Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol77.Issue3.Feb2000:

(1)

Post-dispersal weed seed predation in Michigan crop fields as a function

of agricultural landscape structure

Fabián D. Menalled

a,∗

_{, Paul C. Marino}

_{, Karen A. Renner}

_{, Douglas A. Landis}

a a_{Department of Entomology and Pesticide Research Center, Michigan State University, East Lansing, MI 48824-1311, USA}

b_{Department of Biology, University of Charleston, Charleston, SC 29424-0001, USA} c_{Department of Crop and Soil Science, Michigan State University, East Lansing, MI 48824-1325, USA}

Received 13 October 1998; received in revised form 11 May 1999; accepted 10 June 1999

Abstract

Weed seed predation by invertebrates and vertebrates was compared between a simple (large crop fields embedded in a matrix of widely scattered woodlots and hedgerows) and a complex (small crop fields embedded in a matrix of numerous hedgerows and woodlots) agricultural landscape in southern Michigan. The structural differences between landscapes were evaluated by analysis of aerial photographs and digital land-use data. Seed predation experiments were conducted in four conventional tillage corn (Zea mays L.) fields within each landscape type. Trials included four common agricultural weed species, i.e., crabgrass (Digitaria sanguinalis), giant foxtail (Setaria faberii), pigweed (Amaranthus retroflexus), and velvetleaf

(Abutilon theophrasti). Treatments to exclude vertebrates, invertebrates+vertebrates and no exclusion were established at

27 m from hedgerows. Fields in the complex landscape were 75% smaller, had 63% more wooded perimeter, and 81% more wide hedgerow perimeter than fields in the simple landscape. Fields in the simple landscape were surrounded mainly by herbaceous roadside and crops, whereas the complex landscape had fields surrounded primarily by wide hedgerows. In both the landscape types there was considerable post-dispersal weed seed removal with a tendency towards higher removal rates in the complex landscape. Although there were no differences in the rate of seed removal among the four weed species, seed predation showed a high degree of variability within and among fields. ©2000 Elsevier Science B.V. All rights reserved.

Keywords: Agroecosystems; Landscape structure; Weed biocontrol; Post-dispersal seed predation; Southern Michigan

1. Introduction

In many agricultural landscapes herbicide use may result in ground and surface water contamination and the development of herbicide-resistant weeds. Thus, profitable crop production with little or no herbicide use could be desirable and a reappraisal of alterna-tive weed management strategies is needed. These alternative strategies include: delaying weed

emer-∗_{Corresponding author. Tel.: 517-432-5282; fax: 517-353-5598.}

E-mail address: menalled@pilot.msu.edu (F.D. Menalled).

gence, reducing seedling densities, reducing resource consumption, shifting of weed species to less com-petitive species, and reducing weed seed production and survival (Aldrich, 1984). This paper explores the last of these strategies, seed survival, as affected by post-dispersal weed seed predation. Specifically, it as-sess the influence of agricultural landscape complexity on weed seed removal by vertebrates and invertebrate predators.

The effect of weed seed predation in agricultural fields can be examined at several spatial scales. On a within-field scale, seed predation could be compared

(2)

between crop field edges and crop field interiors (Marino et al., 1997), among years (Cardina et al., 1996), or as a function of management practices (Brust and House, 1988). At the within-field scale, Marino et al. (1997) found the effect of post-dispersal seed predators to be patchy and not consistently re-lated to the relative location of hedgerows. This led Marino et al. (1997) to suggest that future studies of seed predation in agroecosystems should evaluate weed seed predation at a larger scale of analysis.

In agricultural ecosystems, most potential post- dis-persal seed predators such as small mammals (Pol-lard and Relton, 1970; Castrale, 1987), birds (Lewis, 1969; Best, 1983) and insects (Thomas et al., 1991, 1992) are found in non-crop habitats. Carabid bee-tles, which are important seed consumers in temperate agroecosystems (Johnson and Cameron, 1969; Best and Beegle, 1977; Lund and Turpin, 1977; Kjellsson, 1985; Brust and House, 1988; Manley, 1992) are also known to use non-crop habitats as over-wintering sites (Desender, 1982; Sotherton, 1984, 1985; Wallin, 1985; Thomas et al., 1991, 1992; Lys and Nentwig, 1992; Lys et al., 1994; Zangger et al., 1994). Because of the linkage between seed predators and non-crop habi-tat, it follows that the relative abundance of non-crop habitats in an agricultural landscape may have an ef-fect on weed seed predation within crop fields. As such, it should be expected that predation on weed seeds would be lower in simplified agricultural land-scapes than in complex agricultural landland-scapes. This study assesses weed seed removal by seed predators in Michigan maize fields at the landscape scale. It com-pares weed seed loss in complex agricultural land-scapes (small crop fields embedded in a matrix of nu-merous hedgerows and woodlots) with that in sim-ple agricultural landscapes (large crop fields embed-ded in a matrix of widely scattered woodlots and hedgerows).

2. Methods

2.1. Landscape and hedgerow characterization

The 3.2 by 12.9 km study region was located in Onondaga Township, Ingham County, Michigan, USA (42◦25′30′′N, 84◦29′00′′W). This area was se-lected because it encompassed a gradient between

two typical agricultural landscapes of southern Michi-gan and was previously used by Marino and Landis (1996) and Menalled et al. (1999b) to analyze the influence of landscape structure on insect parasitism

and parasitoid diversity. The southernmost 3.2 km2

contains a highly heterogeneous mixture of crop and non-crop habitats (hereafter ‘complex landscape’).

The northernmost 3.2 km2is a homogenous area with

low crop diversity and few non-crop habitats (here-after ‘simple landscape’). The central area comprised a transitional landscape.

Landscape structure was characterized and quan-tified using black and white aerial photographs (1 : 2000) and digital land-use data. Photos were taken on 12 June 1988 and were scanned at 150 dpi (dots per inch) and analyzed with ERDAS (Earth Re-source Data Analysis System) 7.5 (ERDAS, Atlanta, Georgia, USA). All agricultural fields within each landscape were identified and 30 fields from each landscape type selected randomly for more intensive analysis. The following attributes of each selected field were measured: area, perimeter, distance from the center of the field to the closest field edge, num-ber of edge types per field, area to perimeter index, percentage of field perimeter comprising late succes-sional habitats (woodlots, wide hedgerows [>10 m], narrow hedgerows [5–10 m], shrublands, and old fields) and percentage of field perimeter comprising early successional habitats (herbaceous roadside and crops) and residential areas. Digital land use data from the Michigan Department of Natural Resources Inventory System (MIRIS) were used to obtain an overall evaluation of land use patterns at a multi-field scale. Using MIRIS data, the percentage of crop-land and deciduous habitats was determined for each landscape type and contrasted between landscapes. Analysis of differences between landscape types was conducted using t-tests with significance levels corrected using a sequential Bonferroni adjustment (Holm, 1979; Rice, 1988). Further details used to assess the structure of these landscapes can be found in Marino and Landis (1996) and Menalled et al. (1999b).

2.2. Seed removal experiments

Four conventional tillage corn (Zea mays L.) fields in the complex and four conventional tillage maize

(3)

fields in the simple landscape were chosen for the seed removal experiments. Fields were selected to repre-sent typical cropping systems within each landscape. Because of the labor-intensive nature of this study it was not possible to replicate simple and complex land-scapes. Also, crop rotation impeded the temporal repli-cation of this study. Therefore, conclusions regarding landscape influences on weed seed removal are limited to the areas under study.

Four agricultural weed species commonly found in Michigan were used to examine the effect of agricultural landscape complexity on post-dispersal seed removal: crabgrass (Digitaria sanguinalis L. [Scop.]), giant foxtail (Setaria faberii Herrm.), pig-weed (Amaranthus retroflexus L.), and velvetleaf (Abutilon theophrasti Medicus). Species used in this study were chosen to represent a wide range

of seed size (seed size: D. sanguinalis=2–2.2 mm,

S. faberii=2.5–3 mm, A. retroflexus=1–1.2 mm, A.

theophrasti=3–3.5 mm; Radford et al., 1968) and morphology (Davis, 1993). Three treatments were employed to quantify seed predation: (1) no ex-closure, allowing both vertebrate and invertebrates to remove seeds (2) vertebrate exclosures, which prevented vertebrates from removing seeds but al-lowed invertebrates to remove them, and (3)

ver-tebrate+invertebrate exclosure, which prevented

both vertebrates and invertebrates from removing seeds and were used to determine unknown losses of seeds. Vertebrate exclosures were constructed with cages of 1.25 sq. cm mesh rigid hardware cloth

(34 cm×34 cm×7 cm) (length×width×height)

sunk 3 cm into the soil. Vertebrate+invertebrate

exclosures consisted of vertebrate exclosure cages enclosing plastic rings (28 cm diameter, 5 cm high) sunk 3 cm into the ground. Rings were painted with FluonTM, a slick material that prevents invertebrates from climbing the barrier and excludes them from reaching the seeds placed within rings (Mittelbach

and Gross, 1984). Vertebrate+invertebrate

exclo-sures were utilized to estimate recovery efficiency and unknown sources of seed losses such as trans-portation to and from the fields. For each treatment, 50 seeds of each species were placed on separate

11 cm×14 cm×0.5 cm (length×width×height)

waterproof pads (3M Metallic Finishing Pad). Seed bank density in Michigan corn-soybean-wheat fields ranges between 1873 seeds m−2and 5000 m−2Renner

et al., 1998). Fifty seeds were placed on each pad (3246 seeds m−2) to resemble natural occurring seed densities. Our laboratory observations indicated that invertebrates walk freely on pads and that seeds were concealed in the rough surface of the pads, resem-bling the situation observed for freshly shed seeds on soil. Pads were placed with one side flush with the soil surface and were used to reduce seed losses from wind and facilitate recovery of uneaten seeds. Each treatment was covered with a clear plastic roof to reduce seed losses from rain. In each field, one repetition of each species-treatment combination was established at 27 m from the center of each one of three randomly chosen field edges. This distance was chosen because this was the distance from the field edge to the center in the smallest field. The order of the four species and three treatments within each repetition was completely randomized with sample units 2 m apart. Thus, in each field, 36 sample units (four species, three treatments, and three repetitions) were established using a total of 14,400 seeds in each trial.

Field experiments were done twice during the period corresponding to the peak of abundance of potential invertebrate seed predators such as carabid beetles (Kirk, 1973). This period is usually associ-ated with the period of natural weed seed production and dispersal. The first experiment was started on 3 September, 1996 and the second was started on 23 September, 1996. During the first trial, weather conditions were dry with no heavy rains or winds, whereas it rained during the first 2 days of the second trial. Seeds were left in the field for 1 week, recov-ered and the number of seeds remaining on all pads was determined in the laboratory. Because seed coats and rodent fecal pellets were observed on a high proportion of pads, seed removal was assumed to be primarily due to predation (Schupp and Frost, 1989; Myster and Pickett, 1993). Results were analyzed using a four factor (landscape, field, species, and treatment) ANOVA model with fields nested within landscapes and proportion of seeds removed as the dependent variable using Proc GLM, SAS software (SAS Institute, 1996). To normalize the data and to increase homoscedasticity, the proportion of seeds removed was modified using the arcsin Freeman and Tukey transformation prior the ANOVA (Sokal and Rohlf, 1995).

(4)

3. Results

3.1. Landscape characterization

The selected simple and complex agricultural landscapes differed in several key structural vari-ables. The simple landscape contained more farmland (71.4% versus 59.4%) and slightly less decidu-ous habitat (11.2% versus 14.3%) than the com-plex landscape. In the 3.2 km2 representative areas of the simple and complex landscape there were a total of 61 and 139 crop fields, respectively. A random sample of 30 fields per landscape revealed that fields in the complex landscape had statisti-cally smaller area (x±1 SD, simple: 12.4±10.3 ha;

complex: 3.4±3.1 ha; t=4.62, p<0.0001), less

perimeter (x±1 SD, simple: 1638±720 m;

com-plex: 776±303 m; t=6.04, p<0.0001), shorter

dis-tance to field edge (x±1 SD, simple: 101±54 m;

complex: 63±39 m; t=3.16, p=0.003), and a

longer perimeter of wooded field edge per unit of field area (x±1 SD, simple: 8.7±19.1 m; complex:

23.5±24.4 m; t= −2.61, p=0.012), than those lo-cated within the simple landscape. Also, whereas the complex landscape had fields surrounded primarily by

wide hedgerows (mean% of field perimeter±1 SE,

simple: 3.5±1.7; complex: 18.7±5.2; t=3.16,

p<0.01), those from the simple landscape were

en-Table 1

Results of the nested ANOVA for the first trial testing the effect of landscape structure, fields nested within landscape, treatment, and species on the percentage post-dispersal seed removala

Source of Variation df SS F p

Landscape 1 613271.8 6.81 0.0349

Species 3 102029.9 1.24 0.3252

Treatment 2 15526652.9 140.43 0.0001

Landscape x Species 3 159826.8 1.94 0.1595

Landscape x Treatment 2 206493.9 1.87 0.1967

Species×Treatment 6 648959.1 4.85 0.0010

Landscape×Species×Treatment 6 238625.2 1.79 0.1299

Field(Landscape) 7 644987.4 3.51 0.0015

Treatment×Field(Landscape) 12 663993.0 2.10 0.0185

Species×Field(Landscape) 18 500278.9 1.06 0.3990

Species×Treatment×Field(Landscape) 36 802049.7 0.85 0.7158

Error 184 4837075.5

a_{Landscape was tested using Field(Landscape) as the error term, Species and Landscape}_×_{Species were tested using Species}_× Field(Landscapes) as error term, Treatment and Landscape×Treatment were tested using Treatment×Field(Landscapes) as the error term, and Species×Treatment and Landscape×Species×Treatment were tested using Species×Treatment× Field(Landscape) as the error term. Remaining terms were tested using the residual error term.

compassed mainly by herbaceous roadsides (mean% of field perimeter±1 SE, simple: 9.4±2.4; complex: 2.4±1.5; t= −2.48, p<0.05), and crops (mean% of

field perimeter±1 SE, simple: 28.4±5.0; complex: 13.4±3.9; t= −2.38, p<0.05).

3.2. Seed removal in crop fields

In both the landscapes and for all seed species, there was a significant difference among exclosure treat-ments (Tables 1 and 2). This indicates that cages effec-tively reduced seed removal and that there was consid-erable post-dispersal weed seed removal by both verte-brate and inverteverte-brate seed predators. Percentage seed removal was highest in the no exclosure treatment, intermediate in the vertebrate exclosure treatment, and lowest in the vertebrate+invertebrate exclosure treatment (Fig. 1). Analysis of the first and second trial showed no differences in the rate of seed removal among the four weed species (Tables 1 and 2). Al-though it was possible to observe a tendency towards higher removal rates in the complex landscape than in the simple landscape (Fig. 2), only in the first trial were these differences statistically significant (Tables 1 and 2). For both the trials, it was possible to detected significant differences among fields suggesting that within the simple and complex landscapes, fields may support different numbers of beneficial vertebrates

(5)

Table 2

Results of the nested ANOVA for the second trial testing the effect of landscape structure, fields nested within landscape, treatment, and species on the percentage post-dispersal seed removal

Source of Variationa _df _SS _F _p

Landscape 1 447105.9 2.01 0.1988

Species 3 132286.1 1.44 0.2649

Treatment 2 97008933.1 94.05 0.0001

Landscape×Species 3 66988.9 0.73 0.5487

Landscape×Treatment 2 152616.3 1.48 0.2667

Species×Treatment 6 465955.3 2.30 0.0558

Landscape×Species×Treatment 6 149155.9 0.74 0.6243

Field(Landscape) 7 1553891.3 6.75 0.0001

Treatment×Field(Landscape) 12 619394.7 1.57 0.1033

Species×Field(Landscape) 18 552312.4 0.93 0.5389

Species×Treatment×Field(Landscape) 36 1216744.2 1.03 0.4336

Error 186 6112466.5

a_{See Table 1 for explanation of the statistical tests.}

and invertebrates (Fig. 3). Moreover, in both

anal-yses there were significant species×treatment and

treatment×field interactions (Tables 1 and 2). An

analysis of the differential seed loss in the exclosure treatments suggests that invertebrates and vertebrates had no preferences for removing different species (Fig. 4). Fig. 4 also suggests that the significant species×treatment and treatment×field interactions were the result of lower recovery rates of D.

san-guinalis and A. retroflexus seeds from the

verte-Fig. 1. Percentage seed removed per day (mean±1 SE) per exclusion treatment averaged across field, species and landscape type.

brate+invertebrate exclosure treatment. The small

size of these two species seeds may have affected recovery efficiency.

4. Discussion

The main objective of this study was to determine whether agricultural landscape complexity influences removal of weed seeds from crop fields. A clear

(6)

Fig. 2. Percentage seed removed per day (mean±1 SE) per landscape type averaged across field, species and exclusion treatment.

difference in seed predation between the complex and simple landscapes was expected because of an-ticipated higher numbers of both vertebrate (Best, 1983, Castrale, 1987) and invertebrate (Desender, 1982; Sotherton, 1984, 1985; Wallin, 1985, 1986; Thomas et al., 1991, 1992; Lys and Nentwing, 1992; Lys et al., 1994; Zangger et al., 1994) seed predators in the complex landscape. As predicted, in the first trial significantly more seeds were removed in the

Fig. 3. Percentage seed removed per day (mean±1 SE) per field averaged across species and exclusion treatment.

complex landscape than the simple landscape. How-ever, in the second trial although there was a clear trend for higher seed removal in the complex land-scape, the proportion of seeds removed did not differ significantly between the two types of landscapes. Although seeds were protected with a plastic roof, heavy rainfall during the first two days of the second trial may have moved non-eaten seeds away from pads and resulted in being responsible for the lack of

(7)

statistical differences between landscapes. Because portion of southern Michigan has no extremely sim-ple agricultural landscapes (those consisting almost entirely of crop fields), results may simply reflect that a relatively simple to a relatively complex land-scape rather than a highly simple to a highly complex agricultural landscape was compared. It is logical to predict differences in seed removal to be more profound in agricultural landscapes that have more extreme differences in complexity.

Surprisingly and contrary to the patterns observed in previous studies (e.g. Borchert and Jain, 1978; Mittel-bach and Gross, 1984, Brust and House, 1988; Myster and Pickett, 1993), this study did not detect differences in the overall rate of seed predation among the four weed species. Seeds were left in the field for 7 days and high levels of post-dispersal seed removal were observed in both landscape types. It is possible that the high rates of seed predation detected in field con-ditions coupled with 1-week trials might have masked inter-specific differences in seed removal. Spatially, there was very high variation in weed seed removal

Fig. 4. Percentage seed removed per day and exclusion treatment (mean±1 SE) of each of the four weed species. Values averaged across landscape and field.

among fields, and the fields differed in the numbers of seeds removed by vertebrates and invertebrates. Part of the difficulty in detecting a landscape effect may be caused by the large amount of spatial variability in seed removal (Mittelbach and Gross, 1984; Dessaint et al., 1991; Thompson et al., 1991; Forcella et al., 1992; Marino et al., 1997). Concordantly, significant differ-ences among fields in the rate of seed removal were observed. As such, this study arrives at the same con-clusion as Marino et al. (1997) that the high variabil-ity in seed predation probably reflects realistic spatial and temporal variability in the foraging behaviors of seed predators in agroecosystems.

This study was not designed to identify preda-tors removing seeds and the actual list of predapreda-tors responsible for removal of weed seeds under field conditions is difficult to determine (Lys, 1995). Most seed predators are nocturnal (Cardina, et al., 1996) and our field observations indicated that the light necessary to make observations may interfere with normal feeding activities. Potential seed predators responsible for the observed results include

(8)

inverte-brates such as ground beetles, crickets, gastropods, millipedes and worms (Cardina et al., 1996; Carmona et al., 1999) and small vertebrates such as birds and rodents (Hulme, 1994, 1998). With some limitations, laboratory feeding studies may help to understand the causal relationship between field observations of predator abundance and weed removal (Clark et al., 1994; Menalled et al., 1999a). Future studies should complement laboratory-feeding trials with an evaluation of variations in predator abundance as a function of landscape complexity to provide with a mechanistic explanation of the observed tendencies,

Relatively few studies have documented seed pre-dation in agricultural systems (Pearson, 1964; Best and Beegle, 1977; Lund and Turpin, 1977; Brust and House, 1988; Manley, 1992; Cardina et al., 1996; Marino et al., 1997). Most have studied predation in deserts, forests, old-fields, or grasslands (e.g. Mit-telbach and Gross, 1984; Thompson, 1985; Louda, 1989; Louda et al., 1990; Crawley, 1992; Terborgh et al., 1993; Hulme, 1994; Boman and Casper, 1995; Notman et al., 1996). In non-agricultural ecosystems, the magnitude of seed predation ranges from 1 to 20% per day in undisturbed fields, and up to 60% per day in prairies (Crawley, 1992). Yet, even in these ecosys-tems, little is known about the degree to which seed predators influence the population dynamics of annual plants over the long term. In non-agricultural ecosys-tems, for example, it has been suggested that the ef-fect of seed predation is buffered by recruitment from the bank of dormant seeds or by the immigration of wind-borne propagules. Thus, quite large changes in the seed predation rate may have no measurable effect on plant recruitment (Crawley and Nachapong, 1985; Wellington and Noble, 1985). On the other hand, Maron and Simms (1997) reported that seed preda-tion by rodents exert a strong, but habitat-dependent influence on seed bank size and seedling recruitment. The extent to which seed bank recruitment and seed immigration affect weed population dynamics in agri-cultural setting is largely unknown. In a simulation analysis of crop rotation effects on weed seed banks, Jordan et al. (1995) demonstrated that winter survivor-ship in the upper-seed bank (0–10 cm) was the most influential parameter on green foxtail (S. viridis) and velvetleaf (A. theophrasti) population dynamics. The high rates of seed removal observed in both the com-plex and simple landscape suggests that seed

preda-tors may have a significant effect on weed population dynamics in cropping systems. This finding highlights the value of weed management measures to reduce the survival of overwintering seed, such as residue burn-ing or delayed tillage that expose seeds to high rates of mortality (such as through seed predation) on the soil surface (Louda, 1989; McFadyen, 1998). These techniques should be coupled with habitat manage-ment approaches that favor the establishmanage-ment of vi-able and effective natural enemy populations. There is growing evidence that increasing landscape complex-ity by including stable habitats such as natural vegeta-tion bordering cultivated areas, unplowed refuges and hedgerows interact with highly disturbed annual crop fields in determining the within-field diversity and abundance of natural enemies (Sotherton, 1984, 1985; Kajak and Lukasiewicz, 1994; Zangger et al., 1994; Vitanza et al., 1996). Clearly, in agricultural systems the importance of control strategies such as increased seed predation and conservation of viable communi-ties of seed predators require longer-term studies ad-dressing hypothesis at several scales of analysis.

Acknowledgements

We thank M. Haas, D. Carmona, J. J. Forester, and G. Hellmann for assistance in the field and R. Isaacs for his editorial comments and suggestions. A. Ziegler, of Michigan State University, Entomology Spatial Analysis Laboratory assisted us in the land-scape characterizations. We acknowledge S. Hawkins Jr., L. Eldred, and D. Swiler, for the use of their fields. This research was funded by a USDA SARE grant LWF 62-016-03508.

References

Aldrich, R.J., 1984. Weed-Crop Ecology: Principles in Weed Management. Breton Publishers, North Scituate, MA, pp. 399–435.

Best, L.B., 1983. Bird use of fencerows: Implications of contemporary fencerow management practices. Wild. Soc. Bull. 11, 343–347.

Best, R.L., Beegle, C.C., 1977. Food preferences of five species of carabids commonly found in Iowa cornfields. Environ. Entomol. 6, 9–12.

(9)

Boman, J.S., Casper, B.B., 1995. Differential postdispersal seed predation in disturbed and intact temperate forest. Am. Midl. Nat. 134, 107–116.

Borchert, M.I., Jain, S.K., 1978. The effect of rodent seed predation on four species of California annual grasses. Oecologia 33, 101–113.

Brust, G.E., House, G.J., 1988. Weed seed destruction by arthropods and rodents in low-input soybean agroecosystems. Am. J. Alter. Agric. 3, 19–35.

Cardina, J., Norquay, H.M., Stinner, B.R., McCartney, D.A., 1996. Postdispersal predation of velvetleaf (Abutilon theophrasti) seeds. Weed Sci. 44, 534–539.

Carmona, D. F., Menalled, F.D., Landis D.A., 1999. Northern field cricket, Gryllus pennsylvanicus Burmeister (Orthoptera: Gryllidae): laboratory weed seed predation and within field activity-density, Econ. Entomol. in press.

Castrale, J.S., 1987. Wildlife use of cultivated fields set aside under the payment of kind (PIK) program. Indiana Acad. Sci. 97, 173–180.

Clark, M.S., Luna, J.M., Stone, N.D., Youngman, R.R., 1994. Generalist predator consumption of armyworm (Lepidoptera: Noctuidae) and effect of predator removal on damage in no-till corn. Envrion. Entomol. 23, 617–622.

Crawley, M.J., 1992. Seed predators and plant population dynamics. In: Fenner, M. (Ed.), Seeds: the Ecology of Regeneration in Plant Communities. CAB International, Wallingford, pp. 157–191.

Crawley, M.J., Nachapong, M., 1985. The establishment of seedlings from primary and regrowth seeds of ragwort (Senecio

jacobaea). J. Ecol. 73, 255–261.

Davis, L.W., 1993. Weed Seeds of The Great Plains: A Handbook For Identification. Publisher Lawrence, Kansas.

Desender, K., 1982. Ecological and faunal studies on Coleoptera in agricultural land II. Hibernation of Carabidae in agro-ecosystems. Pedobiologia 23, 295–303.

Dessaint, F., Chadoeuf, R., Barralis, G., 1991. Spatial pattern analysis of weed seeds in the cultivated soil seed bank. J. App. Ecol. 28, 721–730.

Forcella, F.R., Wilson, G., Renner, K., Dekker, A.J., Harvey, R.G., Alm, D.A., Buhler, D.D., Cardina, J., 1992. Weed seedbanks of the US cornbelt: Magnitude, variation, emergence and application. Weed Sci. 40, 636–644.

Holm, S., 1979. A simple sequentially rejective multiple test procedure. Scandinavian J. Stat. 6, 65–70.

Hulme, P.E., 1994. Post-dispersal seed predation in grassland: Its magnitude and sources of variation. J. Ecol. 82, 645–652. Hulme, P.E., 1998. Post-dispersal seed predation and seed bank

persistence. Seed Sci. Res. 8, 513–519.

Johnson, H.E., Cameron, R.S., 1969. Phytophagous ground beetles. Ann. Ent. Soc. Am. 62, 909–914.

Jordan, N., Mortensen, D.A., Prenzlow, D.M., Curtis Cox, K., 1995. Simulation analysis of crop rotation effects on weed seedbanks. Am. J. Bot. 82, 390–398.

Kajak, A., Lukasiewicz, J., 1994. Do semi-natural patches enrich crop fields with predatory epigean arthropods?. Agr. Ecosyst. Env. 49, 149–161.

Kirk, V.M., 1973. Biology of ground beetle Harpalus pensylvanicus. Ann. Ent. Soc. Am. 66, 513–518.

Kjellsson, G., 1985. Seed fate in a population of Carex pilulifera L II. Seed predation and its consequences for dispersal and the seedbank. Oecologia 67, 424–429.

Lewis, T., 1969. The distribution of insects near a low hedgerow. J. Appl. Ecol. 6, 443–452.

Louda, S.M., 1989. Predation in the dynamics of seed generation. In: Leck, M.A., Parker,V.T., Simpson, R.L. (Eds.), Ecology of Soil Seed Banks. Academic Press, New York. pp. 25–51. Louda, S.M., Potvin, M.A., Collinge, S.K., 1990. Predispersal

seed predation, postdispersal seed predation , postdispersal seed predation and competition in recruitment of seedlings of a native thistle in sandhills prairie. Am. Midl. Nat. 124, 105–113. Lund, R.D., Turpin, F.T., 1977. Carabid damage to weed seeds

found in Indiana cornfields. Econ. Entomol. 6, 695–698. Lys, J., 1995. Observation of epigeic predators and predation on

artificial prey in a cereal field. Entomol. Exp. Appl. 75, 265– 272.

Lys, J.A., Nentwig, W., 1992. Augmentation of beneficial arthropods by strip-management. 4. Surface activity, movements, movements and activity density of abundant carabid beetles in cereal fields. Oecologia 92, 373–382.

Lys, J.A., Zimmerman, M., Nentwig, W., 1994. Increase of activity density and species number of Carabid Beetles in cereals as result of strip-management. Entomol. Exp. Appl. 73, 1–9. Maron, J.L., Simms, E.L., 1997. Effect of seed predation on seed

bank size and seedling recruitment of bush lupine (Lupinus

arboreus). Oecologia 111, 76–83.

McFadyen, R.E.C., 1998. Biological control of weeds. Ann. Rev. Entomol. 43, 369–393.

Manley, G.V., 1992. Observations on Harpalus pensiylvanicus (Coleoptera: Carabidae) in Michigan seed corn fields. News Michigan Entomol. Soc. 37, 1–2.

Marino, P.C., Landis, D.A., 1996. Effects of landscape structure on parasitoid diversity in agroecosystems. Ecol. Appl. 6, 276–284. Marino, P.C., Gross, K.L., Landis, D.A., 1997. Weed seed loss due to predation in Michigan maize fields. Agr. Ecosyst. Env. 66, 189–196.

Menalled, F.D, Lee, J., Landis, D.A., 1999a. Manipulating carabid beetle abundance alters prey removal rates in corn fields, Biocontrol, in press.

Menalled, F.D, Marino, P.C., Gage, S., Landis, D.A., 1999b. Does agricultural landscape structure affect parasitism and parasitoid diversity? Ecol. Appl., in press.

Mittelbach, G.G., Gross, K.L., 1984. Experimental studies of seed predation in old fields. Oecologia 65, 7–13.

Myster, R.W., Pickett, S.T.A., 1993. Effects of litter, distance, density, density and vegetation patch type on postdispersal tree seed predation in old fields. Oikos 66, 381–388.

Notman, E., Gorchov, D.L., Cornejo, F., 1996. Effect of distance, aggregation, and habitat on levels of seed predation for two mammal - dispersed neotropical rain forest tree species. Oecologia 106, 221–227.

Pollard, E., Relton, J., 1970. Hedges: V. A study of small mammals in hedges and uncultivated fields. J. Appl. Ecol. 7, 549– 557.

Pearson, O.P., 1964. Carnivore- mouse predation: an example of its intensity and bioenergetics. J. Mammalogy 45, 177–188.

(10)

Radford, A.E., Ahles, A.E., Bell, C.R., 1968. Manual of the Vascular Flora of the Carolinas. University of North Carolina Press, Chapel Hill, NC.

Renner, K.A., Halstead, S.J., Gross, K.L., 1998. Weed seed bank dynamics at the LTER (Long term ecological research) agroecosystems site. Abstracts of the 1998 Meeting of the Weed Science Society of America. Chicago, IL.

Rice, W.R., 1988. Analyzing tables of statistical tests. Evolution 43, 223–225.

SAS Institute, 1996. SAS/STAT User’s guide for personal computers, version 6th ed., vol. 2, SAS Institute, Cary, NC. Schupp, E.W., Frost, E.J., 1989. Differential predation of

Welfia-georgii seeds in treefall gaps and forest understory.

Biotropica 21, 200–203.

Sokal, R.R., Rohlf, F.J., 1995. Biometry, 3rd ed., W.H. Freeman, New York.

Sotherton, N.W., 1984. The distribution and abundance of predatory arthropods over wintering on farmlands. Ann. Appl. Biol. 105, 423–429.

Sotherton, N.W., 1985. The distribution and abundance of predatory Coleoptera over wintering in field boundaries. Ann. Appl. Biol. 105, 17–21.

Terborgh, J., Riley, E., Bolaños Riley, M.P., 1993. Predation by vertebrates and invertebrates on the seeds of five canopy tree species of the Amazonian forest. Vegetatio 107/108, 375– 386.

Thomas, M.B., Wratten, S.D., Sotherton, N.W., 1991. Creation of island habitats in farmland to manipulate populations of beneficial arthropods: Predator densities and emigration. J. Appl. Ecol. 28, 906–917.

Thomas, M.B., Wratten, S.D., Sotherton, N.W., 1992. Creation of island habitats in farmland to manipulate populations of beneficial arthropods: Predator densities and species composition. J. Appl. Ecol. 29, 524–531.

Thompson, J.N., 1985. Postdispersal seed predation in Lomatium spp. (Umbelliferae): variation among individuals and species. Ecology 66, 1608–1616.

Thompson, D.B., Brown, J.H., Spencer, W.D., 1991. Indirect facilitation of granivorous birds by desert rodents: Experimental evidence from foraging patterns. Ecology 72, 852–863. Vitanza, S., Sorenson, C.E., Bailey, W.C., 1996. Impact of warm

season grass strips on arthropod populations in Missouri cotton fields. Proc. Beltwide Cotton Conf. Memphis Tn. 1, 174–176. Wallin, H., 1985. Spatial and temporal distribution of some abundant carabid beetles (Coleoptera: Carabidae) in cereal fields and adjacent habitats. Pedobiologia 28, 19–34.

Wallin, H., 1986. Habitat choice of some field inhabiting carabid beetles (Coleoptera: Carabidae) studied by recapture of marked individuals. Ecol. Entomol. 11, 457–466.

Wellington, A.B., Noble, I.R., 1985. Seed dynamics and factors limiting recruitment of the mallee Eucalyptus incrassata Labill. in semi-arid, southeastern Australia. Australian J. Ecol. 73, 657–666.

Zangger, A., Lys, J.A., Nentwig, W., 1994. Increasing the availability of food and reproduction of Poecilus cupreus in a cereal field by strip-management. Entomol. Exp. Appl. 71, 111–120.

(1)

Table 2

Results of the nested ANOVA for the second trial testing the effect of landscape structure, fields nested within landscape, treatment, and species on the percentage post-dispersal seed removal

Source of Variationa _df _SS _F _p

Landscape 1 447105.9 2.01 0.1988

Species 3 132286.1 1.44 0.2649

Treatment 2 97008933.1 94.05 0.0001

Landscape×Species 3 66988.9 0.73 0.5487

Landscape×Treatment 2 152616.3 1.48 0.2667

Species×Treatment 6 465955.3 2.30 0.0558

Landscape×Species×Treatment 6 149155.9 0.74 0.6243

Field(Landscape) 7 1553891.3 6.75 0.0001

Treatment×Field(Landscape) 12 619394.7 1.57 0.1033

Species×Field(Landscape) 18 552312.4 0.93 0.5389

Species×Treatment×Field(Landscape) 36 1216744.2 1.03 0.4336

Error 186 6112466.5

a_{See Table 1 for explanation of the statistical tests.}

and invertebrates (Fig. 3). Moreover, in both

anal-yses there were significant species

×

treatment and

treatment

×

field interactions (Tables 1 and 2). An

analysis of the differential seed loss in the exclosure

treatments suggests that invertebrates and vertebrates

had no preferences for removing different species

(Fig. 4). Fig. 4 also suggests that the significant

species

×

treatment and treatment

×

field interactions

were the result of lower recovery rates of D.

san-guinalis and A. retroflexus seeds from the

verte-Fig. 1. Percentage seed removed per day (mean±1 SE) per exclusion treatment averaged across field, species and landscape type.

brate

+

invertebrate exclosure treatment. The small

size of these two species seeds may have affected

recovery efficiency.

4. Discussion

The main objective of this study was to determine

whether agricultural landscape complexity influences

removal of weed seeds from crop fields. A clear

(2)

Fig. 2. Percentage seed removed per day (mean±1 SE) per landscape type averaged across field, species and exclusion treatment.

difference in seed predation between the complex

and simple landscapes was expected because of

an-ticipated higher numbers of both vertebrate (Best,

1983, Castrale, 1987) and invertebrate (Desender,

1982; Sotherton, 1984, 1985; Wallin, 1985, 1986;

Thomas et al., 1991, 1992; Lys and Nentwing, 1992;

Lys et al., 1994; Zangger et al., 1994) seed predators

in the complex landscape. As predicted, in the first

trial significantly more seeds were removed in the

Fig. 3. Percentage seed removed per day (mean±1 SE) per field averaged across species and exclusion treatment.

complex landscape than the simple landscape.

How-ever, in the second trial although there was a clear

trend for higher seed removal in the complex

land-scape, the proportion of seeds removed did not differ

significantly between the two types of landscapes.

Although seeds were protected with a plastic roof,

heavy rainfall during the first two days of the second

trial may have moved non-eaten seeds away from

pads and resulted in being responsible for the lack of

(3)

statistical differences between landscapes. Because

portion of southern Michigan has no extremely

sim-ple agricultural landscapes (those consisting almost

entirely of crop fields), results may simply reflect

that a relatively simple to a relatively complex

land-scape rather than a highly simple to a highly complex

agricultural landscape was compared. It is logical

to predict differences in seed removal to be more

profound in agricultural landscapes that have more

extreme differences in complexity.

Surprisingly and contrary to the patterns observed in

previous studies (e.g. Borchert and Jain, 1978;

Mittel-bach and Gross, 1984, Brust and House, 1988; Myster

and Pickett, 1993), this study did not detect differences

in the overall rate of seed predation among the four

weed species. Seeds were left in the field for 7 days

and high levels of post-dispersal seed removal were

observed in both landscape types. It is possible that

the high rates of seed predation detected in field

con-ditions coupled with 1-week trials might have masked

inter-specific differences in seed removal. Spatially,

there was very high variation in weed seed removal

Fig. 4. Percentage seed removed per day and exclusion treatment (mean±1 SE) of each of the four weed species. Values averaged across landscape and field.

among fields, and the fields differed in the numbers of

seeds removed by vertebrates and invertebrates. Part

of the difficulty in detecting a landscape effect may

be caused by the large amount of spatial variability in

seed removal (Mittelbach and Gross, 1984; Dessaint et

al., 1991; Thompson et al., 1991; Forcella et al., 1992;

Marino et al., 1997). Concordantly, significant

differ-ences among fields in the rate of seed removal were

observed. As such, this study arrives at the same

con-clusion as Marino et al. (1997) that the high

variabil-ity in seed predation probably reflects realistic spatial

and temporal variability in the foraging behaviors of

seed predators in agroecosystems.

This study was not designed to identify

preda-tors removing seeds and the actual list of predapreda-tors

responsible for removal of weed seeds under field

conditions is difficult to determine (Lys, 1995). Most

seed predators are nocturnal (Cardina, et al., 1996)

and our field observations indicated that the light

necessary to make observations may interfere with

normal feeding activities. Potential seed predators

responsible for the observed results include

(4)

inverte-brates such as ground beetles, crickets, gastropods,

millipedes and worms (Cardina et al., 1996; Carmona

et al., 1999) and small vertebrates such as birds and

rodents (Hulme, 1994, 1998). With some limitations,

laboratory feeding studies may help to understand

the causal relationship between field observations

of predator abundance and weed removal (Clark et

al., 1994; Menalled et al., 1999a). Future studies

should complement laboratory-feeding trials with an

evaluation of variations in predator abundance as a

function of landscape complexity to provide with a

mechanistic explanation of the observed tendencies,

Relatively few studies have documented seed

pre-dation in agricultural systems (Pearson, 1964; Best

and Beegle, 1977; Lund and Turpin, 1977; Brust and

House, 1988; Manley, 1992; Cardina et al., 1996;

Marino et al., 1997). Most have studied predation in

deserts, forests, old-fields, or grasslands (e.g.

Mit-telbach and Gross, 1984; Thompson, 1985; Louda,

1989; Louda et al., 1990; Crawley, 1992; Terborgh

et al., 1993; Hulme, 1994; Boman and Casper, 1995;

Notman et al., 1996). In non-agricultural ecosystems,

the magnitude of seed predation ranges from 1 to 20%

per day in undisturbed fields, and up to 60% per day

in prairies (Crawley, 1992). Yet, even in these

ecosys-tems, little is known about the degree to which seed

predators influence the population dynamics of annual

plants over the long term. In non-agricultural

ecosys-tems, for example, it has been suggested that the

ef-fect of seed predation is buffered by recruitment from

the bank of dormant seeds or by the immigration of

wind-borne propagules. Thus, quite large changes in

the seed predation rate may have no measurable effect

on plant recruitment (Crawley and Nachapong, 1985;

Wellington and Noble, 1985). On the other hand,

Maron and Simms (1997) reported that seed

preda-tion by rodents exert a strong, but habitat-dependent

influence on seed bank size and seedling recruitment.

The extent to which seed bank recruitment and seed

immigration affect weed population dynamics in

agri-cultural setting is largely unknown. In a simulation

analysis of crop rotation effects on weed seed banks,

Jordan et al. (1995) demonstrated that winter

survivor-ship in the upper-seed bank (0–10 cm) was the most

influential parameter on green foxtail (S. viridis) and

velvetleaf (A. theophrasti) population dynamics. The

high rates of seed removal observed in both the

com-plex and simple landscape suggests that seed

preda-tors may have a significant effect on weed population

dynamics in cropping systems. This finding highlights

the value of weed management measures to reduce the

survival of overwintering seed, such as residue

burn-ing or delayed tillage that expose seeds to high rates

of mortality (such as through seed predation) on the

soil surface (Louda, 1989; McFadyen, 1998). These

techniques should be coupled with habitat

manage-ment approaches that favor the establishmanage-ment of

vi-able and effective natural enemy populations. There is

growing evidence that increasing landscape

complex-ity by including stable habitats such as natural

vegeta-tion bordering cultivated areas, unplowed refuges and

hedgerows interact with highly disturbed annual crop

fields in determining the within-field diversity and

abundance of natural enemies (Sotherton, 1984, 1985;

Kajak and Lukasiewicz, 1994; Zangger et al., 1994;

Vitanza et al., 1996). Clearly, in agricultural systems

the importance of control strategies such as increased

seed predation and conservation of viable

communi-ties of seed predators require longer-term studies

ad-dressing hypothesis at several scales of analysis.

Acknowledgements

We thank M. Haas, D. Carmona, J. J. Forester,

and G. Hellmann for assistance in the field and R.

Isaacs for his editorial comments and suggestions.

A. Ziegler, of Michigan State University, Entomology

Spatial Analysis Laboratory assisted us in the

land-scape characterizations. We acknowledge S. Hawkins

Jr., L. Eldred, and D. Swiler, for the use of their fields.

This research was funded by a USDA SARE grant

LWF 62-016-03508.

References