23
The effects of fragmentation on food webs have been surprisingly little-studied. In terrestrial
24
systems we can envisage fragmented networks in the classical biogeography sense when they
are situated within islands within an aquatic matrix. An example of this comes from recent
1
work carried out in Ireland (McLaughlinet al.,2010). The Gearagh woodland, located in the
2
floodplain of the River Lee in County Cork, is composed of a complicated braided river
3
system composed of approximately 13 channels, each 1–7 m wide. The main channels are
4
stabilized by tree roots, which create a mosaic of small islands due to the accumulation of
5
detrital material and fallen trees over time. A food web study, examining the trophic structure
6
of the invertebrate community on series of 16 islands, ranging in size from 4.5 to 40.8 m2
7
found that, on average, the larger islands contained more species and links than the smaller
8
islands, and network structure consequently differed markedly among fragments (Figure 15).
9 10
<<Figure 15 near here>>
11 12
Fragmentation of food webs can also occur in other lateral (i.e. across landscape) and
13
temporal dimensions, as well as via fractal branching pattern dimensions (e.g. in river
14
networks) (Text Box 1, Figure 16). Additionally, vertical fragmentation, which is even more
15
rarely considered, can occur, such as in mountainous regions (Text Box 8, Figure 17).
16
The loss of large consumers at higher trophic levels due to habitat fragmentation should result
17
in a decreased overall trophic height of the food web, driven by shorter food chains (e.g.
18
O’Gorman and Emmerson, 2009; Byrneset al., 2011; Woodward et al. 2012). This could also
19
lead to an increase in the proportion of top consumers relative to intermediate species, as the
20
latter are effectively promoted to the termini of food chains as the largest higher-level
21
predators are lost (see O’Gorman and Emmerson, 2010; Woodward et al. 2012). Loss of large
22
species at high trophic levels is also likely to result in reduced linkage density (Montoyaet 23
al., 2005; O’Gormanet al., 2010) and connectance (O’Gorman and Emmerson, 2010) within
24
local networks, as well as reduced compartmentalisation, which could make the web less
robust to secondary extinctions (Dunneet al., 2002a), although this is not necessarily the case
1
if there is high redundancy in the system (Woodward et al. 2012). Large species may have
2
weak per unit biomass interactions with their prey and high functional uniqueness
3
(O’Gormanet al., 2011), so their extinction could increase the overall interaction strength
4
within the system. This may reduce stability (see McCannet al., 1998; Neutelet al., 2002),
5
while loss of functional trait diversity will alter ecosystem process rates and functioning
6
(Petchey and Gaston, 2006).
7 8
<Figure 16 near here>
9
<Figure 17 near here>
10
<Text Box 8 near here>
11 12
Body-mass-driven extinctions due to habitat fragmentation may cause an overall increase in
13
the predator–prey body mass ratio, assuming that larger predators eat prey closer to their own
14
body mass (Broseet al., 2006). Smaller predator–prey body mass ratios have been linked to
15
longer food chains due to their stabilising properties (Jonsson and Ebenman, 1998; Jennings
16
and Warr, 2003; however see Mulderet al., 2009), so increases could raise the probability of
17
catastrophic phase shifts or total collapse. Conversely, in systems where large predators are
18
considerably larger than their prey (e.g. fish eating invertebrates versus invertebrates feeding
19
on other invertebrates) the loss of these consumers could increase stability of the food web, as
20
appears to be the case in headwater streams where fish are lost due to habitat loss and
21
fragmentation arising from chemical and/or physical barriers (Layeret al., 2010, 2011).
22 23
<Figure 18 near here>
24 25
The response of freshwater food webs to fragmentation by droughts (Text Box 1), have been
1
characterised recently by manipulating flows in a series of artificial stream mesocosms
2
(Ledgeret al.,2008, 2011, 2012a, b; Woodward et al. 2012; Figure 18). These model systems
3
reflected the abiotic conditions, biodiversity and food web properties of natural streams
4
(Brownet al.,2011; Harriset al.,2007; Ledgeret al.,2009). The results of this fragmentation
5
experiment revealed some dramatic impacts on the food webs: consistent with thehigher 6
trophic rank hypothesis(e.g. Holt, 1996), top predators’ production declined by >90%.
7
Among the primary consumers, production of shredder detritivores was also suppressed (by
8
69%), whereas the base of the food web was relatively unaffected (Ledgeret al.,2011;
9
2012a, b). Contrasting responses were evident among functional groups, ranging from
10
extirpation to irruptions in the case of small midge larvae, although production of most
11
species was suppressed. The ratio of production to biomass (P/B) increased, reflecting a shift
12
in production from large, long-lived, taxa to smaller taxa with faster life cycles (Ledger et al.
13
2011). Fragmentation by drought caused high mortality and the partial collapse of the food
14
web from the top-down (Ledgeret al.,2012a, b) as well as reveresing successional dynamics
15
of benthic algal assemblages (i.e. basal resources), with effective colonists replacing
16
competitive dominants (Ledgeret al.,2008, 2012). The general shift in biomass flux from
17
large to small species, could not fully compensate for the overall biomass flux. Many other
18
network characteristics (e.g. connectance) were, however, conserved, suggesting some
19
higher-level properties might be conserved even when exposed to extreme perturbations
20
(Woodward et al. 2012).
21 22
Fragmentation can also affect marine food webs (Text Box 1). Coral bleaching creates
23
fragments of surviving coral surrounded by reef pavement and coral rubble, with
24
consequences for top-down control as average food chains shorten, generalist species
proliferate, and phase shifts may occur (Hughes, 1994). Simulations of fragmentation
1
processes in Caribbean coral reefs indicate that species losses due to body size or diet
2
constraints will lead to decreases in number of links and changes in connectance and food
3
chain length (Figure 19). Human-induced fragmentation in seagrass food webs could further
4
lead to fewer trophic groups and top predators, lower maximum trophic levels, shorter food
5
chains, and prey-dominated communities (Collet al.,2011). In kelp forests, habitat loss and
6
fragmentation due to storms simplify marine food webs, mainly by decreasing diversity and
7
complexity at higher trophic levels, resulting in shorter food chains (Byrneset al.,2011). The
8
effects of habitat fragmentation on food webs, although little-studied, can be pronounced.
9 10
<Figure 19 near here>
11 12