Community Ecology Processes, Models, and Applications

assembly processes are different between temper-

ate and tropical areas. Similarly, Ryberg and Chase

(2007) showed both theoretically and empirically

how the presence of top predators can reducez.

Other community stressors with effects similar to

predators (e.g. increasing local extinction rates)

should have similar effects.

5.6 Effects of dispersal rates on local communities

As noted above, variation in dispersal rates is a

primary factor that can influence metacommunity

structure and the partitioning of diversity. It is well

known that islands that are closer to the mainland,

and thus have higher rates of immigration, have

higher diversity than comparable islands farther

from the mainland, which receive lower rates of

dispersal (e.g. MacArthur and Wilson 1967). Fur-

ther, in a conservation context, experimental data

are emerging that show higher levels of diversity in

habitat patches that are connected by habitat corri-

dors than in habitat patches without such corridors

(Gilbertet al. 1998; Damschenet al. 2006).

There will often be a limit to the positive effect of

dispersal on local diversity, such that diversity can

show an asymptotic (MacArthur and Wilson 1967)

or hump-shaped (Mouquet and Loreau 2003) rela-

tionship with increasing dispersal rates. Using

meta-analysis, Cadotte (2006) compared standar-

dized dispersal rates with their influence on local

species diversity among animals and plants.

Among animals, there was a positive, but potential-

ly asymptotic or hump-shaped relationship be-

tween the enhancement of local diversity relative

to the control with increasing dispersal rates,

whereas there was generally a positive effect of

dispersal on local diversity (most treatment effects

relative to the control are greater than zero), but this

did not seem to vary with the rates of enhanced

dispersal.

Higher dispersal through habitat connections

does not always increase local diversity. For exam-

ple, Hoyle and Gilbert (2004) used an experimental

landscape of moss and its microarthropod inhabi-

tants similar to that used by Gilbertet al.(1998),

but found no positive effects of habitat corridors

on species diversity during a year with favourable

weather conditions. A possible explanation for

this difference is that, during drought years, res-

cueeffectsviacorridorsmaybemoreimportantin

preventing local extinctions. Similarly, Kneitel and

Miller (2003) found strong effects of dispersal on

the diversity of a protist community in the absence

of predatory mosquitoes, but much weaker dis-

persal effects in the presence of mosquitoes.

While dispersal’s effects on diversity have

been well-studied at local scales (e.g. reviewed

in Cadotte 2006), its effects onb-diversity are

less studied. However, both theoretical and em-

pirical results show that rates of dispersal often

result in lowerb-diversity (and lowerz), homo-

genizing communities, and potentially ameliorat-

ing or reversing the local-scale positive effects of

dispersal, when diversity is measured at the re-

gional scale (Harrison 1997, 1999; Amarasekare

2000; Forbes and Chase 2002; Chase 2003; Mou-

quet and Loreau 2003; Chase and Ryberg 2004;

Fukami 2004; Cadotte and Fukami 2005). Finally,

the influence of dispersal on b-diversity and

scale-dependent patterns of diversity will also

depend on the mechanisms that allow species

to coexist both locally and regionally. Among

experimental communities treehole-dwelling

protists, Ostmanet al. (2006) found that dispersal

homogenized local communities under benign

conditions (reducing b-diversity) and thus re-

duced overall g-diversity. However, when

drought disturbance was imposed on those

1.0

0.6

0.2

02040

Slope (

Z

) of the

species–area relationship

60

Latitude (
)

80

–0.2

Figure 5.2Decline in the slope of the species area curve
(z) with decreasing latitude. Redrawn from Drakare
et al.(2006).

64 SPACE AND TIME