Community Ecology Processes, Models, and Applications

they are more productive. It would be very interest-
ing to see whether similar patterns would hold in
the data used to infer large-scale diversity–invasi-
bility patterns.
Experimental approaches can also be very useful
in addressing some of the many different factors
that can potentially influence the success of invad-
ing species. For example, Lockwoodet al. (2005)
have emphasized the underappreciated role of
propagule pressure, the number of invaders arriv-
ing at a community per unit time, in affecting inva-
sion success. Using approaches similar to those
outlined above, it should be relatively easy to vary
either propagule size – the number of invaders
arriving per unit time and area – during any inva-
sion event, or the frequency of invasion events to
see how these factors influence potential establish-
ment of invaders in communities with different
initial properties. Those properties could include

diversity, productivity or any of a number of fea-

tures of interest.

Another potentially interesting approach would

be to explore how the trophic position of an invader

interacts with diversity at multiple trophic levels in

a community. Most inferred effects of local diversi-

ty on invasion success invoke some sort of biotic

resistance operating within the trophic level that

the invader is attempting to enter. The arguments

essentially rely on increasing competition within a

diverse trophic level, guild or functional group to

reduce invasion success. However, differences in

diversity on other trophic levels could conceivably

have either positive or negative effects. For exam-

ple, consider the situation depicted in Fig. 14.3, in

which an invader on the second trophic level, a

herbivore, potentially encounters differences in

species diversity of its prey (trophic level 1) or its

predators (trophic level 3). Greater prey diversity

Productivity

Invader abundance

Productivity

Local diversity

Local diversity

Invader abundance

low <- productivity -> high

2.5

2.0

1.5

1.0

0.5

0.0

2468

Resident species richness

Invader density [log

10

(abundance/ml+1)]

Invader density [log

10

(abundance/ml+1)]

Nutrient concentration (gram protozoan pellets/L)

Nutrient concentration (gram protozoan pellets/L–1)

10

0.0

0.0

0.5

1.0

1.5

2.0

0

0.0 0.2 0.4 0.6

Pre-invasion species richness

0.8

2

4

6

8

10 (a)

(c)

(d)

0.2 0.4 0.6 0.8

Figure 14.2Positive relations between productivity and diversity and productivity and invader success in experimental
microbial communities (modified from Jiang and Morin 2004). These separate patterns combine to produce an apparent
positive correlation between diversity and invader success, but the correlation is spurious and not indicative of a cause—
effect relationship.

198 FUTURE DIRECTIONS