Biological Invasions

The spread of species beyond their natural ranges due to human activities and the subsequent homogenization of the world’s biota is of interest to many ecologists. In some cases, the “invaders” may be native species expanding their ranges as a result of anthropogenic climate change. As far as humans are concerned, the net consequence of this biotic homogenization is under debate, since some invaders are highly destructive, while others can provide services in the form of food, raw materials, or compensation for extinct native species. We are interested in what influences invasion success and what impacts invasions have on native communities and ecosystems.

Invasions Cause Loss Of Oyster Reefs in California

Invasive non-native species may look a lot like their native counterparts, but they don’t always act like them, and their presence can be incredibly disruptive to normal predator-prey interactions within native food webs. This can have profound ecological consequences when the species most affected is a ‘foundation species’ that supports biodiversity by creating extensive habitat. Such is the case for the Olympia oyster Ostreola conchaphila, a native species in California estuaries that provides critical nursery habitat for a suite of species including crabs, anemones, and fishes. Even though these remnant reefs of Olympia oysters are normally preyed upon by native predatory snails, they are protected to some extent by the presence of native rock crabs that both consume native snails and force all others to spend more time hiding from crabs than eating oysters. Enter invasive snails and crabs that were accidentally introduced from the Atlantic to the Pacific coast, and the whole dynamic changes.

As a graduate student at UC Davis, I worked with the Grosholz lab to demonstrate that the presence of invasive species, which take the place of the native crabs and snails in the less saline parts of the estuary, completely destroyed the native oyster population. The root cause is the change in interactions between oysters, snails, and crabs. The invasive snail, also known as the Atlantic oyster drill, is unfamiliar with crab predators, and thus fails to avoid them as the native snails do.  Moreover, the invasive European green crab, which is smaller than the native crab, cannot effectively control the snail population. This artificial and devastating mis-match between predator and prey then allows high numbers of invasive snails to negatively impact the biological diversity of a pristine California estuary by consuming and eliminating critical oyster habitat. While the native predator-prey interactions that benefit oyster reefs are intricate and took a long time to develop, the invasive crab and snail do not have the historical exposure that is necessary to recreate these important interactions.

Like a long-married couple, native crabs and snails have figured out how to coexist, leaving oyster populations intact.  But the lack of experience that introduced crabs and snails have with each other led to the destruction of parts of the oyster population. In that respect, the study serves as a warning that predicting consequences of adding new species to our ecosystems will not be easy.

As human activities increasingly move species beyond their natural borders, predator-prey mismatches between native and exotic species may lead to further losses of critical habitat that support marine biodiversity and ecosystem function.

Kimbro, D. L. E. D. Grosholz, A. Baukus, N. Nesbitt, N. Travis, S. Attoe, and C. Coleman-Hulbert. 2009. Invasive species cause large-scale loss of native California oysters by disrupting trophic cacades. Oecologia 160: 563-575.

A Meta-analysis of Biotic Resistance in Marine Systems

Native predators provided a powerful explanation for why invasions were resisted from certain oyster reefs I studied in California, so I recently teamed back up with the Grosholz lab to search for generalities and trends in the strength of biotic resistance.

The success of invasions has been addressed by a diversity of hypotheses and ecological mechanisms (e.g. enemy release, novel weapons, invasional meltdown), but these different hypotheses all share a common outcome: either the native community biotically resists the invader, or it does not. Therefore, understanding the patterns and causes of biotic resistance is fundamental to a general understanding of invasion. We highlighted that under the common framework of biotic resistance, predictions from a diversity of invasion hypotheses can be linked.

To explore this question at a broader level, I used meta-analytic techniques in order to synthesize the results of 30 years of terrestrial and marine experiments on invasive species. This effort produced three valuable insights into how biological invasions are changing species interactions in food webs. First, I found that native marine competitors are not outcompeting autotrophic invaders unless the native autotrophic community is biologically diverse. Interestingly, this diversity effect is typically weaker in marine than in terrestrial systems. Second, although consumer pressure in non-invaded communities generally affects marine autotrophs more than terrestrial autotrophs, invasive autotrophs have altered this ecosystem distinction by promoting equally strong consumer pressure on land and in the sea. Third, within marine experiments, native species are more strongly and negatively affecting invasive invertebrates (through competition and consumption) than invasive autotrophs. Thus, the trophic attenuation of negative species interactions that generally occurs in native food webs still operates within invaded food webs. 

Kimbro, D.L., Cheng, B.S., Grosholz, E.D. (2013). Biotic resistance in marine environments. Ecology Letters 16(6): 821-833.

Temperature dependency of intraguild predation between native and invasive crabs

In this study, my graduate student (Tanya Rogers) and I examined how temperature affects the strength of competitive and predatory interactions between two co-occurring Atlantic crab species: the native blue crab (Callinectes sapidus) and the invasive green crab (Carcinus maenas). Blue and green crabs consume similar prey resources and large (but not small) blue crabs can consume green crabs, an interaction known as size-structured intraguild predation. In outdoor mesocosm experiments, we quantified interactions between blue crabs, green crabs, and shared prey (mussels) at 3 temperatures reflective of those across their range, using 2 size classes of blue crab. We found that green crabs had a competitive advantage at low temperatures, whereas blue crabs had a competitive advantage at high temperatures. Predation by blue crabs on green crabs also increased with temperature.  
To evaluate how these differences in competitive and predatory rates might influence the coexistence of these species across the temperature gradient spanning U.S. Atlantic coast, Tanya used a dynamical model of the blue crab-green crab interaction with parameter values taken from the mesocosm experiments. In the model, we found that green crabs were likely to competitively exclude blue crabs at low temperature, whereas blue crabs were likely to competitively and consumptively exclude green crabs at higher temperatures, particularly when resource productivities and rates of predation on green crabs were high. These results suggest that temperature-dependent interactions have the potential to influence local coexistence and are worth considering when developing mechanistic models of species distribution and predicting responses to environmental change.

Rogers, T. L., T. C. Gouhier, D. L. Kimbro. 2018. Temperature dependency of intraguild predation between native and invasive crabs. Ecology 99(4), 885-895.