds did not overlap for all species.
In southern Arizona, the native plant community is dominated by several grass species and herbaceous forbs; most species produce relatively large seeds. Vegetative cover is generally low, but varies in a patchy fashion. The rodent communities in uninvaded native grasslands are dominated by seed-eaters that prefer sparse cover.
Invasion of these grasslands by Lehmann lovegrass (Eragrostis lehmanniana) results in increased vegetative cover but the grass produces very small seeds that probably provide little to no food for rodents. Another result is a decrease in overall abundance of arthropods. The new conditions favor different rodent species from those most common in uninvaded habitat.
Two more specialized seed-eating rodent species, which seek both lower cover and larger seeds, decreased in abundance. A rodent species which favors lower vegetative cover and feeds on larger invertebrates also declined. In contrast, abundance increased for two other rodent species that prefer more dense cover and are more opportunistic in their feeding. One species surprised the scientists: Dipodomys merriami increased in abundance, despite the fact that this species favors more open environments. Perhaps other functional traits or biotic interactions are important to this species? There was no apparent change in abundance for three other species, suggesting either a lack of statistical power (2 were less abundant) or that these rodents were able to persist through a balance of positive and negative changes in food and habitat characteristics.
Warblers in Riparian Habitats in the Southwest
Riparian habitats in the same desert region have been aggressively invaded by the exotic shrub saltcedar (Tamarix spp.). Litt and Pearson consider the findings of Mahoney et al. of this invasion’s impact on two ecologically similar warbler species. One, the yellow warbler (Setophaga petechia), is very widely distributed across North America; it is considered a generalist. The other, Lucy’s warbler (Oreothlypis luciae), is endemic to a small region of the southwest United States and northern Mexico.
The two species have similar feeding behaviors but differ in their nesting requirements. The yellow warbler constructs open cup nests in the branches of shrubs and trees. Lucy’s warbler nests in cavities in larger trees excavated by others. Hence, these species were expected to respond similarly to changes in food resources and foraging habitat, but differ in their responses to changes in nesting substrate.
Native vegetation in the region consists primarily of willows and cottonwoods in the riparian corridors, with oak and mesquite woodlands in the adjacent uplands. Saltcedar invasion rapidly displaces the willows; it takes much longer to displace cottonwoods since are large and long-lived. Upland vegetation is uninvaded and unaffected. While saltcedar is structurally similar to native willows, its leaf architecture allows more light to penetrate in saltcedar stands. This can exacerbate heat stress on nestlings in these hot, arid environments, as well as expose the nestlings to nest predation. These effects are exacerbated by the presence of a biocontrol leaf beetle (Diorhabda spp.), which cause widespread defoliation of saltcedar during nesting season. Meantime, the cavity nests used by Lucy’s warbler are barely affected.
The study by Mahoney et al. showed that in low-invasion riparian sites, the two warblers occur at comparable abundances. When saltcedar invasion replaces willows, yellow warblers decline by ~50% while there is no apparent change in abundance of Lucy’s warblers.
Litt and Pearson point out that their framework is based on two key assumptions that establish the context for its efficacy.
The first is that bottom-up forces fuel ecological processes. Plants are key to making the sun’s energy available to consumer animals and – thence to predators. Consumers’ and predators’ top-down effects are secondary. The authors’ framework thus provides better predictions of community outcomes when systems are predominantly structured by bottom-up forces. As top-down forces increase or when invasive plants differentially affect multiple dimensions of the consumer niche space, it will be more challenging to track and predict outcomes, as our rodent example demonstrates.
The second assumption is that exotic plant invasions will most strongly influence bottom-up processes. Invasive plants displace native plants and their plant traits, thus directly affecting consumers by altering the quality and quantity of food and habitat resources. However, plant community changes caused by plant invasions can also affect predators directly and indirectly via several interactions. These changes in predators’ abundance and/or their per capita effects on prey might create feedbacks that can complicate interpreting and predicting invasion outcomes.
Litt and Pearson concluded that their approach is promising but has inherent limitations linked to the dynamic nature of ecological systems.
[Ecologists continue to evaluate the impacts of saltcedar eradication efforts on another bird species, the federally endangered southwestern willow flycatcher (Empidonax extimus trailii). See, for example, Goetz, A., I. Moffit and A.A. Sher. 2022. Recovery of a native tree following removal of an invasive competitor with implications for endangered bird habitat. Biological Invasions Vol. 24, pp. 2769-2793.]
SOURCE
Litt, A.R. and D.E. Pearson. 2022. A functional ecology framework for understanding and predicting animal responses to plant invasion. Biol Invasions https://doi.org/10.1007/s10530-022-02813-7
& Supporting Information [warblers in riparian ecosystems invaded by tamarisk]
Posted by Faith Campbell
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For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
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