While non-native species have devastated many ecosystems, some invasions are relatively benign. The trick is to figure out which ecosystems are most vulnerable to invasion. Theory says species-poor ecosystems are more likely to be invaded but new research bolsters the growing body of evidence suggesting the opposite: in southern California’s coastal sage scrub, sites with more native spiders also had more non-native spiders.
“Our data support the hypothesis that non-native species are more common in areas of high biodiversity,” say Jutta Burger and three co-authors in the April issue of Biological Conservation.
Southern California’s coastal sage scrub has been highly fragmented by development and so is particularly vulnerable to invasion. For instance, the Argentine ant has replaced native ants along urban edges in coastal sage scrub valleys.
Burger and her colleagues compared the abundance of 140 native and five non-native spiders at 60 sites in undisturbed coastal sage scrub in coastal San Diego County. The researchers found that the non-native spiders were widespread and abundant. Non-native spiders were caught at 51 sites and comprised more than half of all spiders collected at eight sites. Moreover, the non-native Oecobius annulipes was the most common spider overall, at 16 percent of all spiders caught.
To their surprise, Burger and her colleagues found that non-native spiders had invaded sites where native spiders were more abundant and diverse. Interestingly, two of the non-native spiders (Dysdera crocata and Oecobius annulipes) were most abundant at sites with the most native spiders. How might non-native spiders benefit native ones? Many spiders eat other spiders, and non-natives may simply provide the natives with more prey.
“Our results support the contention that both invasibility and resilience are higher in diverse, highly linked communities with high resource availability,” say Burger and her colleagues. “Colonization by non-natives does not necessarily mean that native species, communities, or ecosystems are adversely affected.” This means we need to be able to predict where non-native invasions will be harmful.
Due to the rise in both accidental and intentional biological introductions, the researchers call for better models of their respective impacts. This will include determining how non-native species affect native systems and identifying traits of ecosystems that are particularly vulnerable to invasion. Knowing which species may invade a given ecosystem next could help managers prevent the invasion.
For more Information
Burger, J.C. 2001. Evidence for spider community resilience to invasion by non-native spiders. Biological Conservation 98:241-249.
