DOI: 10.5061/dryad.ghx3ffbvh
title: Plant, grasshopper and spider food web responses to bird exclusion

description: Ecosystems are frequently considered to be controlled by predation
 (top-down). Experiments examined this in four
 bird/spider/grasshopper/prairie habitats over 34 years, employing in each
 habitat three 100 m2 bird exclosures and controls (121 habitat/year cases)
 where plant, grasshopper and spider abundances were measured. 
 Top-down control (plants decrease and grasshoppers increase with bird
 exclusion) was observed in only 13.2% of cases, while plants increased and
 grasshoppers decreased in 33.1% of cases, plants decreased and
 grasshoppers decreased in 25.6% of cases, and plants increased and
 grasshoppers increased in 28.1% of cases. Therefore, top-down control was
 not common and system responses were not constant, but varied among sites,
 years and directionally over time with climate change. This diversity of
 responses is expected given the variety of underlying processes in complex
 ecosystems.  For example, Decision Tree/Discriminant Analysis
 found that plant decreases and increases with bird exclusion were
 correctly identified in 78.3% of cases by grasshopper hatchling abundance,
 plant cover and annual net primary production (ANPP), while grasshopper
 decreases and increases with bird exclusion were correctly identified in
 76.7% of cases by edible plant biomass per grasshopper hatchling,
 grasshopper hatchling abundance and large grasshopper abundance. Analysis
 of other system-wide terrestrial trophic experiments indicates that the
 variety of responses observed by us over time and space may be common, so
 that system-wide trophic responses may, in general, be more variable than
 either top-down or bottom-up as often considered.
description: 1) AVIAN EXCLUSION – Birds are the major predator of grasshoppers
 in this system. a)  Avian exclosure and control
 areas were used at each site to measure the impact of avian predation
 (Joern 1986; Belovsky & Slade 1993).  Belovsky & Slade
 (1993) demonstrated the absence of experimental artifacts. b)  At each site, three 100
 m<sup>2</sup> areas have birds excluded using nylon avian
 netting (5 cm squares) and each was paired with a 100
 m<sup>2</sup> control area where birds had access. Exclosures
 and controls at each site were randomly located and erected in early June
 of each year. c) 
 In mid-September of each year, each exclosure and matching control area
 was surrounded by insect netting (1.25 m wide) and the avian netting was
 removed. The insect netting either contained escaping grasshoppers or
 forced them to fly high enough, so that their departure was
 observed. d) Two
 researchers with insect nets immediately caught grasshoppers and spiders
 (abdomen &gt; 5mm) in each exclosure and matching control for three –
 15-20 min periods with a 10 min period between each catch period. The
 grasshoppers and spiders were preserved in 70% ethyl alcohol for later
 identification to species (Brooks 1958; Brusven 1972; Handford 1946;
 Hebard 1928, 1934; Helfer 1963; Otte 1981, 1984; Pfadt 2002; Scoggin
 & Brusven 1972; Scott 2010) and sex. Large grasshoppers have an
 adult body mass &gt;500mg and small grasshoppers have an adult body
 mass &lt;250mg). e) Grasshopper density was
 estimated using a “catch-effort” technique (Southwood 1978).  The sum of
 all grasshoppers caught in an area prior to each 15-20 min catch period
 was the independent variable, and the number caught in the 15-20 min
 period was the dependent variable.  These 3-4 pairs of values for each
 area were used in a linear regression, where the regression’s x-intercept
 provided an estimate of population size with a coefficient of variation
 (CV observed: 5-20%). 2)  ENVIRONMENTAL TRAITS – After
 censusing grasshoppers, these measures were made: a)  Live (green) plant biomass
 inside each exclosure and matching control area was measured by clipping
 the plants in five 0.1 m2 plots that are randomly located.  Clipped
 vegetation was separated between grasses and forbs, dried at
 60<sup>o</sup>C, and weighed. b)  Plant N content and solubility
 are measured on the dried clipped grass and forb samples.  N content was
 measured using micro-Kjeldahl methods in 1994 – 2000 (AOAC 1984) and
 combustion in an elemental analyzer (©Costech) after 2000 (Robertson et
 al. 1999). Solubility was determined by digestion in HCl and pepsin (an
 index of digestibility to grasshoppers: Belovsky and Slade
 1995). c) 
 Vegetation and bareground cover was measured using 25 toe-points
 (Daubenmire 1947) in each exclosure and matching control that were located
 randomly.
description: # Plant, grasshopper and spider food web responses to bird exclusion
 National Bison Range (NBR, MT U.S.A.) BIRD EXCLUSION METADATA: A. COLOR –
 1. Gray = measurement 2. Rose = Site A (UTM 706453 EAST 5249980 NORTH) 3.
 Light Blue = Site B (UTM 708205 EAST 5244024 NORTH) 4. Gold = Site C (UTM
 711317 EAST 5242500 NORTH) 5. Green = Site D (UTM 713570 EAST 5248100
 NORTH) B. YEAR – 1. Site A – experiments started in 1985 and every year
 thereafter except 1988 through 2019. 2. Site B – experiments started in
 1989 and every year thereafter through 2019. 3. Site C – experiments
 started in 1990 and every year thereafter through 2019. 4. Site D –
 experiments started in 1994 and every year thereafter through 2019. C.
 DATA – 1. INITIAL GRASSHOPPERS – estimate of grasshopper hatchling density
 starting the population for the site and year (Belovsky and Slade 2024).
 2. TOTAL GRASSHOPPERS – estimate of total grasshoppers sampled in
 exclosure and matching control. 3. LARGE GRASSHOPPERS – estimate of
 large-size grasshoppers (adults &gt; 500 mg) sampled in exclosure and
 matching control. 4. MEDIUM GRASSHOPPERS – estimate of medium-size
 grasshoppers (adults &lt; 500 mg and &gt; 250 mg) sampled in exclosure and
 matching control. 5. SMALL GRASSHOPPERS – estimate of small-size
 grasshoppers (adults &lt; 250 mg) sampled in exclosure and matching
 control. 6. SPECIES RICHNESS – number of grasshopper species caught in the
 exclosures and controls. 7. SPECIES DIVERSITY – Shannon diversity index
 for exclosures and controls. 8. SPIDERS – number of spiders (&gt;5 mm
 abdomen) caught in exclosure and matching control. 9. BIRDS – passerine
 bird density reported by U.S.F.W.S. for the NBR in a year (vacant cells
 reflect years in which U.S.F.W.S. did not survey birds). 10. ANPP – annual
 net primary production reported for the site and year (Belovsky and Slade
 2020). 11. PLANT BIOMASS – estimate of plant abundance in the exclosure
 and matching control. 12. EDIBLE BIOMASS – product of PLANT BIOMASS and
 its solubilty in the exclosure and matching control. 13. COVER – plant
 cover (toe points) in the pen and matching control.