Abstract
Nutrient availability in ecosystems has increased dramatically over the last century. Excess reactive nitrogen deposition is known to negatively impact plant communities, e.g. by changing species composition, biomass and vegetation structure. In contrast, little is known on how such impacts propagate to higher trophic levels. To evaluate how nitrogen deposition affects plants and herbivore communities through time, we used extensive databases of spatially explicit historical records of Dutch plant species and Orthoptera (grasshoppers and crickets), a group of animals that are particularly susceptible to changes in the C:N ratio of their resources. We use robust methods that deal with the unstandardized nature of historical databases to test whether nitrogen deposition levels and plant richness changes influence the patterns of richness change of Orthoptera, taking into account Orthoptera species functional traits. Our findings show that effects indeed also propagate to higher trophic levels. Differences in functional traits affected the temporal-spatial dynamics of assemblages of Orthoptera. While nitrogen deposition affected plant diversity, contrary to our expectations, we could not find a strong significant effect of food related traits. However we found that species with low habitat specificity, limited dispersal capacity and egg deposition in the soil were more negativly affected by nitrogen deposition levels. Despite the lack of significant effect of plant richness or food related traits on Orthoptera, the negative effects of nitrogen detected within certain trait groups (e.g. groups with limited disperse ability) could be related to subtle changes in plant abundance and plant quality. Our results, however, suggest that the changes in soil conditions (where many Orthoptera species lay their eggs) or other habitat changes driven by nitrogen have a stronger influence than food related traits. To fully evaluate the negative effects of nitrogen deposition on higher trophic levels it is essential to take into account species life-history traits.
Highlights
With the expansion of agriculture and increased use of fossil fuels, reactive nitrogen availability in ecosystems has increased dramatically over the last century [1]
Between 1956–1970 (P1) and 1976–1990 (P2) pronounced plant richness increases were detected at finer scales and no changes were detected at country level
When looking in more detail to the fine scale (10 km) changes in richness, our results show that increases in richness found between P2 and P3 were negatively related with ammonia
Summary
With the expansion of agriculture and increased use of fossil fuels, reactive nitrogen availability in ecosystems has increased dramatically over the last century [1]. Nutrient enrichment of ecosystems may cause the decline or increase of plant species through direct toxic effects, changes in their abilities to compete for light and changes in their susceptibility to secondary stress and disturbance [3]. Such changes become visible in the vegetation in two ways: changes of species composition and changes in vegetation structure, biomass and density. [4,5], little information is available on how such changes propagate through trophic levels Higher trophic levels, such as herbivores and predators, may be impacted via the loss of plant species, via changes in vegetation structure or, in the case of herbivores via changes in food quality. Elevated nitrogen deposition can impact food quality by changing the internal C:N balance of plants which in turn is known to affect both the nutritional value to herbivores as well as the level of plant defence
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