Spatially variable habitat quality contributes to within-population variation in reproductive success

Abstract

Variation in habitat quality is common across terrestrial, freshwater, and marine habitats. We investigated how habitat quality influenced the reproductive potential of mud crabs across 30 oyster reefs that were degraded to different extents. We further coupled this field survey with a laboratory experiment designed to mechanistically determine the relationship between resource consumption and reproductive performance. We show a >10-fold difference in average reproductive potential for crabs across reefs of different quality. Calculated consumption rates for crabs in each reef, based on a type II functional response, suggest that differences in reproductive performance may be attributed to resource limitation in poor quality reefs. This conclusion is supported by results of our laboratory experiment where crabs fed a higher quality diet of abundant animal tissue had greater reproductive performance. Our results demonstrate that spatial variation in habitat quality can be a considerable contributor to within-population individual variation in reproductive success (i.e., demographic heterogeneity). This finding has important implications for assessing population extinction risk.

Keywords: Demographic heterogeneity; Oyster reef; Panopeus herbstii; habitat quality; individual variation.

Figure 1

Oyster reef in North Inlet, South Carolina where this study was conducted. Inset: Mud crab Panopeus herbstii.

Figure 2

The density of small bivalves (oysters and mussels <4 cm shell length) increased with oyster reef height.

Figure 3

Distribution of carapace widths for large crabs (>16 mm CW) across reefs of different height, demonstrating that larger crabs generally resided in taller reefs.

Figure 4

The number of reproductive (gravid+vitellogenic) crabs increased with the density of small bivalves (part A) and decreased with time throughout our sampling period (part B).

Figure 5

Average reproductive potential (gonado-hepatosomatic index, GHSI) of all large crabs (>16 mm CW) in a reef increased with the density of small bivalves in that reef. Circle size indicates the relative number of crabs captured in each reef.

Figure 6

Increasing consumption rate as a function of small bivalve density calculated using a type II functional response and published empirically determined search efficiencies and handling times for this system.

Figure 7

Total calculated egg production by all mature crabs across 30 sampled reefs as a function of reef height. Symbols represent calculated consumption rates given in Figure6, rounded down to the nearest whole bivalve consumed per day (square = 0, circle = 1, triangle = 2, cross = 3, × = 4, and diamond = 5 per day). Symbol size represents the relative number of large crabs captured on each reef. Inset: Relationship between the mean number of eggs per crab and the variance in the number of eggs per crab at each reef. Circle size is relative number of large crabs captured on each reef (symbol sizes on main figure and in inset figure are at different scales).

Figure 8

Asymptotically increasing reproductive potential (GHSI) with the average daily amount of animal tissue consumed per gram of crab in a 10-week laboratory feeding experiment. Triangles represent three crabs that molted during the experiment.