Functional consequences of realistic biodiversity changes in a marine ecosystem

Abstract

Declines in biodiversity have prompted concern over the consequences of species loss for the goods and services provided by natural ecosystems. However, relatively few studies have evaluated the functional consequences of realistic, nonrandom changes in biodiversity. Instead, most designs have used randomly selected assemblages from a local species pool to construct diversity gradients. It is therefore difficult, based on current evidence, to predict the functional consequences of realistic declines in biodiversity. In this study, we used tide pool microcosms to demonstrate that the effects of real-world changes in biodiversity may be very different from those of random diversity changes. Specifically, we measured the relationship between the diversity of a seaweed assemblage and its ability to use nitrogen, a key limiting nutrient in nearshore marine systems. We quantified nitrogen uptake using both experimental and model seaweed assemblages and found that natural increases in diversity resulted in enhanced rates of nitrogen use, whereas random diversity changes had no effect on nitrogen uptake. Our results suggest that understanding the real-world consequences of declining biodiversity will require addressing changes in species performance along natural diversity gradients and understanding the relationships between species' susceptibility to loss and their contributions to ecosystem functioning.

Fig. 1.

Seaweed species composition in tide pools containing different numbers of macroalgal species. For each level of richness, the proportion of pools containing each species is indicated by the size of the dot. Changes in tide pool seaweed species composition were nonrandom; low-diversity (one to two species) tide pools contained predominately Cladophora, whereas all seven species were commonly found in high-diversity (six to seven species) pools.

Fig. 2.

Influences of random versus nonrandom diversity changes on the relationship between species richness and ammonium use by experimental assemblages of tide pool seaweeds. Values are ammonium uptake coefficients (L h⁻¹ g⁻¹). Open circles show the monoculture uptake coefficients for all species, filled circles show the values for our experimental assemblages, and dotted horizontal lines indicate the weighted averages of all seven monocultures. (a) Realistic, nonrandom increases in species richness were associated with enhanced use of water-column ammonium (F_1,283 = 71.4, P < 0.001), and uptake of the single-species experimental assemblages was lower than the average of the seven monocultures (P = 0.014). (b) In contrast, there was no relationship between diversity and ammonium uptake when assemblages were composed of random sets of species (F_1,282 = 0.3, P = 0.619), and uptake values of experimental assemblages were never different from the average of the seven monocultures (P > 0.239). Error bars are ±1 standard error.

Fig. 3.

Effects of random versus nonrandom diversity changes on the relationship between species richness and ammonium use by model assemblages of tide pool seaweeds. Values are ammonium uptake coefficients (liter h⁻¹ g⁻¹). Open circles show the monoculture uptake coefficients for all species, filled circles show the values for the model assemblages, and dotted horizontal lines indicate the average uptake of the seven monocultures. Uptake coefficients of model assemblages were calculated as the average uptake of the constituent seaweed species. Results are based on 500 assemblages at each level of richness. (a) When assemblages were constrained based on field survey results, increases in species richness were associated with enhanced use of water-column ammonium (χ² = 2542.13, P < 0.001). (b) In contrast, there was no relationship between diversity and ammonium uptake when assemblages were composed of random sets of species (χ² = 1.86, P = 0.173).