An anthropogenic habitat within a suboptimal colonized ecosystem provides improved conditions for a range-shifting species

Abstract

Many species are shifting their ranges in response to the changing climate. In cases where such shifts lead to the colonization of a new ecosystem, it is critical to establish how the shifting species itself is impacted by novel environmental and biological interactions. Anthropogenic habitats that are analogous to the historic habitat of a shifting species may play a crucial role in the ability of that species to expand or persist in suboptimal colonized ecosystems. We tested if the anthropogenic habitat of docks, a likely mangrove analog, provides improved conditions for the range-shifting mangrove tree crab Aratus pisonii within the colonized suboptimal salt marsh ecosystem. To test if docks provided an improved habitat, we compared the impact of the salt marsh and dock habitats on ecological and life history traits that influence the ability of this species to persist and expand into the salt marsh and compared these back to baselines in the historic mangrove ecosystem. Specifically, we examined behavior, physiology, foraging, and the thermal conditions of A. pisonii in each habitat. We found that docks provide a more favorable thermal and foraging habitat than the surrounding salt marsh, while their ability to provide conditions which improved behavior and physiology was mixed. Our study shows that anthropogenic habitats can act as analogs to historic ecosystems and enhance the habitat quality for range-shifting species in colonized suboptimal ecosystems. If the patterns that we document are general across systems, then anthropogenic habitats may play an important facilitative role in the range shifts of species with continued climate change.

Keywords: Aratus pisonii; climate change; habitat analog; mangrove; salt marsh; thermal habitat.

Figure 1

Map of the location of study sites, northernmost Aratus pisonii (Riley, Johnston et al., 2014), and northernmost black (Avicennia germinans) and red (Rhizophora mangle) mangroves (Williams, Eastman et al., 2014; Williams, Lundholm et al., 2014). The map also displays a point delineated as the extent of the mangrove‐dominated ecosystem. While the transition from mangrove to salt marsh exists as a mosaic‐like ecotone, this location represents an area with roughly 50:50 mangrove:salt marsh coverage (Rodriguez, Feller, & Cavanaugh, 2016; IC Feller pers. com.). North of this line, mangroves can still be found but are progressively more isolated and exist as individuals or small patches within a salt marsh‐dominated ecosystem

Figure 2

The proportion of time spent feeding ± SE by Aratus pisonii in the mangrove, salt marsh, and dock habitats before and after slack high tide. Groups that are significantly different are denoted by different letters

Figure 3

(a) Box plots comparing the proportion of time spent in sun by Aratus pisonii between the three habitats. Groups that are significantly different are denoted by different letters. In each box plot, and in all other box plots represented in this paper, the median is represented by a heavy line, the box represents the upper and lower quartiles, while the whiskers represent 95% of the data and circles show outliers. (b) Thermal logger data of loggers placed in the shade under a dock (dashed line) and in the open in the salt marsh (solid line) from 8 September 2016 to 11 September 2016

Figure 4

(a) Average body temperature ± SE of crabs in each habitat. Groups that are significantly different are denoted by different letters. (b) Differences between average crab body temperature and ambient air temperature ± SE in each habitat. Groups that are significantly different are denoted by different letters

Figure 5

Box plots showing the gut fullness of Aratus pisonii by tidal period in the mangrove, salt marsh, and dock habitats. Groups that are significantly different are denoted by different letters

Figure 6

Box plots comparing the gut‐width:carapace‐width ratios of Aratus pisonii between the mangrove, salt marsh, and dock habitats. Groups that are significantly different are denoted by different letters. A lower gut‐width:carapace‐width ratio suggests a relatively higher proportion of animal material in the long‐term diet of the individual

Figure 7

Box plots comparing the investment in long‐term energy storage, calculated as hepatosomatic index, of male, gravid female, and nongravid female Aratus pisonii between the three habitats. Groups that are significantly different are denoted by different letters