Low river flow alters the biomass and populationstructure of a riparian predatory invertebrate

MICHELLE J . GREENWOOD*, † AND ANGUS R. M CCINTOSH*

*School of Biological Sciences, University of Canterbury, Christchurch, New Zealand†National Institute for Water and Atmospheric Research, Ltd, Christchurch, New Zealand

SUMMARY

1. Low flows in rivers are predicted to increase in extent and severity in many areas in the

future, yet the consequent impacts of river drying on terrestrial communities via (i)

changes to riparian microclimatic conditions and (ii) the identity and abundance of

emerging aquatic insects available to riparian predators have not been quantified.

2. We investigated the influence of low river flow on a riparian fishing spider, Dolomedes

aquaticus, in five New Zealand rivers containing permanently flowing and drying reaches

and, in one river, along a longitudinal drying gradient.

3. The biomass of aquatic insects, potential prey for D. aquaticus, declined with low river

flows while the abundance of potential terrestrial prey remained similar at all sites. In the

replicate rivers, and along the longitudinal drying gradient, spider biomass was lower, and

size classes were skewed towards more small and fewer large spiders, in drying sites. A

desiccation experiment in the laboratory indicated high sensitivity of the spiders, with

prey presence increasing spider survival.

4. Differences in the spatial distribution, biomass and population size structure of spiders

were observed along the longitudinal drying gradient and disappeared within 16 days of

the water returning to all sites.

5. In total, low river flow affected the biomass of D. aquaticus, as well as their size class

structure and spatial distribution. This indicates that low river flows have the potential to

affect adjacent terrestrial ecosystems.

Keywords: distribution, Dolomedes aquaticus, drought, size structure, spider

Introduction

Flow regime has an overriding influence on the

physical environment of river channels and the

adjacent riparian zone (Bendix, 1997; Amoros &

Bornette, 2002; Stromberg et al., 2007). Flow-related

abiotic factors, such as substratum movement and

hydrological connectivity, also often have strong

influences on the composition and stability of aquatic

communities (Winterbourn, Rounick & Cowie, 1981;

Resh et al., 1988; Williams, 1996; Hart & Finelli, 1999;

Amoros & Bornette, 2002). Furthermore, flow regimes

can impact terrestrial communities via changes in the

abundance and identity of potential prey and the

suitability of the riparian zone as a habitat for

terrestrial predators (Bell, Petts & Sadler, 1999;

Greenwood & McIntosh, 2008; Sperry & Weather-

head, 2008). Many riparian predators, such as insec-

tivorous birds, bats, lizards, beetles and spiders, live

and ⁄or forage within the riparian zone, often depend-

ing on emerging winged aquatic insects for a large

proportion of their diet (Nakano & Murakami, 2000;

Collier, Bury & Gibbs, 2002; Sanzone et al., 2003;

Baxter, Fausch & Saunders, 2005). A subsidy of

aquatic prey can increase the abundance of terrestrial

predators and sometimes result in strong top-down

Correspondence: Michelle J. Greenwood, National Institute of

Water and Atmospheric Research, P.O. Box 8602, Christchurch

8011, New Zealand. E-mail: michelle.j.greenwood@gmail.com

Freshwater Biology (2010) 55, 2062–2076 doi:10.1111/j.1365-2427.2010.02462.x

2062 � 2010 Blackwell Publishing Ltd

between-river differences in the rate and direction of

river drying and the degree of change in wetted width

will have on the response of D. aquaticus to low river

flows. Further research is needed to clarify how

terrestrial predators will respond to the pattern and

speed of river drying, and to investigate whether

aquatic and terrestrial communities, by becoming

adapted, can maintain higher abundances in rivers

that dry regularly or predictably. The fact that we saw

significant and consistent effects of river drying on

D. aquaticus biomass in five different drying rivers

suggests that low flows are a strong determinant of the

spider biomass that can be supported. Moreover, our

study design highlights the value of multiriver com-

parisons, as opposed to the traditional approach of

studying one river intensively.

Dolomedes aquaticus population size structure and

spatial aggregation

We predicted that the general decline in aquatic

invertebrate biomass (and, by implication, food avail-

ability) would affect the size structure of D. aquaticus

populations. Larger individuals (or those in better

condition) should have been able to survive longer

under food stress. However, drying sites were dom-

inated by smaller, juvenile spiders, with very few or

no large spiders occurring. A strategy of depending

on damp soil near the river to resist desiccation is

used commonly in riparian spiders (Carico, 1973;

DeVito & Formanowicz, 2003), and smaller spiders

can presumably use smaller or deeper interstitial

refuges and remain in contact with damp rocks for

longer. In addition, desiccation mortality is likely to

vary with body size, as body surface area to volume

ratios alter (Willmer, Stone & Johnston, 2000), with

smaller spiders often surviving longer than adults

(DeVito & Formanowicz, 2003). However, our desic-

cation mortality experiment showed no evidence of

size-selective desiccation mortality where death was

very rapid for all size classes at a moderately high

temperature (24 �C) and low humidity (�30%). In

addition, during the field survey, water was still

present within the river channel at all sites, and

D. aquaticus was often under rocks very close to the

water’s edge, which presumably allows them to avoid

desiccation. Thus, it is unlikely that any effects of size-

dependent refuge use on D. aquaticus population size

structure were occurring at the time of sampling.

Other size-specific sources of mortality that may

occur at drying river reaches include increased size-

specific predation as D. aquaticus and other terrestrial

predators congregate around the decreasing aquatic

habitat. Dolomedes aquaticus became aggregated as

sites on the Selwyn River dried. This may also have

occurred at the drying river reaches on the replicate

rivers because the river drying had not progressed far

enough to encourage spider aggregations. Spider

aggregations at the drying sites on the Selwyn River

disappeared only 1 day after the water returned,

possibly indicating that high small-scale densities

incur a cost, such as an increased likelihood of

intraspecific encounters and competition. Size-selec-

tive predation on large D. aquaticus by other predators

congregating around the drying pools of the river

may also lead to the low proportion and number of

large spiders found at drying reaches.

In conclusion, the changes in biomass and size class

structure of D. aquaticus found at river reaches that

flowed permanently, or had declining flow, were

probably related to a reduction in aquatic prey

abundance as the river dried. Desiccation ⁄ thermal

stress may also play a role in rivers that dry

completely, as D. aquaticus was shown to be prone

to desiccation mortality. Furthermore, the combina-

tion of aggregated spider distributions and low

aquatic food availability provided ideal conditions

for increased predatory or competitive interactions

between conspecifics. Predicted alterations to the

global climate, including an increased occurrence of

droughts in many areas (Arnell et al., 1996), mean that

understanding the impact of river low flows on

adjacent ecosystems is of considerable importance

(e.g. Harper & Peckarsky, 2006). Our results indicate

that the drying of rivers has serious consequences, not

only for the aquatic organisms but also for the

abundance and biomass of terrestrial consumers that

are supported (at least in part) by the river reach. The

nature of the drying regime, the relative dependence

of the consumer on aquatic prey and water, and its

scale of dispersal relative to the scale of drying will

largely determine specific consumer responses and

merits further investigation.

Acknowledgments

Jon Harding, Russell Death, Pete McHugh, Mary

Power, Mike Winterbourn and two anonymous

2074 M. J. Greenwood and A. R. McIntosh

� 2010 Blackwell Publishing Ltd, Freshwater Biology, 55, 2062–2076