Disturbance and Exotic Species Success in Urban Environments

Vishva Nalamalapu

            When an exotic organism is introduced to an environment, it is not guaranteed success. What, then, determines whether or not it will succeed? To answer this question, research is needed that compares exotics that are invasive (species that are widespread and dominant) and those that are non-invasive (isolated species with low abundances). Lake and Leishman did just that (2004).

            Lake and Leishman compared specific traits in invasive exotic, non-invasive exotic, and native species in order to understand which traits promote exotic species success, and whether those traits change with disturbance type. One of the traits was specific leaf area, which they expected to be greater in invasives because it is an indicator of relative growth rate, and exotics need higher growth rates in order to succeed. They also expected exotics to have more glabrous, soft leaves because exotics experience less herbivory (herbivores are not adapted to prey on exotics when they are first introduced) and, consequently, do not need to allocate as much to defense. Lake and Leishman expected invasives to have less seed mass because there is a tradeoff between seed mass and number, and exotics are more successful if they have more seeds to maximize their colonization potential. Lastly, they expected invasives to have herb, grass, climber, and vine growth forms and lower canopies because exotics are more successful if they grow more quickly.

How They Did It

To test these hypotheses, Lake and Leishman sampled 400m² of vegetation in three types of urban bushland in Sydney, Australia. Sites where there was stormwater runoff were considered water and nutrient disturbed, riparian sites next to creeks receiving urban runoff were considered nutrient disturbed, and sites with disturbance from clearing, vehicles, and/or trampling were considered physically disturbed.

What They Found

Lake and Leishman found exotics only where there was nutrient enrichment, and they found only one exotic species in physically disturbed areas. They also found that invasives had higher specific leaf areas and more soft leaves than non-invasives. In physically disturbed sites, invasives had high colonization abilities (small seed masses) and short times to maturity (low canopy heights and herb or grass growth forms). In nutrient enriched sites, invasives had greater allocations to growth (climbing or vine growth forms) and long-distance seed dispersals (vertebrate seed dispersal). Therefore, higher specific leaf areas and more soft leaves are associated with exotic species success. High colonization ability and short time to maturity are particularly important in physically disturbed sites, while maximum allocations to growth and long-distance seed dispersals are particularly important in nutrient enriched disturbed sites. Thus, traits that promote exotic species success depend on the sites that the exotic species are invading.

Why It's Important

These findings have significant implications. To inhibit exotic species success, stormwater runoff should be stopped from reaching low-nutrient soils and physical disturbance should be minimized. More generally, ecological restoration cannot attempt to restore previous communities without first restoring previous environmental conditions (nutrients, water, disturbance, etc.). In sum, against our human tendency to simplify issues and disregard their root cause, we must manage the root cause of exotic species success.

What's Next?

            To further our understanding of physical disturbance and species success, along with a fellow Ecology classmate, I am studying desire paths in Grinnell College's campus. Desire paths are paths that have been developed and maintained by students repeatedly walking on them (rather than formal paths). We are considering this walking a type of physical disturbance, and are surveying the dominant species along the disturbance gradient (at different distances from the paths' centers). With the findings of Lake and Leishman in mind, we expect successful species will have high colonization abilities and short times to maturity. By carrying out this study, we hope to further the findings of Lake and Leishman and to understand which species and, thus, which traits cause species to be successful in this type of physical disturbance.

References

Lake, J.C., & Leishman, M.R. (2004). Invasion success of exotic plants in natural 

            ecosystems: the role of disturbance, plant attributes and freedom from herbivores. 

            Biological Conservation, 117, 215-226.