Competition in Marine Invasions

Competition is a negative interaction between two or more species that utilize the same shared, limiting resource (Connell 1983). Although competition can have large local, immediate effects, (e.g. on demography, resource use, etc.), competition in many marine systems is often assumed to have minimal effect on population persistence, primarily due to characteristics of the dominant life histories of marine organisms. Notably, a large proportion of marine species have pelagic larvae and thus often reside in open populations where the supply of progeny is decoupled from progeny production. Thus, although competition can still affect adults, future generations are supplied from distant populations that can “rescue” populations of inferior competitors from being excluded. Even in relatively closed marine habitats, e.g., bays or estuaries, a constant influx of larvae in ballast water (Verling et al. 2005) may make many populations effectively open, subsidizing populations of species that would otherwise be excluded. The open nature of larval production and delivery applies to food resources as well. The preponderance of filter feeders, which feed on a food resource that is typically replenished frequently (e.g., with tidal cycle) and whose supply is often decoupled from consumptive pressure by resident organisms, may reduce the occurrence of resource competition.

Bringing evidence to bear on the frequency and strength of competition in marine species is not easy. Experimental manipulations are usually logistically difficult. For example, planktonic species are extremely hard to track because of their small size and fluidity. Also, dramatic ontogenetic changes and concomitant dietary and habitat shifts are common as well, meaning that even if competition between some life stages can be elucidated, its relative importance on populations overall may be difficult to assess. Thus, particularly in marine systems where logistical and common life history characteristics can make competition hard to study, it is important to assess what has been done, how well it has been done, and what future research needs are.