Plant, grasshopper and spider food web responses to bird exclusion

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.

Methods
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 m2 areas have birds excluded using nylon avian netting (5 cm squares) and each was paired with a 100 m2 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 > 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 >500mg and small grasshoppers have an adult body mass <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 60oC, 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.