The Impact of Invasive Species: Managing Threats to Forest Ecosystems

• The Impact of Invasive Species in forests with closed canopy is more resistant to invasion by alien plant species due to limited light in the understory.
• Invasives’ success depends on species-specific and site-specific factors.
• Competition for space, water, and nutrients also keeps invasives away.
• Natural forests are more resistant to invasives than planted or managed forests.

The rapid spread of invasives into natural plant communities is a major ecological threat due to its impact on species and ecosystems. Understanding the mechanisms that make invasives successful and those that increase resistance to them is necessary to develop appropriate control strategies.

Impact of Invasive Species

Invasive species that establish beyond their natural range, where they multiply and spread rapidly, is called biological invasion. The invasives spread unchecked as natural enemies like pests, herbivores, and diseases may be missing in their new homes, giving them an advantage over native species.

Invasive alien plant species (IAPS) are considered a threat as they spread fast, outcompete native species, and displace them. The impact of invasive species on biodiversity includes a reduction in species diversity, alteration of natural succession, and reduction of forest regeneration. Other impacts can be changes in soil microbial communities, litter quality, habitat degradation, and the overall functioning of the forest ecosystem. Invasives change nutrient, carbon, and hydrological cycles and increase the risk of fires.

Therefore, invasives are now classified as a serious ecological threat.

Forest Canopy Disturbance Increases Invasives

Forests and rangelands suffer from biological invasion of herbs and woody species. Reports from several parts of the world reveal that invasives are more abundant in open-canopy habitats than in closed-canopy sites. For example, the invasion of Lantana camara (Lantana) is common in the open, forest edges, and forest gaps.

Natural forests are less prone to invasives than modified ecosystems because their canopy cover is more. Forest regrowth and restoration, which increase cover and close the canopy, suppress the spread of invasives. Species richness and canopy of invasives decreased when tree canopy cover and basal area increased, see Figure 1. It is reported as an unexpected benefit of managing degraded Shorea robusta forests in Nepal.

Forest species diversity does not influence invasives’ spread. For instance, Chromolaena odorata cover across a native species richness gradient in Sal forests doesn’t change.

Figure 1: “Variation of invasive alien plant species (IAPS) richness (#species/plot) with tree canopy and basal area as well as the IAPS cover with tree canopy and basal area,” Khaniya and Shrestha, 2020. (Image credits: https://doi.org/10.1186/s41610-020-00158-7)

Conversely, when forests are disturbed by human interventions to create gaps, invasive easily colonize the open areas. In gaps and forest edges, invasives become dominant, leading to the local extinction of native species. Areas with more edges in smaller canopy gaps or forest edges are particularly prone to invasives.

Environmental Factors Favoring Invasives

Invasives succeed due to a combination of environmental and biological factors. Invasives grow because of changes in microclimate and competition due to the absence of the canopy.

When canopy cover reduces, several growth conditions change in favor of invasives, including light intensity, temperature, water and nutrient availability, etc.

• Light availability is by far the most crucial change that occurs due to canopy openings in forests. Higher light intensity favors the germination of invasives. Most invasive species record significantly higher germination rates under higher light intensity than in shade. For example, Cortaderia jubata’s germination rate is three times higher than in the open than under closed canopy.
• Higher temperature is vital for some but not all invasives for establishment. Higher or fluctuating temperatures can break the seed dormancy of invasives.
• Seed burial depths can be crucial by limiting access to light. Seeds buried in deeper soils have lower rates of germination. So, if canopy disturbance is accompanied by soil disturbance that brings seeds to the surface, light can increase germination.
• Nutrient and water availability increases when larger trees fall in gaps, providing more resources for establishing invasives.
• Space is freed for the emergence of new plants.
• The size of the disturbance determines the species that can establish. For example, larger clearings due to fire will facilitate fire-tolerant species, while smaller gaps encourage many weedy species due to less competition.

The effect of these factors is specific to species and site. Andropogon gayanus germinates in Australian grasslands by taking advantage of disturbed canopy and soil, but seedling survival depends on soil disturbance that reduces competition for nutrients and water. For Chromolaena odorata in Nepal’s Sal forests, canopy disturbance and more light were enough for successful establishment.

Biological Factors Limiting Invasion

Biological factors affecting establishment are competition, and invasive’ plant traits and high seed dispersal.

Competition from Natural Forests

Less competition from dominant trees due to disturbances and gaps is a significant biotic mechanism for invasive success. The ability of standing tree communities to limit access to resources varies across communities and on the traits of the invasives. More than species diversity, the canopy cover, density of trees, amount of biomass, and number of forest strata (of trees, understory, and shrubs) are more crucial to resist invasion. These factors will also determine if gaps become available for invasive exploits.

Invasives’ Traits

A study found that the number of invasive species and their traits were also decisive. A single dominant species with long-distance seed dispersal, fast growth, and tolerance to water stress can survive extreme conditions in large open spaces. It can establish itself by forming compact clonal patches in its new home and exclude native species by taking up space and resources, such as Aster lanceolatus in European lowland forests.

Impatiens parviflora establishes itself in sandy areas in a central European forest to avoid competition within a forest. Multiple invasive species that are not dominant and have a small canopy cover survive in closed-canopy forests. These kinds of invasives do not reduce native species numbers and could increase species numbers by co-existing with resident plants.

Fruiting and Seed Dispersal

Invasives spread because they produce several seeds, many of which are dispersed long distances.

Abiotic Factors

Seed dispersal is also affected by biotic and abiotic factors. Crucial abiotic factors like irradiance, rainfall, temperature, and photoperiod can influence fruiting intensity, determining seed numbers. Fruiting patterns increase with light intensity even when all other conditions (precipitation, temperature, and photoperiod) are similar. Hence, a higher fruit crop in open areas results in more seeds being dispersed. Therefore, seed dispersal will differ between sites.

Biotic Factors

Also, nearly 60 percent of invasive shrubs are dispersed by animals that are frugivores. The number and types of dispersers, such as birds and small mammals, will depend on the fruit load on invasives and site characteristics.

Site features could also be the nature of the surrounding and neighboring areas. Whether these areas have fruit-bearing plants or habitats that could attract dispersers. If the habitat type differs, the number of frugivore types and visits to the habitat will change. For instance, Lantana under low canopy had more fruits and were visited more often by bulbuls, the primary dispersers of lantana seeds. Bulbuls’ visit to Lantana under a closed canopy was less. Therefore, there was more seed dispersal in the open than in the closed canopy.

Maintain Canopy Cover

Anthropogenic activities that change forest canopy cover can influence invasive numbers. Maintaining a closed canopy in forests will resist invasives. In contrast, activities like logging, lopping, and grazing that disturb forests and open the canopy will create conditions suitable for invasives to germinate, establish, and propagate. The Impact of Invasive Species is exacerbated by canopy cover reduction due to climate change or drought, making controlling the invasives more challenging.

Measuring forest canopy cover with precise, rapid instruments like the CI-110 Plant Canopy Imager from CID Bio-Science Inc. can help scientists and foresters develop management strategies to reduce the number of invasive species.

Sources

Gómez P, Murúa M, San Martín J, Goncalves E, Bustamante RO (2019) Maintaining close canopy cover prevents the invasion of Pinus radiata: Basic ecology to manage native forest invasibility. PLoS ONE 14(5): e0210849. https://doi.org/10.1371/journal.pone.0210849

Khaniya, L., Shrestha, B.B. (2020). Forest regrowth reduces richness and abundance of invasive alien plant species in community-managed Shorea robusta forests of central Nepal. J ecology environ 44, 12. https://doi.org/10.1186/s41610-020-00158-7

Kołodziejek, J., & Patykowski, J. (2015). Effect of environmental factors on germination and emergence of invasive Rumex confertus in Central Europe. The Scientific World Journal, 2015.

Lanta, V., Liancourt, P., Altman, J. et al. (2022). Determinants of invasion by single versus multiple plant species in temperate lowland forests. Biol Invasions 24, 2513–2528. https://doi.org/10.1007/s10530-022-02793-8

Najar, M.U.I, & Rahim, A. (2018) Effect of Canopy Cover on Understory Invasive Alien Species in the Wayanad Wildlife Sanctuary, Kerala, India. J Biodivers Manage Forestry 7:1. doi: 10.4172/2327-4417.1000194

Setterfield, S. A., Douglas, M. M., Hutley, L. B., & Welch, M. A. (2005). Effects of Canopy Cover and Ground Disturbance on Establishment of an Invasive Grass in an Australia Savanna. Biotropica, 37(1), 25–31. http://www.jstor.org/stable/30045503

Sharma, L. N., Adhikari, B., Watson, M. F., Shrestha, B. B., Paudel, E., Karna, B., & Rijal, D. P. (2022). Forest canopy resists plant invasions: a case study of Chromolaena odorata in Sal (Shorea robusta) forests of Nepal. Journal of Tropical Ecology, 38(2), 49–57. doi:10.1017/S0266467421000456